PAMELA's coming-out

Yesterday, PAMELA finally posted on ArXiv her results on the cosmic-ray positron fraction. In the last months there was a lot of discussion whether it is right or wrong to take photographs of PAMELA while she was posing. Here at CERN, people were focused on less philosophical aspects: a few weeks ago Marco Cirelli talked about the implications for dark matter searches, and Richard Taillet talked about estimating the positron background from astrophysical processes in our galaxy. Finally, PAMELA had her coming-out seminar two days ago. PAMELA is a satellite experiment that studies cosmic-ray positrons and anti-protons. She has a better energy reach (by design up to 300GeV, although the results presented so far extend only up to 100 GeV) and much better accuracy than the previous experiments hunting for cosmic anti-matter. Thanks to that, she was able to firmly establish that there is an anomaly in the positron flux above 10 GeV, confirming the previous hints from HEAT and AMS.

Here are the PAMELA positron data compared with the theoretical predictions. The latter assume that the flux is dominated by the secondary production of positrons due to collisions of high-energy cosmic rays with the interstellar medium. The two lines are almost perpendicular to each other :-). In fact, the discrepancy below 10 GeV is not surprising, and is interpreted as being due to solar modulation. It turns out that the solar wind modifies the spectrum of low-energy cosmic rays, and in consequence the flux depends on solar activity which changes in the course of the 22-years solar cycle. Above 10 GeV the situation is different, as solar modulation is believed to produce negligible effects. Even though the secondary production of positrons has large theoretical uncertainties, one expects that it decreases with energy. Such a power-law decrease has been observed in the flux of anti-protons who also may originate from secondary production. The positron fraction, instead, significantly increases above 10 GeV.

Thus, PAMELA shows that the secondary production is not the dominant source of high-energy positrons. The excess can be due to astrophysical sources, for example young near-by pulsars have been proposed as an explanation. But what makes particle physicists so aroused is that dark matter annihilation is a plausible explanation too. It might be that PAMELA is a breakthrough in indirect dark matter searches. It is less known that there are other experiments that see some excess in the cosmic ray flux. Most interestingly, two balloon-borne experiments called ATIC and PPB-BETS see an excess in the total electron+positron flux (they cannot distinguish the two) with a peak around 700 GeV. This adds to the EGRET gamma-ray excess at a few GeV, and to the WMAP haze - an excess of diffuse microwave background from the core of our galaxy.

A dark matter candidate that fits the PAMELA excess, must have rather unexpected properties. If the observed dark matter abundance has a thermal origin, the dark matter annihilation cross section naively seems too small to explain the observed signal. As usual, theorists have magic tricks to boost the annihilation rate today. One is using the so-called boost factor: if dark matter clumps, its density is locally higher than average, and then the average annihilation rate also increases with respect to the case of a uniform distribution. However, this does not save the day for the most popular dark matter candidates. For example, the MSSM neutralino typically requires a boost factor of order a few hundred, which is probably stretching the point. The latest trick is called the Sommerfeld enhancement: if the dark matter particle feels some attractive long range forces (other than electromagnetism, of course), a pair of particles may form a bound state, which enhances the annihilation rate.

Another challenge for particle models is the fact that PAMELA sees no excess in the antiproton flux. This means that the dark matter particle must be hadrophobic, that is to say, it should decay preferentially into leptons. Again, the most popular dark matter candidates, like the MSSM neutralino, do not satisfy this criterion. However, particle models compatible with the PAMELA data do exist, for example Minimal Dark Matter (though this one is not compatible with the ATIC/PPB-BETS peak), or recent Exciting Dark Matter.

So, it seems, we have to wait and see till the smoke clears up. Certainly, a single indirect detection signal has to be taken with all due scepticism (so many have died before). Only combined efforts of several experiments can lead to a convincing conclusion. As for the moment, if somebody pointed a gun to my face and made me choose one answer, I would probably go for an astrophysical explanation. On the other hand, if the PAMELA excess is really a manifestation of dark matter, the LHC could concentrate on more interesting issues than discovering and undiscovering the MSSM. It seems that astrophysicists have at least one more year to sort this thing out by themselves.

It's All Decay

Contrary to what you might think from the title, this is not about Christmas. This is yet another PAMELA/ATIC related post. If it continues like that I'll have to rename the blog from Resonaances to Bumps.

By now you know the story too well: PAMELA and ATIC have observed an excess of cosmic-ray positrons that may are may not be a manifestation of TeV scale particles that constitute dark matter. Most of the subsequent theoretical activity was focused on explaining the signal via dark matter annihilation. However, there has also been a number of papers pursuing a different scenario in which the dark matter particle is unstable, and the excess positrons are produced while it decays. In fact, there are quite good reasons, both theoretical and phenomenological, to seriously consider this possibility.

On the theoretical side, there are some difficulties with the annihilation scenario. If the dark matter is a thermal relic, the inferred annihilation rate is some 100 times too small to explain the positron excess. In order to boost the annihilation rate today one needs some awkward modeling. One possibility is to assume that the dark matter distribution today is very inhomogeneous, and the average annihilation rate is boosted due to the existence of high-density regions. Another trick is to cook up new forces mediated by some mysterious 1 GeV particles. Of course, one could also drop the assumption of thermal equilibrium and invoke some non-thermal mechanism to explain the present dark matter abundance, in which case the annihilation rate can be high enough.

But who cares about theory? In the end, witty theorists can always find a way out. Indeed, the approach called I-will-fit-PAMELA-to-my-cherished-model-against-all-odds is very popular these days. However, there are purely phenomenological reasons to reserve some skepticism toward the annihilation scenario. That's what can be inferred from the recent paper by Bertone and al.

The observation is that production of 1 TeV electrons and positron inevitably leads to production of high-energy photons via the bremsstrahlung process. Thus, annihilation of dark matter should lead not only to a positron excess but also to a gamma-ray excess. Best limits on the cosmic gamma-ray flux are set by the Namibia-based Cherenkov telescope called HESS (as homage to Rudolf Hess [or maybe another Hess]) who covers the 100 GeV - 100 TeV energy range. It turns out that, assuming the annihilation hypothesis, the parameter space suggested by PAMELA and ATIC (the red region in the plot) is incompatible with HESS. A word of caution is in order here. The results of that analysis depend on the dark matter density profile for which we have only more or less educated guesses. The plot I included here assumes the most popular NFW profile, whereas the bounds are less severe if the density profile is less steep than NFW in the region close to the galactic center. For example, using the Einasto profile (which seems to be preferred by numerical simulations) the bounds are weaker and the PAMELA/ATIC region is only marginally excluded, while for the isothermal profile (less preferred by numerical simulations) the PAMELA/ATIC region is marginally allowed. It is fair to say, however, that there is a tension between the annihilation interpretation of the positron excess and the gamma-ray data. Moreover, observations in radio waves (which should be produced by the synchrotron radiation of the positrons) also seem to be incompatible with the annihilation scenario.


On the other hand, this tension disappears if the PAMELA/ATIC results are explained by an unstable dark matter particle with the life-time of order $10^{26}$ seconds and the mass of order 2 TeV. The plot, taken from this paper, shows that the PAMELA/ATIC region safely satisfies the HESS and radio bounds, even for the NFW profile. The simple reason is that the decay rate depends on the dark matter density as $\rho^1$, unlike the annihilation rate that depends on $\rho^2$. Thus, the growth of the decay rate toward the galactic center is effectively less steep.

Coming back to theory, a very long-lived unstable particle is by no means unusual (I mean in theory, in practice we haven't seen any so far). If the (meta-)stability of the dark matter particle is due to some accidental global symmetry, it is natural that this symmetry is broken at some high-scale. That is to say, the symmetry is broken by higher dimensional non-renormalizable operators suppressed by that high scale, and these operators could be responsible for the slow decay. It was pointed out here that the life-time of $10^{26}$ seconds and the mass of 1 TeV is compatible with dimension-six operators suppressed by the GUT scale. Which is inspiring... Note that, by exactly the same token, we expect that the global baryon symmetry is broken at the GUT scale and that protons are long-lived but eventually decay.

HESS has the potential to improve the bounds, or see the gamma-ray signal if it lurks behind the corner. Unfortunately, astrophysicists are more interested in astrophysical backgrounds. It seems that we need to wait for GLAST-now-FERMI to learn more. Unless...If you're a pirate off the Africa coast reading this blog here's the plan for you: 1) hijack the HESS crew, 2) force them to point the telescope into "nothing", 3) submit the results to ArXiv. The ransom will be paid in Stockholm.

Fermi confirms PAMELA

The annual sabbat of the Fermi collaboration took place last week in Rome. One of the new results presented there was the measurement of the positron fraction in the cosmic rays. This is a very hot observable given PAMELA's claim that the positron fraction at high energies is larger than the one predicted by models of cosmic ray propagation. The PAMELA excess can be interpreted as a signature of dark matter, although boring astrophysical explanations are also possible. But a skeptic could doubt the PAMELA result. Measuring the positron fraction requires discriminating between positrons and much numerous protons; one needs the proton rejection power at the level of 1 in 100 000. PAMELA claims to have that rejection power, but the possibility of an unaccounted for systematic effect did exist.

Now the PAMELA result has been confirmed by a cute measurement performed by the Fermi satellite. Fermi, unlike PAMELA, does not have a magnet to distinguish positively charged particles from negatively charged ones. That's why, until now, they were presenting only the combined flux of electrons and positrons. Nevertheless, they are able to some extent separate electrons and positrons by borrowing the magnet from the Earth. The configuration of the Earth magnetic field lines happens to be such that for certain energies and certain arrival directions only electrons or only positrons are expected, see the picture. Using this effect, Fermi was able to produce the following measurement of the positron fraction:
The black data points are from PAMELA, and the grey band is the Fermi measurement with their systematical uncertainties. The two are nicely consistent. So, we still don't know whether the positron excess is due to dark matter or pulsars or old newspapers, but at least we know for sure it is real.

Another result presented in Rome deserves some advertisement. One of the main goals of Fermi is to search for gamma rays produced by annihilation of dark matter. A good strategy is to look in the direction of one of the dwarf galaxies. These are small satellites of our galaxy that are vastly dominated by dark matter, therefore the annihilation signal can be significant while the astrophysical backgrounds are less pesky. So far, no anomalous gamma ray flux from dwarf galaxies has been detected. From that, Fermi is able to put quite stringent limits on the annihilation cross section for various hypotheses about the final state into which dark matter annihilates:
The point is that for certain hypotheses, like for light dark matter annihilating into tau leptons or b-quarks, they are already excluding the cross sections expected if the dark matter is a thermal relic. So they're really closing in on the parameter space where a signal may be lurking if the WIMP paradigm is true. No luck so far, but they'll try again next year :-)

Hitchhiker's Guide to Anomalies in Astroparticle Physics

I wrote recently a related post in which I collected anomalous experimental results in particle physics. That list was pathetic. I had to search deep to find a few results worth mentioning. None of those items could pass for a convincing argument for physics beyond the Standard Model. It is very likely that all of these anomalies are in fact unaccounted for systematic error, or bread-and-butter physics improperly understood.

The situation in astrophysics is completely different. Almost every respectable experiment can boast of an unexplained excess, a mysterious bump or a striking anomaly. Part of the reason is that, in astrophysics, backgrounds are often as good as unknown while the error bars are estimated by throwing dice. But, hopefully, this is not the whole story. In the end, the only clear evidence for physics beyond the Standard Model comes from astrophysical observations that have established the existence of dark matter. There is actually more dark than ordinary matter in the sky, and it is quite likely that some of the puzzling results below are in fact messages from the dark sector.

Here is a collection of astroparticle anomalies directly or indirectly related to dark matter searches. In my subjective order of relevance.

1. PAMELA

It finally happened: SUSY is no longer the favorite hottie, all eyes are now on PAMELA (although some attempt dating both). PAMELA is a satellite experiment who measures the cosmic flux of anti-protons and positrons. While the former flux is roughly consistent with theoretical estimates, the positron flux displays a steep rise at energies above 10 GeV, contrary to expectations based on the secondary production of positrons by cosmic rays scattering on interstellar matter. The simplest interpretation of the PAMELA excess is that the background is not properly estimated, and for the moment this remains a perfectly viable option. Another possibility is that the positron spectrum is contaminated by a nearby astrophysical source like a pulsar or a micro-quasar. Finally, the excess could be a manifestation of dark matter.
The PAMELA positron excess can be explained by a dark matter particle who is heavier than 100 GeV and annihilates preferentially into the Standard Model leptons, with the annihilation into hadrons suppressed down to the rate of 10 percent or less. A slightly more exotic scenario is that dark matter is not stable but decays into leptons, which amounts to pretty much the same from the point of view of indirect detection.

2. ATIC

If one naively continues the slope in the PAMELA spectrum beyond 100 GeV, the positron fraction above a few hundred GeV becomes of order one. That energy range is probed by ATIC - a balloon experiment detecting cosmic electrons and positrons (without being able to distinguish the two). And indeed, ATIC observes an excess of electrons at positrons at energies between 100 and 800 GeV. The size of the effect nicely fits with the PAMELA excess, and it is very likely that both observations have the common origin (whether it is dark matter or not). Moreover, ATIC observes a clear feature in the spectrum - a bump around 600 GeV followed by a sharp decline above 800 GeV (the latter recently confirmed by HESS). If this features are indeed signals of dark matter, the ATIC observation pinpoints the mass scale of the dark matter particle to be around 1 TeV. A good news for the LHC.

It may be worth mentioning that the ATIC peak is inconsistent with another experiment called EC who studied the similar energy range but found no excess. On the other hand, ATIC is consistent with the results from PPB-BETs, but that experiments is generally dismissed due to its miniature size (it was manufactured by Japanese). The rumor is that the new ATIC-4 data will confirm the peak and reduce the error bars by a factor of two.

3. HAZE

The WMAP satellite made its name studying the primordial microwave spectrum produced at the early hot stage of the Universe. The microwave emission from our galaxy is an annoying background (or foreground, depending which way you look) and has to be carefully studied too. Our galaxy pollutes the CMB via thermal dust emission, thermal bremsstrahlung, synchrotron radiation and spinning dust. Subtracting these known contributions revealed the presence of an additional component that extends some 30 degrees around the galactic center. This excess can be interpreted as the synchrotron radiation of electrons and positrons produced by dark matter in the galactic center (that's where the dark matter density is the largest). By itself, the Haze is maybe not an overwhelming evidence for dark matter, but in the light of PAMELA and ATIC it is another indication that too many positrons and/or electrons are flying around. Besides, it has a cool name.

4. EGRET

EGRET was a cosmic telescope that studied diffuse gamma-ray emission in the 30 MeV - 100 GeV range. Excessive emission from the galactic center at energies between 10 and 50 GeV was concluded in this paper. The excess can be interpreted as another manifestation of dark matter annihilation or decay that produces high-energy electrons and positrons. The latter produce high-energy photons via inverse-Compton scattering of starlight or of the microwave background.

5. INTEGRAL

The INTEGRAL satellite detected the 511 keV gamma-ray line from the galactic center. Photons carrying 511 keV energy arise from the e+e- annihilation at rest. If dark matter annihilation is the origin of this line, the dark particle must have rather non-trivial properties to produce electrons and positrons nearly at rest: either its mass is in the MeV range, or it has an excited state with a 1-2 MeV splitting. The most recent results from INTEGRAL weakened the case for dark matter. The new observations display an asymmetry of the emission with respect to the central axis of the galaxy which seems to be correlated with the distribution of low mass X-ray binaries - systems including a neutron star or a black hole that accretes matter from its companion. At this point a conventional astrophysical explanation seems far more likely, but the case is not closed yet.

6. DAMA

All the previous observations, when interpreted in terms of dark matter, fall into the class of indirect detection, that is observations of the final products of dark matter annihilation or decay. The complementary technique, called direct detection, consists in searching for signals of dark matter particles scattering on a target made of ordinary Standard Model particles. There are many direct detection experiments going on: CDMS, XENON, CRESST, DAMA to name a few active ones. The last one actually claims a detection. Unfortunately, this is the one we trust the least. The reason is that DAMA's detection technique cannot effectively distinguish dark matter particles from a huge background of ordinary particles scattering on the target. Instead, the claim is based on observing the annual variation of the signal which may be induced by a variation of the dark matter flux due to the motion of the Earth around the Sun. The size of the effect observed by DAMA is however in conflict with other direct detection experiments, unless the dark matter particle has some contrived properties (for example, an excited state with a 100 keV splitting). Another viable interpretation of the DAMA signal is that the Italian mafia dumps radioactive waste near Gran Sasso every year in June. Or there is some other regular effect with an annual period. Most likely, DAMA will share the fate of LSND: we will never what went wrong.

That's it. As a homework, try to fit all six anomalous results in a single theory of dark matter (solution here). But be careful. A wise man once said that if a theory can explain all experimental results then it is certainly wrong. Because some experiments are always wrong.

Life After FERMI

I mean, after FERMI's first electron data that shed new light on the currently hottest topic in astroparticle physics - the origin of the cosmic-ray positron excess measured by PAMELA. The PAMELA anomaly trumpeted last summer, combined with the data from ATIC (who claims a spectacular excess of electrons at few hundred GeV), prompted zillions of publications that speculate of its dark matter origin. A few weeks ago FERMI revealed their first measurement of the electron+positron cosmic ray spectrum up to 1 TeV. Here is my summary of what that implies for the models of dark matter.

Let's begin with a handful of facts:

• Thanks to FERMI and HESS we now have a pretty accurate picture of the cosmic-ray electron+positron spectrum. Up to 1 TeV, the spectrum is well approximated by power law, $\sim E^{-3}$, while above TeV it becomes softer (larger power).
• ATIC screwed: there is no clear feature in the electron spectrum. In principle, one should describe the present situation as "two experiments giving inconsistent results", given that ATIC's brand new data continue displaying a distinct bump with even smaller errorbars. But, somehow, the public opinion prefers a fancy high-tech satellite over a dirty leaking balloon somewhere in the cold Antarctica. More seriously, FERMI simply beats ATIC with a hundred times more statistics.
• The electron spectrum near TeV is above the background predicted by conventional cosmic-ray propagation models (which prefer a larger power, $\sim E^{-3.3}$) in which the electrons are produced by scattering of cosmic ray protons. But that alone is hardly an anomaly, and the spectrum can be easily fitted by cranking up the propagation model.
• However, combining FERMI with the PAMELA positron data strongly indicates the presence of a new primary component of electrons/positrons (as opposed to the secondary production by protons) .
• The primary component could be injected by nearby pulsars, or by another so far unidentified astrophysical process, or by dark matter annihilating or decaying in our galaxy. We have to wait for the jury to reach the verdict.
• But, light (less than 1 TeV) dark matter particle as a source of the PAMELA and FERMI excess is now strongly disfavored. The reason is that a light particle would lead to sharp spectral features at energies comparable to its mass, whereas FERMI sees none of that. If dark matter is the cause of the excess, it has to be relatively heavy, 1 TeV or more.

In fact, the new FERMI data did not really lead to a slaughter of the dark matter models or its authors. New papers keep appearing in which the excess is fitted with axions, neutralinos, winos, KK particles, F-theory or three little pigs. But Occam is waving his razor menacingly, and we are reaching the point where boring astrophysics becomes the simplest explanation of all available data. The dark matter models, although still viable, have to be intelligently designed to yield observable signals in the positron and electron channels, but none in antiprotons, gamma rays or neutrinos. Incidentally, the electron channel is the one where the astrophysical background is most difficult to control...

The cool thing is that by the end of this summer we may further disfavor dark matter or return it to grace. The new crucial piece of information will be FERMI's measurement of the diffuse gamma ray spectrum that will extend its previous measurement to larger energies. If the electrons and positrons observed by PAMELA and FERMI originate from dark matter it means that they are produced all over the galaxy. Once produced, the electrons lose their energy by scattering on starlight and on the CMB or by synchrotron radiation in the galactic magnetic fields, which leads to a diffuse flux of photons at energies of a few hundred GeV. Given the number of electrons needed to explain PAMELA and FERMI, the diffuse signal should be detectable by 1-year FERMI data. The important thing is that boring astrophysics cannot easily fake that signal. On the other hand, the absence of features in diffuse gamma would be a huge setback for the dark matter interpretation. The experimental data are expected on August 12, so just a little patience please...

For more details and plots see the slides of Alessandro Strumia's talk at Planck'09; check also for the connection between dark matter and dialectic materialism.

Hot Photos of PAMELA

There's been some buzzing earlier this month about the new results from the satellite experiment PAMELA who measures the cosmic flux of anti-particles. Nature had a news story which said that positrons in the 10-60 GeV range show a large excess over the background. If the background is correctly estimated (rather unheard of in astrophysics), this could be an indirect sign of dark matter annihilation in our galaxy. The results from PAMELA were flashed during ICHEP and other conferences. However, due to the fact that the collaboration is publishing in Nature, there’s an embargo on releasing the plots in advance of publication. Congratulations to the Nature team for the idea, though they should acknowledge the Chinese government for the inspiration.

Fortunately, everyone can now find out what PAMELA's results are thanks to the authors of this paper. A footnote explain that

the data points shown in our figure have been graphically extracted from a photo of a slide shown by M. Boezio at the IDM08 conference, Stochkolm, 20/08/2008

Clearly, this adds some James Bond flavor to astroparticle research. Here are the pictures in all their beauty:
Thanks to Bob for the news.

Update: The story unfolds. Yesterday PAMELA's talk at CERN was "postponed due to Nature embargo". In retaliation, the latest volume of Nature laid out in the TH common room has been stained with coffee. Read also here and here.

ATIC ATAC

I'm slowly recovering from the shock of starting a new life in a new time zone in a new haircut. Time to kick off with no-longer-from-CERN blogging. As a warm-up, I have an overdue rant. Some inspiration came from Tommaso's post who shares a few warm remarks about theorists teaching experimentalists. Here I elaborate on the opposite case.

The Nature magazine recently published a paper from the ATIC collaboration. ATIC is a balloon-borne experiment that studies high energy electrons and positrons (they cannot distinguish the two) coming from the cosmos. Many of these electrons and positrons are created by known astrophysical processes, mainly by cosmic rays scattering on interstellar matter (the secondary production). Astrophysicists can roughly estimate the flux due to the secondary production, although these estimates are subject to many uncertainties and should be taken with a whole container of salt. Anyway, assuming that the background estimates are correct, ATIC observes an excess of electrons at positrons at energies between 100 and 800 GeV. This fits well together with the positron excess between 10 and 100 GeV reported recently by the PAMELA satellite. Things are even more interesting. Rather than a mere excess, ATIC sees a distinct feature in the spectrum: a bump between 300 and 800 GeV. Astronomers are excited because this could be a signature of an interesting astrophysical object (a young nearby pulsar?, a microquasar?). Particle physicists are even more excited because the PAMELA and ATIC observations could be the first clear signals of annihilating or decaying dark matter particles with TeV scale masses.

The results from ATIC may turn out an important piece in the puzzle of what is the nature of dark matter. However, the collaboration must consider their results so uninteresting that they have to provide us with a cavalier theoretical interpretation. The signal is interpreted in the context of the so-called LKP - the lightest Kaluza-Klein particle in the Universal Extra Dimensions (UED) scenario. The names of Kaluza and Klein appear in the abstract and $e^N$ times in the main text. In case you missed it, they stress in conclusion that "if the Kaluza–Klein annihilation explanation proves to be correct, this will necessitate a fuller investigation of such multi-dimensional spaces, with potentially important implications for our understanding of the Universe." All in all, ATIC claims to have found in their data some hints toward the presence of extra dimensions of spacetime. What is their reason for such an extraordinary claim? It looks like they applied the modern tertium non datur: whatever is not the MSSM must be the UED.

That's philosophy. Physics, on the other hand, does not support ATIC's interpretation. From the theoretical point of view one can complain that the UED is an artificial and poorly motivated construction, that it does not address any problems of the SM (except for dark matter) while creating new problems of its own, and so on. But the main point here is that the LKP interpretation is not consistent with all available experimental data. Firstly, the LKP annihilation cross section is not large enough to explain the ATIC and PAMELA signals (if the dark matter abundance is of thermal origin). ATIC shrugs this off with a bla bla, where the former bla stands for a 100 boost factor from dark matter clumpiness (which does not come out of numerical simulations) and the latter for "other kids do it too". There is yet another serious problem that has to do with the fact that PAMELA observes no excess in cosmic anti-protons. Even though the LKP couples more strongly to leptons than to quarks (because the former have larger hypercharges to which the LKP couples), the decay rate into hadrons in the UED is still far too large. This issue is not addressed at all in ATIC's paper because it was submitted before PAMELA's data came out.

To summarize, it's such a pity to mar a beautiful experiment with a crappy theory.

AMS is on

AMS-02 is up and running, and first events have already been twitted to the Earth. AMS is a full fledged particle detector attached to the ISS whose goal is to measure the cosmic ray spectra. The mission has been plagued by ill fate (delay due to the Columbia crash, scrapping of their superconducting magnet), now the road seems to be clear at last. The final preparations and the launch have been widely reported in the mainstream media, however my impression was that the actual science that AMS may accomplish was not clearly exposed. Here is my understanding of what AMS could teach us.

The official page of AMS lists the following scientific goals

• Search for primordial antimatter
• Search for dark matter
• Search for exotic forms of matter
• Study of the cosmic ray composition

The first point situates somewhere between Sam Ting's fixation and crackpottery. AMS will search for anti-helium nuclei arriving from the outer space. Unlike antiprotons, positrons and anti-deuterium, heavier anti-nuclei are not expected to be produced by cosmic ray collisions; anti-helium would have to be produced by astrophysical objects made of anti-matter. The problem is that we know there is no such thing: all the primordial antimatter annihilated with matter around 1 sec after the big bang. This view is not only the consequence of the current cosmological model, but it is also firmly supported by several independent observations, such as the cosmic gamma-ray spectrum, the cosmic microwave background, and the near perfect agreement between the predictions of nucleosynthesis and the composition of visible matter in the universe. Given the current body of evidence, AMS has a better chance for a 3rd degree encounter than for finding primordial anti-matter.

The situation with dark matter is more subtle. The PAMELA and FERMI satellites launched in the previous decade have been providing us with precise measurements of the high energy cosmic ray spectra. One thing we definitely have learnt is that it is painstaking to search for dark matter this way. Several excesses over theoretical predictions have been reported so far: PAMELA's positrons, Fermi's electrons, Fermi's photons from the galactic centre. They all have a plausible interpretation in terms of models of dark matter and an equally plausible interpretation in terms of boring astrophysical phenomena. AMS may provide more input regarding the high energy spectra. As can seen in the plots of the projected sensitivity, after 10 years of data taking they expect to extend the measurement of the positron and antiproton spectra up to almost TeV (compared to the current reach of PAMELA of about 200 GeV). It's hard to say if these projections are realistic, since it is not clear how much the resolution at high energies is degraded due to the replacement of the superconducting magnet by a weaker permanent one. Assuming they are realistic, particle physicists will be able to refine their models of dark matter, and astrophysicists to refine their models of pulsars. In any case, the chances for a smoking gun signal of dark matter appear slim at this point.

Nevertheless, there is one area where AMS is clearly superior to all previous experiments. The instrumentation of AMS includes a calorimeter, trackers, a Cherenkov detector and a time-of-flight detector to measure the energy, charge and mass of incoming particles. All this gives them very good particle identification, in particular they can easily separate heavier nuclei from much more numerous protons and helium nuclei. Flux ratios of various heavy nuclei, for example the boron-to-carbon ratio, are an important input for the models of cosmic ray production and propagation. Furthermore, if there exists exotic matter with distinct charge-to-mass ratio, for example the hypothetical strangelets with small Z/A, AMS is well equipped to identify it.

In summary, high energy astrophysics is a crowded field, and AMS is unlikely to turn it upside down. Their best shot for a spectacular discovery is exotic forms of matter with distinct Z/A ratio, provided they exist. Furthermore, if AMS and the ISS last long enough, and if the performance of the detector is as good as they promise, they should be able to extend PAMELA and FERMI measurements of the antiproton and positron spectra to higher energies, which may or may not clarify the origin of the positron and electron excess in PAMELA and Fermi. In the worst case AMS will sort out the spectra of heavier cosmic ray nuclei, providing valuable input for cosmic ray propagation model. Critics may complain that 2 billion dollars for tuning GALPROP is a lot. Optimists may stress that so far it's the only hope for returns from the 200 billion dollars sunk into the ISS.

Figures are taken from the talk of Andrei Kounine at TeVPA'10.

Dark matter or pulsars? AMS hints it's neither.

Yesterday AMS-02 updated their measurement of cosmic-ray positron and electron fluxes. The newly published data extend to positron energies 500 GeV, compared to 350 GeV in the previous release. The central value of the positron fraction in the highest energy bin is one third of the error bar lower than the central value of the next-to-highestbin.  This allows the collaboration to conclude that the positron fraction has a maximum and starts to decrease at high energies :]  The sloppy presentation and unnecessary hype obscures the fact that AMS actually found something non-trivial.  Namely, it is interesting that the positron fraction, after a sharp rise between 10 and 200 GeV, seems to plateau at higher energies at the value around 15%.  This sort of behavior, although not expected by popular models of cosmic ray propagation, was actually predicted a few years ago, well before AMS was launched.  

Before I get to the point, let's have a brief summary. In 2008 the PAMELA experiment observed a steep rise of the cosmic ray positron fraction between 10 and 100 GeV. Positrons are routinely produced by scattering of high energy cosmic rays (secondary production), but the rise was not predicted by models of cosmic ray propagations. This prompted speculations of another (primary) source of positrons: from pulsars, supernovae or other astrophysical objects, to  dark matter annihilation. The dark matter explanation is unlikely for many reasons. On the theoretical side, the large annihilation cross section required is difficult to achieve, and it is difficult to produce a large flux of positrons without producing an excess of antiprotons at the same time. In particular, the MSSM neutralino entertained in the last AMS paper certainly cannot fit the cosmic-ray data for these reasons. When theoretical obstacles are overcome by skillful model building, constraints from gamma ray and radio observations disfavor the relevant parameter space. Even if these constraints are dismissed due to large astrophysical uncertainties, the models poorly fit the shape the electron and positron spectrum observed by PAMELA, AMS, and FERMI (see the addendum of this paper for a recent discussion). Pulsars, on the other hand, are a plausible but handwaving explanation: we know they are all around and we know they produce electron-positron pairs in the magnetosphere, but we cannot calculate the spectrum from first principles.

But maybe primary positron sources are not needed at all? The old paper by Katz et al. proposes a different approach. Rather than starting with a particular propagation model, it assumes the high-energy positrons observed by PAMELA are secondary, and attempts to deduce from the data the parameters controlling the propagation of cosmic rays. The logic is based on two premises. Firstly, while production of cosmic rays in our galaxy contains many unknowns, the production of different particles is strongly correlated, with the relative ratios depending on nuclear cross sections that are measurable in laboratories. Secondly, different particles propagate in the magnetic field of the galaxy in the same way, depending only on their rigidity (momentum divided by charge). Thus, from an observed flux of one particle, one can predict the production rate of other particles. This approach is quite successful in predicting the cosmic antiproton flux based on the observed boron flux. For positrons, the story is more complicated because of large energy losses (cooling) due to synchrotron and inverse-Compton processes. However, in this case one can make the  exercise of computing the positron flux assuming no losses at all. The result correspond to roughly 20% positron fraction above 100 GeV. Since in the real world cooling can only suppress the positron flux, the value computed assuming no cooling represents an upper bound on the positron fraction.

Now, at lower energies, the observed positron flux is a factor of a few below the upper bound. This is already intriguing, as hypothetical primary positrons could in principle have an arbitrary flux,  orders of magnitude larger or smaller than this upper bound. The rise observed by PAMELA can be interpreted that the suppression due to cooling decreases as positron energy increases. This is not implausible: the suppression depends on the interplay of the cooling time and mean propagation time of positrons, both of which are unknown functions of energy. Once the cooling time exceeds the propagation time the suppression factor is completely gone. In such a case the positron fraction should saturate the upper limit. This is what seems to be happening at the energies 200-500 GeV probed by AMS, as can be seen in the plot. Already the previous AMS data were consistent with this picture, and the latest update only strengthens it.

So, it may be that the mystery of cosmic ray positrons has a simple down-to-galactic-disc explanation. If further observations show the positron flux climbing  above the upper limit or dropping suddenly, then the secondary production hypothesis would be invalidated. But, for the moment, the AMS data seems to be consistent with no primary sources, just assuming that the cooling time of positrons is shorter than predicted by the state-of-the-art propagation models. So, instead of dark matter, AMS might have discovered models of cosmic-ray propagation need a fix. That's less spectacular, but still worthwhile.

Thanks to Kfir for the plot and explanations. 

First results of AMS-02

Today we saw the first physics results from the AMS-02 collaboration. AMS is a particle detector attached to the International Space Station where it collects more than 10 billion cosmic ray events per year. The data released today concern the energy spectrum of cosmic ray positrons. Before discussing the AMS results it's worth taking a historical detour to understand the wider context.

The Universe we see is made of matter, but some small amounts of antimatter are being constantly produced by all sorts of violent processes: the scattering of high energy cosmic rays on the interstellar medium, the creation of  electron-positron pairs in the electromagnetic field of pulsars,  proton-proton collisions at the LHC, etc. Another possible production mechanism is annihilation of dark matter in the center of our galaxy, hence the interest of particle physicists in the subject.  Dark matter may show up as an excess of high-energy positrons over the background predicted from common astrophysical processes. Assuming we understand the background.

Until a few years ago the common lore was that the dominant production of positrons in our galaxy is via the scattering of high energy cosmic protons off particles in the galactic disc. This predicts the positron fraction decreasing with energy. For this reason, when back in summer 2008 PAMELA  reported a sharp rise of the positron fraction between 10 and 100 GeV we thought  for a moment we had a smoking-gun signal of dark matter. Later, the Fermi satellite confirmed the excess and showed that the rise extends at least up to 200 GeV. However, now we don't consider the excess an evidence of dark matter. One reason is that models of dark matter that quantitatively explain the PAMELA and Fermi signal are rather baroque. Firstly, one needs a large annihilation cross section, of order 10^-24 cm^3/sec, 2 orders of magnitude larger than the one required for dark matter to be a thermal relic. Moreover, dark matter needs to annihilate mostly into leptons  and, unlike what happens in typical models, very little into hadrons (as no excess in the cosmic ray antiproton spectrum is observed).  Another reason for skepticism is  that any dark matter model explaining PAMELA and Fermi is in tension with constraints on the gamma ray flux from the galactic center and from the dwarf galaxies. In the meantime, astrophysicists went back to the drawing board and proposed more ordinary sources of high energy positrons. The current lore is that a few nearby pulsars could be responsible for the observed rise of the positron fraction. Thus, after the initial excitement, things have settled down in a limbo: we're sure the  positron excess is real, but we cannot prove that it's a signature of dark matter, and neither we can prove that it isn't.

So, what have we learned today? Qualitatively, not much, quantitatively, a bit. AMS, with its full-fledged multi purpose detector, has better particle identification capabilities compared to the previous missions, which allows them to reduce systematic errors in the positron fraction down to 1% at low energies (compared to 2% in PAMELA) and explore a larger energy range. Currently, their positron measurement extends up to 350 GeV, so almost a factor of 2 beyond the highest data point from Fermi. AMS shows that the rise continues at least up to 250 GeV. The flux of high energy positrons seems to be isotropic, although their current constraints on the dipole component do not yet exclude a local (pulsar) origin of the positron flux. They also see a hint of flattening of the positron fraction above 250 GeV, although at this point this is not significant. If the positron excess originates from annihilation of dark matter particles with the mass of several hundred GeV one should see a drop in the positron spectrum at energies above the dark matter mass, but it is not said pulsars or other astrophysical phenomena could not produce a similar drop. (Note also that  a sharp drop in the positron spectrum will typically be accompanied by a similar feature in the electron+positron spectrum, but according to the measurements by Fermi nothing dramatic is happening up to 1 TeV.)

So, AMS-02 made some bold claims today. Dark matter is mentioned 9 times in the  press release, supersymmetry twice. They say that “...over the coming months, AMS will be able to tell us conclusively whether these positrons are a signal for dark matter...”. However this is just a lot of smoke without fire. There's absolutely no way that measurements of the  positron spectrum may give us a reliable evidence for dark matter: not now, and not anytime soon. We simply have no robust way of telling a dark matter signal from a boring astrophysics background in that channel, because we don't know the shape nor the normalization of the background.  It doesn't mean that AMS cannot provide a tantalizing signature of dark matter in the future. The most important thing we learned today is that AMS works and exceeds in precision the previous instruments (which wasn't that obvious: it's the first time a serious experiment is performed on a space station, and besides the mission underwent a dramatic downgrade shortly before the launch). We're waiting most eagerly on the AMS measurements of the antiproton  and anti-deuterium spectra. A correlated excess in several channels could give us more confidence in the dark matter origin. Until that happens, the history has taught us to be skeptical about any evidence of dark matter from astrophysics experiments.

You can find the AMS paper here. See also Matt's blog. Reading the mainstream press it seems that Sam Ting with some help from CERN succeeded in fooling the journalists. I'm glad  that CERN already shook off the faster-than-light neutrinos trauma and is ready for another hoopla.... life is going to be  more interesting for bloggers :-) 

HEP 2008 chart

Symmetry Breaking just posted the 2008 topcites according to SPIRES. That list is a bit depressing to look at, as it is dominated by papers from the last century. Recent ideas cannot stand ground against Maldacena, Randall-Sundrum, ADD. To check what's new in town, instead, I compiled another ranking of most cited papers FROM 2008. I counted the ratio of the number of citations to the number of months the paper has been out. (I was too lazy to count days, which would be more adequate. Also, I included only the papers that have already acquired 50+ citations). Here is what's revealed.

1. 70 citations/month, Five-Year WMAP Observations: Cosmological Interpretation, by WMAP Collaboration, arXiv:0803.0547
   The absolute leader, even though they were hardly the news last year: one needs very large telescopes to tell the 5-year results from the 3-year ones. Nevertheless, quite deservedly, they keep receiving the credit for turning cosmology into a precision science.(Actually, there should be two more WMAP clones in this list: arXiv:0803.0586 and arXiv:0803.0732 who have received 32 and 22 citations/month respectively, but these two got disqualified for obscuring the view.)
2. 26 citations/month, N=6 superconformal Chern-Simons-matter theories, M2-branes and their gravity duals, by Aharony, Bergman, Jafferis, Maldacena, arXiv:0806.1218.
   So this is the most cited 2008 theory paper... It constructs novel gravity duals that employ the notion of M2 branes. I have no idea how important this is. Go ask Jacques, who has a post on the Bagger-Lambert construction of M2 branes, and another one that includes a brief comment on the Aharony et al. paper.
3. 26 citations/month, Observation of an anomalous positron abundance in the cosmic radiation, by PAMELA collaboration, arXiv:0810.4995.
   PAMELA was definitely the star of the last summer: the paparazzi affair was more widely discussed than Britney Spears' new look. On top of that, PAMELA's observation of a cosmic-ray positron excess in the energy range 10-100 GeV was readily interpreted as an indirect signature of dark matter annihilation/decay. This spawned a whole lot of theoretical activity, including the paper below.
4. 19 citations/month, A Theory of Dark Matter, by Arkani-Hamed, Finkbeiner, Slatyer, Weiner, arXiv:0810.0713.
   Once again Nima set the direction. The paper proposes a model that explains the PAMELA excess and a few other mysterious astrophysical results. The crucial feature is the hidden sector hosting dark photons (cool name) with GeV scale masses.
5. 16 citations/month, Superconformal Chern-Simons Theories and AdS(4)/CFT(3) Correspondence, by Benna, Klebanov, Klose, Smedback, arXiv:0806.1519.
   It is a new development related to the Aharony et al (number 2 on this list) who in turn is a new development related to the Baggert-Lambert construction of M2 branes...
6. 13 citations/month, Averages of b-hadron and c-hadron Properties at the End of 2007, By Heavy Flavor Averaging Group, arXiv:0808.1297
   That must be very important, even if Heavy Flavor Averaging Group sounds a bit ominously.
7. 12 citations/month, M2 to D2 revisited, by Ho, Imamura, Matsuo, arXiv:0805.1202.
   It's a pity there is no R-branes yet in string theory; R2-D2 would make an even better title.
8. 12 citations/month, M2-branes on M-folds, by Distler, Mukhi, Papageorgakis, Van Raamsdonk, arXiv:0804.1256I'm afraid I have run out of jokes on M2 branes. Given that I have no intelligent comments in store, I'll just skip to the next one.
9. 11 citations/month, Three-flavour neutrino oscillation update, by Schwetz, Tortola, Valle, arXiv:0808.2016.
   Nobody expects the Spanish Inquisition...people out there are still working on neutrinos...
   Congratulations to Thomas - the only CERN theorist (now ex-) who has made it to this ranking.
10. 11 citations/month, Bagger-Lambert Theory for General Lie Algebras, by Gomis, Milanesi, Russo, arXiv:0805.1012.
    Heeeelp....

In summary, 2008 in particle theory was the Baggert-Lambert year :-|. Way down below (6 citations/month) is the paper of D.T. Son on AdS/cold atoms correspondence, which I personally consider the most interesting one in the last year.

Update: It was pointed out to me that R2D2 is already on ArXiv :-/

FERMI is seeing something?

It has become a tradition that release of new astrophysical data proceeds in the atmosphere of scandal, sex, and intrigues. Less than two weeks ago in this blog I was whining that the FERMI collaboration is guarding their secrets too effectively. Not any more. Not even guns and barbed wire fences could keep theorists off, once they have smelled real data.

Once again the story is related to the searches of indirect signals of dark matter in cosmic rays. In the previous episodes, the PAMELA satellite reported an excess of cosmic ray positrons between 10 and 100 GeV, and FERMI announced that the spectrum of electrons and positrons is harder (falls off more slowly with energy) than predicted by conventional cosmic ray propagation models. Although there exist plausible explanations in terms of mundane astrophysics, the excess positrons and electrons can also be understood as products of annihilation or decay of dark matter in our galaxy. If that is the case, there is one robust consequence. The electrons and positrons produced by dark matter throughout the galaxy should interact with the photons of the cosmic microwave background and starlight in the process known as the inverse Compton scattering, ICS in short. A high energy electron scattering off a photon transfers most of its energy to the photon. Thus, dark matter models explaining the PAMELA and FERMI results also predict an excess of gamma ray photons from the galactic center at energies 100 GeV and more. That's why the astroparticle community has been eagerly awaiting the release of FERMI measurements of gamma rays from the galactic center.

A week ago FERMI announced some new results at the TeV Particle Astrophysics conference held at SLAC. The new data included measurements of the gamma ray spectrum from the galactic center. The results from the one-by-one degree square around the galactic center, while providing new constraints on dark matter models, do not show any exciting features, see the upper plot. However, the data from a larger portion of the sky referred to as the inner galaxy do show an excess, or a hardening of the spectrum, starting at 100 GeV, see the plot on the left. The hardening occurs exactly where the dark matter models predict it! The FERMI collaboration did not want to post the latter result because it is still contaminated with poorly understood backgrounds. But somehow, mysteriously, the plot made it into the summary talk given by Persis Drell and the slides were posted at the conference page. These slides have now been removed; too late alas too late. Today there is a new paper on arXiv that interprets the new FERMI data in terms of the PAMELA/FERMI motivated models dark matter. The plot below reproduced from that paper shows the FERMI data together with expected backgrounds and predictions from dark matter models.

So is FERMI seeing dark matter? Most likely not. Members of the FERMI collaboration suspect that the feature in the gamma ray spectrum around 100 GeV is due to an unexpected background from other cosmic ray particles. Further analysis should clarify the situation. What is definitely true is that we're living in interesting times...

Farewell to the Noughties - Experiment

Year beginnings are always lazy in both theory and experiment (except for most important decisions being taken, but others write about it). So it's a perfect moment for writing all sorts of summaries. Like, for example, summarizing the entire decade. Here I would like to give the list of the most important experiments in the last decade from the point of view of a particle physicist. The other day I named the noughties the most depressing decade ever, and experiment is one of the main reasons. But not for the lack of trying. The pain is that all these beautiful experiments kept confirming the old truths rather than showing new horizons.

Auger

Cosmic rays tickle our imagination because of the huge energies involved: the most energetic beasts reach the stunning energies of order $10^{8}$ TeV in the Earth rest frame. (this translates to hundreds of TeV in the center of mass frame of the collision, still much more than what we can achieve in present colliders). Some earlier experiments suggested that some of these cosmic rays are TOO energetic. Above the energy threshold known as the GZK cutoff a cosmic particle should quickly lose its energy due to interactions with the cosmic microwave background. Observation of cosmic ray events above the GZK cutoff would defy the foundations of physics, maybe pointing to modifications of such fundamental symmetries as the Lorentz symmetry. Auger killed these reveries. The cosmic ray spectrum displays the superboring GZK cutoff at $5x10^{7}$ TeV, more or less where it should be. What a disappointment.

SNO

Neutrinos are the only subfield of particle physics that enjoyed experimental progress in the last decade. Of course, the really groundbreaking discovery of atmospheric neutrino oscillations falls into the previous decade. The SNO experiment only swiped the floor in the early noughties, by obtaining a solid proof that the solar neutrinos also oscillate. They demonstrated that the total number of neutrinos arriving from the Sun is more or less what we expect from our solar models, but that some of the neutrinos change the identity from electron to muon ones.

PAMELA

The true legacy of this experiment is still unclear at the moment of writing. One certain thing is that it turned out very influential, pushing particle theorists in a new direction. PAMELA has made precise measurements of cosmic ray protons, electrons, and their antiparticles, at energies extending to hundreds of GeV. The experiment reached the celebrity status after announcing that the positron spectrum displays a completely different shape than that predicted by standard models of our galaxy. Dark matter particles floating in our galaxy and annihilating into light SM particles provide one tantalizing explanation of that discrepancy. But huge astrophysical uncertainties involved in theoretical predictions make any strong conclusions impossible. It might be that in the future PAMELA will be promoted to the first harbinger of new physics. But more likely, downgraded to yet another false lead.

BaBar

Generally, flavor physics is best suited for botanists. Yet new physics hunters cannot afford the comfort of ignoring it. Because of approximate symmetries of the SM that suppress certain transitions between the generations of quarks, flavor physics is very sensitive to contributions from new hypothetical heavy particles. BaBar and its twin sister Belle produced kilograms of upsilon mesons (the ones made of a b-quark and a b-antiquark), which allowed them to precisely measure their properties. The results showed no major deviations from the predictions of the standard model, apart from a few glitches here and there that, maliciously, occur in observables under poor theoretical control. These results provide a strong hint that, apart from the Higgs boson, there is no new particles in the near energy reach. Scaring.

CDMS

Everybody hates them now: theorists for playing such a cruel game on them, while other experimentalists for getting too much attention. Yet they have been the leader in the field of dark matter direct detection for most of the decade. Depressingly, their leadership consisted in setting more and more stringent limits on the dark matter-nucleon cross section. One solid fact that has been established is that the dark matter particle is not a WIMP in its simplest form. That is to say, it cannot be a weak scale particle interacting via Z boson exchange with the weak coupling strength. But many other options are still wide open, so the hunt continues.

Tevatron Run-2

Great expectations, beautiful performance, gazillion events, hundreds of clever physicists devising clever tricks to extract tiny signals from the data. And nothing that would raise an eyebrow. Precise measurement of the W mass, or discovering omega bee baryons, is not the kind of story our grandchildren will want to listen. But the most depressing must be the that thousands of Higgs bosons have probably been produced at the Tevatron, maybe hundreds have been written on tape, but we just could not see it in all this hadronic mess. After LEP and B-factories, the Tevatron gave us yet another hint that the physics of electroweak symmetry breaking might be less rich than we hoped for.

WMAP

For more than 7 years WMAP has been making precise measurements of the anisotropies in the Cosmic Microwave Background. The only surprise was that our shaky theoretical models describe the data so well. The experiment turned cosmology into precision physics, bringing it dangerously close to Lord Kelvin's nightmare. But there is a glimmer of hope. WMAP's greatest achievement is a precise determination of the amount of various forms of matter in the universe. In particular, it solidly established that dark matter does exist. Which is the most tangible proof we have that the current standard model of particle physics is not the whole story. Maybe this decade we'll find out what's beyond.

Weekend Plot: BaBar vs Dark Force

BaBar was an experiment studying 10 GeV electron-positron collisions. The collider is long gone, but interesting results keep appearing from time to time.  Obviously, this is not a place to discover new heavy particles. However, due to the large luminosity and clean experimental environment,  BaBar is well equipped to look for light and very weakly coupled particles that can easily escape detection in bigger but dirtier machines like the LHC. Today's weekend plot is the new BaBar limits on dark photons:

Dark photon is a hypothetical spin-1 boson that couples to other particles with the strength proportional to their electric charges. Compared to the ordinary photon, the dark one is assumed to have a non-zero mass mA' and the coupling strength suppressed by the factor ε. If ε is small enough the dark photon can escape detection even if mA' is very small, in the MeV or GeV range. The model was conceived long ago, but in the previous decade it has gained wider popularity as the leading explanation of the PAMELA anomaly.  Now, as PAMELA is getting older, she is no longer considered a convincing evidence of new physics. But the dark photon model remains an important benchmark - a sort of spherical cow model for light hidden sectors. Indeed, in the simplest realization, the model is fully described by just two parameters: mA' and ε, which makes it easy to present and compare results of different searches.

In electron-positron collisions one can produce a dark photon in association with an ordinary photon, in analogy to the familiar process of e+e- annihilation into 2 photons. The dark photon then decays to a pair of electrons or muons (or heavier charged particles, if they are kinematically available). Thus, the signature is a spike in the e+e- or μ+μ- invariant mass spectrum of γl+l- events. BaBar performed this search to obtain world's best limits on dark photons in the mass range 30 MeV - 10 GeV, with the upper limit on ε in the 0.001 ballpark. This does not have direct consequences for the explanation of the  PAMELA anomaly, as the model works with a smaller ε too. On the other hand, the new results close in on the parameter space where the minimal dark photon model  can explain the muon magnetic moment anomaly (although one should be aware that one can reduce the tension with a trivial modification of the model, by allowing the dark photon to decay into the hidden sector).

So, no luck so far, we need to search further. What one should retain is that finding new heavy particles and finding new light weakly interacting particles seems equally probable at this point :)

On CDF Anomaly

Mhm, it seems that I chose the wrong side of the Atlantic. First, the LHC produces a big firework display instead of small black holes, and then the CDF collaboration at the Tevatron discovers new physics. About the CDF anomaly in multi-muon events, see Tommaso's post or the original paper. Together with a few CERN fellows we had an impromptu journal club today, and we have reached the conclusion that, well, we don't know :-). The anomaly occurs in a theoretically difficult region, the B-baryon spectrum is poorly known, the local Monte Carlo magicians are very sceptical about modeling the b-quarks, etc, etc. It does not mean, of course, that one should shrug it off. Whether we want it or not, the CDF anomaly will dominate particle model-building for the next few months.

Meanwhile, there is already one model on the market that, incidentally:-), looks relevant for the anomaly: SuperUnified Theory of Dark Matter. One can immediately cook up $e^N$ variations of that model, but there seem to be 3 basic building blocks:
1) The "visible" sector that consists of the usual MSSM with the supersymmetry breaking scale $M_{MSSM} \sim$ few hundred GeV.
2) The dark sector with a smaller supersymmetry breaking scale $M_{dark} \sim $ GeV. It includes a dark gauge group with dark gauge bosons and dark gauginos, a dark Higgs that breaks the dark gauge group and gives the dark mass to the dark gauge bosons of order 1 dark GeV. In fact it's all dark.
3) The dark matter particle that is charged under the dark group and has a large mass, $M_{DM} \sim $ TeV. Unlike in a typical MSSM-like scenario, dark matter is not the lightest supersymmetric particle, but rather some new vector-like fermion whose mass is generated in the similar fashion as the MSSM mu-term.

The dark group talks to the MSSM thanks to a kinetic mixing of the dark gauge bosons with the Standard Model photon, that is via lagrangian terms of the type $f_{\mu\nu} F_{\mu \nu}$. Such mixing terms are easily written down when the dark group is U(1), although for non-abelian gauge groups there is a way to achieve that too (via higher-dimensional operators). Once the dark gauge boson mixes with the photon, it effectively couples to the electromagnetic current in the visible sector. Thanks to this mixing, the dark gauge boson can decay into the Standard Model particles.

The SuperUnified model is tailored to fit the cosmic-ray positron excess PÀMELA and ATIC/PPB-BETS. The dark matter particle with a TeV scale mass is needed to explain the positron signal above 10 GeV (as seen by PAMELA) all the way up to 800 GeV (as suggested by ATIC/PPB-BETS), see here. The dark gauge bosons with a GeV mass scale play a two-fold role. Firstly, they provide for a long range force that leads to the Sommerfeld enhancement of the dark matter annihilation rate today. Secondly, the 1 GeV mass scale ensures that the dark matter particle does not annihilate into protons/antiprotons or heavy flavors, but dominantly into electrons, muons, pions and kaons. The second point is crucial to explain why PAMELA does not see any excess in the cosmic-ray antiprotons. Supersymmetry does not play an important role in the dynamics of dark matter, but it ensures "naturalness" of the 1 GeV scale in the dark sector, as well as of the electroweak scale in the visible sector. I guess that analogous non-supersymmetric constructions based, for example, on global symmetries and axions will soon appear on ArXiv.

What connects of this model to the CDF anomaly is the prediction of "lepton jets". In the first step, much as in the MSSM, the hadron collider produces squarks and gluinos that cascade down to the lightest MSSM neutralino. The latter mixes into the dark gauginos, by the same token as the dark gauge boson mixes with the visible photon. The dark gaugino decays to the dark LSP and a dark gauge boson. Finally, the dark gauge boson mixes back into the visible sector and decays into two leptons. At the end of this chain we obtain two leptons with the invariant mass of order 1 GeV and a small angular separation, the latter being due the Lorentz boost factor $\gamma \sim M_{MSSM}/M_{dark} \sim 100$.

The perfect timing of the "lepton jets" prediction is unlikely to be accidental. A new spying affair is most welcome, now that the paparazzi affair seems to by dying out. While waiting for CDF to find the traitor and hang him on the top pole, I keep wondering if the SuperUnified model does indeed explain the CDF excess. If you take a look at the invariant mass distribution of the anomalous muon-pair events (right panel) it does not resemble a 2-body decay of a narrow-width particle (for comparison, admire the J/Psi peak in the left panel), which it should if the muons come from a decay of the dark gauge boson. Or am I missing something? Furthermore, it has been experimentally proved that bosons are discovered in Europe, while only fermions can be discovered in the US. This is obviously inconsistent with the Tevatron finding the dark gauge boson ;-)

Thanks to Bob, Jure and Tomas for the input.
See also Lubos' post on the SuperUnified model.
For more details and explanations on the CDF anomaly, see the posts of Peter and Tommaso and John.