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.

Christmas Play '09

This blog is no longer from CERN but it'll take time to shake off the nostalgia... End of year at CERN TH is traditionally marked by the Christmas play. This year's play is focused on the two events that recently sent a shudder through the planet: the credit meltdown at Wall Street and the LHC meltdown at CERN. Detective Holes (Ellis) investigates the connection between the two. Although the script falters at times, there are enough good gags to take you through 37 minutes of the play. A few must-sees include the Hawaiian nuts (Giddings and Mangano) hula-hopping, DG Cauchemar (Grojean) as Louis XIV/XV, and Evans the Accelerator (Lesgourgues) helium-squeaking. Also starring Fat Wall Street Bastard and Carla Bruni. For more inquisitive minds, there is something extra about the dimensions of Gia Dvali. Enjoy.

If it does not work, try downloading the movie from this page.

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.

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.

Expelled

Today I left CERN. I can never stay long in paradise - I always get expelled for some ridiculous reason. This time it might be because

• I lost hope that the LHC would be running soon
  
• I spilled coffee on Witten the other day at CosmoCoffee
• I discovered that John Ellis' beard is fake
• My contract has expired
  

Whatever the reason, Luis (depicted above as an archangel) has told me to leave. This means I cannot run a blog from CERN anymore. From now on this is a no-longer-from-CERN blog. Reports from CERN seminars will give place general broodings about particle physics. However, CERN-gossiping should continue thanks to the spy network I established in the last years.

I suppose blogging will be perturbed until I settle in a new vacuum.