Scherzo

Last week I was stranded at a conference far in Italy. There was no internet to speak of; instead, the venue was offering alternative commodities. Like for example a chapel:

It striked me as a really good idea. They say that science and religion cannot go hand in hand, but that's obviously not true. Take the first example in a row: the Higgs searches at Tevatron.
According to the plot on the right, Tevatron has roughly a 30 percent chance of finding a 3 sigma evidence for the Higgs in the most interesting region between 115 and 130 GeV. Since we speak of chances the matter is open for prayers. Therefore chapels should be available not only at conferences but also at every accelerator facility.

In that case one would expect the following pattern:

• Tevatron would pray that they find the Higgs
• The LHC would pray that Tevatron does not find the Higgs
• Graduate students at the LHC would pray for any data at all before they turn fifty
• Everybody would pray that the magnets do not explode
• Except for Hawaii surfers and Holger Nielsen who would pray for the contrary

And, as usual, gods will cast dice to decide whose requests should be granted.

New ideas in dark matter

I keep receiving complaints about my meager blogging output. Concerning the last few weeks I have a good excuse: I was studying Mont Blanc (that narrow resonance visible at the LHC on a clear day); and after that I was being tired; and after that I was being lazy; and in any case nothing happens during summer months.

But the coming autumn is going to be exciting. The approaching LHC start-up is one but not the unique reason. On a completely different frontier, huge progress is expected in the area of direct detection of dark matter. The XENON100 experiment in Gran Sasso is going to kick off next month, while its bitter enemy - the LUX experiment in the Homestake mine - is hoping to follow before the end of the year. Within one year experimental sensitivity to the WIMP-nucleon cross section should be improved by two orders of magnitude, biting deep into the parameter space region where numerous popular theories predict a signal.

In the best case scenario the very first months or even days can lead to a discovery. This is the prediction of the dark matter models designed to resolve the DAMA puzzle. Recall that the DAMA experiment in Gran Sasso claims to have detected dark matter by observing the annual modulation of the count rate in their sodium-iodine detector. Particle theorists struggle to reconcile the DAMA signal with the null results from a dozen of other, in principle more sensitive, experiments. That is not quite impossible because various experiments use different targets and different detection techniques, in particular, the masses of the target nuclei as well as the range of the observable recoil energies are specific to each experiment. The game is thus to arrange the properties of dark matter such that the nuclear recoils due to scattering of dark matter particles could have been observed only by DAMA.

The standard WIMP is not an option here: DAMA would require large cross section at the level that has been excluded by CDMS, XENON10 (the little brother of XENON100) and others. But theorists are not easily discouraged and they are trying to come up with alternative ideas. Recently the so-called inelastic dark matter has gained a lot of publicity. In that scenario, dark matter particles scatter inelastically off nuclei into a slightly (hundred of keV) heavier state. Thus, one needs to provide enough energy to produce the heavier state which implies a minimum velocity of the initial dark matter particle for the scattering to occur. The splitting can be tuned such that DAMA is able to see the signal while the others are not. That of course requires some amount of conspiracy. Fortunately, the inelastic dark matter theory predicts a thunderstorm of events in the upcoming runs of XENON100 and LUX.

Until recently inelastic dark matter was the only plausible explanation of DAMA. But this week there was a paper exploring a different idea. In this new scenario, the scattering of dark matter on nucleons is elastic, but the scattering amplitude depends in a non-trivial way on the momentum transfer, hence the name form factor dark matter. If the form factor is suppressed outside the window to which DAMA happens to be sensitive, the null results of other experiments can be explained.

Non-trivial form factors can be arranged by some dirty model building tricks. The paper presents an example of a scalar dark matter particle with dipole-type ($D_\mu X^\dagger D_\nu X F_{\mu\nu}$) interactions with some new hidden vector fields. The latter mix with the photon which provides a coupling to the ordinary matter. To explain the DAMA phenomenology one needs at least two vector fields with opposite couplings to the photon and comparable masses, which makes the whole construction a bit contrived. Again, the model predicts a characteristic recoil spectrum, and a large number of events in XENON100 and LUX.

What seems to be most valuable in these constructions is that they demonstrate that dark matter can have very different properties than the standard WIMP. That should encourage the experimenters to extend the scope of their searches; so far their search algorithms have been tailor-made for the standard WIMP case, and they could have easily missed something interesting. The fantastic experimental progress makes the dark matter models testable well before the LHC can offer us any interesting data. If you have a good idea in store now is the good time to come out.

10? 6.66? Eleventeen?

Today (Wednesday) the CERN management is going to reach a decision that will affect the life of everybody on this planet. Namely, the operating energy of the LHC machine in the first year will be decided today. Senior readers may remember that the LHC used to be a 14 TeV collider. However that energy cannot be achieved in near future due to poor quality of the magnets provided by industry. Reaching the nominal energy will require a long process of magnet training, and the prospect for upgrade seem unlikely within the next 3 years. For this reason, 10 TeV was the energy planned for the last year false start, as well as for the restart scheduled for mid-November.

However, the rumor is that even this smaller energy will not be achieved next year due to the well known problems with bad splices. The hundreds of individual magnets around the LHC ring are connected using a process called soldering - an advanced cutting-edge technique whose many aspects are clouded in mystery. There are in fact two separate problems with soldering that have been detected at the LHC. One is a poor quality of interconnections between the superconducting magnets. That leads to excessive resistance (like nanoohms) and, in consequence, the current flowing through the interconnection generates heat that triggers a quench of the superconductor. The other problem are faulty interconnections between copper bus bars who are supposed to carry the current when the superconductor quenches. It is suspected that the solder in the bus bars was sometimes accidentally melted during subsequent soldering of the superconducting cable connections. In fact, it was a combination of the two above mentioned problems that triggered the fireworks of September 19.

Bad splices are known to be present in the LHC ring, and those residing in cold sectors cannot be repaired without a considerable slip to the schedule. So the alternative is to either postpone the LHC restart or run at slightly lower energies (the latter implies slightly smaller currents running through magnets and thus a slightly smaller risk of another catastrophe). During the last few months numerous simulations and experiments have been performed to determine the maximum safe current.

After a careful study of the plot above, listening to the experts, and weighing all pros and cons, the director general is going to roll a pair of dice, and the sum of dots will determine the LHC energy for the coming restart. As for the rumors, I have heard any rational number between 4 and 10 TeV. So, now is the last moment place your bets. Theoretical analysis of the 2-dice experiment suggests that 7 is the most likely outcome :-).

Once we know the operating energy, we will have a better idea what kind of results to expect in the first year. It is already certain that for a while the LHC cannot compete with the Tevatron in the area of Higgs searches. In fact, almost all reasonable new physics signatures require at least one inverse femtobarn of integrated luminosity, much more than the 100 inverse picobarns expected in the first year. This leaves boring standard model signatures, including slightly less boring top quark physics (but even in the case of the top quark competing with the Tevatron results may be tough if the center-of-mass energy is lower than 10 TeV). However, some spectacular (and unlikely) signatures like a new 1 TeV Z' gauge boson or light superparticles may be within reach if the center of mass energy is not much less than 10 TeV. But realistically, we have to keep patient until at least 2012.

And the winner is...

Seven!

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...

Bullets Fly

I'm sure that everybody has heard of the Bullet cluster aka 1E 0657-56:

This picture from August 2006 made the headlines because it offers a new way to see the presence of dark matter in the universe (the key of course is to paint it blue). The depth of the gravitational potential deduced from gravitational lensing is marked blue, while the matter that shines ordinary photons is marked red. The picture is interpreted as showing two clusters of galaxies that have recently undergone a head-on collision. The dark matter components (and also most of the ordinary stars in the galaxies) just passed through each other with little or no interaction, while the interstellar gas made of familiar protons and electrons collided and got left behind. The observation of the Bullet cluster gave another blow to dark matter alternatives like MOND-type modified gravity theories: in the latter context it is hard to explain why the gravitational potential is not spatially correlated with the ordinary matter distribution.

What might be a little less known is that the Bullet cluster is not the only one. In August 2007 Abell 520 aka Train Wreck was revealed:

This one is much more messy. In fact, in this case the dark matter interpretation is less straightforward. The reason is that the galaxies seems to have been removed from the densest core of dark matter, and it is not clear what mechanism could have caused it. Then in August 2008 we had a pleasure to meet MACS J0025.4-1222 aka Baby Bullet:
which is another pretty clear evidence in favor of dark matter. Apparently, galactic collisions happen every year in August, so next month we should be presented with another picture of this kind :-)

The most important thing about these observations is that they look cool in pictures. But they also carry some practical consequence for particle theorists who sweat to construct models of dark matter. From the fact that the dark matter components pass through each other so easily one can derive a constraint on the self-interaction cross section of dark matter. The paper of Randall et al (not that Randall) based on the analysis of the Bullet cluster quotes the limit

$\sigma/M \leq 3 \cdot 10^3/GeV^3$.

That's an order of magnitude better than the so-called Spergel-Steinhard bound that can be deduced from the dynamics of our galaxy. While this bound is irrelevant for a standard 100 GeV WIMP, it might be a useful constraint for recently popular theories of dark matter where the dark sector consists of strongly interacting particles bound by some new unknown GeV scale forces.

That's Another One for the Fire

It's a lazy summer season: everybody's on the beach and nothing's much going on. To stay in business I have to feed you with some microwaved news. Last week the FERMI collaboration uploaded a pile of papers on arXiv, one of which caught my attention. FERMI is a space gamma-ray observatory, but first of all he is a ruthless terminator with a mission to eliminate other astrophysical experiments. A while ago in May the widely publicized measurement of the cosmic-ray electron+positron spectrum pierced the ATIC balloon that had been pumped for several months. Earlier this year FERMI made another kill: it shot down EGRET, its direct predecessor in cosmic gamma-ray observations. That result has been presented at conferences for a few months, but only last week it made it to arXiv (there's also a longer PRL paper announced).

EGRET was a sattelite gamma-ray observatory that in particular studied the diffuse gamma ray emission in the 30 MeV-100 GeV range. Diffuse means spread over the sky rather than originating from point sources. The main source of diffuse radiation is scattering of the cosmic rays on the milk of the Milky Way. Dark matter annihilation into standard model particles can also contribute to the diffuse flux. The EGRET results showed an excess of gamma rays above 1 GeV, which was quickly hailed as the harbinger of dark matter and supersymmetry. But FERMI's measurement now demonstrates that there's nothing exciting going on below 10GeV: the experimental curve nicely follows the theoretical prediction of the standard propagation model. No exotic physics in sight. ATIC, EGRET...who's next?

FERMI's goal is to measure the diffuse gamma-ray spectrum up to some 300 GeV. The high energy data around are even more interesting for theorists as many popular models of dark matter - especially those that explain the PAMELA positron excess - predict a large signal peaking around a few hundred GeV. More results are expected in August since on August 12th the collaboration is supposed to make all their photon data public. If you hear cries and squealing later this summer that's the dark matter models being slaughtered. Or maybe FERMI sees an excess in which case all hell will break loose? The rumor is...the weird thing is that there are no rumors. Last time, quite accurate descriptions of the electron spectrum circulated in the community months before publication of the FERMI data, and theory papers from outside of the collaboration were out days after FERMI's publication (or even violating causality in one case). Probably because of that experience the collaboration has now entrenched in its camp with barbed wire fences, dogs, booby traps to keep off the theorists. Oh come on, dont be so serious, we also wanna know ;-)

Angles and Demons

Since writing down the standard model back in the summer of love the only progress in particle theory has been the discovery that neutrinos have masses. This fact makes the leptons similar in spirit to the quarks in the sense that transitions between different flavors are possible. In both cases the flavor eigenstates, that is the states to which the W bosons couple, are not the same as the mass eigenstates but linear combinations thereof. This fact opens the door to a fascinating endeavor of measuring the angles in the unitary matrix that relates the flavor and the mass eigenstates.

The angles in the quark sector have been measured from every angle. The final conclusion is that what they taught at school was right: the sum of the angles in a (unitarity) triangle is equal to 180 degrees. In the lepton sector, experiment is lagging behind: so far we know only two of the angles. The one known as the atmospheric or $\theta_{23}$ angle (responsible in particular for the transitions of atmospheric muon neutrinos into tau ones) is close to 45 degrees. The solar or $\theta_{12}$ angle (responsible for the vanishing of electron neutrinos produced in the Sun) turned out to be a bit smaller, about 30 degrees. For the last angle, at the moment we have only an upper bound from the CHOOZ reactor experiment: $\theta_{13} < 11$ degrees at 90 percent C.L.


Now what have demons to do with it? I recently came across a talk from the MINOS collaboration a few month ago in Fermilab. MINOS, when on leave from Hades, studies the muon neutrino beam sent over the distance of 735 kilometers from Fermilab to a far detector located in the Soudan mine in Minnesota. One reason to bother (just imagine what it takes to dig such a long tunnel to send the neutrino beam over several states) is a precise measurement of the angle $\theta_{23}$ which controls the fraction of muon neutrinos that disappear on the way. But there may be more fun than that. Most of the muon neutrinos that vanish turn into tau neutrinos who escape detection. However, if $\theta_{13}$ is non-zero then a small fraction of the muon neutrinos should turn into electron neutrinos, and those receive a warm welcome in Soudan. Thus, MINOS is one in a long queue of experiments trying to pinpoint $\theta_{13}$.


In February this year MÌNOS announced their first results concerning the electron neutrino appearance. They see 35 electron event, roughly 1.5 sigma above the expected background of $27$ events. Not too significant, but already tantalizing. Moreover, if the MINOS data are combined with all available neutrino data the hint for a non-zero $\theta_{13}$ is strengthened to 2 sigma. The central value for $\theta_{13}$ inferred from the overall fit is 8 degrees (plus minus 4) - just below the CHOOZ limit.

If the current hints converge to a full-fledged measurement of $\theta_{13}$ in the 5-10 degrees ballpark then there are some far reaching consequences. First of all, measuring the $\theta_{13}$ angle paves the way to measuring yet another angle (isn't particle physics exciting?), more precisely the CP violating phase in the neutrino mixing matrix. Secondly, it would appear that the mixing angles in the lepton sector are pretty random numbers with no structure, in stark contrast to the quark sector where the CKM matrix displays a very hierarchical structure. In other words, neutrinos would prove to be anarchic. That would mean that anarchy is at rule, at least in the lepton sector, for the first time since Barcelona'36.