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SUSY and Higgs: romance or drama?

Posted by Jester.

At the beginning of 2012, particle physicists are in such a confusing state of mind: Higgs has been practically discovered but we're not allowed to celebrate yet. It's like when your football team is on top of the league, playing in the last round agai...

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Higgs chased away from another hole

Posted by Jester.

The hunt for the Higgs continues. Tevatron is running at full steam hoping to catch a glimpse of the sucker before the LHC joins in the game. If the standard model is correct, the entire range of allowed Higgs masses will be covered within next 3-4 years. But there is one disturbing puzzle: indirect measurement indicate that we should have already found the Higgs! Indeed, precision measurements at LEP and Tevatron - mostly lepton asymmetries of Z decay and the value of the W boson mass - are best explained if the Higgs mass is some 80-90 GeV, whereas the direct limit from LEP implies that it must be heavier than 115 GeV.

There is one more reason, this time purely theoretical, to expect that the Higgs may be lighter than the naive LEP bound. If supersymmetry is relevant at the weak scale it is in general very uncomfortable with a heavy Higgs. Well, they keep telling you that the upper limit in the MSSM is 130 GeV. But that requires stretching the parameters of the model to the point of breaking, while the natural prediction is 90-100 GeV. Indeed, not finding the Higgs at LEP is probably the primary reason to disbelieve that supersymmetry is relevant at low energies.

Is it possible that Higgs is lighter than 115 GeV and LEP missed it? The answer is yes, because the LEP searches have left many loopholes. Sensitivity of LEP analyses deteriorates if the Higgs decays into a many-body final state, which is possible in some extensions of the standard model. One popular theory where this could happen is the NMSSM - the 2.0 version of the MSSM with an additional singlet. Roughly, the Higgs could first decay into the new singlet, who in turn decays into two tau leptons, which amounts to Higgs decaying into four tau leptons. This funny decay topology could escape LEP searches even if the Higgs is as light as 86 GeV! That is the case not because of deep physical reasons, but simply because LEP collaborations were too lazy to search for it (in comparison, Higgs decaying into four b-quarks, which was studied by LEP, is excluded for the Higgs mass up to 110 GeV).

But not anymore - this particular gaping hole has been recently sealed. A group of brave adventure-seekers ventured into CERN caverns, excavated the ancient LEP data and analyzed them lookig for the Higgs-to-4tau signal. The results were presented this week at the ALEPH meeting celebrating the 20th anniversary and 9th anniversary of its demise. Of course, there is nothing there, in case you had any doubts. The new limit for the Higgs-to-4tau channel excludes the Higgs mass smaller than 105-110 GeV. Yet the beautiful thing in that analysis is that going back to the LEP data is still possible, if only there is reason, and will, and cheap work force.
So, is the idea of the hidden light Higgs dead? It has definitely received a serious blow, but it can still survive in some perverse models where Higgs decays into four light jets, at least until someone ventures to kill that too. Anyway, never say dead; there is no experimental results that theorists could not find a way around ;-)

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Hail to Freedom

Posted by Jester.

Experimental collaborations display vastly different attitudes toward sharing their data. In my previous post I described an extreme approach bordering on schizophrenia. On the other end of the spectrum is the Fermi collaboration (hail to Fermi). After one year of taking and analyzing data they posted on a public website the energy and direction of every gamma-ray photon they had detected. This is of course the standard procedure for all missions funded by NASA (hail to NASA). Now everybody, from a farmer in the Guangxi province to a professor in Harvard, has a chance to search for dark matter using real data.

The release of the Fermi data has already spawned two independent analyses by theorists. One is being widely discussed on blogs (here and here) and in popular magazines, whereas the other paper passed rather unnoticed. Both papers claim to have discovered an effect overlooked by the Fermi collaboration, and both hint to dark matter as the origin.

The first (chronologically, the second) of the two papers provides a new piece of evidence that the center of our galaxy hosts the so-called haze - a population of hard electrons (and/or positrons) whose spectrum is difficult to explain by conventional astrophysical processes. The haze was first observed by Jimi Hendrix ('Scuse me while I kiss the sky). Later, Doug Finkbeiner came across the haze when analyzing maps of cosmic microwave radiation provided by WMAP; in fact, that was also an independent analysis of publicly released data (hail to WMAP). The WMAP haze is supposedly produced by synchrotron radiation of the electrons. But the same electrons should also produce gamma rays when interacting with the interstellar light in the process known as the inverse Compton scattering (Inverse Compton was the younger brother of Arthur), the ICS in short. The claim is that Fermi has detected these ICS photons. You can even see it yourself if you stare long enough into the picture.

The second paper also takes a look at the gamma rays arriving from the the galactic center, but uncovers a completely different signature. There seems to be a bumpy feature around a few GeV that does not fit a simple power-law spectrum expected from the background. The paper says that a dark matter particle of mass around 30 GeV annihilating into b quark pairs can fit the bump. The required annihilation cross section is fairly low, of order $10^{-25} cm^3/s$, only a factor of 3 larger than that needed to explain the observed abundance of dark matter via a thermal relic. That would put this dark matter particle closer to a standard WIMP, as opposed to the recently popular dark matter particles designed to explain the PAMELA positron excess who need a much larger mass and cross section.

Sadly, collider physics has a long way to go before reaching the same level of openness. Although collider experiments are 100% financed by public funds, and although acquired data have no commercial value, the data remains a property of the collaboration without ever being publicly released, not even after the collaboration has dissolved into nothingness. The only logical reason to explain that is inertia - a quick and easy access to data and analysis tools has only quite recently become available to everybody. Another argument raised on that occasion is that only the collaboration who produced the data is able to understand and properly handle them. That is of course irrelevant. Surely, the collaboration can make any analysis ten times better and more reliably. However, some analyses are simply never done either due to lack of manpower or laziness, and others are marred by theoretical prejudices. The LEP experiment is a perfect example here. Several important searches have never been done because, at the time, there was no motivation from popular theories. In particular, it is not excluded that the Higgs boson exist with a mass accessible to LEP (that is less than 115 GeV), but it was missed because some possible decay channels have not been studied. It may well be that ground breaking discoveries are stored on the LEP tapes rotting on dusty shelves in CERN catacombs. That danger could be easily avoided if the LEP data were publicly available in an accessible form.

In the end, what do we have to lose? In the worst case scenario, the unrestricted access to data will just lead to more entries in my blog ;-)

Update: At the FERMI Symposium this week in Washington the collaboration trashed both of the above dark matter claims.