NEUTEL is a series of conferences oriented on neutrino physics but for me, like for most theorists working on physics beyond the Standard Model, neutrinos are boring. Of course, this sentiment cannot be supported by logical arguments. Neutrinos provides one of the few experimental hints that the Standard Model of particle physics is not a final theory. Moreover neutrino physics has a number of interesting questions left unanswered (absolute mass scale, 1-3 mixing angle, CP violation), it connects to exciting phenomena in cosmology and astrophysics, it enjoys a rich experimental program, and it holds a promise of new discoveries in near future. But what can I help… it’s boring. I am looking at NEUTEL with interest for another reason: Wednesday at high noon the Xenon100 collaboration is expected to reveal new results of dark matter searches. Either dark matter will be discovered or new, much more stringent limits will be set.
The field of direct detection of dark matter has already a long history. The idea to look for scattering of dark matter particles on ordinary matter was put forward back in 1986 in a theoretical paper by good man Witten (this Witten). First experimental limits on the cross section of dark matter on nucleons soon followed. The remaining history is a painful process of improving the exclusion limits with a few entertainment breaks thanks to unconfirmed positive signals. In a sense, being a decades-long series of disappointments, the field is similar to collider physics. The important difference between the 2 fields is the much quicker rate of progress of the former. Here are 2 plots, one from 1997, one from 2010:
The left-hand side shows the experimental situation in the late 90s, with the solid red line denoting the limits from the DAMA experiment (the lower lying green lines refer to a projected sensitivity); the right-hand shows the current limits from CDMS and Xenon100. Thus the gain in sensitivity was 2 orders of magnitude in a course of the decade. New Xenon100 limits are expected to move the limits down by another order of magnitude, if no signal is not found. For comparison, in collider physics it took 8 years to vamp up the energy by a meager factor of 3.5, from 1.96 TeV in Tevatron’s run II to 7 TeV in the LHC.
When should the dark matter experiments expect to see the signal? Or when should they quit? Unfortunately, the answer to these questions is not clear at present. The most appealing possibility – a weak scale dark matter particle interacting, like neutrinos, via Z-boson exchange – leads to the cross section of order 10^-37 cm^2 which has been excluded in the 80s. There exists another natural possibility for WIMP dark matter: a particle interacting via Higgs boson exchange. This would lead to the cross section in the ballpark of 10^-43cm^2 (however strongly depending on the Higgs mass, and also on the coupling of dark matter to the Higgs) which is currently being probed by experiment. If Xenon100 see nothing, it is a good moment to seriously start worrying whether the WIMP paradigm corresponds to reality, although models that predict even lower cross sections do exist. In the worst case dark matter may be very weakly interacting (axions, gravitinos) or very light (keV-MeV scale dark matter), in which case the direct detection searches are doomed from the start.
For the next decade or so the direct detection will go in the direction of monster detectors that will allow us to improve the sensitivity down to 10^-47 cm^2. Preparations for a 1 ton xenon detector are already well underway, while 10 ton xenon detectors are on the drawing board. Actually, it’s not the size of the experiment that is the limit here. When the sensitivity reaches 10^-48cm^2, probably sometime in the next decade, these searches will encounter the background of atmospheric neutrinos and diffuse supernova neutrinos. Thus, asymptotically, dark matter detection will merge into neutrino physics, which is probably why dark matter results are presented at NEUTEL 😉 . Well, that does not necessarily have to be the nightmare scenario. Recall that Kamiokande was actually meant to look for proton decay; instead, it got the Nobel for discovering neutrino oscillations.
(posted by Jester)