What a year... Previously I had to think hard to make up a blogging subject that would not be too boring. But these days there's hardly a week without a new discovery, a new rumor of a discovery, or a refutal of the previous week's rumor. This year the particle community was already electrified by the CDF forward-backward asymmetry, the CDF W+2j bump, the would-be Higgs decaying to photons in ATLAS, and now there is CoGeNT... The rumor that CoGeNT observes the annual modulation of the signal has been circulating for a while, but only recently it was officially announced, first at the APS April Meeting in Disneyland last Monday, and today at the symposium in STSI Baltimore.
CoGeNT is a dark matter experiment located in the Soudan mine in Minesotta. In spite of a relatively small size and limited background rejection its germanium detector has certain advantages, e.g. a low threshold (0.4 keVee, corresponding to true recoil energy of about 2 keV) and a very good energy resolution. This makes it particularly sensitive to light GeV-scale dark matter whose scattering cannot produce nuclear recoils far above keV. CoGeNT was running continuously since December 2009 until March 2011 when the power was cut off due to a fire in the Soudan mine. The new results based on 145 kg.day of data continue to show an excess of events at low recoil energies which can be interpreted as the scattering of light dark matter particles in the detector. The preferred parameter region has shrunk and now points to 7-8 GeV particle with the cross section on nucleons around 10-40 cm2. More importantly they were able to measure the annual modulation of the signal. Because the velocity of the Earth with respect to the dark matter sea changes anually due to the orbital motion around the Sun, the event rate of dark matter scattering is expected to oscillate with a peak in June and a minimum in December. And here is what CoGeNT observes.
The solid line is the expectation from dark matter, and the dashed line is the best modulation fit to the signal. The phases of the two are within 1 sigma. CoGeNT estimates that the modulation hypothesis is preferred at 2.8 sigma. The modulation is most pronounced in the 0.5-2 keV region while it is absent for surface events.
Well, I don't know what to think about it. The parameter region consistent with the CoGeNT signal is naively excluded by CDMS, Xenon10 and Xenon100. One would have to assume that these 3 experiments are terribly wrong about their energy scale in order to reconcile their limits with the CoGENT signal. Maybe CoGeNT is just wrong. On the other hand, the observed modulation is very intriguing, especially in combination with the long-standing DAMA modulation signal and the oxygen band excess in CRESST. On the third hand, maybe nobody is wrong, but dark matter is simply different than what we've expected it to be. Prepare for a new wave of dark papers on arXiv.
The video of CoGeNT's presentation is here. See also this post on Cosmic Variance.