They say that il n'y a que Paris. This is roughly true, however Paris last week was not the best place in France to learn about the latest dark matter news. Simultaneously to ICHEP'10 in Paris down south in Montpellier there was the IDM conference where most of the dark matter community was present. One especially interesting result presented there concerns the hunt for light dark matter particles.
Some time ago the CoGeNT experiment noted that the events observed in their detector are consistent with scattering of dark matter particles of mass 5-10 GeV. Although CoGeNT could not exclude that they are background, the dark matter interpretation was tantalizing because the same dark matter particle can also fit (with a bit of stretching) the DAMA modulation signal and the oxygen band excess from CRESST.
The possibility that dark matter particles could be so light caught experimenters with their trousers down. Most current experiments are designed to achieve the best sensitivity in the 100 GeV - 1 TeV ballpark, because of prejudices (weak scale supersymmetry) and some theoretical arguments (the WIMP miracle), even though certain theoretical frameworks (e.g asymmetric dark matter) predict dark matter sitting at a few GeV. In the low mass region the sensitivity of current techniques rapidly decreases. For example, experiments with xenon targets detect scintillation (S1) and ionization (S2) signals generated by particles scattering in a detector. Measuring both S1 and S2 ensures very good background rejection, however the scintillation signal is the main showstopper to lowering the detection threshold. Light dark matter particles can give only a tiny push to much heavier xenon atoms, and the experiment is able to collect only a few, if any, resulting scintillation photons. On top of that, the precise number of photons produced at low recoils (described by the notorious Leff parameter) is poorly known, and the subject is currently fiercely debated with knives, guns, and replies-to-comments-on-rebuttals.
It turns out that this debate may soon be obsolete. Peter Sorensen in his talk at IDM argues that xenon experiments can be far more sensitive to light dark matter than previously thought. The idea is to drop the S1 discrimination, and use only the ionization signal. This allows one to lower the detection threshold down to ~1 keVr (while it's order 10 times higher when S1 is include) and gain sensitivity to light dark matter. Of course, dropping S1 also increases background. Nevertheless, thanks to self-shielding, the number of events in the center of the detector (blue triangles on the plot above) is small enough to allow for setting strong limits. Indeed, using just 12.5 day of aged Xenon10 data a preliminary analysis shows that one can improve on existing limits for the dark-matter-nucleon scattering cross section in the 5-10 GeV mass interval:Most interestingly, the region explaining the CoGeNT signal (within red boundaries) seems by far excluded. Hopefully, the bigger and more powerful Xenon100 experiment will soon be able to set even more stringent limits. Unless, of course, they will find something there...