What if dark matter is no WIMP?

There is a dozen of dark matter direct detection experiments currently in operation all over the world. Most of them are hunting the so-called WIMP: a particle weakly interacting with nuclei and having  the mass in the  GeV-to-TeV range. This WIMP chauvinism has two reasons. One is that, not so long ago, everybody knew for a fact that the dark matter particle is the lightest neutralino in the constrained MSSM, therefore its mass and interactions were constrained to a small window. Things have changed in theory since: we're aware of many more well-motivated dark matter candidates in a wider mass range. The other reason is practical. Today the dark matter particles in our galaxy move very slowly, about 200 km/sec on average. A 10 GeV WIMP hitting a nucleus with the mass close to that of the Higgs produces a recoil of the latter on the order of 1 keV, barely detectable above the instrumental noise. For this reason searching for dark matter lighter than 10 GeV is challenging, and at some point becomes impossible using current techniques.

If dark matter is not a WIMP the current direct detection efforts are doomed. But not all is lost. This new paper by Essig et al argues that, using available detection techniques,  the hunt of dark matter can be taken to a completely new territory. Namely, it turns out that it is possible to search for an MeV-scale dark matter particle, provided it  interacts with electrons.

When a light dark matter particle scatters on a heavy nucleus, the fraction of the initial energy transferred to the nucleus is suppressed by m_DM/m_N, which leads to negligible recoil energies when dark matter is sub-GeV. However, the kinetic energy involved in the reaction is larger; for example, it is of order keV for a GeV dark matter particle which is more than enough to ionize the target atom.  Now, in some dark matter experiments it is possible to detect that a single atom in the target became  ionized.  In particular, the Xenon10 and Xenon100 experiments use a tank filled with liquid xenon to which an electric field is applied. Thanks to that, an electron kicked out from a xenon atom drifts up, and once it reaches the gaseous xenon at the top of the tank it accelerates and produces a lightning flash registered by the photomultipliers.  Thus, light dark matter interacting with electrons should produce events with a small number of ionization electrons. Such events are usually discarded by the experimenters, because the typical WIMP produces more ionization and also a strong scintillation signal when the recoiled atom is de-excited. Nevertheless there exist old Xenon10 calibration data where the trigger was sensitive to even a single ionization electron. These data can be recycled to produce bounds on dark matter in the MeV-GeV mass range. Based on events where 1, 2, and 3 ionization electrons were registered one can obtain the following limits on the scattering cross section of dark matter on (free) electrons.  
 
As one can see, the constraints bite into the virgin parameter space of models where MeV dark matter interacts with us via a "dark photon". What is most encouraging here is that just 12 days of data collected for a different purpose can give useful constraints on light dark matter. Hopefully, similar analysis will be done in the future by the Xenon100 and LUX collaborations, leading to a better bound (or a signal :-).  And, who knows, maybe one day we'll have an experiment completely dedicated to direct detection of light dark matter. Although, for the moment, the WIMP remains the theoretically favored dark matter candidate, it's high time to take the alternatives more seriously.