The search for dark matter weakly interacting massive particles has probed masses down and below a GeV/c2. The ultimate limit is represented by the experimental threshold on the energy transfer to the nuclear recoil. Currently, the experimental sensitivity has reached a threshold equivalent to a few ionization electrons. In these conditions, the contribution of a Bremsstrahlung photon or a so-called Migdal electron due to the sudden acceleration of a nucleus after a collision might be sizeable. These effects are expected theoretically, but have not been observed yet. In the present project, we propose to study how these effects can be observed and develop a technique based on high precision TPC to carry out the first measurement of such processes.
Dark Matter represents one of the most fascinating and challenging task for fundamental physics today. Given the lack of direct evidences the WIMP sub-GeV mass region has lately received renewed attention and represent today a new frontier in direct DM searches. The search for light WIMPs implies demanding experimental requirements due to the very low recoil energy deposited in the detector, typically smaller than detection thresholds. To overcome this limitation, some new promising detection strategies also involving new signatures have been proposed recently. Among these, the Migdal effect, which could contribute an additional ER excitation/ionisation signal above threshold accompanying the (undetectable) NR, can be particularly useful to facilitates a DM signal detection by effectively lowering detection threshold and improving the sensitivity in the sub-GeV WIMP mass region.
Despite its robust theoretical grounds, the Migdal effect has never been observed experimentally to these days. This raises doubts about the validity of exploiting it in DM searches, especially since the published DM limits on this subject are calculated adopting theoretical assumptions that may break down in liquid or solids targets like the ones employed in these experiments. It appears therefore evident how the experimental assessment of the Migdal effect is remarkably beneficial and well-timed at the present moment, and can generate a significant short and long term impact on current and future low mass DM searches, in a regime particularly difficult to access with current technologies.