R&D to Detect the Migdal Effect in a Negative-Ion TPC

Dinesh Loomba, Professor, University of New Mexico, Albuquerque, NM (Principle Investigator)

An exciting avenue for placing constraints on dark matter mass has recently come from the application of an obscure quantum mechanical prediction made by Arkady Migdal in the 1940’s. In the Migdal effect an atomic nucleus receiving a small kick can emit an atomic electron or other detectable signals. Migdal originally considered the nuclear recoil in α- and β-decay, where the effect has been observed, but recent theoretical work applied to dark matter interactions has not been validated. Nevertheless, a number of experiments have recently invoked this effect to improve their sensitivity to lower dark matter masses, some by almost 2 orders of magnitude. Given how these and future experiments could impact the dark matter landscape, an experimental verification of the effect under these conditions is very much needed.

This proposal is for research and development of a liter-scale negative ion drift (NID) time projection chamber (TPC) detector operating in low-pressure gas with a small admixture of an electronegative component. The latter results in NID operation of the detector, which provides the high resolution, 3D reconstruction of low energy ionization tracks needed to detect and study the Migdal effect. The TPC will employ a fine-grained 2D strip readout instrumented with electronics designed at Brookhaven National Labs for use in liquid argon TPCs. The slow NID speed enables exquisite measurements of the arrival time of ionization at the readout plane, which is used to reconstruct the 3rd dimension of the particle track. Once the detector is constructed we will characterize its performance for the Migdal effect by using ionization tracks produced with electrons, alpha particles and nuclear recoils emitted from radioactive sources. We will conduct these measurements in low-pressure NID gas mixtures containing elements of interest for dark matter searches, e.g., helium, fluorine, argon and xenon. These studies will determine the optimal gas mixtures, pressures and other experimental parameters for a Migdal experiment.

The TPC R&D proposed here will impact other research areas beyond searches for the Migdal effect. Applications that could benefit from the high 3D resolution tracking of the NID TPC include directional dark matter searches, measurements of coherent neutrino-nucleus scattering, X-ray polarimetry and directional neutron detection.