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 aand b-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 renewal proposal is for research and development of a low-pressure negative-ion drift (NID) time projection chamber (TPC) with the sensitivity to detect and study the Migdal effect at low energies. The detector is based on our prior work on low-pressure TPCs used to measure and reconstruct very low energy ionization tracks for directional dark matter searches. The liter-scale TPC will use a fine-grained readout consisting of gas electron amplifiers (GEMs) and 2 dimensional strips with 200 µm pitch. The electronics to instrument the strips are based on a BNL design that has been used for liquid argon TPCs. The electronics and TPC are ready to integrate and test. In this proposal we will characterize the detector’s performance for the Migdal effect by using various radioactive sources that produce low energy tracks from electrons, alpha particles and nuclear recoils. We will conduct these studies in low-pressure gas mixtures containing elements of interest for dark matter searches with small admixtures of a NID gas. Such gas mixtures containing helium, argon and xenon have been optimized and demonstrate low diffusion. Our measurements will be used to validate the TPC’s sensitivity for Migdal searches at low energies. An important aspect of this work includes developing simulation and analysis pipelines, including novel Machine Learning algorithms developed at UNM, to optimize the extraction and use of track parameters for Migdal searches.

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