Enhancing The MIGDAL Experiment's Sensitivity in the Low Energy Regime Relevant to Dark Matter Searches

Over the past few years, an obscure quantum mechanical prediction made by Arkady Migdal in the late 1930’s has revolutionized the search for dark matter (DM). The Migdal effect has been invoked, based on theoretical arguments alone, to improve the DM mass-sensitivity of the world-leading DM experiment by 2 orders of magnitude. The effect predicts that an atomic nucleus receiving a small “kick” can result in the emission of an atomic electron.

When the kick is from very light DM particles, the energy deposited in the detector by the recoiling nucleus is too small to detect, but the accompanying electron emission is detectable. The effect, however, has not been observed in this regime, raising questions on the validity of these interpretations. Given the potential impact of the Migdal effect on DM detection, an experimental verification of the effect is very much needed.

This award will be used to make an unambiguous detection of the Migdal effect for a number of atoms of interest for DM searches. The method leverages recent progress in the PI’s group on developing detectors to image and resolve low energy particle tracks. With this capability, a Migdal event can be clearly identified by reconstructing the electron and nuclear recoil tracks diverging from a common interaction point.

As a member of the international MIGDAL collaboration, the PI will participate in experiments to detect the effect at a specially designed facility in the UK. The detailed studies based on the first detection will be used to constrain theoretical predictions at energies above those being exploited in DM experiments. Novel detectors needed to extend the search to lower energies are being developed at UNM and, if successful, will be deployed in the second phase of this project.

The project will train and involve undergraduate and graduate students in all aspects of the experiment. This will include hands-on experience in designing, testing and operating small table-top detectors and developing novel image processing techniques to simulate and analyze the data.

This project will evaluate the validity of using the Migdal effect for setting dark matter (DM) limits by detecting and studying it in neutron-induced nuclear recoils in target atoms of interest for DM. It leverages recent progress in the PI’s group demonstrating high spatial resolution and high signal-to-noise imaging of low energy ionization tracks in an optical time-projection chamber (OTPC), which enables particle identification and reconstruction of its direction of motion along the track.

With this capability, the Migdal event topology can be clearly identified by reconstructing the electron and nuclear recoil tracks diverging from the interaction point. These preliminary findings are the basis of a funded European MIGDAL experiment to begin in the Fall. This award will provide support to actively participate in all phases of the experiment, from data taking to simulations and analysis.

A key goal of the project is to expand the search to lower energies, approaching those of interest to DM. This will involve developing novel image processing techniques and a novel negative ion OTPC with the exquisite spatial resolution needed for the low energy regime. If successful, this technology will be implemented in the second phase of the MIGDAL experiment.

This award will also support training and a wide range of research experience to a diverse set of undergraduates and graduate students in increasingly rare small-scale experiments. With its novel capabilities, the NID OTPC promises to impact a broad range of applications across the sciences. These include X-ray polarimetry, dark-photon and double-beta searches, rare nuclear decays, low background alpha and neutron measurements and others.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.