Opening a new path for light dark matter detection: Chinese scientists directly observed the Migdal effect induced by neutron bombardment

As one of the profound unsolved mysteries in modern physics, dark matter detection has driven numerous physicists to dedicate themselves to finding conclusive evidence that has yet to be found. In addition to weakly interacting massive particles, the search for light dark matter (with masses ranging from MeV to GeV) has recently become a focus of dark matter detection. However, these particles leave very faint signals in experiments, requiring substantial reductions in the detection thresholds of existing detectors. The Migdal effect, a promising approach to enhancing light dark matter detection sensitivity, has received increasing attention in theoretical discussions since the mid-2000s but, again, lacks direct experimental observation.

The Migdal effect was first predicted in 1939 by Soviet physicist Arkady Migdal.2,3 It describes a process in which an atomic nucleus, struck by a particle, such as dark matter or a neutron, can receive a sudden “kick” that transfers energy to its surrounding electron via electromagnetic interaction, causing its ionization or excitation. The energy of the ejecting electron is usually on the keV scale. While the former measurements of the Migdal effect have mainly been made in nuclear decay processes involving α-decay or β-decay without observing the Migdal electrons, its possible occurrence in nuclear scattering, particularly with neutral particles interacting with the nucleus, remains just an elegant theoretical curiosity without direct observational results. Confirming the Migdal effect requires simultaneous observation of the recoil nucleus and the Migdal electron, with the two tracks forming a topological structure that shares a common vertex. By identifying this unique signature, researchers have to distinguish “Migdal events” from background signals such as gamma rays and cosmic radiation. So this imposes stringent requirements on the imaging ability and position resolution of the detector.

The direct observation of the Migdal effect induced by neutron bombardment highlights the multifaceted innovations of Chinese scientists. This is a remarkably ingeniously designed experiment with a clear division of labor. Firstly, the experimental approach adopted by the Chinese team is completely different from those used by previous research groups from the United States, Japan, and the United Kingdom in their studies of the Migdal effect of neutral particles. Yangheng Zheng from the University of Chinese Academy of Sciences (UCAS) has led a group focused on the frontier research in particle physics experiments for many years. One of his team members, Qian Liu, also from UCAS, has been investigating the Migdal effect in the experimental verification of light dark matter since 2022. Liu is responsible for the overall coordination of the experiment, including aspects such as the experimental scheme, detection method, neutron source, and theoretical calculations. Secondly, the key technical basis enabling this breakthrough is a specially developed ultra-sensitive detection system that combines a micro-pattern gas detector with a pixelated readout chip (Topmetal-II chip). The micro-pattern gas detector and pixelated readout chip (Topmetal-II chip) was primarily designed by Hongbang Liu at Guangxi University and Xiangming Sun at Central China Normal University, respectively. Featuring a broad energy detection range, excellent vertex resolution ability, low noise levels, and advanced imaging abilities, the system functions like a “camera,” producing a high-precision image of the recoil nucleus and the Migdal electron. Moreover, the team employed a compact deuterium-deuterium generator, designed by a team from Lanzhou University, to produce 2.5 MeV neutrons, which then bombard the mixed gas in the detector to generate the recoil nucleus and Migdal electrons, thus providing an excellent platform for the experiment (Figure 1). Finally, the theoretical calculation also presents notable innovation. Two theoreticians, Lei Wu from Nanjing Normal University and Bin Zhu from Yantai University, accurately treated many-body electron correlation effects from first principles, enabling precise calculations of many-electron wave functions and transition matrix elements and yielding reliable theoretical predictions.