Chinese scientists unlock possible key to dark matter after almost 90 years

Chinese scientists have made the first direct observation of a quantum effect that was proposed almost 90 years ago and could help detect dark matter, the universe’s invisible “glue”.\nSoviet physicist Arkady Migdal developed a theory in Leningrad in 1939 about what would happen when a neutral particle – such as dark matter – collided with an atomic nucleus.\nMigdal believed that the collision would cause the nucleus to recoil and trigger a secondary electronic recoil, generating a detectable signal.\nBut for 87 years it remained purely theoretical until the team led by researchers from the University of the Chinese Academy of Sciences (UCAS) directly observed the Migdal effect using cutting-edge equipment.\n“Dark matter, an invisible yet gravitationally interacting component of the universe, remains one of the most profound unsolved mysteries in modern physics,” the team wrote in a paper published in the peer-reviewed journal Nature on Wednesday.\nThe team said theoretical discussions of the Migdal effect in relation to dark matter detection dated back to the mid-2000s.\n“However, for more than 80 years, the Migdal effect in neutral particle collisions had never been directly confirmed by experiments,” Zheng Yangheng, corresponding author of the paper and a professor at UCAS, told the official newspaper Science and Technology Daily.\nThis has led to an “ongoing questioning” of the lack of experimental support behind the theoretical assumptions made in experiments that relied on the Migdal effect, Zheng added.\n“In this study, we present the direct evidence of the Migdal effect in neutron-nucleus scattering – a phenomenon predicted more than 80 years ago but confirmed only now with a statistical significance exceeding 5 sigma,” the Chinese team said.\nFive sigma is a standard statistical threshold that indicates a high level of confidence that the observed result is not a false positive.\n“By validating the Migdal effect, we address a long-standing gap in the scientific understanding of fundamental interactions and offer a potential approach for the detection of light dark matter,” the researchers said.\nThe research team developed a gaseous pixel detector designed for the high-precision imaging of nuclear recoil and Migdal electron tracks, using a mixture of 40 per cent helium and 60 per cent dimethyl ether as an imaging medium.\nA neutron generator bombarded the gas mixture to induce the Migdal effect, which was captured by a charge-sensitive pixel array chip.\nThe simultaneous observation of the recoil nucleus and Migdal electron was necessary to confirm the effect, the team said.\nAfter collecting data for around 150 hours, they recorded more than 100 events, which they then narrowed down to six candidate events that met their selection criteria.\n“This result establishes a crucial benchmark for nuclear and particle physics, providing an experimental foundation for future theoretical and experimental investigations,” the team said.\nThe team – which also included researchers from Central China Normal University, Guangxi University, Lanzhou University, Nanjing Normal University and Yantai University – said future work could build on their findings to develop refined detection strategies.\nDark matter is thought to make up 27 per cent of the universe, compared with only 5 per cent for visible matter – which includes all the stars and planets. The rest of the universe is thought to be composed of dark energy, according to the European Organisation for Nuclear Research.\n\n\nUnlike ordinary matter, dark matter does not emit, absorb or reflect any light, making it detectable only through its gravitational pull on galaxies and stars. Without this added gravity, galaxies would fly apart, which has seen dark matter described as a cosmic glue.\nUnderstanding dark matter would revolutionise our understanding of the universe, but scientists have yet to directly detect a dark matter particle.\nPrevious searches for dark matter have focused on massive particles, though these experiments have not yielded conclusive evidence.\nThe search has recently shifted to light particles, which have lighter masses.\nThese particles would leave very faint signals experimentally, requiring enhanced detection sensitivity and an amplified signal. The Migdal effect offers an avenue to achieve this.\n