New era of dark matter particle detection

Abstract

The astronomical evidence of the existence of dark matter is overwhelming. However, the physical nature of dark matter is still unknown. It is widely believed that dark matter is some kind of new particles beyond the standard model of particle physics. Alternative candidates include the massive compact halo objects such as primordial black holes. Among all these candidates, the class of so-called weakly interacting massive particles (WIMPs) is the most natural and well theoretically motivated one. The WIMP scenario can naturally explain the abundance of dark matter in the universe via a canonical thermal production mechanism. It also gives a practical way of detecting those particles via various kinds of particle physics experiments. The direct detection method measures the momentum exchange when a dark matter particle scatters off a nucleus. The sensitivities of direct detection increase remarkably during the past 30 years since it was first proposed in the 1980s. China has operated two direct detection experiments, the CDEX and PandaX experiments, in the Jinping underground laboratory. Through the efforts of all the direct detection experiments, including CDEX and PandaX, the current upper limit about the spin-independent scattering cross section between the dark matter particle and the nucleon reaches the level of 10⁻⁴⁷ cm² which is only about two orders of magnitude higher than the neutrino-induced background. Although no convincing signal has been detected yet, such null results can constrain severely many kinds of theoretical models of dark matter. Several experiments are upgrading now, with the goal of a fiducial detector mass of several tons, which are expected to further improve the direct detection sensitivities. The indirect detection method is to search for the annihilation or decay products of dark matter in cosmic rays and gamma-rays, which is complimentary to the direct detection. The main indirect detection experiments in operation include the alpha magnetic spectrometer and the calorimeter electron telescope on the International Space Station, the Fermi gamma-ray space telescope, and China’s dark matter particle explorer. Several anomalies have been reported by the indirect detection experiments, such as the positron excess, the excess of the total electron plus positron spectrum, the gamma-ray excess in the Galactic center region. Interpretations of these excesses are still in debate. Nevertheless, these anomalies offer us targets for further investigation with increased statistics and improved understanding of the astrophysical backgrounds. Both the direct detection and indirect detection experiments will have significantly upgrading in the coming years. Particularly, China’s PandaX experiment is currently building a 4-ton scale detector which is expected to start to operate in 2021. The long-term plan of the CDEX experiment is also a ton-scale detector for multi-purpose studies including the dark matter searches. For the indirect detection, the HERD payload onboard the Chinese Space Station is in the prototype phase, and the next generation very large area gamma-ray space telescope is under design. On the ground we are building the large high-altitude air shower observatory for high-sensitivity sky survey in the very high energy gamma-ray band. All these efforts will make China play a significant or even leading role in the campaign of dark matter detection. We are looking forward to the breakthroughs in the exploration of the nature of dark matter.