The Particle and Nuclear Physics Division at Shanghai Tsung-Dao Lee Center consists of an experimental group and a theory group. The experimental group’s research projects include direct dark matter detection (PandaX), neutrinoless double beta decay, and others. The theory group studies tests of the Standard Model (SM), new physics beyond the Standard Model (BSM), and their connections to cosmology and astrophysics.
Dark Matter
There are several reliable and independent pieces of evidence for the existence of dark matter in the universe. Dark matter contributes about 23% of the current energy density of the universe. Apart from gravitational interactions, almost nothing is known about the properties of dark matter, such as its mass, its interactions with Standard Model particles, its self-interactions, the number of species, and so on. Studying the nature of dark matter remains one of the most important tasks in modern physics. There are three traditional strategies for dark matter detection: direct detection, indirect detection, and collider searches. Members of Shanghai Tsung-Dao Lee Center have made important contributions in all three directions. The PandaX experiment is one of the world-leading collaborations for direct dark matter detection. It provides one of the best classes of constraints on the interaction strength between dark matter and nucleons. In addition to the traditional dark matter searches mentioned above, members of Shanghai Tsung-Dao Lee Center are also actively exploring new strategies for dark matter searches.
TeV-Scale Physics
After decades of exploration, the Higgs boson (125 GeV) was finally discovered at the Large Hadron Collider (LHC) in 2012, marking a turning point in particle physics. The particle content of the Standard Model (SM) appears complete, but this is far from the end of the story. The discovery of the Higgs boson has opened a new door for exploring new physics beyond the Standard Model. The vacuum expectation value of the Higgs field is determined by the Fermi constant v = 246 GeV, which sets the mass scale for all Standard Model particles. However, from naive field theory expectations, the probability of observing a Higgs particle with a mass of 125 GeV is surprisingly low. This is commonly referred to as the gauge hierarchy problem. Moreover, the mass spectrum of matter particles in the Standard Model is highly spread out, ranging from the top quark mass of 173 GeV to the electron mass of 0.511 MeV, and further to neutrino masses of O(0.1 eV). Understanding the mass puzzle is one of the major challenges facing modern physics. The Large Hadron Collider provides an ideal environment for studying the gauge hierarchy problem, since most of its explanations require new particles beyond the Standard Model to appear at the TeV scale. Through a top-down approach, one can start from some ultraviolet theory and study the predicted new particles or new phenomena. On the other hand, through a bottom-up approach, one can explore deviations of Higgs boson properties from the Standard Model. Understanding the newly discovered Higgs boson (125 GeV) and searching for new physics beyond the Standard Model are key tasks in particle physics today. Members of Shanghai Tsung-Dao Lee Center are actively working at this frontier.