With the discovery of the charm quark in 1974, there opened a new era of studying the heavy quark in the particle physics. Up to now, the Standard Model (SM) has been widely recognized as the theory of fundamental particles and the involved interactions.
In the SM, fundamental particles include six quarks and six leptons categorized into three generations, which are building blocks of matter, as well as five bosons, which are mediators of the strong, weak and electromagnetic interactions. Among the six types of quarks, the charm quark stands in a unique position in exploring the fundamental physics. Its effective mass is about 1.5 GeV, which locates in the transition region of the perturbative and non-perturbative energies in the QCD theory, which models the dynamics of the strong interaction.
In addition, the charm quark decays via the weak interaction and its studies are directly connecting to over-constrain the CKM matrix elements, which are used to characterize the transition rates between different quarks. Therefore, experimental precision studies of the charmed hadrons are crucial to test the SM and search for new physics beyond the SM.
The Beijing Electron-Position Collider (BEPCII) accelerates two head-on beams of electrons and positrons, and the two beams of matters and anitmatters can annihilate into pairs of charmed hadrons at the energies near the threshold of the sum of their masses. The Beijing Spectrometer (BESIII) records the information of the whole reactions and measures the relevant physical observables. At BESIII, three types of the ground-state charmed mesons (D0, D+, and Ds) and one charmed baryon Λc, which contain one charm quark and different constitution light quark(s), are produced copiously and their properties can be measured in a unprecedentedly precise way.
This review article, written by two spokespersons of the BESIII collaboration Dr. Hai-Bo Li from Institute of High Energy Physics and Dr. Xiao-Rui Lyu from University of Chinese Academy of Sciences, presents the achievements on the weak decays of charmed hadrons based on analyses of the threshold data at BESIII since its first charm data set taken in 2011. Three types of decay modes: purely leptonic decays, semi-leptonic decays and hadronic decays, are explored systematically.
With the unique feature of the pair production and quantum coherence the two charmed hadrons near threshold, a charmed-hadron tagging technique is implemented, facilicating studies of the decay modes with invisible particles, such as neutrino, neutron or the KL meson, and extracting strong phases in the neutral D meson decays, and probing the decay dynamics of the charmed baryon.
Based on the high statistics data at the center-of-mass energies of 3.773 GeV, 4.18 GeV and 4.6 GeV, high-precision tests of the SM have been carried out in different aspects, such as the most precise determinations of the CKM matrix elements Vcs and Vcd, extracting the strong phase differences in the hadronic decays of neutral D mesons as crucial inputs to the world-wide compaign of determing CP-violating phase gamma angle in the CKM unitary triangle, as well as the lepton universality test via leptonic decays.
In addition, the semi-leptonic decays can be used to calibrate the Lattice QCD calculations on the transition form factors, and various symmetries and non-perturbative QCD effects can be verified by measuring different hadronic decay rates of charmed hadrons. In particular, BESIII has systematically investigated the production and decays of the lightest charmed baryon Λc for the first time using near-threshold data, and became a new research frontier in the BESIII project.
In the coming few years, the charm study continues as an important physics topic at BESIII. About seven times more D mesons will be accumulated during the years of 2022-2023 and much improved precisions of the charm results will be foreseen. Dedicated upgrade of the BEPCII on the luminosity and maximum energy has been deployed, which aims for studying all the ground-state charmed baryons near threshold. These efforts are critical as part of the worldwide flavor physics program and more accurate inputs from future BESIII analyses based on larger data samples will deepen our understanding of the universe and matter.