Chunhui Chen

Research Projects:

I carry out my research in the area of experimental elementary particle physics, also referred to as high-energy physics (HEP). High-energy physicists study the subatomic particles (subatomic particles are particles much smaller than atoms) that are the fundamental constituents of matter. We try to understand the properties of these particles and their interactions in order to understand how our universe was formed and is evolving. The following is a list of experimental projects that I have worked on.

• CDF II experiment (1999-2003): I received my Ph.D. from University of Pennsylvania in 2003 working on the CDF II (Collider Detector at Fermilab) experiment. My thesis work is the measurement of prompt charm meson production cross-section in proton-antiproton collision at a center-of-mass energy of 1.96TeV, which is the first Fermilab Tevatron Run II result published in Physics Review Letter. A brief description of the measurement can be also found in Fermilab Today.

• BaBar experiment (2003-present): At the beginning of the Universe, the big-bang theory predicts the creation of an equal amount of particles and anti-particles. But in everyday life, we do not encounter anti-particles. The question, therefore, is "What has happened to the anti-particles?" The primary motivation of the BaBar experiment is to study the violation of charge and parity (CP) symmetry. This violation manifests itself as different behavior between particles and anti-particles and is the first step to explain the absence of anti-particles in everyday life. To achieve this goal, the BaBar experiment exploits the 9.1 GeV electron beam and the 3 GeV positron beam of the PEP-II accelerator at the SLAC National Accelerator Center, near Stanford University, in California. The two beams collide in the center of the experiment, producing Y(4S) mesons that decay into equal numbers of B and anti-B mesons and allows us to perform precise measurement of CP violation effect using B-meson decays. In 2008, half of the Nobel Prize in Physics was awarded to Makoto Kobayashi and Toshihide Maskawa for their theory which simultaneously explained the source of matter/antimatter asymmetries in particle interactions and predicted the existence of the third generation of fundamental particles. The BaBar experiment together with the Belle experiment at KEK in Japan, provided experimental confirmation of the theory, some thirty years after it was published, through precision measurements of matter/antimatter asymmetries. For detail, please see articles at Babar statement  SLAC Today  Nature  Science  Symmetry Magazine.

• ATLAS experiment (2009 - 2020): ATLAS (A Toroidal LHC ApparatuS) is the largest, general-purpose particle detector experiment at the Large Hadron Collider (LHC), a particle accelerator at CERN (the European Organization for Nuclear Research) in Switzerland. The experiment is designed to take advantage of the unprecedented energy available at the LHC and observe phenomena that involve highly massive particles which were not observable using earlier lower-energy accelerators. ATLAS was one of the two LHC experiments (ATLAS and CMS) involved in the discovery of the Higgs boson in July 2012.  As a result of the discovery of the Higgs boson by the ATLAS and CMS experiments, the 2013 Nobel Prize was awarded to the two physicists, Dr. Francois Englert and Dr. Peter W. Higgs, who proposed the theory independently of each other in 1964 (Englert together with his now-deceased colleague Dr. Robert Brout). In addition, the ATLAS experiment was also designed to search for evidence of theories of particle physics beyond the Standard Model.

• Belle II experiment (2019 - present): Belle II is a new electron-positron collider experiment designed to study the disparity between matter and antimatter observed in the Universe.  With 50 times more data than the previous B-factories (BaBar and Belle experiments) combined, the Belle II experiment is able to perform many precision measurements in flavor physics that are not accessible to experiments at hadron colliders due to their background and trigger limitations. The Quantum Loop effects allow for precision measurements in heavy flavor physics to probe potential new physics at the TeV or even higher energy scale beyond the reach of the LHC experiments. Here the Quantum Loop effect refers to corrections of the standard model predictions (e.g. particle decay probability, CP violation effects) due to the presence of virtual particles in the loop-level Feynman diagram. The size of the correction is often proportional to the particle mass and thus can be used as an indirect probe to infer the existence (or not) of a heavy new particle. The existences of the top quark and Higgs boson, and their masses, were predicted by such measurements based on the Quantum Loop effects well before they were directly produced and observed in experiments. Thus, the Belle II physics program is competitive and complementary to direct searches for new physics at hadron colliders.