Overview: The main research focus of the particle physics group is the fundamental particles that make up the material world and their interactions. They conduct theoretical and experimental research on major scientific issues in the fields of heavy flavor physics, neutrinos, and dark matter, aiming to precisely test the Standard Model and search for new physics beyond the Standard Model.

Members: Kai Chen, Shaolong Chen, Ryotaro Watanabe, Chang Gong, Xinqiang Li, Yanbing Wei, Yuehong Xie, Yadong Yang, Hang Yin, Xingbo Yuan, Dongliang Zhang, Xiao-Kang Zhou.

Introduction:The Standard Model of particle physics is the most comprehensive framework for understanding the fundamental structure of matter and the basic interactions. Despite its great success, there are still some fundamental questions that remain unanswered, such as: Why is the universe dominated by matter? Why are there three generations of quarks? How do fermions acquire mass? What is the nature of dark matter and dark energy? Precisely testing the Standard Model and searching for new physics beyond it are among the most important frontier issues in current particle physics research. Flavor physics research, which aims to understand the properties of quarks and leptons, provides an ideal way to explore new physics and helps answer two important fundamental questions: Is there a mechanism for CP violation beyond the Standard Model? Are there new particles or interactions beyond the Standard Model that are sensitive to flavor structure?

To address these questions, the particle physics research team at Central China Normal University (CCNU) participates in the LHCb experiment at the Large Hadron Collider (LHC) at CERN and the BESIII experiment at the Beijing Electron-Positron Collider (BEPC), conducting systematic research on related issues. By combining theoretical and experimental approaches, they fully utilize high-precision experimental measurements from international large-scale scientific facilities such as LHCb and Belle II, which focus on heavy flavor physics, along with theoretical calculations and model predictions to explore these issues, thereby precisely testing the Standard Model and searching for new physics beyond it.

Currently, the particle physics team consists of 10 members, forming a theoretical team led by Professor Yadong Yang(including Professors Shaolong Chen, Xinqiang Li, Associate Professors Xingbo Yuan, and Ryotaro Watanabe) and an experimental team led by Professor Yuehong Xie(including Professors Hang Yin, Kai Chen, Associate Professors Dongliang Zhang, and Xiao-Kang Zhou). They focus on heavy flavor physics theory and phenomenology, neutrino and dark matter physics phenomenology, physics research of the LHCb experiment at CERN, physics research of the BESIII experiment in China, and the development of advanced particle detectors.

1.Heavy Flavor Physics Research:Heavy flavor physics research has always been at the forefront and a hot topic in international high-energy physics research. It is a powerful means to precisely test the Standard Model and indirectly search for new physics signals beyond it. It is also the main physics goal of LHCb and Belle II. The team led by Professor Yadong Yang(including Professors Xinqiang Li, Associate Professors Xingbo Yuan, and Ryotaro Watanabe) conducts systematic research combining theory and experiment on the mixing and decay processes of various heavy flavor hadrons, deeply understanding the subtle differences in properties between matter and antimatter, and searching for new physics effects beyond the Standard Model.

2.Neutrino and Dark Matter Research:The Standard Model of particle physics is an extremely successful theory but not complete. Two important issues requiring new degrees of freedom and interactions are the origin of neutrino mass and the nature of dark matter. Neutrinos and dark matter are two theoretical hypotheses proposed in the 1930s to explain the "missing energy" in beta decay and the "missing mass" in galaxy clusters, respectively, and have a history of nearly a century. Neutrino oscillations during propagation confirm that neutrinos have extremely small masses and mixing, pointing to the need for new degrees of freedom or interactions beyond the Standard Model. Numerous astronomical and cosmological evidence strongly suggests that more than 80% of the matter in the universe is non-baryonic, mainly composed of so-called cold dark matter. Although the nature of dark matter is still unclear, it is certain that the majority of dark matter does not come from the Standard Model of particle physics. The origin of neutrino mass and mixing, the nature of dark matter, and related physics are among the most important frontier research areas in current particle physics. As two clear new physics categories beyond the Standard Model, they provide important guidelines and scientific questions for exploring additional new particles and interactions. Professor Chen Shaolong's team mainly focuses on these scientific issues.

3.LHCb Experiment Research:The LHCb experiment is a detector designed and built specifically for heavy flavor physics research at the Large Hadron Collider. Its goal is to precisely measure the charge-parity (CP) violation and rare decays of heavy flavor hadrons (hadrons containing bottom or charm quarks), rigorously test the theoretical calculations of the Standard Model, and seek indirect evidence of new physics. Central China Normal University joined the LHCb international collaboration in 2013 and became a formal member in 2018. Currently, the LHCb experiment team includes five faculty members: Professors Yuehong Xie, Hang Yin, Kai Chen, Associate Professor Dongliang Zhang, and Associate Professor Xiao-Kang Zhou. The main research focuses on CP violation and rare decays of heavy flavor hadrons, including time-dependent CP violation of (B_s) mesons, flavor-changing neutral current processes (b \rightarrow s/d), and measurements of CKM phase angles. Additionally, they conduct unique electroweak physics research in the forward rapidity region, including studies on the production cross-section, mass, and rare decays of Z/W bosons, and measurements of the weak mixing angle.

4.BESIII Experiment Research:Central China Normal University was one of the earliest institutions to join the BES experiment. Currently, three faculty members participate: Professors Feng Liu, Yuehong Xie, and Associate Professor Xiao-Kang Zhou. Their research focuses on the strong interaction, weak interaction, mixing, and CP violation of charm mesons and charm baryons. Utilizing the unique quantum-correlated charm meson pairs produced at the threshold in the BESIII experiment, they have precisely measured the strong phase parameters of neutral (D^0) meson decays to (K_s/K_sKK). The (D^0) decay to (K_s/K_sKK) is considered the "golden channel" for measuring phase angles in the LHCb experiment. These strong phase parameters provide crucial experimental input for the precise measurement of the CKM matrix triangle's phase angles. Accurately testing the unitarity of the CKM matrix is a significant research direction in particle physics, allowing for the verification of the Standard Model and the search for new physics beyond it. They also collaborate with the LHCb experiment on joint measurements of angles and explore new methods for angle measurement, such as the model-independent unbinned Fourier method, which is expected to further improve the measurement accuracy over traditional methods.

5.Detector Development:The hardware team consists of Professors Yuehong Xie, Kai Chen, and Associate Professor Xiao-Kang Zhou. To meet the needs of the LHCb experiment's Phase II upgrade, the particle group at Central China Normal University is involved in the development, production, and testing of the upstream tracking detector (UP). They are setting up a related research and teaching laboratory at Central China Normal University, approximately 90 square meters, which has already been renovated. Currently, they have one set of scintillator cosmic ray detection equipment and two sets of silicon microstrip laser detectors for teaching purposes. They have supervised an undergraduate thesis and are involved in the training programs for reserve talents in scientific and technological innovation for middle school students (the "Elite Program" and "Chu Talent Program"). They also plan to offer related experimental courses for graduate and undergraduate students.

The particle physics team will leverage their existing research strengths to further integrate theory and experiment, focusing on key scientific issues in this research field for collaborative studies. They aim to develop the particle physics discipline in a balanced manner, achieving leading levels in theory, experiment, and detection technology research domestically, with significant international influence, striving to create a world-class particle physics discipline.

Highlights of Achievements:

1.Theory:

2.High-Precision Measurement of CP Violation in B Mesons:The CP violation provided by the Standard Model is far from sufficient to explain the matter-antimatter asymmetry in the universe. Finding new sources of CP violation in the LHCb experiment is of significant scientific importance. The team at Central China Normal University has systematically studied CP violation in various decay processes of bottom mesons, including measuring the mixing phase angle ( \phi_s ) of ( B_s ) mesons in the "golden decay channel" ( B_s \rightarrow J/\psi \phi ), maintaining the most precise measurement results in the world over the past decade. They have also achieved the world's only high-precision measurement of CP violation parameters in the ( B_s \rightarrow \phi \phi ) process. Additionally, they observed signs of direct CP violation for the first time in the ( B^+ \rightarrow J/\psi \pi^+ ) process, providing the first evidence of direct CP violation in bottom hadron to charmonium decays.

3.Discovery and Property Study of Double-Charm Baryons:In 2017, the LHCb experiment discovered double-charm baryons for the first time. The LHCb team at Central China Normal University made significant contributions to this discovery and, together with the Chinese team, was awarded the "Top Ten Scientific Advances of China in 2017" by the Ministry of Science and Technology's Basic Research Management Center. The team subsequently conducted the first measurement of the lifetime of this particle, showing that the discovered double-charm baryon primarily decays through weak interactions. Based on this, the team also observed a new decay mode of the double-charm baryon ( \Xi_{cc}^{++} \rightarrow \Xi_c^+ \pi^+ ), independently verifying the previous detection and mass measurement results of the double-charm baryon.

4.Electroweak Physics Research in the Forward Region:The LHCb detector has a unique geometric acceptance, complementing the detection ranges of the ATLAS and CMS experiments. The Z bosons collected by the LHCb experiment are accelerated into the forward rapidity region, making them very sensitive to partons with large or small ( x ). Based on LHCb Phase II data, the team obtained the most precise production cross-section of Z bosons in the forward region at 13 TeV, providing important input for the global fitting of proton parton distribution functions. They also precisely measured the polarization parameters of Z bosons in the forward region for the first time, aiding in the precise testing of the Standard Model of particle physics. Using LHCb Phase II data, they achieved the most precise measurement of the weak mixing angle in the forward region.

Appendix:

LHCb Section

The European Organization for Nuclear Research (CERN) is located on the border between France and Switzerland and is the world's center for particle physics experimental research. CERN's Large Hadron Collider (LHC) is a double-ring superconducting hadron accelerator that accelerates protons to nearly the speed of light before colliding them. It is the highest-energy collider ever built by humans and will provide important data for studying the origin of mass, charge-parity (CP) violation, and more. The LHC accelerator utilizes the circular tunnel constructed for the Large Electron-Positron Collider (LEP), with a circumference of 26.7 kilometers. During operation, proton beams are accelerated in opposite directions within the circular accelerator to nearly the speed of light and collide at collision points, with relevant data collected by particle detectors. There are four main large detectors on the LHC: ATLAS, Alice, CMS, and LHCb. Since the LHC began operation, the proton beam energy has successively been 3.5 TeV, 4 TeV, 6.5 TeV, and 6.8 TeV, corresponding to multiple running periods. In the future, the beam energy is expected to reach 7 TeV, with a collision energy reaching the design goal of 14 TeV.

The LHCb detector is a single-arm forward detector, covering the forward region of 2.0 < η < 5.0. At higher collision energies, a significant proportion of heavy flavor hadrons are produced in the forward region, which is why the LHCb detector is designed as a forward detector. Initially, it was designed to study heavy flavor physics involving bottom and charm quarks, searching for CP violation and rare decays in heavy flavor mesons or baryons. Notably, due to the LHCb detector's excellent reconstruction and detection efficiency, its research scope has expanded beyond heavy flavor physics to include QCD studies, precise measurements in electroweak physics, and more, achieving numerous important results.

BES Section

The particle experiment group at Central China Normal University participates in the Beijing Spectrometer (BESIII) experiment at the Beijing Electron-Positron Collider (BEPCII).

Beijing Electron-Positron Collider (BEPC)The BEPC was completed in October 1988 at the Institute of High Energy Physics, Chinese Academy of Sciences. It is located on the east side of Babao Mountain in the western suburbs of Beijing, covering an area of 50,000 square meters. The diagram below shows the overall layout of the BEPC. It consists of several parts: the injector (BEL), the transport line, the storage ring, the Beijing Spectrometer (BES), and the synchrotron radiation facility (BSRF). The injector is a 200-meter-long linear accelerator that provides positron and electron beams with energies ranging from 1.1 to 1.55 GeV for the storage ring. The transport line connects the injector and the storage ring, delivering the positron and electron beams from the injector to the storage ring. The storage ring is a circular accelerator with a circumference of 240.4 meters, which accelerates and stores the positron and electron beams to the required energy. The large detector for high-energy physics research, the Beijing Spectrometer, is located at the collision point on the south side of the storage ring. The synchrotron radiation facility is located in the third and fourth sections of the storage ring, where synchrotron radiation light emitted by electrons passing through bending magnets and wigglers is directed to various synchrotron radiation experimental stations through the front-end area and beamlines.

The main scientific goals of the BEPC are to conduct research on tau lepton and charm physics and synchrotron radiation. Therefore, the BEPC operates in two modes: a mixed mode optimized for high-energy physics collision experiments while also providing synchrotron radiation light, and a dedicated mode solely for synchrotron radiation research.

Overall Layout of BEPC

With the care and support of the central government and Comrade Deng Xiaoping, the BEPC project, funded by the state, achieved its first electron-positron collision in October 1988. The project mainly includes the collider (BEPC), the Beijing Spectrometer (BES), and the synchrotron radiation facility (BSRF). In 1991, the State Planning Commission officially approved the establishment of the National Laboratory for the Beijing Electron-Positron Collider. Since its operation in 1990, the BEPC has achieved a number of significant research results in the international high-energy physics community, such as the precise measurement of the tau lepton mass, the precise measurement of the hadronic reaction cross-section (R value) in the 2-5 GeV energy range, the discovery of new resonant states at the proton-antiproton mass threshold, and the discovery of the new particle X(1835), attracting widespread attention from the high-energy physics community both domestically and internationally.

At the end of 2003, the state approved the major upgrade project of the BEPC (BEPCII). The BEPCII project is one of the most challenging and innovative projects in China's major scientific engineering. The project started in early 2004, completed construction tasks in July 2008, and passed national acceptance in July 2009. The acceptance opinion stated that the BEPCII project completed all construction tasks with high quality according to the indicators, plan, and budget, receiving high praise from the international high-energy physics community. It is a successful example of China's large scientific engineering construction and another significant milestone in the development of high-energy physics in China.

BEPCII is an internationally leading collider in the charm physics energy region and a high-performance synchrotron radiation facility. It mainly conducts charm physics research, with expected breakthroughs in the search for and characterization of multi-quark states, glueballs, and hybrids, ensuring China's leading position in international charm physics experimental research. At the same time, it can also serve as a synchrotron radiation source providing vacuum ultraviolet to hard X-rays for interdisciplinary research in condensed matter physics, materials science, biology and medicine, environmental science, geology and mineral resources, and microfabrication technology, achieving "dual-use."

Beijing Spectrometer (BES)The Beijing Spectrometer is an important part of the major upgrade project of the BEPC. As the "eye" of the BEPC, it studies the fundamental composition and properties of matter by measuring the secondary particles produced in electron-positron collisions.

The BESIII upgrade, with a total investment of 240 million RMB, consists of the following sub-detectors:

Overall Layout of BESIII

The cylindrical core of the BESIII detector includes a helium-based multilayer drift chamber (MDC), a plastic scintillator time-of-flight system (TOF), and a CsI(Tl) electromagnetic calorimeter (EMC), all enclosed in a superconducting solenoid magnet providing a 1.0T (0.9T in 2012) magnetic field. The detector has a 93% acceptance for charged particles and photons, covering a 4π solid angle. The momentum resolution for charged particles is 0.5% at 1 GeV/c, and the dE/dx resolution for Bhabha scattering electrons is 6%. The EMC measures the energy resolution of 1 GeV photons to be 2.5% in the barrel region and 5% in the endcap region. The TOF system has a time resolution of 68 ps in the barrel and 110 ps in the endcap. In 2015, the endcap TOF system was upgraded to multi-gap resistive plate chamber technology, achieving a time resolution of 60 ps.

The BESIII collaboration currently consists of approximately 600 members from 85 institutions in 17 countries.

Scintillator Detector Cosmic Ray Experiment Diagram