Recently, the LHCb international collaboration at the Large Hadron Collider (LHC) of the European Organization for Nuclear Research (CERN) has, for the first time, utilized LHCb experimental data to complete a precise measurement of the Z boson mass. This marks the first measurement of this physical observable at the LHC. The related research findings have been published in Physical Review Letters (PRL) under the title "Measurement of the Z-boson mass" and have been selected as an "Editor's Suggestion." The LHCb team from our school made outstanding contributions to this achievement.
The Z boson is one of the fundamental particles that mediate the weak interaction in the material world. As an electrically neutral gauge boson, it holds a central position in particle physics research. The Z boson was experimentally discovered in 1983, and its existence provided key evidence for the electroweak unification theory, earning the related scientists the 1984 Nobel Prize in Physics. Precise measurement of the Z boson mass holds significant scientific value. The Z boson mass is one of the most fundamental parameters in the Standard Model, and its measurement accuracy directly affects the reliability of electroweak precision tests. At the same time, there is a close correlation between the Z boson mass and other fundamental parameters such as the Higgs boson mass and the top quark mass; any deviation in measurement might hint at the existence of new physics.
The paper analyzed proton-proton collision data from the LHC's 2016 run at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 1.7 fb⁻¹. By analyzing the final state process of Z boson decay into muon pairs and employing a template fitting method to analyze the invariant mass distribution of the muon pairs, the experimental team obtained a precise measurement of the Z boson mass: m_Z = 91185.7 ± 8.3 (statistical) ± 3.9 (systematic) MeV.
This measurement result is highly consistent with previous measurements from other experiments and the theoretical predictions of the Standard Model based on global electroweak fits, providing a new independent and precise experimental input for testing the Standard Model at higher energies. The total uncertainty of this measurement is primarily contributed by statistical error, indicating significant potential for data scalability in this measurement. Currently, the data recorded by the LHCb experiment is approximately one order of magnitude larger than the sample used in this analysis, and after the High-Luminosity LHC (HL-LHC) phase begins, the total data volume is expected to increase by about two more orders of magnitude. LHCb is expected to surpass the LEP experiment in the precision of Z mass measurement in the future, achieving the world's most precise measurement result.
Professor Yin Hang and Dr. Xu Menglin, who was trained at our university, played leading roles in this research. Dr. Xu Menglin is the corresponding author of the paper. Dr. Xu is currently engaged in postdoctoral research at CERN. She pursued her doctoral studies at Central China Normal University under the supervision of Professor Yin Hang, conducting research on electroweak physics in the LHCb experiment, which laid a solid foundation for the successful execution of this study.
The LHCb experiment is one of the four major experiments at CERN's Large Hadron Collider. Its international collaboration consists of nearly 1,500 researchers from over 80 institutes and universities worldwide. Central China Normal University joined the LHCb international collaboration in 2013 under the leadership of Professor Xie Yuehong, making it one of the earliest Chinese universities to join the collaboration. Over the past decade, our university's team has deeply participated in multiple frontier physics research directions of the LHCb experiment, continuously producing internationally influential research results in areas such as charge-parity (CP) violation, rare decays, and electroweak precision measurements, and has cultivated a group of young high-energy physics talents with international perspectives. The team was deeply involved in the discovery of the "double-charm baryon," which was selected as one of China's "Top 10 Scientific Advances" in 2017. They also co-led the first discovery of "baryon CP violation" with domestic collaborative institutions, providing key experimental evidence for understanding the mystery of the matter-antimatter asymmetry in the universe. These systematic achievements signify the continuous enhancement of our university's research capabilities in the frontier field of high-energy physics and its irreplaceable role in international large-scale scientific collaborations.
Related link: https://journals.aps.org/prl/abstract/10.1103/ydn7-qx1d
Preprint article link: https://arxiv.org/abs/2505.15582
