CMS实验中利用双光子事例寻找额外的小质量希格斯等新共振态

The Higgs boson discovered at the LHC has added further credence to the Standard Model of particle physics, in which the final missing ingredient was the Higgs boson. Measurements of its properties from LHC have not excluded the declaration that this Higgs boson may be part of a much larger and more complex Higgs sector than that envisioned in the Standard Model. Many beyond Standard Model (BSM) theories, such as the Next-to-Minimal Supersymmetric Standard Model (NMSSM) and general Two Higgs Doublet Model (2HDM), foresee the existence of a Higgs boson to be consistent with the observed 125 GeV Higgs boson and possibly with additional lighter Higgs particle. Some excesses of about 2-3σ at the mass around 95-98GeV have been observed by LEP and CMS experiments, which is already publically discussed in the community. These slight excesses need to be confirmed by analyzing more data taken by the LHC in the future. In this program, we propose to use and combine all the CMS Run2 data and part of CMS Run3 data to search for a second and low mass Higgs with the diphoton events in the proton-proton collisions. This research will confirm the slight excesses observed in the public results of LEP and CMS experiments, and search for any other new resonance which is from BSM and lighter than the observed 125 GeV Higgs boson. If any new excess found with at least 5σ, it will be a new milestone of particle physics and change our knowledge of the nature of matter and our understanding of the law of the fundamental forces of elementary particles. If nothing new can be found, we can set the upper limit on the production cross section times branching ratio to diphoton of the SM-like low-mass Higgs boson at different mass hypotheses. Using the measured results， we can try to constrain the phase space of some BSM theories such as NMSSM and 2HDM.

LHC上希格斯粒子的发现，使得粒子物理标准模型更加完善。但LHC的测量结果不能排除这个希格斯粒子属于其他超标准模型的可能性。一些超标准理论模型，比如次最小超对称模型或者双希格斯二重态模型，在保证125GeV希格斯存在的同时在小于125GeV质量区间可以存在额外的小质量希格斯粒子。LEP和CMS实验95-98GeV质量区间大约2-3σ超出的测量结果引起了粒子物理界的关注和讨论，急需LHC更多的数据积累和分析来进行检验。本项目将利用CMS所有Run2数据以及结合部分Run3数据，以更多的数据量分析双光子事例寻找额外的小质量希格斯等新共振态，检验现有实验公布的2-3σ的超出结果和寻求对标准模型的突破。如果有超过5σ的发现结果，将改变人类对物质的本质及其相互作用规律的认识。否则给出其产生截面乘以衰变分支比的上限测量结果，同时利用实验结果来限制诸如次最小超对称模型或者双希格斯二重态模型等超标准理论。

LHC上希格斯粒子的发现，使得粒子物理标准模型更加完善。但LHC的测量结果不能排除这个希格斯粒子属于其他超标准模型的可能性。一些超标准理论模型，比如次最小超对称模型NMSSM或者双希格斯二重态模型2HDM，在保证125GeV希格斯存在的同时在小于125GeV质量区间可以存在额外的小质量希格斯粒子。LEP实验给出了98GeV质量区间大约2σ超出的测量结果。本项目利用CMS第二阶段（Run2）即2016-2018年取得的13TeV质子-质子对撞数据，通过分析双光子事例在双光子不变质量区间70-110GeV寻找额外的小质量希格斯等新共振态。2016年的数据观测到的最大超出为95.3GeV的2.9sigma，2016年数据与之前2012年数据联合测量得到的产生截面乘以衰变分支比的上限，归一到标准模型希格斯假设的产生截面乘以衰变分支比最小上限为103.0GeV处的0.17,最大上限为90GeV处的1.15,文章发表在PLB杂志，实验结果引起关注。基于全部Run2数据，通过引入VBF事例类和改进的Drell-Yan压制方法等新策略，产生截面乘以衰变分支比的上限最小值的测量灵敏度比2016年结果提升了大约50%，没有观测到明显超出（<3sigma），全部Run2的结果在CMS内部公开待发表。该项目分析中开展的光子能量重建和刻度方面的工作，对该分析至关重要，相关研究结果发表在JINST杂志。利用CMS寻找额外的小质量希格斯等新共振态13TeV的公开结果，利用NMSSM和Georgi-Machacek模型这两个超标准理论模型来对实验测量结果进行了理解以及开展利用实验测量结果来限制理论模型的唯象分析，研究结果分别发表在中国物理C杂志上，有助于对相关超标准模型理论的限制和发展。

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