ATLAS “observed” the Higgs Boson decaying into bottom quark pairs

On July 9th, 2018, ATLAS collaboration, being one of the main experiments at LHC CERN and consisting of ~3000 scientists all over the world, announced, together with its sister CMS experiment, the first “observation” of the signal of Higgs Boson decaying into bottom quark pairs (Hàbb).

 

 

http://atlas.cern/updates/physics-briefing/higgs-observed-decaying-b-quarks

 

 

 

The Higgs Boson decaying into bottom quark pairs is the decay channel with highest branching ratio, ~58%. The observation of it is a very important knowledge enhancement of Higgs boson properties, which essentially helps to further understanding of the Higgs mechanism, probing and measuring the Higgs to Fermion Yukawa couplings, test of the Standard Model predictions of Higgs boson as well as search of the corresponding new physics in such decay final states.

 

The ATLAS China Group member institutes/universities have been strongly involved in this milestone achievement. Out of all, the TDLI and SJTU-SPA-INPAC ATLAS group members Shu Li, Chikuma Kato and Yanfeng Hang have made crucial contributions to the Hàbb discovery, via the Z(àll)H(àbb) channel.

 

ZH(bb) is a very important analysis topic in ATLAS experiment. It is an associate production mode of Higgs boson (in association with a Z boson), being one of the most sensitive channel in Hàbb decay signal searches. Shu Li, Chikuma Kato and Yanfeng Hang creatively introduced constrained fit of llbb kinematic system in the transverse plane to obtain a very high precision b-jet energy correction so that the Higgs boson decaying into bottom quark pairs can be reconstructed with a much better mass resolution (improved by 40%) and reduce the measured experimental uncertainty. Hàbb measurement is now systematic dominant, so such work is a very critical contribution to the 5-sigma observation.

 

Dr. Chikuma Kato obtained his PhD degree in 2018 from Univ. of Tokyo and during his PhD, he participated in many important ATLAS high energy frontier research projects and essentially contributed to the 2017 Hàbb first 3-sigma evidence paper. He is now a T.D. Lee postdoctoral research fellow of TDLI and is responsible for Muon Condition Data project of ATLAS in general, on behalf of TDLI and SJTU-SPA-INPAC ATLAS group.

 

Prof. Shu Li did his undergraduate and graduate study at USTC, PhD degree in 2012, under the supervision of Prof. Zhengguo Zhao, academician of CAS. From 2013 to 2017, he worked as a postdoc at Duke University, deeply involved in the ATLAS experiment. He joined TDLI as the first T.D. Lee Junior Fellow, co-appointed by INPAC, SJTU-SPA in 2017. Shu’s remarkable research achievements consist of: precise estimate of the SM background of HiggsàWW greatly helping the Higgs discovery in 2012; 1st observation of triboson Zγγ rare process at LHC and 1st probe of Z/γàZγγ aQGC; 1st ATLAS measurement of electroweak Zγ VBS; search for Zγ resonance which greatly helped to resolve the 750GeV puzzle in 2016; 1st LHC search for Higgs+γ resonance. With the high recognition of his contribution, he has been appointed by the ATLAS collaboration (~3000 members in the world) to several important leadership positions such as: 2015/12-2017/01, the convener of the Monte Carlo Validation Group; 2017/04-2018/03, the convener of the Standard Model Electroweak (SM-EWK) Physics Group; 2018/04-Present, the convener of the Monte Carlo Generator Development and Tuning Group. The SM-EWK group has ~200 participants from ~50 institutions including Havard, UC Berkeley, Oxford, Michigan etc., working on dozens of multiboson measurement and anomalous coupling search topics.

 

Refernece:

https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/CONFNOTES/ATLAS-CONF-2018-036

http://atlas.cern/updates/physics-briefing/higgs-observed-decaying-b-quarks

 

 

ATLAS实验首次发现希格斯粒子到正反底夸克对衰变道信号

 

        欧洲核子中心(CERN)大型强子对撞机实验(LHC)上的ATLAS大型国际实验合作组于7月9日在ICHEP2018国际高能物理会议上公布了首次发现希格斯粒重要的衰变过程——正反底夸克对衰变(H→bb),详见:

http://atlas.cern/updates/physics-briefing/higgs-observed-decaying-b-quarks

    希格斯玻色子衰变成一对底夸克,是希格斯粒子最频繁发生的衰变过程,占比高达58%。观察到这一衰变过程将填补对希格斯粒子认知的一大空白,对于进一步证实解释夸克质量的希格斯机制、深入理解并测量希格斯粒子到费米子的汤川耦合系数、验证标准模型汤川耦合系数的预期及探索可能存在的超越标准模型的新物理具有重要的科学意义。

        中国ATLAS组各单位对此次H→bb衰变道的发现做出了重要贡献。其中,上海交通大学李政道研究所粒子物理与核物理分部和物理与天文学院粒子物理与核物理研究所ATLAS团队的李数、Chikuma Kato、杭岩峰在H→bb测量灵敏度最高的分析道之一:Z(→ll)H(→bb)双轻子分析道做出了关键性贡献。

        ZH(bb)是ATLAS实验中重要的物理课题,是希格斯玻色子与标准模型适量玻色子伴随产生的重要物理过程之一、也是H→bb测量灵敏度最高的分析道之一。李数、Chikuma Kato、杭岩峰团队创新性地在llbb衰变末态横向动力学平衡系统作出限制性拟合(constrain fit),得到b-强子喷注的能量修正与刻度的更高测量精确度,从而大幅提高了希格斯粒子在双底夸克衰变道的质量重建分辨率达40%,进而极大地改善了最终H→bb测量的系统误差不确定度。H→bb测量目前是系统误差主导,故该项工作对其最终实现5倍标准偏差统计显著性测量打下坚实基础。

        Chikuma Kato博士2018年毕业于日本东京大学,博士期间直接参与了多项重要ATLAS实验前沿物理课题,对于2017年H→bb的首次3倍标准偏差显著性测量作出重要贡献,目前任李政道研究所博士后学者,并担任ATLAS合作组Muon Condition Data的负责人之一。

    李数研究员2008年本科毕业于中国科学技术大学,2012年获得博士;2013-2017年在美国杜克大学从事博士后研究;2017年起作为首位“李政道青年学者”加盟李政道研究所和上海交通大学物理与天文学院核与粒子物理研究所(INPAC)。2009年起从事LHC/ATLAS实验前沿课题研究,历任ATLAS合作组蒙特卡洛验证组负责人(2015-2017),标准模型电弱物理组负责人(2017-2018),蒙特卡洛产生子研发组负责人(2018.4-)。重要的研究成果还包括:1)WW玻色子产生截面和反常耦合测量、2)精确测量希格斯粒子的WW衰变道标准模型本底、3)LHC首次发现三玻色子(Z和双光子)稀有过程及反常耦合测量、4)ATLAS首篇Z+光子+双喷注纯电弱相互作用散射过程测量、5)有效排除750GeV疑似共振态的Z+光子重质量共振态寻找。

参考文献:https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/CONFNOTES/ATLAS-CONF-2018-036

 

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