LHC “Observed” the Vector Boson Scattering Processes

LHC “Observed” the Vector Boson Scattering Processes

 

On July 5th, 2018, ATLAS collaboration, being one of the main experiments at LHC CERN and consisting of ~3000 scientists all over the world, announced the first “observation” of the SM rare processes: the Vector Boson Scattering in WZàWZ (5.6-sigma)and same-sign WWàWW (6.9-sigma)

 

left: LHC experiments; middle: BDT outputs of WZ channel; right: di-jet invariant mass distribution in same-sign WW channel.

 

http://atlas.cern/updates/physics-briefing/weak-lightsabers

https://twitter.com/ATLASexperiment/status/1014883135392305153

 

Simultaneously, this results were chosen by ATLAS as one of its annual physics highlights to be presented at the ICHEP2018 conference (the world highest level high energy physics conference biennially), with dedicate emphasis as “first observation”.

https://indico.cern.ch/event/686555/contributions/2975622/attachments/1681180/2701001/Pleier-ICHEP-2018.pdf

 

 

Vector Boson Scattering (VBS, in 2à2 scattering mode) together with Vector Boson Fusion (VBF, in 2à1fusion model) has been very important high energy frontier research topics, especially in TeV collider experiments. In the absence of Higgs mechanism, the longitudinal polarization component of the VBS process will have its amplitude infinitively increase as a function of center-of-mass energy and violate the unitarity at sufficiently high energy. VBS/VBF processes are, therefore, not only useful for the test of Standard Model theory and electroweak symmetry breaking, but also a very unique and important test of Higgs Mechanism and its role in the electroweak interactions and unitarity preserving effects. Moreover, it provides also important test of any alternative new physics mechanism that would play a role in replacement of  or in addition to Higgs mechanism in the VBS/VBF processes, as well as examining the new high mass resonances that can generate vector boson pairs via VBF production mode. All of these define VBS/VBF processes high physics values and importance.

 

The TDLI and SJTU ATLAS group has made critical contributions to those new “observations”:

Elena Yatsenko and Shu Li made essential contributions to the WZàWZ VBS analysis, responsible for the analysis framework design/development, signal and background modeling and estimation, theoretical uncertainty estimation, … Elena Yatsenko takes the role of the analysis contact, in charge of the WZ analysis topic in general. The “observation” of WZàWZ scattering process is for the “1st time at the LHC experiments”.

 

Monika Mittal, Elena Yatsenko, Haijun Yang, Shu Li and Jun Guo made important contributions to the “observation” of the same-sign WWàWW scattering, responsible for the electroweak high order and QCD high order interference effect estimation and corresponding theoretical uncertainty estimation. Elena Yatsenko has been the member of the Editorial Board for this analysis. Same-sign WWàWW scattering process is “observed” for the 1st time at the ATLAS experiment, and in good consistency with the previously published CMS result.

 

Dr. Elena Yatsenko graduated from DESY and Univ. of Hamburg in the year 2015 and did her first postdoc at LAPP, CNRS/IN2P3, France. She contributed greatly and continuously to the ATLAS Standard Model Measurements at high energy frontier, and is an expert of Standard Model high precision test. Elena is now a postdoc at SJTU-SPA (sponsored by Prof. Shu Li), working as the WZ analysis contact for ATLAS collaboration and same-sign WW analysis Editorial Board.

 

Dr. Monika Mittal graduated from Panjab Univ. in India in the year 2016. She worked for CMS experiment during PhD and obtained the CMS collaboration Achievement Award. She is now a postdoc at SJTU-SPA (sponsored by Prof. Haijun Yang and Jun Guo), contributing to the SM electroweak physics and beyond SM new physics research.

 

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.

 

Prof. Haijun Yang obtained his PhD from Institute of High Energy Physics (IHEP) and ETHZ, Switzerland. He worked for twelve years for the Department of Physics, University of Michigan, USA from 2000 to 2012. Starting from September 2012, he joined SJTU-SPA and responsible for the initialization of its collider physics group. He led the SJTU ATLAS group to join the collaboration in 2005 and has been the mass data processing and analysis coordinator for ATLAS from 2009 to 2014. Starting from 2016, he has been co-leading the USTC-SDU-SJTU ATLAS cluster of China. Starting from 2018, he has been the Collaboration Board Advisory Committee member. He introduced the BDT method for particle identifications which has been widely used in LHC experiments’ physics analyses and helped greatly the discovery of Higgs Boson and its property measurements as well as the new physics searches. Haijun is now a full professor of SJTU-SPA and associate fellow of TDLI, the director of INPAC at SJTU-SPA and the Collider Group Leader (member of ATLAS, CEPC and BESIII experiments), responsible for calorimeter R&D project for future circular collider experiment in China.

 

Prof. Jun Guo did his undergraduate study at USTC and obtained his PhD degree from Stony Brook Univ. of New York State in 2009. He conducted his postdoc research at the high energy physics group at Colombia Univ. from 2009~2014 and joined INPAC, SJTU-SPA in 2014. Jun has been a member of D0 experiment during his PhD and contributed greatly the high precision measurement of W boson mass. During his postdoc, he was responsible for ATLAS Calorimeter Monitoring and Data Quality, and moreover published several very important new physics search results. Jun is now responsible on behalf of SJTU ATLAS group for the Muon RPC upgrade project and contribute to TeV new physics searches.

 

Refernece

https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/CONFNOTES/ATLAS-CONF-2018-033/

https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/CONFNOTES/ATLAS-CONF-2018-030/

http://atlas.cern/updates/physics-briefing/weak-lightsabers

 

LHC实验上发现矢量玻色子散射过程

 

    欧洲核子研究中心(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5日在ICHEP2018国际高能物理会议上公布了首次以超过5倍标准偏差的统计显著性发现了标准模型稀有产生过程,即 WZàWZ(5.6-sigma)和同号电荷WWàWW (6.9-sigma)“矢量玻色子散射”过程。

 

左图:LHC,中图:BDT输出变量,右图:双喷注的质量分布

并为此撰写了物理摘要Quarks observed to interact via minuscule ‘weak lightsabers’作专题报导:

http://atlas.cern/updates/physics-briefing/weak-lightsabers

 

  矢量玻色子散射(Vector Boson Scattering, 简称VBS,一般对于2à2过程而言)与矢量玻色子聚合(Vector Boson Fusion,简称VBF,一般对于2à1过程而言)是TeV能区对撞机物理中重要的前沿研究课题。在无希格斯机制作用假设下的标准模型预言中,纵向极化的W/Z矢量玻色子散射振幅将会随能量增加而发散,最终在TeV的高能区破坏幺正性。VBS与VBF过程既可以用来检验标准模型电弱相互作用的物理预期及电弱破缺机制,又可以用来检验希格斯机制在电弱融合、散射过程中的幺正化影响。同时可以用来探索可能存在的、除标准模型希格斯机制外的其他新物理机制的存在性,譬如新共振态、规范玻色子反常耦合及对VBS/VBF过程幺正性的影响等。具有重要的物理意义。

     上海交通大学和李政道研究所ATLAS研究团队对该发现做出重要的贡献。Elena Yatsenko和李数对WZ分析道作出了关键性贡献,负责了分析框架的搭建、信号与本底模拟和计算、理论不确定度计算等重要分析任务,博士后Elena Yatsenko担任WZ分析的课题负责人。WZàWZ散射过程的发现为LHC实验中的首次。Monika Mittal、Elena Yatsenko、杨海军、李数、郭军对同号电荷WWàWW散射过程作出了重要贡献,负责了电弱高阶过程和QCD高阶过程相干性的物理研究及理论不确定度估计,Elena Yatsenko任该物理分析的Editorial Board。同号WWàWW散射过程的发现为ATLAS实验中的首次,与之前发表的CMS实验结果一致。

    

    Elena Yatsenko博士2015年毕业于德国同步加速器研究所(DESY)与汉堡大学,曾于法国国家科学研究中心安纳西粒子物理研究所从事博士后研究,长期参与多项重要ATLAS实验前沿物理课题,尤其是对于标准模型的精确检验做出了突出贡献。目前任上海交通大学物理天文学院博士后,ATLAS合作组WZ物理分析课题负责人,同号电荷WW物理分析课题Editorial Board。

    Monika Mittal博士2016年博士毕业于印度Panjab大学,博士期间参与了CMS实验并获合作组Achievement Award,目前任上海交通大学物理天文学院博士后,从事电弱物理和超越标准模型新物理研究。

    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。

    郭军研究员2009年毕业于美国纽约州立大学石溪分校,获博士学位,2009~2014年于美国哥伦比亚大学作博士后研究,2014年加入上海交通大学物理与天文学院。博士期间参加D0实验,对W质量作出精确测量,博士后期间负责ATLAS Calo Monitoring和Data Quality工作并有多篇新物理寻找的重要物理结果发表。目前承担上海交大ATLAS团队缪子探测器升级项目和TeV新物理寻找课题。

    杨海军教授2000年博士毕业于高能物理研究所和瑞士苏黎世联邦理工学院,2000-2012年在美国密歇根大学任博士后和研究科学家。2012年9月起任职上海交通大学物理与天文学院,创建对撞机实验组。2005年起参与ATLAS实验组,2009-2014年担任ATLAS大批量数据处理和分析联络人。2016年起担任ATLAS 中国组(USTC-SDU-SJTU)的负责人,2018年起担任ATLAS大型国际合作组顾问委员会成员。最先提出用BDT方法作粒子鉴别,已经广泛应用于大型强子对撞机实验,对希格斯粒子的发现和性质测量,以及新粒子的寻找做出了重要贡献。

 

参考文献:

https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/CONFNOTES/ATLAS-CONF-2018-033/

https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/CONFNOTES/ATLAS-CONF-2018-030/

http://atlas.cern/updates/physics-briefing/weak-lightsabers

 

 

 

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