本文關鍵詞：ATLAS實驗上ZZ雙玻色子產(chǎn)生的物理研究

  更多相關文章： 粒子物理 ATLAS實驗 ZZ 標準模型 Higgs粒子 新物理

【摘要】：在大型強子對撞機LHC上的質子-質子對撞中,有多種物理機制可以產(chǎn)生ZZ雙玻色子末態(tài)。利用Z玻色子的輕子衰變(Z→ll)和中微子衰變(Z→vv),即雙輕子和丟失橫動量的實驗末態(tài),本論文研究了ATLAS實驗上在質心系對撞能量為8TeV下采集的ZZ事例,總積分亮度為20.3fb-1。 首先,利用ZZ→llvv衰變道,我們測量了標準模型下ZZ產(chǎn)生的反應截面。這一產(chǎn)生過程在粒子物理標準模型下可以進行精確的理論計算,通過實驗測量,我們可以與標準模型的預言相比較,驗證理論計算。實驗測得的8TeV能量下ZZ產(chǎn)生的總截面為σZZtot=8.84-0.88+1.04(stat.)-0.85+0.87(syst.)-0.28+0.33(lumi.)pb；而在NLO精度下的理論預言結果為6.58-0.28+0.30pb；兩者相差為1.8個標準差。在最新的NNLO QCD修正下,理論計算結果為7.74-0.31+0.35pb,與我們的測量結果符合的更好。我們還進一步利用實驗數(shù)據(jù)尋找了三玻色子異常耦合對ZZ產(chǎn)生的可能貢獻。由于在實驗上沒有觀察到與標準模型預言的明顯偏差,我們給出了異常耦合參數(shù)的上限值。與ATLAS先前發(fā)表的實驗結果相比,我們獲得了對異常耦合最嚴格的限制。 2012年在LHC上發(fā)現(xiàn)的希格斯(Higgs)粒子被發(fā)現(xiàn)同樣具有離殼效應(off-shell),這一效應在Higgs質量高于2倍于Z玻色子質量(2mz)的范圍內(nèi)可以被觀察到,并對ZZ事例的產(chǎn)生增加額外的貢獻。因此H*→ZZ衰變提供了一個獨特的機制來測量Higgs粒子的離殼耦合強度。利用ZZ→llvv末態(tài),在95%的置信水平下,Higgs離殼信號強度的上限被確定為在9.6-12.4的范圍內(nèi),預期的上限區(qū)間為8.8-13.3。進一步與ZZ→4l及WW→evμv衰變道的實驗結果相結合,我們觀察到(預期)的約束范圍為5.1-8.6(6.7-11.0)。在每種情況下,上限范圍是通過改變未知gg→ZZ和gg→WW本底的高階QCD修正因子與已知的gg→H→ZZ和gg→H→WW信號的高階QCD修正因子之間的比值來確定的,并且這一比值的變化區(qū)間為0.2-2。假設相關的Higgs的耦合是獨立于Higgs生產(chǎn)的能標,與在殼(on-shell)測量的耦合強度相結合,我們可以進一步限制Higgs粒子的總的質量寬度ΓH。在95%的置信水平下,我們獲得的觀察(預期)的ΓH/THSM范圍是4.5-7.5(6.5-11.2)。假定未知的gg→VV本底的高階QCD修正因子與信號的相同,這一結果可以轉化成Higgs總寬度在95%的置信水平下的上限,即觀察(預期)結果為22.7(33.0)MeV。 最后,基于H→ZZ→llvv衰變,我們尋找了質量在240和1000GeV之間的新的Higgs粒子。實驗數(shù)據(jù)與本底假設相符合,我們設定了在雙Higgs二重態(tài)模型(2HDM)中的額外Higgs粒子產(chǎn)生的截面的上限。在這一模型中,tanβ的很大一部分范圍被排除了,并且排除范圍取決于cos(β-α)；對于cos(β-α)=±0.1,tanβ1的區(qū)域,在95%的置信水平下,Higgs質量在250-350GeV區(qū)間內(nèi)被排除了。我們還提供了獨立于任何物理模型的關于Higgs產(chǎn)生截面與H→ZZ分支比的乘積的上限。在膠子-膠子融合(ggG)反應道下,對于一個質量為400GeV的Higgs粒子,σggF×BΥH→ZZ在95%的置信水平下的上限為227fb,預期的上限為209fb。在矢量玻色子融合(VBF)反應道下,σggF×BΥH→ZZ在95%的置信水平下的上限為248fb,預期的上限為136fb。
【關鍵詞】：粒子物理 ATLAS實驗 ZZ 標準模型 Higgs粒子 新物理
【學位授予單位】：中國科學技術大學
【學位級別】：博士
【學位授予年份】：2015
【分類號】：O572.214
【目錄】：

• 摘要6-8
• 英文摘要8-10
• acknowledgements10-13
• 致謝13-15
• Contents15-20
• 附件20-21
• 1 Introduction21-25
• 2 Theory25-55
• 2.1 The Standard Model of Particle Physics25-31
• 2.1.1 Elementary particles26-27
• 2.1.2 Electroweak theory27-29
• 2.1.3 Electroweak symmetry breaking and the Higgs mechanism29-31
• 2.2 Beyond the Standard Model31-34
• 2.3 Dark Matter34-38
• 2.3.1 Dark matter particle candidates34-35
• 2.3.2 Effective field theories35
• 2.3.3 Higgs portal35-37
• 2.3.4 Dark matter detections37-38
• 2.4 Phenomenology for the Large Hadron Collider38-44
• 2.4.1 Hadronic collision39-42
• 2.4.2 Monte Carlo event generators42-44
• 2.5 Diboson physics at the LHC44-47
• 2.5.1 Diboson production44
• 2.5.2 Anomalous triple gauge couplings44-47
• 2.6 Higgs physics at the LHC47-54
• 2.6.1 Higgs production at LHC47-48
• 2.6.2 Higgs decays48-50
• 2.6.3 BSM Higgs benchmark models50-54
• 2.7 Physics with ZZ production54-55
• 3 The Large Hadron Collider and the ATLAS Experiment55-73
• 3.1 The Large Hadron Collider55-59
• 3.1.1 Design parameters and machine layout55-58
• 3.1.2 Operation and performance58-59
• 3.2 The ATLAS Experiment59-73
• 3.2.1 Overview59-62
• 3.2.2 Physics requirements62-63
• 3.2.3 Inner Detector63-65
• 3.2.4 Calorimeters65-68
• 3.2.5 Muon Spectrometer68-70
• 3.2.6 Trigger70-73
• 4 The ATLAS Detector Simulation and Event Reconstruction73-105
• 4.1 Event Simulation73-77
• 4.1.1 Simulation framework73-75
• 4.1.2 Fast simulation75-77
• 4.2 Event Reconstruction77-105
• 4.2.1 Track77-80
• 4.2.2 Primary vertex80-81
• 4.2.3 Electron81-91
• 4.2.4 Muon91-96
• 4.2.5 Jet96-101
• 4.2.6 Missing Transverse Energy101-105
• 5 Measurement of the SM ZZ Production Cross Section105-215
• 5.1 Introduction105-111
• 5.1.1 Theory and Experimental status107-108
• 5.1.2 Analysis Overview108-111
• 5.2 Theoretical calculations and uncertainties111-120
• 5.2.1 Cross Section Definitions111-112
• 5.2.2 Cross Section Prediction112-120
• 5.3 Data and MC Samples120-122
• 5.3.1 Data Samples120
• 5.3.2 Signal MC modeling120
• 5.3.3 Background MC modeling120-122
• 5.4 Trigger122-123
• 5.5 Physics Object Selection123-129
• 5.5.1 Electrons123-125
• 5.5.2 Muons125-127
• 5.5.3 Jets127
• 5.5.4 Missing Transverse Momentum127
• 5.5.5 Primary Vertex and Pileup Reweighting127-128
• 5.5.6 Object Overlap Removal128-129
• 5.6 Event Selection129-136
• 5.6.1 Preselection129-130
• 5.6.2 l+l-vv Selection130-131
• 5.6.3 Signal cut flow131-136
• 5.7 Backgrounds136-160
• 5.7.1 W Z background136-140
• 5.7.2 tt,tW,WW and ττ background140-145
• 5.7.3 Z background145-151
• 5.7.4 Data-driven estimate of W and multijet background151-158
• 5.7.5 The other backgrounds158-160
• 5.8 Systematic Uncertainties160-171
• 5.8.1 Electrons160-161
• 5.8.2 Muons161-162
• 5.8.3 Jets162-164
• 5.8.4 Missing transverse energy164
• 5.8.5 Luminosity and pileup164
• 5.8.6 Theoretical uncertainties164-169
• 5.8.7 Summary of systematic uncertainties169-171
• 5.9 Selection Results171-177
• 5.9.1 Final Numbers of Observed and Expected Events171
• 5.9.2 Kinematic Distributions171-177
• 5.10 Cross Section Extraction177-184
• 5.10.1 Efficiency Correction Factor and Acceptance177-179
• 5.10.2 Cross Section Extraction Procedure179-181
• 5.10.3 Cross Section Results181-184
• 5.11 Extraction of the Anomalous Triple Gauge Couplings(aTGC)184-201
• 5.11.1 Introduction184-185
• 5.11.2 MC samples and Observables Sensitive to aTGC's185-187
• 5.11.3 TGC Parametrization and Matrix-element Reweighting187-191
• 5.11.4 TGC Limits Setting191-194
• 5.11.5 Binning Optimization194-196
• 5.11.6 Results196-201
• 5.12 Conclusions201-203
• 5.13 Appendix203-215
• 5.13.1 Background mc samples203-204
• 5.13.2 Performance packages204
• 5.13.3 Jet veto acceptance204-215
• 6 Measurement of the Off-shell Higgs Signal Strength215-287
• 6.1 Introduction215-217
• 6.2 Analysis idea and theoretical considerations217-220
• 6.3 Simulation220-244
• 6.3.1 Simulation and corrections to the gg→VV→4f processes220-230
• 6.3.2 Simulation and corrections to the qq→VV→4f background230-237
• 6.3.3 Correlations between gg→VV→4f and qq→VV→4f237-238
• 6.3.4 Simulation of VV final states in electroweak production modes238-242
• 6.3.5 Dependence of the off-shell signal and the background interference on the signal strength242-244
• 6.4 Analysis strategy in the ZZ→2l2v channel244-251
• 6.4.1 Physics Object Selection244-248
• 6.4.2 Cut Optimization248-251
• 6.5 Background estimation251-262
• 6.5.1 ZZ and WZ Backgrounds251-253
• 6.5.2 WW,tt,Wt,and Z→ττ Backgrounds253-256
• 6.5.3 Z→ee,μμ Backgrounds256-261
• 6.5.4 W+jets and Multijet Backgrounds261-262
• 6.6 Systematic uncertainties262-269
• 6.6.1 Experimental Uncertainties262-263
• 6.6.2 Theoretical Uncertainties263-269
• 6.7 Event Selection Results269-271
• 6.8 Extraction of off-shell couplings and Higgs total width271-280
• 6.8.1 Extraction of off-shell couplings in H~*-→ZZ→llvv271-272
• 6.8.2 Combination with H~*→ZZ→llll and H~*→WW→lvlv Channel272-275
• 6.8.3 Constrains on the Higgs Total Width275-280
• 6.9 Summary280-281
• 6.10 Appendix281-287
• 6.10.1 Additional study of theoretical uncertainties281-287
• 7 Search for Heavy Higgs Bosons287-381
• 7.1 Introduction287-289
• 7.2 Data and Monte Carlo Samples289-291
• 7.2.1 Data sample289
• 7.2.2 Signal samples289
• 7.2.3 SM Higgs samples289-290
• 7.2.4 Other Background Samples290-291
• 7.3 Selection of Physics Objects291-292
• 7.4 Event Selection292-303
• 7.4.1 Trigger292
• 7.4.2 Event Preselection292-295
• 7.4.3 H→ZZ→llvv Selection295-303
• 7.5 Backgrounds303-333
• 7.5.1 Di-boson background303-304
• 7.5.1.1 ZZ background303
• 7.5.1.2 WZ background303-304
• 7.5.2 Z background304-314
• 7.5.3 Wand multijet backgrounds314-326
• 7.5.4 WW,tt,Wt and Z→ττ326-331
• 7.5.5 Other backgrounds331
• 7.5.6 Summary331-333
• 7.6 Systematic Uncertainties333-337
• 7.6.1 Experimental systematic uncertainties333-335
• 7.6.2 Theoretical uncertainties335-337
• 7.7 Results337-340
• 7.8 Statistical interpretation340-358
• 7.8.1 Likelihood definition340-341
• 7.8.2 Fit inputs341
• 7.8.3 Statistical uncertainties341
• 7.8.4 Pruning of the Systematic Uncertainties341-342
• 7.8.5 Nuisance parameter pulls and constraints342
• 7.8.6 Nuisance parameter correlations342-345
• 7.8.7 Nuisance parameter ranking345
• 7.8.8 Postfit plots345-358
• 7.9 Exclusion Limits358-364
• 7.9.1 Exclusion limits on narrow-width Higgs358-359
• 7.9.2 Esclusion limits on 2HDM359-364
• 7.10 Conclusion364-365
• 7.11 Appendix365-381
• 7.11.1 MC samples for the Higgs signal365
• 7.11.2 Expected Higgs signal yields365
• 7.11.3 Signal acceptance uncert ainty365-373
• 7.11.4 WZ Theory Uncertainties373-376
• 7.11.5 Pruning of systematics and MC statistical uncertainties376-380
• 7.11.6 2HDM Interpretation380-381
• 8 Summary381-383
• 9 The Expected Sensitivity for Run 2383-393
• 9.1 Introduction383-385
• 9.2 Expected Sensitivity for Invisible Higgs Search385-387
• 9.3 Expected Sensitivity for Higgs Off-shell Signal Strength Measurement387
• 9.4 Expected Sensitivity for Heavy Higgs Search387-391
• 9.5 Summary391-393
• Bibliography393-413

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本文編號：909637