Higgs Boson


Higgs Boson

希 格 斯 玻 色 子 的 发 现 和 属 性 测 量杨 海 军( 上 海 交 通 大 学 , 代 表 ATLAS 合 作 组 )中 国 物 理 学 会 秋 季 会 议2013 年 9 月 13-15 日

Outline Brief Introduction of Standard Model ATLAS Experiment at LHC Higgs Production and decays at LHC Major challenge for Higgs Searches Observation of Higgs-like particle Update results for Higgs searches (20.7fb -1 at 8 TeV) Higgs Properties (Spin, CP, Couplings) Measurements Summary and ConclusionsCPS2013 H. Yang - Discovery of the Higgs Boson 2

Standard Model of Elementary Particles Elementary Particles> 100 years’ discoveries The SM is in excellentagreement with the numerousexperimental measurements. The only missing SM particle isthe Higgs boson which isproposed to responsible for theelectroweak symmetry breaking,particles acquire mass wheninteracting with the Higgs field.1964: F. Englert, R. Brout, P. Higgs, G.S. Guralnik, C.R. Hagen and T.W.B. KibbleHunting for the Higgs boson is one of main goalsin particle physics (LEP, Tevatron, LHC)CPS2013 H. Yang - Discovery of the Higgs Boson 3

Large Hadron Collider at CERNATLASALICECERNCMSLHCbLHC: 27 km, the world’s largestproton-proton collider (7-14 TeV)Where the WWW was born …CPS2013 H. Yang - Discovery of the Higgs Boson 4

Proton-proton Collisions at LHCCPS2013 H. Yang - Discovery of the Higgs Boson 5

The ATLAS Detector: Huge CameraTaking pictures at arate of 40 Millon/s andrecording pictures at arate of ~1000/second46 x 25 x 25 m, 7000 tons~3000 collaboratorsCPS2013 H. Yang - Discovery of the Higgs Boson 6

Particle Detection Different particles have different signatures in detectorsMuon Spectrometer:muon identification andmomentum measurementHadronic calorimeter:Measurement of jets andmissing energyElectromagnetic calo:e/ identification andenergy measurementTracking system:Charged particlemomentum, vertexingCPS2013 H. Yang - Discovery of the Higgs Boson 7

Higgs Boson Production at LHCYukawacouplingGluon-gluon fusion ggH and vector-bosonfusion qqqqH are dominantGaugecouplingInelastic pp cross section at 7 TeV is ~ 60 mbCPS2013 H. Yang - Discovery of the Higgs Boson 8

Higgs decay branchingratio at m H =125 GeV‣bb: 57.7% (huge QCDbackground)‣WW: 21.5% (easyidentification in di-leptonmode, complex background)‣tt: 6.3% (complex final stateswith t leptonic and/or hadronicdecays)‣ZZ*: 2.6% (“gold-plated”,clean signature of 4-lepton,high S/B, excellent mass peak)‣: 0.23% (excellent massresolution, high sensitivity)Higgs Boson DecayHiggs boson production rate:1 out of 10 12 collision eventsCPS2013 H. Yang - Discovery of the Higgs Boson 9

ATLAS Data Samples 7 TeV data samples (2011)– 4.8 fb -1 for physics analysis– Peak luminosity 3.6×10 33 cm -2 s -1 8 TeV data samples (2012)– 20.7 fb -1 for physics analysis– Peak luminosity 7.7×10 33 cm -2 s -1 Data-taking efficiency: ~94% Significant pileup eventsHCP 2012ICHEP 2012Moriond 2013CPS2013 H. Yang - Discovery of the Higgs Boson 10

Major Challenge (Large Pileup) Large pileup events result in big challenge to the detector,reconstruction and particle identification !HZZ4mmmmmCPS2013 H. Yang - Discovery of the Higgs Boson 11

Observation of a new Particle (July 4, 2012)H H WWH ZZCPS2013 H. Yang - Discovery of the Higgs Boson 12

Observation of a new Particle (5s) !Phys. Lett. B 716 (2012) 1-29 (ATLAS)Phys. Lett. B 716 (2012) 30-61 (CMS)CPS2013 H. Yang - Discovery of the Higgs Boson 13

New - After the Higgs Discovery• H→ , ZZ*, WW* analysisupdates based on full2011-2012 dataset(4.6 fb -1 @ 7TeV,20.7 fb -1 @ 8TeV)New ATLAS Higgs PapersarXiv:1307.1427 Sub. Phys. Lett. B(Mass, Couplings)arXiv:1307.1432 Sub. Phys. Lett. B(Spin-parity)• Higgs mass from H→ andH→ZZ*→ 4l• Signal strengths (m = s/s SM )• Sensitivity to vector boson fusion(VBF)• Higgs Couplings• Higgs Spin and parityNew ATLAS Higgs Pub NotesATLAS-CONF-2013-012 (γγ)ATLAS-CONF-2013-013 (ZZ*)ATLAS-CONF-2013‐031 (WW*)ATLAS-CONF-2013-040 (Spin)ATLAS-CONF-2013-079 (VH → bb)ATLAS-CONF‐2012‐160 (H → tt)ATLAS-CONF-2013‐075 (WW*)ATLAS-CONF-2013-029 (γγ)ATLAS-CONF-2013-009(Z)ATLAS-CONF-2013-010(mm)ATLAS-CONF-2013-067(HMHWW)ATLAS-CONF-2013-072 (diff s H)ATLAS-CONF-2013-075(VHWW)ATLAS-CONF-2013-080(tt +H)ATLAS-CONF-2013-081(tcH)PropertymeasurementSearchesCPS2013 H. Yang - Discovery of the Higgs Boson 14

Update of H Higgs Significance- Expected 4.1s- Observed 7.4sBest fitted mass:M H = 126.8 ±0.2(stat) ± 0.7(syst) GeVBest fittedSignal strengthCPS2013 H. Yang - Discovery of the Higgs Boson 15

Update of HZZ*4lCPS2013H. Yang - Discovery of the Higgs BosonBest fit mass:M H = 124.3 ±0.6(stat) ± 0.3(syst) GeVBest fit signal strength:m = 1.7 + 0.5 (– 0.4) @ 124.3 GeVm = 1.5 ± 0.4 @ 125.5 GeV16

Update of H WW* lnlnDue to spin correlation between W + and W - ,The signal has the following properties:Large P T (ll), small m ll , small Df llATLAS best-fit signal strength:ICHEP(4.6+5.8 fb -1 ): m = 1.3 ±0.52012 (4.6+20.7 fb -1 ): m = 1.0 ± 0.3CPS2013 H. Yang - Discovery of the Higgs Boson 17

Higgs Mass MeasurementsBest fit mass for H and 4lM H ()= 126.6 ±0.2(stat) ± 0.7(syst) GeVM H (4l)= 124.3 ±0.6(stat) ± 0.3(syst) GeVBest fit mass for combination:ATLAS: 125.5 ±0.2(stat) ± 0.6(syst) GeVCMS:125.7±0.3(stat) ± 0.3(syst) GeVMass compatibility: 1.2%, 2.5sCPS2013H. Yang - Discovery of the Higgs Boson18

Search for High Mass H WW, ZZExtend the Higgs search to high mass assume SMlikewidth and decay.ATLAS-CONF-2013-067WW* lnlnZZ* 4l95% C.L. exclusion of a SM-like heavy Higgs up to ~ 650 GeVCPS2013 H. Yang - Discovery of the Higgs Boson 19

Update of Higgs Signal StrengthSignal strength: m = 1.3 ±0.2 (ATLAS)m = 0.8 ±0.14 (CMS)CMS-HIG-13-005ATLAS-CONF-2013-034CPS2013 H. Yang - Discovery of the Higgs Boson 20

Strong Evidence for a New Particle 2012 ICHEPSignificance 5.9s (exp 5.0s)M H =126.0±0.4±0.4 GeV 2012 Full DatasetsSignificance 9.9s (exp 7.5s)M H =125.5±0.2±0.6 GeVCPS2013 H. Yang - Discovery of the Higgs Boson 21

Is it the SM Higgs Boson? Higgs production (m H = 125 GeV) Higgs decaysHW, ZHfW, Zf Couplings (new force!) Spin and Parityg F (Yukawa coupling) =√2 x m F /ng V (Gauge coupling) = 2m V2 /n(n is the vacuum expectation value)CPS2013 H. Yang - Discovery of the Higgs Boson 22

Spin for H Using events in signal mass window [123.6, 128.6] GeV The photon polar angle |cosq*| in the resonance rest frame(Collins-Soper frame) is sensitive to the spin of Higgs. Data agree with spin 0 +hypothesis (1-CL b ) ~ 58.8%. Spin 2 is disfavored at99.3% C.L. (or 2.9s).CPS2013 H. Yang - Discovery of the Higgs Boson 23

Spin for H WW Combine several variables in a MVA discriminant (BoostedDecision Trees, BDT) Variables used: The ATLAS data favors spin 0 with CP even(+).CPS2013 H. Yang - Discovery of the Higgs Boson 24

HZZ*4l : Spin and CP Fully reconstructed final state allows measuring Spin/CP:– Five kinematic angles (production, decay)– Invariant mass of the primary Z and the secondary ZDiscriminate 0 + (SM) hypothesis against:– 0 - (CP odd), 1 + , 1 -– 2 - (pseudo-tensor)– 2 + m (graviton-like tensor, minimal coupling)CPS2013 H. Yang - Discovery of the Higgs Boson 25

HZZ*4l : Spin and CP0 +0 -0 -MVA: m Z1 ,m Z2 + decayangles0 +BDT analysis variables:m Z1 , m Z2 from Higgs --> ZZ* 4l+ production and decay anglesExclusion (1-CL s ):Observed 0 - exclusion 97.8%Observed 1 + exclusion 99.8%CPS2013 H. Yang - Discovery of the Higgs Boson 26

Higgs Boson SpinCPS2013 H. Yang - Discovery of the Higgs Boson 27

Higgs Production: ggF vs.VBFμ VBF+VH vs μ ggF+ttH potentially modified by B/B SMμ VBF+VH / μ ggF+ttH = 1.4+0.4-0.3(stat)+0.6-0.4(sys)CPS2013 H. Yang - Discovery of the Higgs Boson 28

Fermion and Vector Couplings2-parameter benchmark model:k V = k W =k Z (>0)k F =k t =k b =k c =k t =k g(Gluon coupling are related to top, b,and their interference in tree level loopdiagrams)Assume no BSM contributionsto loops: gg H and H,and no BSM decays (noinvisible decays)‣ k F = 0 is excluded (>5s)Double minimum frominterference between vector(W)and fermion (top) in HCPS2013 H. Yang - Discovery of the Higgs Boson 29

Constraints on BSM LoopsNew heavy particles may contribute to loops• Introduce effective k g , k to allow heavy BSMparticles contribute to the loops• Tree-level couplings: k W ,k Z, k t ,k b , k t etc set to 1• Absorb all difference into loop couplings• Indirectly fixed normalization of Higgs widthk g =1.04 ± 0.14 k =1.20 ± 0.152D Compatibility with SM: 14%Couplings tested for anomalies w.r.t.fermion and boson, W/Z and vertex loopcontributions at ±10-15% precisionCPS2013 H. Yang - Discovery of the Higgs Boson 30

Summary and Conclusions A SM-like Higgs particle was observed and confirmedMass: m H = 125.5 ± 0.2 (stat) ± 0.6 (syst) GeVSignal strength @ 125.5 GeV : m = 1.3 ± 0.2 Higgs decays to , ZZ* and WW* (Gauge coupling)are established. The spin-1 is excluded due to observation of H. Spin/CP: data favour 0 + (spin 0 and CP even, SM) Uncertainties of couplings parameters ~10-20% , nosignificant deviations from SM couplings are observed.CPS2013 H. Yang - Discovery of the Higgs Boson 31

Backup Slides Higgs couplings: https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/CONFNOTES/ATLAS-CONF-2013-034 Higgs mass: https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/CONFNOTES/ATLAS-CONF-2013-014 Higgs spin: https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/CONFNOTES/ATLAS-CONF-2013-040 γγ: https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/CONFNOTES/ATLAS-CONF-2013-012 γγ spin : https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/CONFNOTES/ATLAS-CONF-2013-029 ZZ: https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/CONFNOTES/ATLAS-CONF-2013-013 WW: https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/CONFNOTES/ATLAS-CONF-2013-030 WW spin: https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/CONFNOTES/ATLAS-CONF-2013-031 ττ: https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/CONFNOTES/ATLAS-CONF-2012-160 bb: https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/CONFNOTES/ATLAS-CONF-2012-161 μμ: https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/CONFNOTES/ATLAS-CONF-2013-010 Zγ: https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/CONFNOTES/ATLAS-CONF-2013-009 ZH→invisible: https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/CONFNOTES/ATLAS-CONF-2013-009CPS2013 H. Yang - Discovery of the Higgs Boson 32

Summary of what we know now ?Higgs Mass125-126 GeV – agree with SM rough predictionSpin Spin 0 fits well, spin 1 unlikely, spin 2 + excluded at 99.9%ParityChargeLifetimeInteraction with W/ZInteraction with fermionsInteraction with gluonsReasonable evidence it is symmetricZero, as it should beUnknown, but narrow resonance and no obvious flightRates in WW and ZZ look as expectedDirect evidence weak, gluon rate implies ~SM coupling,Tevatron +CMS have evidence for HbbTotal rate suggest it’s as expectedInteraction with photons 1.6 ±0.2(stat) ±0.2 (syst), < 2s deviation from SM !ConclusionSM Higgs with reasonable statistical fluctuationsCPS2013 H. Yang - Discovery of the Higgs Boson 33

Fermion and Vector CouplingsTwo coupling scale factors k F forfermions and k V for bosons Vector coupling (k V ) measured in channels (H, WW, ZZ) Fermion coupling (k F ) measured:– Directly in Hbb and Htt– Indirectly via loop ggH l FV =k F /k V , k VV =k V·k V /k H‣Measured ratio of fermion tovector couplings:‣Fermion & vector couplingsnon-zero, consistent with SM.‣2D compatibility of the SMhypothesis with the best fitpoint is 8%.CPS2013 H. Yang - Discovery of the Higgs Boson 34

Probing custodial symmetry of the W/Z Coupling Similar to previous benchmark model, but k V k W and k Z ,so there are three free parameters k W , k Z and k F . Identicalcouplings scale factors for the W and Z are required withintight bounds by SU(2) custodial symmetry and r parameter. The VBF process is parametrized with k W and k Z accordingto the Standard Model.‣4D compatibility of the SMhypothesis with the best fitpoint is 9%.CPS2013 H. Yang - Discovery of the Higgs Boson 35

Probing Potential Non-SM Particle Contributions For H and ggH vertices, effective scale factors k andk g are introduced (two free parameters). Non-SM particles cancontribute to H and ggH loops or in new final states.Compatibility10%CPS2013 H. Yang - Discovery of the Higgs Boson 36

Consistency Check of Higgs Mass DiscrepancyThe probability for a single Higgs boson-like particle to produce a value ofthe L test statistic disfavoring the DM H =0 hypothesis more than observedis found to be 1.2% or 2.5s .CPS2013 H. Yang - Discovery of the Higgs Boson 37

Coupling Measurements The compatibility of themeasured yields for the studiedchannels with the prediction forthe SM Higgs boson is testedunder various benchmarkassumptions proving salientfeatures of the couplings. For the different testedbenchmarks the compatibilitywith the SM Higgs expectationranges between 10%-20%. No significant deviation fromthe SM prediction is observedin any of the fits performed.CPS2013 H. Yang - Discovery of the Higgs Boson 38

Higgs Observation of H excludes spin-1 Higgs has two types of couplings– “Gauge” couplings (to bosons)– Yukawa couplings (to fermions) Explore tension between SM value and observation fromdifferent Higgs production modes: m VBF+VH vs. m ggF+ttHCPS2013 H. Yang - Discovery of the Higgs Boson 39

Higgs WW* lnln Model independent couplingstudies which are directly relatedto experimental observables.2D contour: m VBF+VH vs. m ggF+ttHThe signal strength ratioscancel the branching ratios ofdifferent channels so that theresults can be compared directly.CPS2013 H. Yang - Discovery of the Higgs Boson 40

Higgs Coupling StructureDepending on the benchmark model, k g , k and k H are eitherfunctions of other couplings or independent parameters. Notation for ggHCPS2013 H. Yang - Discovery of the Higgs Boson 41

HZZ*4l CandidateCPS2013 H. Yang - Discovery of the Higgs Boson 42

Higgs CPS2013 H. Yang - Discovery of the Higgs Boson 43

Higgs Boson DecaysThe decay properties of the Higgs boson are fixed,if the mass is known:HW + , Z, t, b, c, g, t + , W - , Z, t, b, c, g, t - , Higgs Boson:‣ it couples to particlesproportional to their masses‣ decays preferentially in theheaviest particles kinematicallyallowedCPS2013 H. Yang - Discovery of the Higgs Boson 44

Higgs MechanismThe potential in (a) is symmetricThe potential in (b) the potential is still symmetric,but the symmetry of the ground state is spontaneously broken.ZZSpontaneously symmetry breaking Nambu-Goldstone bosons (no spin, mass)Peter Higgs showed that Goldstone bosons need not occur when a local symmetryis spontaneously broken in a relativistic theory. Instead, the Goldstone modeprovides the third polarisation of a massive vector field. The other mode of theoriginal scalar doublet remains as a massive spin-zero particle – the Higgs boson.CPS2013 H. Yang - Discovery of the Higgs Boson 45

History of the Higgs Search 1964 Brout & Englert, Higgs, Guralnik, Hagen & Kibble,– Not taken too seriously until...1973 Experimental acceptance of the Standard Model1983 Discovery of W and Z bosons– Closely linked to the Higgs boson1993 CERN/LEP1 studies Z's and rules out m H 300 GeV2000 CERN/LEP2 lower limit reaches 114.4 GeV2011 CERN/LHC excludes 130-550 GeV2011 Fermilab/Tevatron excludes 156-175 GeV2012 Fermilab/Tevatron observed 2.5s excess at [120,130]2012.7.4 New particle found at ~125 GeV– 5s for ATLAS/CMS, consistent with the SM HiggsCPS2013 H. Yang - Discovery of the Higgs Boson 46

Higgs Boson Width‣ Strong mass dependent H = 3.5 MeV @ 120 GeV1.4 GeV @ 200 GeV8.4 GeV @ 300 GeV68.0 GeV @ 500 GeV‣At low mass region (

Higgs Z Loop decay like H– Could be enhanced if radion not Higgs (hep-ph/9907447) Four channels combined to make limits– ee, mm at 7 TeV and 8 TeV– Observed limit 18 SM Higgs– Expected limit 13 SM HiggsCPS2013 H. Yang - Discovery of the Higgs Boson 48

Higgs mmExpect suppressed by (1.7778/0.1056) 2 ~280 w.r.t. ttGood efficiency and mass resolution improves things- But SM sensitivity needs considerably more data- Observed limit 9.2 SM- Expected limit 8.2 SMCPS2013 H. Yang - Discovery of the Higgs Boson 49

Update of Z4lSingle resonant Z4lenhanced by relaxingmass, P T requirementsBR of Z4l is measured to be (4.2±0.4 )10 -6 ,consistent with SM prediction (4.37±0.03) 10 -6 .CPS2013H. Yang - Discovery of the Higgs Boson50

Is it the SM Higgs Boson? Verify the new observed particle Spin-0 particle Spin-1: excluded by H Spin-2: look at angular correlations CP-natureSM Higgs CP-even, extended Higgs sectors has CP-odd or mixed statesLook at angular correlations CouplingsGauge / Yukawa couplings g vvH , g ffH mUnitarity in W L W L scattering g WWH m WHiggs self-couplings, determine shape of Higgs potential via trilinear andquartic couplings, V = m 2 |F |2 + l|F| 4 + constantCPS2013 H. Yang - Discovery of the Higgs Boson 51

Higgs Mechanism (1964)‣ J. J. Sakurai Prize for Theoretical Particle Physics (2011)Peter W. HiggsPhys. Lett. 12 (1964.9.15) 132PRL 13 (1964.10.19) 508F. Englert, R. BroutPRL 13 (1964.8.31) 321G.S. Guralnik, C.R. Hagen andT.W.B. Kibble, PRL 13 (1964.11.16) 585CPS2013 H. Yang - Discovery of the Higgs Boson 52

Search for Higgs boson at LEPResults: exclude m H < 114.4 GeV/c 2 at 95% CL(Physics Letters B 565 (2003) 61-75)CPS2013 H. Yang - Discovery of the Higgs Boson 53

Search for Higgs boson at Tevatron Results (arXiv:1207.0449): 2.5s excess at m H =120-130 GeV~2.5s excessCPS2013 H. Yang - Discovery of the Higgs Boson 54

Searches for Higgs Boson at LEP and LHC Direct searches at LEP (2000):m H > 114.4 GeV @ 95% C.L. Direct search at LHC (2012.3)m H < 127 GeV @ 95% C.L. Precision electroweak data aresensitive to Higgs mass, globalfit mass:M 2 W=M 2 Z(1-sin 2 q w )(1+Dr)Radiative correction: Dr(m t ,m H ,a,…)CPS2013 H. Yang - Discovery of the Higgs Boson 55

Evolution of Higgs ZZ* 4l CandidatesCPS2013 H. Yang - Discovery of the Higgs Boson 56

Higgs Signal StrengthSignal strength μ = σ/σ SMCombination of diboson final statesH → γγH → ZZ(*) → 4lH → WW(*) → lνlνmeasured at combined m H =125.5 GeV• Variation due to m H uncertainty:±3%• Compatibility with SM (μ=1): 7%• Largest deviation μ γγ : 1.9 sIncluding preliminary μ bb , μ ττ : μ=1.23 ±0.18ATLAS also sets preliminary (95%CL)limits:H → μμ: μ

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