1. Higgs Properties Measurement based on H  ZZ*  4 with ATLAS 杨海军 （上海交通大学） 中国物理学会高能物理分会 第九届会员代表大会暨学术年会 华中师范大学，武汉 2014 年 4 月 19-22 日 1

2. Outline o Standard Model and Discovery of Higgs Boson o Higgs Production and Decays at LHC o Major Challenge for Higgs Searches o Event Selection of H  ZZ*  4 o Measurement of Properties : mass, spin, CP, couplings o Summary 2 中国科学技术大学和上海交通大学参与 H  ZZ*  4l 的发现和希格斯特性的测量 References: PLB 726 pp.88-119, pp. 120-144 ATLAS-CONF-2013-013 ， ATLAS-CONF-2013-034 ATLAS-CONF-2014-009

3. Standard Model and Discovery of the Higgs 3  The Higgs boson was discovered by ATLAS and CMS at LHC in July, 2012.  F. Englert and P. Higgs won the Nobel Prize in 2013. Higgs boson is proposed to responsible for the electroweak symmetry breaking, particles acquire mass when interacting with the Higgs field.

4. Higgs Boson Production at LHC 4 Gluon-gluon fusion gg  H and vector-boson fusion qq  qqH are dominant Gauge coupling Yukawa coupling 87% 0.5% 5% 7%

5. Higgs Boson Decay Higgs decay branching ratio at m H =125 GeV  bb: 57.7% (huge QCD background)  WW: 21.5% (easy identification in di-lepton mode, complex background)   : 6.3% (complex final states with  leptonic and/or hadronic decays)  ZZ*: 2.6% (“gold-plated”, clean signature of 4-lepton, high S/B, excellent mass peak)   : 0.23% (excellent mass resolution, high sensitivity) 5 Higgs boson production rate: 1 out of 10 12 collision events

6. ATLAS Data Samples  7 TeV data samples (2011) – 4.6 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: ~95.5%  Significant pileup events ICHEP HCP 6

7. Major Challenge (Large Pileup)  Large pileup events result in big challenge to the detector, reconstruction and particle identification ! 7 H  ZZ  4     

8. H  ZZ*  4 Overview  Extremely clean – “Gold-plated” channel o Fully reconstructed final states o Good mass resolution (~ 1.6-2.4 GeV) o High S/B ratio (~ 1-2) o Low decay branching fraction  Currently statistically limited o 4.6 fb -1 @ 7 TeV + 20.7 fb -1 @ 8 TeV o Expect 68 SM H  ZZ*  4  (e,  ) events  Properties measurement o Higgs mass, spin, parity, couplings etc. o Critical to determine whether it is fully compatible with the SM Higgs boson 8

9. Event Selection  Trigger match with single and/or di-lepton trigger  Four sub-channels: 4e, 2e2 , 2  2e, 4  9

10. Background Estimation 10

11. Selected Higgs Candidates 11 BR(H  ZZ*) = 2.63%, BR(ZZ*  4l)=0.45% ~ 68 H  ZZ*  4l events produced

12. H  ZZ*  4 :Mass Calibration BR(meas.)=(3.20 ±0.25±0.13)×10 -6 BR(SM NLO)=(3.33 ±0.01)×10 -6  (PS) =107.3 ± 8.8 ± 4.0 ± 3.0 (fb) (Submitted to PRL, arXiv:1403.5657) 12

13. Higgs Mass and Signal Strength  > 5  discovery in H  ZZ*  4 channel 13 Signal Significance : 6.6  (4.4  expected)

14. Higgs Mass Measurements 14 Best fit mass for H   and 4 M H (  )= 126.6 ±0.2(stat) ± 0.7(syst) GeV M H (4 )= 124.3 ±0.6(stat) ± 0.3(syst) GeV Best fit mass for combination: ATLAS: 125.5 ±0.2(stat) ± 0.6(syst) GeV Mass compatibility: 1.2%, 2.5 

15. Probing Higgs Production 15

16. Probing Higgs Production Rate 16

17. Fermion and Vector Couplings 17 2-parameter benchmark model:  V =  W =  Z  F =  t =  b =  c =   =  g (Gluon coupling are related to top, b, and their interference in tree level loop diagrams) Assume no BSM contributions to loops: gg  H and H  , and no BSM decays (no invisible decays)   F = 0 is excluded (>5  )

18. H  ZZ*  4l : Spin and CP 18

19. H  ZZ*  4l : Spin and CP 19 Observed 0 - exclusion 97.8% Observed 1 + exclusion 99.8% MVA: m Z1, m Z2 + decay angles 0+0+ 0+0+ 0-0- 0-0- BDT analysis variables: m Z1, m Z2 from Higgs --> ZZ*  4l + production and decay angles Exclusion (1-CL s ): Observed 2 + m exclusion 83.2%

20. Summary 20

21. Backup 21

22. Measurements of Higgs Signal Strength 22  Signal strength:  =1.3±0.2(ATLAS)   = 0.8 ±0.14 (CMS) CMS-HIG-13-005 PLB 726 pp.88-119, 120-144 ATLAS-CONF-2014-009

23. Higgs Production: ggF vs.VBF 23 μ VBF+VH vs μ ggF+ttH potentially modified by B/B SM μ VBF+VH / μ ggF+ttH = 1.4+0.4-0.3(stat)+0.6-0.4(syst)

24. Higgs Mass 24

25. Higgs Couplings 25

26. Is it the SM Higgs Boson? 26  Higgs production (m H = 125 GeV)  Higgs decays H f f H W, Z  Couplings (new force!)  Spin and Parity g F (Yukawa coupling) =√2 x m F / g V (Gauge coupling) = 2m V 2 /  is the vacuum expectation value 

27. Coupling Measurements 27 Coupling strengths  i & ratio:  F =g F /g F,SM,  V =g V /g V,SM, ij =  i /  j (8) Example H →   g,   : loop coupling scale factors  H is the total Higgs width scale factor

28. Spin Analysis with H   28 Polar angle  * of the photon decay in Collines-Soper frame, along with m  0+0+ 2+2+ data Z CS bisects angle between the momenta of colliding hadrons CS

29. Spin Analysis With H  WW* 29 BDT discriminant Observed 2 + (qq=100%) exclusion 99.96% Observed 2 + (qq = 0%) exclusion 95.2% 2+2+ 0+0+ Exclusion (1-CL s ):

30. H  ZZ*  4l : Spin and CP 30

31. ATLAS Trigger 31

32. Higgs Production and Decays 32

33. 33

34. Background Estimation 34

35. Selected Events 35

36. Invariant Mass of 4-lepton 36

37. MVA Discriminant: Higgs Spin and CP 37

38. BR of Z  4 38

39. Search for High Mass H  ZZ, WW 39 ZZ WW*  l l ZZ*  4l Extend the Higgs search to high mass assume SM-like width, and decay to WW/ZZ 95% C.L. exclusion of a SM-like heavy Higgs up to ~ 650 GeV ATLAS-CONF-2013-067

40. Large Hadron Collider at CERN 40 CERN LHC: 27 km, the world’s largest proton-proton collider (7-14 TeV) ATLAS ALICE LHCb CMS

41. The ATLAS Detector DAQ: 40 MHz 46 x 25 x 25 m, 7000 tons ~3000 researchers 41

42. Particle Detection  Different particles have different signatures in detectors 42 Muon Spectrometer: muon identification and momentum measurement Hadronic calorimeter: Measurement of jets and missing energy Electromagnetic calo: e/  identification and energy measurement Tracking system: Charged particle momentum, vertexing

43. Higgs Width (CMS)  https://cds.cern.ch/record/1670066/files/HIG-14-002-pas.pdf https://cds.cern.ch/record/1670066/files/HIG-14-002-pas.pdf 43

44. Higgs Width (CMS)  https://cds.cern.ch/record/1670066/files/HIG-14-002-pas.pdf https://cds.cern.ch/record/1670066/files/HIG-14-002-pas.pdf 44