1. 1 Search for SM Higgs via ATLAS detector May 29th, 2012 方亚泉 威士康辛大学麦迪逊分校 University of Wisconsin, Madison yaquan.fang@cern.ch 山东大学学术交流

2. introduction LHC and ATLAS detector. Standard Model, Higgs Mechanism and its cross-section and branching ratio. Show the most updated results channel by channel. H → γγ H → ZZ → 4l H → ZZ → ll νν,llqq, H → WW, H → bb, H →  (very brief). Combined results for full 2011data. Conclusion and outlook for 2012 3/21/2016 2

3. LHC (Large Hadron Collider) 3 proton synchrotron : 26 GeV Super proton synchrotron : 450 GeV 4 TeV  100 meters underground, ring with radius 4.3 kilometers  CM energy : 2012 ， 8 TeV, 2011, 7 TeV  Four experiment ： ATLAS ， CMS, ALICE ， LHCb 4 TeV in 2012 4.3 kilometers Higgs, “God Particle”

4. ATLAS detector 3/21/2016 4  Long: 44 meters ， 25 meters in diameter ~7000 tons. (One Eiffel tower,~100 jet 747).  components built within 35 countries : Muon Spectrometer, Hadronic Calorimeter, Electromagnetic (EM) Calorimeter ， Inner Detector, Barrel Toroid, Solenoid.

5. Standard Model 3/21/2016 5 Standard Model explains what and how the matter is built at the subatomic level :  Subatomic particle : 6 quarks : u, d, c, s, t, b 3 leptons e, μ,  and 3 neutrino  Three fundamental forces to describe the interactions between particles : Electromagnetic (EM) force Weak Strong  Three sets of mediators to mediate the forces :  (EM). W/Z, Higgs (Weak). gluons (Strong).  NOT covered: Dark matter (energy). Neutrino Oscillations and its non-zero mass. Gravitons not included in the frame (GUT). It cannot tell who I am and where I am going to…..

6. Motivation for Higgs Mechanism 6 Gauge Symmetry Lagrangian is invariant under local phase transformation QED : local gauge invariance → massless photon field A μ QCD: local gauge invariance → 8 massless vector gluon fields Weak interaction: massive W/Z instead of massless (1983). A solution: spontaneous breaking of a local gauge symmetry (introduce mass without breaking gauge invariance) (1960s). Or ignore the experiment factor that massive W/Z mediators have been discovered. Recommendation of this book except its price. U(1) SU(3)

7. Higgs Mechanism 7 We substitute Φ and A μ with : V(   So a vector gauge boson A  and massive scalar h (higgs particle) are produced Similarly, for SU(2), three massive gauge fields (W ±,Z) and one massive scalar H are produced (Where μ 2 0) Higgs particle : The last particle in SM that hasn’t been shown experimentally. correct and promising ? Peter Higgs 2008 at CERN

8. SM Higgs Production and decay for LHC 8 Vector Boson Fusion (VBF): Second largest Associated (small cross-sections) ggFusion : dominant 8  Most sensitive channels are : <130: γγ ; 125-300: ZZ*→4l; 300-600: ZZ→llvv, 125-180: WW*→lvlv.  H→ , H→ are significantly affected by QCD backgrounds. Try associated/VBF mode

9. Previous limits from LEP and TEVATRON 3/21/2016 9  Before LHC’s 2011 results, some Higgs mass regions have been excluded by TEVATRON (July, 2010) and LEP.  LEP : excludes <114.4 GeV.  TEVATRON : excludes 158-175 GeV.

10. Data taken with ATLAS detector in 2011 3/21/2016 10 In 2011, LHC operated successfully with high luminosity. Bunch spacing 50 ns, peak lumi : 3.65X10 33 /cm 2 /s Delivered 5.61 fb -1, Recorded 5.25 fb -1. Precise understanding pile-up effect is crucial for the analyses. Especially for analyses related with E T miss and jets. low lum. high lum.

11. H→ γγ channel 3/21/2016 11 PRL cover Phys. Rev. Lett., 2012,108,11803 Phys. Lett. B, 2011, 705, 452-470 ATLAS-CONF-2011-161 ATLAS-CONF-2011-085 ATLAS-CONF-2011-071 ATLAS-CONF-2011-025 ATLAS-CONF-2011-004

12. Signal and backgrounds for H→ γγ Signal : Higgs decays to diphoton via top/W triangle Small branching ratio as page 9 shows. Expect ~200 events (120 GeV) with 5 fb -1 before any selection. Backgrounds : Irreducible Born, Box Reducible : Photon-jet/di-jet with one/two jets faking as a photon/photons. Advantage : side-band to fit the signal (the most important channel) 3/21/2016 12 diphoton+ ······· BornBox Fragmentation + Photon-jets + ······· Di-jet+ ······· q  g

13. Requirements for H→  channel Need good energy and angular resolution to achieve ~1-2% resolution in the Higgs mass reconstruction. σ/m H ~ 1.4% Need good particle identification : ~85% for real photon and reject the large QCD background (   et al.) with rejection above 1000. (9 EM shower shape variables+ isolation are applied to separate reducible backgrounds. ( γ -jet,jet-jet). 13 Purity: 70%

14. Energy Calibration and vertex correction Energy Calibration: MC-based calibration (experience from beam-test) After that, energy scale correction obtained from electrons using Z → ee events from data. Vertex reconstruction : Unconverted photon : 1 st +2 nd layer EM calorimeter Converted photon : 1 st layer EM calorimeter + track from converted e+/e- Robust against pileup (not use primary vertex). 3/21/2016 14

15. Analysis strategy and selections 3/21/2016 15 Selection : Two photons passing trigger, identification, isolation with p T γ 1, γ,2 >40, 25 GeV. Strategy : Based on different ratio of S/B and resolution, divide events into 9 categories: unconverted – converted pseudorapidity (central, transition, rest) and p Tt lower/higher than 40 GeV. where p Tt is nothing but the transverse component of p T  w.r.t. thrust axis : which provides a better resolution than p T .

16. Signal modeling Signal MC are available at 11 mass points : 100-150 GeV with a 5 GeV step. The shape is described by : Crystal-ball (CB) + Gaussian For 120 GeV, resolution of CB is from 1.4 to 2.3 for different categories with inclusive 1.7. Simultaneous fit is applied for all 9 categories. For those mass points not available, derived from parameterization. Signal events passed the inclusive selection on previous page ~70 events for m H = 110-125 GeV 3/21/2016 m H (GeV) 110115120125130135140145150 Ns69.971.570.968.363.757.549.840.830.6 16

17. Background modeling Background shape is determined by a fit with single-exponential in the mass range from 100 to 160 GeV. The mis-modeling is treated as systematic uncertainty on number of signal events (“spurious” signal). Simultaneous fit on all categories with the same mass. 3/21/2016 17 9 Categories inclusive

18. Systematics 3/21/2016 18 20% 14%

19. Exclusion limit w.r.t SM prediction 3/21/2016 19 Expected limit (110-150 GeV): (1.61-2.87)XSM Observed new exclusion (m H ): 114-115 GeV, 135-136 GeV 95% CL → If there is Higgs, there is 5% chance one will make a claim of exclusion by mistake.

20. Excess around 126 GeV 3/21/2016 20 Observed excess at m H = 126 GeV Significance w/o look-else-where effect (LEE) : 2.8 σ  p 0 : If there is no Higgs, one could make a wrong claim (there is Higgs) with a probability p 0. It has to be very small because nobody wants to make a wrong claim of a discovery by mistake (5  is regarded as a safe one).  In the other words, p 0 tells how much possibility an upward fluctuation of the background can be as large as (or larger than) the signal.

21. Comparison with CMS results 3/21/2016 21 CMS shows similar excess : 3.2 σ w/o LEE. The mass is 124 GeV. So before making this claim, one has to scratch her/his hair and really thinks hard.

22. H→ ZZ channel 3/21/2016 22 4-μ events Phys. Lett. B 710(2012) 383-402 Phys. Lett. B 707(2012) 27-45 Phys. Rev. Lett. 107(2011) 221802 ATLAS-CONF-2012-017 ATLAS-CONF-2012-016 ATLAS-CONF-2011-162 ATLAS-CONF-2011-150 ATLAS-CONF-2011-148 ATLAS-CONF-2011-131 ATLAS-CONF-2011-048 ATLAS-CONF-2011-026

23. H→ZZ*→4l 3/21/2016 23 “Golden Channel” : low cross section : expect 10-25 signal events with 5 fb -1, clean (only leptons (e or  ) in final state). narrow peak. but constrained by natural H width for m H >>200 GeV. Simple and loosen selections: 4 leptons: p T 1,2,3,4 > 20,20,7,7 GeV; m 12 = m Z ± 15 GeV; m 34 > 15-60 GeV (depending on m H ) Backgrounds: ZZ (*) (irreducible) Z+jet (in particular bb), tt (Prompt lepton requirements : isolation and impact parameter). Challenge of the analysis : Good reconstruction and identification of low pt lepton. Reducible backgrounds have to be estimated from data. Low statistics with current luminosity ~ 5 fb -1.

24. Signal reconstruction 3/21/2016 24 The resolution of the m H is fairly good.

25. Control reducible backgrounds 3/21/2016 25 tt contribution: use e  channel as a control region. Zjet, ttbar, ZZ estimation ZZ,WZ from MC No isolation, impact parameter, charge requirement on second lepton pair. Z→ee Z→μμ

26. Exclusion limit w.r.t Standard Model prediction 3/21/2016 26 Main systematic uncertainties Higgs cross-section : ~ 15% Electron efficiency : ~ 2-8% ZZ* background : ~ 15% Zbb, +jets backgrounds : ~ 40% Observed exclusions : 135-156, 181-234, 255-415 GeV Expected exclusions : 136-158, 182-400 GeV

27. The distribution of M 4l and p 0 (for background only hypothesis) 3/21/2016 27 m H (GeV)Local p 0 Obs. significance 1251.8%2.1 2441.1%2.3 5001.4%2.2

28. H→ZZ→llνv 3/21/2016 28  H →ZZ→llvv is more sensitive at high mass region (both Z on shell).  High pile-up and low pile-up analysis are separated. Most sensitive for high mass Observed exclusions : 320-560 GeV Expected exclusions : 260-490 GeV two regions of selections Z mass window, E miss T and ΔΦ ll selections

29. H→ZZ→llqq Highest rate among ZZ decaying with leptons. on-shell is focused here (ZZ : 200-600 GeV). Backgrounds : Z+jets (largest), top estimated from sideband ZZ,WZ (MC) Selection : Two leptons with 83<m ll <99 GeV Two jets with 70<m jj <105 GeV E T miss <50 GeV More selection for m H >300 GeV Divide into two categories : b-tagged and untagged 3/21/2016 29 Observed exclusions : 300-310, 360-400 GeV Expected exclusions : 360-400 GeV

30. 10/5/2009 30 H→WW→lvlv/lvqq channel H →  channel H→bb channel Phys. Rev. Lett. 108, 11802(2012) Phys. Rev. Lett. 107, 231801(2011) ATLAS-CONF-2012-018 ATLAS-CONF-2011-134 ATLAS-CONF-2012-012 ATLAS-CONF-2012-015 ATLAS-CONF-2012-014 ATLAS-CONF-2010-092

31. Limits for H→WW→lvlv/lvqq 10/5/2009 31 For H→WW→lvlv sub-channel : For H →WW→lvqq sub-channel, exclusion of 1XSM Higgs prediction is not achieved yet. No significant excess is observed in H→WW channels. Expected exclusions : 130-260 GeV Observed exclusions : 127-234 GeV

32. H→bb and H→  For H →bb, only W/ZH model (with leptonic decay of W/Z) is considered due to significant QCD background. For H → ,events are divided into three categories : leptonic, hadronic, leptonic-hadronic decays 3/21/2016 32 Not exclude 1XSM Higgs prediction yet.

33. Combined limit and local p 0 3/21/2016 33  The expected excluded region covers from 120 to 555 GeV (including 126 GeV).  The observed excluded regions are :110.0 GeV -117.5 GeV, 118.5 GeV- 122.5 GeV and from 129 GeV to 539 GeV.  With full 2011 data, we observed an excess from both H → γγ and H →ZZ*→4l channels. The largest excess from the combination appears at m H =126 GeV. ATLAS-CONF-2012-019

34. Conclusion and Outlook for 2012 3/21/2016 34 With the full data of 2011, wide mass range of SM Higgs boson mass has been excluded by the ATLAS detector. 110.0 GeV -117.5 GeV, 118.5 GeV- 122.5 GeV, 129 GeV -539 GeV. An excess has been observed around 126 GeV. With expected 2012 data (15-20 fb -1 ), ATLAS expects to exclude the whole mass region ( May 26 th : 3.1 fb -1 ). It is also possible to have a 5 σ discovery with 2012 data. (Of course, we prefer the latter). Thank you

35. backup slides 3/21/2016 35

36. 3/21/2016 36 Masses of the gauge bosons through symmetry breaking No mass prediction for Higgs. It tends to smaller than a few hundred GeV from a meanful perturbation expansion.

37. 3/21/2016 37 Shower shape variables for photon and jet

38. The ATLAS Collaboration 3000 scientists including 1000 graduate students 38 countries 174 universities and research labs 38

39. Physics Analysis LHC Detectors Construction & Commissioning Trigger & Data Acquisition Event Reconstruction & Calibration Event Generation & Simulation Performance of the Reconstruction