Higgs Boson Searches at ATLAS

Transcription

1 Higgs Boson Searches at ATLAS Jianming Qian University of Michigan October 5, 2011 Maryland/John Hopkins Joint Seminar, October 5, 2011 Jianming Qian (University of Michigan) 1

2 Standard Model Standard model does not answer all the questions, but it does describe existing data remarkable well Nevertheless, there are a few known anomalies - ttbar F-B charge asymmetry - µ ± µ ± charge asymmetry The EW symmetry breaking mechanism in the SM is not confirmed Hunting for Higgs boson Maryland/John Hopkins Joint Seminar, October 5, 2011 Jianming Qian (University of Michigan) 2

3 Higgs Boson Mass Constraint Direct searches at LEP: m > % CL H Precision electroweak data are sensitive to Higgs mass Preferred value from global fit: + 32 = 92 GeV m H 26 and 95% CL upper bound m < 161 GeV H Existing data suggests a low mass standard model Higgs Maryland/John Hopkins Joint Seminar, October 5, 2011 Jianming Qian (University of Michigan) 3

4 Higgs Boson Production ggf gluon-gluon fusion gg H and vector-boson fusion qq qqh diagrams dominate VBF σ H GeV, GeV Typical uncertainty: <10% Maryland/John Hopkins Joint Seminar, October 5, 2011 Jianming Qian (University of Michigan) 4

5 Higgs Boson Decay To all particles kinemtically allowed, but two dominant modes: H bb for mh < 135 GeV; H WW for mh > 135 GeV Neither is ideal for the search and the study of properties - bbbar by itself suffers from huge QCD backgrounds - WW: easy identification in dilepton mode, but no full reconstruction Difficult low mass region: - most decay modes suffer from large background; - the γγ mode offers the best promise (full reconstruction and good mass resolution) Maryland/John Hopkins Joint Seminar, October 5, 2011 Jianming Qian (University of Michigan) 5

6 Higgs Boson Width Strong mass dependent Γ H = GeV, GeV, GeV, GeV At low mass (<300 GeV), detector resolution dominates mass resolution. At higher mass, intrinsic width becomes dominant. Γ H 3 3GFMH 16π 2 MH 500 GeV 1 TeV 3 Maryland/John Hopkins Joint Seminar, October 5, 2011 Jianming Qian (University of Michigan) 6

7 Higgs Searches at Tevatron The ggf cross section is x10 smaller than that at the LHC. Main search channels are: WH νbb, ZH νν bb, H WW ν ν,... The combined CDF and DØ searches resulted in a mass exclusion range of GeV at 95% CL Maryland/John Hopkins Joint Seminar, October 5, 2011 Jianming Qian (University of Michigan) 7

8 Large Hadron Collider A Superconducting Proton-Proton Collider Design: 14 TeV with the peak luminosity of cm -2 s -1 Currently running at 7 TeV with the peak luminosity of cm -2 s -1 Two general purpose detectors: ATLAS and CMS Maryland/John Hopkins Joint Seminar, October 5, 2011 Jianming Qian (University of Michigan) 8

9 LHC: Peak Luminosity Maryland/John Hopkins Joint Seminar, October 5, 2011 Jianming Qian (University of Michigan) 9

10 ATLAS Detector Maryland/John Hopkins Joint Seminar, October 5, 2011 Jianming Qian (University of Michigan) 10

11 Data Samples We are having a hard time to keep up the data! Analyses are a bit conference driven right now. EPS dataset: ~ 1.2 fb -1 Most of the results are based on this dataset Lepton-Photon: ~ 2.5 fb -1 Updates only for a few analyses Data-quality efficiency is typically ~85% EPS LP Maryland/John Hopkins Joint Seminar, October 5, 2011 Jianming Qian (University of Michigan) 11

12 Challenge of High Luminosity Multiple interactions! 11 reconstructed vertices 50 ns bunch trains for all 2011 data, significant effect on - Lepton reconstruction and isolation - Primary vertex identification - Jet energy and multiplicity - ETmiss resolution Maryland/John Hopkins Joint Seminar, October 5, 2011 Jianming Qian (University of Michigan) 12

13 Many Negative Results! Higgs is in a league of its own! Maryland/John Hopkins Joint Seminar, October 5, 2011 Jianming Qian (University of Michigan) 13

14 Higgs Search Channels Driven by production rate, trigger and identification capabilities. Low mass region (<140 GeV): H γγ 1 ( 1.08 fb ) H bb in qq VH H ττ in qq qqh High mass (>130 GeV): H H WW ν ν ν qq ZZ νν qq 1 ( 1.7 fb ) 1 ( 1.04 fb ) 1 ( 2.28 fb ) 1 ( 1.04 fb ) 1 ( 1.04 fb ) 1 ( 1.04 fb ) 1 ( 1.06 fb ) Analyzed integrated luminosity varies from channel to channel Maryland/John Hopkins Joint Seminar, October 5, 2011 Jianming Qian (University of Michigan) 14

15 Statistical Methods Construct likelihood from Poisson probabilities : ( ) ( ) ( ) Compare data with background-only and signal+background models using test statistics L( data µθ, ˆ ) µ q 2ln with 0 ˆ µ = µ µ L data ˆ µθ, ˆ ( ) Calculate the ratio of these two p-values The signal model is excluded at 95% CL if ( ) ( ) L data µθ, = Poisson data µ s θ + b θ p θ θ µ : signal strength; θ: 'nuisance' parameters (efficiencies...) CL s ( ) s+ b µ b ( obs ) µ > µ + ( obs ) µ > µ CL P q q s b = = CL P q q b ( ) CL µ = 1 < 5% s Maryland/John Hopkins Joint Seminar, October 5, 2011 Jianming Qian (University of Michigan) 15

16 H γγ: Introduction Very simple signature, but small rate Br(H γγ)~2x10-3 ; Important decay mode for the low mass region ( GeV) σ H ( γγ ) ~ 0.04 pb Br m = 120 GeV H Irreducible background from γγ production σ ( γγ ) ~ 30 pb Reducible background from γj and jj productions ( j) ( jj) σ γ Theoretical uncertainty σ σ~ 30%, not reliable! 5 ~ 2 10 pb 8 ~ 5 10 pb Maryland/John Hopkins Joint Seminar, October 5, 2011 Jianming Qian (University of Michigan) 16 σ

17 H γγ: Event Identification Photon identification - Main background: π 0 from jets; - Longitudinal and lateral shower profiles; - No track or a pair of tracks consistent with photon conversions (~40% γ converts!) - Energy calibration from Z, J/ψ ee events Event selection: - Two isolated photons with pt>40,25 GeV with η <1.37 and 1.52< η <2.37 Selection efficiency ~40% Vertex reconstruction: - Unconverted: Photon pointing of longitudinal samplings; Resolution with pointing: δ z γγ ~ 1.5 cm - Converted: calorimeter and conversion vertex extrapolation Maryland/John Hopkins Joint Seminar, October 5, 2011 Jianming Qian (University of Michigan) 17

18 H γγ: Mass Reconstruction Full reconstruction of the Higgs decay final state, every little else to distinguish signal from backgrounds other than mass: m ( ) = 2E E 1 cos φ γ γ γγ Mass resolution is the key, dominated by the energy resolution. δ m~ 1.7 GeV at m= 120 GeV Taking advantage of different mass resolutions and signal-background ratios, data sample is splitted into five categories - Converted photons vs unconverted photons; - Detector regions: central, transition and forward improve the H γγ sensitivity by ~15% Maryland/John Hopkins Joint Seminar, October 5, 2011 Jianming Qian (University of Michigan) 18

19 H γγ: Backgrounds The γγ, γj and jj contributions can be decomposed through the analysis of photon identification and isolation. - Determine the shape of these variables for real and fake photons from control samples and MC simulation; - Fit the observed distribution to the sum of three components Isolation distribution decomposed mass distribution 72% Maryland/John Hopkins Joint Seminar, October 5, 2011 Jianming Qian (University of Michigan) 19

20 H γγ: Results Fit observed γγ mass distribution with signal + background model: - Exponential spectrum for background; - Crystal-ball function to describe the signal No indication of any significant bump in the mass spectrum: candidates between GeV; - 18 Higgs signal events expected at 120 GeV 95% CL upper limits are set on σ H ( H γγ ) BR arxiv: SM Maryland/John Hopkins Joint Seminar, October 5, 2011 Jianming Qian (University of Michigan) 20

21 W/Z+H bb Good: Large decay branching ratio at low mass, full reconstruction, one of few modes for potential Higgs-fermion coupling measurement; Bad: Huge QCD backgrounds in the dominant gg H bb production mode associated W/Z+H production with lepton Select events with W or Z bosons in leptonic final states and with exactly two b-tagged jets, the main backgrounds are W/Z+jets, top, multijets, Future improvement: highly boosted Higgs Maryland/John Hopkins Joint Seminar, October 5, 2011 Jianming Qian (University of Michigan) 21

22 VBF H ττ - Similar to H bb decay, gg H ττ suffers from insurmountable backgrounds - Backgrounds are manageable for the VBF process - Likely the only mode for Higgs-lepton measurement Select events with at least one leptonically decaying taus and at least one tagging jet, main backgrounds are W/Z+jets, top, Mass reconstruction through collinear approximation τ τ E/ T Maryland/John Hopkins Joint Seminar, October 5, 2011 Jianming Qian (University of Michigan) 22

23 H WW lvlv The SM WW is said to be irreducible σ ( ν ν) 0 35 H WW*. pb ν ν (m H =150 GeV) Main background: WW, W/Z+jets, tt,,... However, WW from the scalar Higgs is expected to have different kinematics W - W - H e - W + ν W + ν The spin correlation leads to a smaller average opening angle between the two leptons μ + Maryland/John Hopkins Joint Seminar, October 5, 2011 Jianming Qian (University of Michigan) 23

24 H WW lvlv Strategies Wide variation of backgrounds in lepton flavors and different jet multiplicity Separate analysis in lepton flavor and jet bin: - Flavor: same (ee and µµ) and opposite (eµ) flavor; - Jet bin: 0 jet (H+0j) and 1 jet (H+1j) Jet reconstruction and selection: - Reconstructed using anti-kt algorithm; - pt>25 GeV with η <4.5 Event pre-selection: - Two isolated leptons (electrons or muons): ( ) ( ) 1 2 p p e T > 25 GeV, > 15 µ 20 GeV; η < 2.5 T - Large momentum imbalance (ETmiss): E/ > Rel T ( ee µµ ) ( eµ ) 40 GeV,, 25 GeV Rel E/ T φ π 2 T E/ T sin φ φ > π 2 E/ = φ = min /, jets ( ET ) { φ } Maryland/John Hopkins Joint Seminar, October 5, 2011 Jianming Qian (University of Michigan) 24

25 H WW lvlv: Z Backgrounds The overwhelming contribution to events with two isolated leptons are Z ee, µµ: σ(z ee/µµ)~1000 pb, σ(h WW ee/µµ)~ 0.1 pb (m H =150 GeV) However, they can be reduced with Z-mass veto and ETmiss cut. Maryland/John Hopkins Joint Seminar, October 5, 2011 Jianming Qian (University of Michigan) 25

26 H WW lvlv: Jet Multiplicity After preselection (dilepton events with large ETmiss) H+0j analysis: SM WW dominates H+1j analysis: SM WW and top contribute about equally H+2j analysis: aimed at VBF production, not ready yet Additional topological requirements to further reduce backgrounds Maryland/John Hopkins Joint Seminar, October 5, 2011 Jianming Qian (University of Michigan) 26

27 H WW lvlv: H+0j Mass dependent topological selections. For Higgs mass below 170 GeV: T = T +/ T T +/ T ( ) 2 ( ) 2 M E E p E p M T > 30 GeV, < 50 GeV; φ < m < M < m H T H Maryland/John Hopkins Joint Seminar, October 5, 2011 Jianming Qian (University of Michigan) 27

28 H WW lvlv: H+1j - Reduce top through vetoing events with b-tagged jets; - Reduce Z ττ contribution through mass reconstruction b-jet veto tot pt < 30 GeV; mττ MZ > 25 GeV 0.75m < M < m H T H Maryland/John Hopkins Joint Seminar, October 5, 2011 Jianming Qian (University of Michigan) 28

29 H WW lvlv: Backgrounds Backgrounds are determined from data as much as possible - Top and WW: scaled from data control regions: est. N SR.. Data Data NSR.. = NCR.. = α MC NCR.. NCR.. MC - W+jets and multijets: from data using bad/nonisolated leptons; Select events with one good lepton and one bad/nonisolated lepton and then scale the number up using the factor from control samples - Z+jets: MC but corrected for data/mc ETmiss mis-modeling; est MC C C+ D NA = NA C+ D C Data - Others (WZ, ZZ, ): MC Small backgrounds with well predicted cross sections MC Maryland/John Hopkins Joint Seminar, October 5, 2011 Jianming Qian (University of Michigan) 29

30 H WW lvlv: Yields H+0j (m H = 150 GeV) H+1j (m H = 150 GeV) Typical selection efficiency <10% Maryland/John Hopkins Joint Seminar, October 5, 2011 Jianming Qian (University of Michigan) 30

31 H WW lvlv: Results Small excesses (~2σ) are observed over a wide mass range at low masses Expected from a true signal due to the poor mass resolution; More data and study are needed Nevertheless, a standard model Higgs boson is excluded at 95% CL: 154 < < 186 GeV m H The expected exclusion: 135 < m H < 196 GeV (The jump at 220 GeV is due to selection change) Maryland/John Hopkins Joint Seminar, October 5, 2011 Jianming Qian (University of Michigan) 31

32 H WW lvqq About 3.5 times the rate of H WW lvlv, but significantly high backgrounds. Have good sensitivities for heavy Higgs (>200 GeV) when both W s are on-shell: m = MW and m = ν qq MW Select events with one high pt lepton, large ETmiss and 2 (or 3) jets; Solve neutrino momentum using the mass constraint; Look for bumps in the M(lvqq) distribution over smooth background Maryland/John Hopkins Joint Seminar, October 5, 2011 Jianming Qian (University of Michigan) 33

33 H ZZ 4l: Introduction The gold-plated channel over a wide range of potential Higgs mass. Clean signature: - 4 isolated leptons, full reconstruction; - Mass peak over backgrounds. Small backgrounds: Irreducible SM ZZ production and reducible Z+jets, top, But even smaller signal rate: ( ) ( ) σ ( H ZZ ) BR ZZ 4 = 0.45%, BR H ZZ = 4% at m = 130 GeV BR 4 = pb at 130 GeV and pb at 200 GeV H Selection efficiency to the 4 th power of lepton efficiency: ~ 0.25 H = e, µ Maryland/John Hopkins Joint Seminar, October 5, 2011 Jianming Qian (University of Michigan) 34

34 H ZZ 4l: Event Selection Two same-flavor and opposite-sign isolated lepton pairs: Each with pt > 7 GeV and η < 2.5, at least two with pt > 20 GeV; Mass combinations: m M < 15 GeV, m < m < 115 GeV 12 Z threshold 34 mthreshold depends on reconstructed A total of 27 events are selected: 6ee, 9eµ, 12µµ compared with 28±4 expected m4 Maryland/John Hopkins Joint Seminar, October 5, 2011 Jianming Qian (University of Michigan) 35

35 H ZZ 4l: Backgrounds Completely dominated by the irreducible SM ZZ* background which unfortunately has to be estimated through MC simulation Top background is also estimated from MC, but checked using eµ control samples in data; Z+jets background is important at low mass, estimated using data control samples: - select Z ee,µµ events; - require two additional same-flavor opposite-sign leptons without isolation and impact parameter requirement; - scale the rate to the signal region using MC simulation Z+jets increases rapidly at low lepton pt Maryland/John Hopkins Joint Seminar, October 5, 2011 Jianming Qian (University of Michigan) 36

36 H ZZ 4l: A Candidate m = GeV, m = 85.9 GeV, m = 85.5 GeV Maryland/John Hopkins Joint Seminar, October 5, 2011 Jianming Qian (University of Michigan) 37

37 H ZZ 4l: Mass Distributions m 12 m 34 m 4 m4 2% Maryland/John Hopkins Joint Seminar, October 5, 2011 Jianming Qian (University of Michigan) 38

38 H ZZ 4l: Results No credible clustering in the observed mass distribution, Fit the observed mass distribution to the signal+background model to determine 95% CL upper limit on the rate Small BR(H ZZ) Need more data Maryland/John Hopkins Joint Seminar, October 5, 2011 Jianming Qian (University of Michigan) 39

39 H ZZ llvv - Six times the rate of H ZZ 4l, but no mass peak; - Topology similar to H WW lvlv similar analysis; - Good S/B ratio when both Z s are on-shell high mass Select events with two leptons consistent from Z decays and with large ETmiss. Main backgrounds are Z+jets, diboson and top Maryland/John Hopkins Joint Seminar, October 5, 2011 Jianming Qian (University of Michigan) 40

40 H ZZ llvv The observation is consistent with the background expectation Limits are extracted by fitting the observed mt distribution Best sensitivity for the range of GeV; A SM Higgs boson is excluded in GeV at 95% CL arxiv: Maryland/John Hopkins Joint Seminar, October 5, 2011 Jianming Qian (University of Michigan) 41

41 H ZZ llqq Highest rate among ZZ decays with leptons (10x 4l rate), full final state reconstruction. But Large Z+jets background and limited dijet mass resolution. Require two on-shell Z s for now high mass ( GeV) Select events with two leptons 76 < m < 106 GeV two or more jets 70 < < 105 GeV m jj without ETmiss. Separate analysis for b-tagged and untagged samples due to different S/B ratio to improve sensitivity Maryland/John Hopkins Joint Seminar, October 5, 2011 Jianming Qian (University of Michigan) 42

42 H ZZ llqq Search for peak in m lljj over continuum background. Typical resolution ~10% at high mass. No significant excess above the expected backgrounds Extract 95% CL upper limit by fitting the m lljj distribution to a signal+background model; The observed limit is a few times of the SM 360 GeV) arxiv: Maryland/John Hopkins Joint Seminar, October 5, 2011 Jianming Qian (University of Michigan) 43

43 Combination: Individual Yields The H ZZ 4l offers the best signal-to-background ratio while H WW lvlv has the largest signal yield for the most of the search range Maryland/John Hopkins Joint Seminar, October 5, 2011 Jianming Qian (University of Michigan) 44

44 Individual Limits No silver bullet, every channel contributes. Low mass region is particularly difficult Maryland/John Hopkins Joint Seminar, October 5, 2011 Jianming Qian (University of Michigan) 45

45 Combined Limits 95% CL mass exclusion ranges: , and GeV Maryland/John Hopkins Joint Seminar, October 5, 2011 Jianming Qian (University of Michigan) 46

46 Combination: p-value Maryland/John Hopkins Joint Seminar, October 5, 2011 Jianming Qian (University of Michigan) 47

47 What s in Store We will likely to have 5 fb -1 this year and >10 fb -1 by end of exclude the entire mass range if no SM Higgs boson - 5σ discovery for Higgs boson mass greater than 130 GeV - difficult low mass region ( GeV), also the most likely region work on further improvement and addition other channels Luminosity extrapolation of current sensitivity Maryland/John Hopkins Joint Seminar, October 5, 2011 Jianming Qian (University of Michigan) 48

48 Summary Search for the standard model Higgs boson is one of the most important physics pursuits at the LHC. The stake is high a discovery will fortify the standard model, But more importantly a complete exclusion will undermine the foundation of the standard model. It has been a wild year so far... From the attempt by an individual group to scoop the Collaboration to the disappearance of a significant excess. No sight yet, but it appears that the Higgs boson (if exists) is likely between GeV. By the end of next year, LHC can completely exclude it or has the potential to discover it. Maryland/John Hopkins Joint Seminar, October 5, 2011 Jianming Qian (University of Michigan) 49

49 Experimental Systematics Maryland/John Hopkins Joint Seminar, October 5, 2011 Jianming Qian (University of Michigan) 50