Higgs Searches with the ATLAS Experiment at the LHC Bruce Mellado University of Wisconsin-Madison HEP Seminar, PSI 11/11/08
Outline Introduction Most relevant observation channels (SM) H →γγ →γγ H →ττ Focus on what we can →ττ do with 10 fb -1 of H → ZZ (*) → 4l data at the LHC H → WW (*) → ll νν νν MSSM Higgs What can the Tevatron tell us? Feasibility of searches Bruce Mellado, PSI 11/11/08 2
Standard Model of Particle Physics Quarks and Leptons interact via the exchange of force carriers quark, lepton force carrier quark, lepton Force Carrier A Higgs boson in predicted and Strong Gluons (g) Electro-weak required to give mass Electro-Weak bosons ( γ ,W,Z) to particles ? Gravitation Bruce Mellado, PSI 11/11/08 3
What is the origin of the particle masses? Why some particles are heavier than others? The discovery of the Higgs boson should answer these questions Bruce Mellado, PSI 11/11/08 4
The Quest for the Higgs Experimentalists have been looking for the Higgs since the 70’s and 80’s in decays of nuclei, π , K, B, Y, etc… yielding mass limit <5 GeV One of the goals of the LEP experiments (e + e - collisions 1989-2000) was to search for a Higgs boson. The most stringent limit to date comes from the LEP experiments Bruce Mellado, PSI 11/11/08 5
LEP Higgs Searches (M H =115) 2.6% Bruce Mellado, PSI 11/11/08 6
First Possible Hint for a Higgs boson (2000) ALEPH observed three golden candidates in the four-jet channel Input M H =115GeV/c 2 Bruce Mellado, PSI 11/11/08 7
The LEP Limit ALEPH observed an excess over background-only prediction with significance of 2.8 σ at 115 GeV/c 2 Overall significance of LEP experiments ~1.8 σ → limit setting M H >114.4 Above 0 favors Background-only hypothesis Below 0 favors Signal+background hypothesis Bruce Mellado, PSI 11/11/08 8
Electro-Weak Fits Experimental constraints so far: Indirect measurements from fitting the EW data using new world average for M top =172.4±1.2 GeV and M w =80.399±0.025 GeV: m H = 84 +34 -26 GeV Data prefers low mass Higgs m H <154 GeV @ 95%CL (including LEP exclusion m H <185 GeV) Bruce Mellado, PSI 11/11/08 9
Present Tevatron Exclusion Limit Bruce Mellado, PSI 11/11/08 10
Present Tevatron Exclusion Limit Bruce Mellado, PSI 11/11/08 11
Center of mass E 14 TeV The LHC Design Luminosity 10 34 cm -2 s -1 Luminosity Lifetime 10 h Bunch spacing 25 ns Bruce Mellado, PSI 11/11/08 12
LHC Discovery Reach Approximate discovery reach Z’ → ee for one Experiment SUSY Higgs W’ 2010 2009 Bruce Mellado, PSI 11/11/08 13
Higgs Production at LHC Leading Process (gg fusion) Sub-leading Process (VBF) Bruce Mellado, PSI 11/11/08 14
Main Decay Modes Djouadi, Kalinowski, Spira Close to LEP limit: For M H >140 GeV: H →γγ →γγ , ττ ττ ,bb H → WW (*) ,ZZ (*) Bruce Mellado, PSI 11/11/08 15
Cross-sections at LHC Search for Higgs and new physics hindered by huge background rates Known SM particles produced much more copiously This makes low mass Higgs especially challenging Narrow resonances Complex signatures Higgs in association with tops and jets. Bruce Mellado, PSI 11/11/08 16
Low Mass Higgs Associated with Jets Slicing phase space in regions with different S/B seems more optimal when inclusive analysis has little S/B Inclusive H+1jet H+2jet Tag jet Tag jet Not tagged Tag jet Tag jet Forward jets Not Tagged Not tagged φ φ Applied to H →γγ →γγ , ττ ττ ,WW (*) Analyses in TDR were mostly inclusive Higgs Decay η η Bruce Mellado, PSI 11/11/08 17
SM Higgs + ≥ 2jets at the LHC Wisconsin Pheno (D.Zeppenfeld, D.Rainwater, et al.) proposed to search for a Low Mass Higgs in association with two jets with jet veto Central jet veto initially suggested in V.Barger, K.Cheung and T.Han in PRD 42 3052 (1990) Tagging Jets Jet Jet ϕ η Central Jet Veto Higgs Decay Products Bruce Mellado, PSI 11/11/08 18
SM Higgs + ≥ 1jet at the LHC 1. Large invariant mass of leading jet and Higgs candidate 2. Large P T of Higgs candidate 3. Leading jet is more forward than in QCD background Higgs Decay Products Tag jet M HJ ϕ Not Tagged Tag jet Loose Central Jet Veto Quasi-central η (“top killer”) Tagging Jet Bruce Mellado, PSI 11/11/08 19
ATLAS 22 m Weight: 7000 t 44 m Bruce Mellado, PSI 11/11/08 20
Low Mass SM Higgs: H →γγ →γγ Bruce Mellado, PSI 11/11/08 21
ATLAS Bruce Mellado, PSI 11/11/08 22
Higgs decay to γγ γγ Reducible γ j and jj γγ Backgrounds γγ Backgrounds q →π →π 0 Bruce Mellado, PSI 11/11/08 23
CMS and ATLAS analyses for 100 fb -1 CMS ATLAS Bruce Mellado, PSI 11/11/08 24
Higgs Mass Reconstruction In ATLAS Expect about 50% of events to have at least one converted photon, but can achieve <1.2% mass resolution High Lumi Low Lumi σ =1.36 GeV σ =1.59 GeV Events with at least one conversion Bruce Mellado, PSI 11/11/08 25
Photon Identification To separate jets from photons is crucial for Higgs discovery Need rejection of > 1000 against quark-initiated jets for ε γ =80% to keep fake background about 20% of total background Expect rejection against gluon-jets to be 4-5 times greater Jet rejection will be evaluated with data ATLAS TDR (1999) Look into sub-leading jets in multi-jet final states with different P T thresholds Avoid trigger bias Apply trigger pre- scaling if needed Correct for contribution from prompt photons Bruce Mellado, PSI 11/11/08 26
Inclusive H →γγ →γγ ATLAS Bruce Mellado, PSI 11/11/08 27
h,A →ττ →ττ ; H ± →τ →τ ± ν Bruce Mellado, PSI 11/11/08 28
Low Mass SM H →ττ →ττ + jets Because of the poor Higgs mass resolution obtained with H →ττ →ττ , inclusive analysis not possible. Need to reduce QCD backgrounds by using distinct topology of jets produced in association with Higgs H →ττ →ττ + ≥ 1 jets H →ττ →ττ + ≥ 2 jets Tagging Jets Higgs Decay Products ϕ ϕ η Loose Central Jet Veto Quasi-central η Higgs Decay Products Central Jet Veto Tagging Jet (“top killer”) Bruce Mellado, PSI 11/11/08 29
H →ττ →ττ Mass Reconstruction In order to reconstruct the Z mass need to use the collinear approximation Tau decay products are collinear to tau direction Fraction of τ momentum carried by visible τ decay νν νν l H h νν νν x τ 1 and x τ 2 can be calculated if the missing E T is known Good missing E T reconstruction is essential Bruce Mellado, PSI 11/11/08 30
Low Mass SM H →ττ →ττ +jets Reconstruct Higgs mass with collinear approxim. H( →ττ →ττ → ll) + ≥ 2jets H( →ττ→ →ττ→ lh) + ≥ 1jet 30 fb -1 M H =120 GeV Bruce Mellado, PSI 11/11/08 31
Two independent ways of extracting Z →ττ →ττ shape Data driven and MC driven Similar procedure has been defined for H → WW ( * ) 85<M ll <95 GeV Determine shape and Control Sample 1 Control Sample 3 normalization of Z →ττ →ττ MC extrap. background is validated Z → ee, µµ µµ Z → ee, µµ µµ Loose cuts on Jets Tight cuts on Jets M ll <75 GeV MC extrap. Control Sample 2 Signal Region Z →ττ →ττ Z → ττ ττ Loose cuts on Jets Tight cuts on Jets Δη JJ M HJ , Δη Bruce Mellado, PSI 11/11/08 32
Normalization of Z →ττ →ττ using Z → ee, µµ µµ Z → ee, µµ µµ offers about 35 times more statistics w.r.t to Z →ττ→ →ττ→ ll Ratio of efficiencies depends weakly with M HJ and can be easily determined with MC after validation with data Bruce Mellado, PSI 11/11/08 33
SM Higgs: H → ZZ (*) → 4l Bruce Mellado, PSI 11/11/08 34
Higgs decay to Z 0 Z 0 Reducible 4l backgrounds Irreducible Z 0 Z 0 backgrounds Z Z Bruce Mellado, PSI 11/11/08 35
l l l l l ν l b b tt WbWb ZZ*/ γ * → 4l b b ν l l l l l l Resonant Continuum Non-Resonant reducible Irreducible reducible ν l Backgrounds ν l τ Higgs → ZZ (*) → 4l ZZ*/ γ * → 2l 2 τ (l=e µ ) τ l ν l ν Bruce Mellado, PSI 11/11/08 36
SM Higgs → ZZ (*) → 4l Able to reconstruct a narrow resonance, with mass resolution close to 1%. Can achieve excellent signal-to-background > 1 Major issue: Lepton ID and rejection of semi-leptonic decays of B decays. Suppress reducible background Zbb,tt → 4l H[ 130 GeV] 2e2 µ H[ 130 GeV] 4 µ H[ 130 GeV] 4e ATLAS TDR Bruce Mellado, PSI 11/11/08 37
ATLAS ATLAS TDR Reducible background Sum of P t of tracks around each µ in GeV ATLAS ATLAS TDR ! of calo energy around each ! in GeV Bruce Mellado, PSI 11/11/08 38
Bruce Mellado, PSI 11/11/08 39
SM Higgs: H → WW (*) → 2l2 ν Bruce Mellado, PSI 11/11/08 40
Higgs decay to W + W - W + W - backgrounds + Single top & non-resonant WWbb Bruce Mellado, PSI 11/11/08 41
SM Higgs H → WW (*) → 2l2 ν Strong potential due to large signal yield, but no narrow resonance. Left basically with event counting experiment H → WW+2j H → WW+0j Transverse Mass (Gev) Δφ ll (rad) Δφ Bruce Mellado, PSI 11/11/08 42
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