Higgs searches in CMS
Ashok Kumar
Delhi University
On behalf of CMS collaboration
PHYSICS AT LHC 2011 – Perugia
June 06-11, 2011
Outline of Talk
Higgs phenomenology, production and decay modes at LHC
Standard Model Higgs searches:
H  gg
H  WW
H  ZZ
BSM Higgs:
SUSY Neutral, SUSY Charged, Doubly charged Higgs
CMS Higgs Projections
Significance and exclusion limits
Summary and Outlook
Current status of SM Higgs search
Theoretical Limits
 mH < 1 TeV from unitarity arguments
 If λ cutoff ≈ 1TeV weaker limits: 50≤ mH ≤ 800 GeV
Indirect Constraints
 Preferred fit value mH =87-26+35 GeV (68% CL)
 mH ≤ 157 GeV (95%CL) & ≤187 GeV (with LEP-2 direct search results)
Experimental Limits
 LEP: mH ≥ 114.4 GeV at 95% CL
 TEVATRON Run II: 158 ≤ mH ≤ 173 GeV
excluded at 95% CL
LEP Electroweak Working Group (LEP EWWG)
http://lepewwg.web.cern.ch/LEPEWWG/
Conference Note: arXiv:1103.3233 [hep-ex]
Cross sections & Branching
Ratios
"Handbook of LHC Higgs Cross Sections: 1. Inclusive Observables" (CERN-2011-002,
arXiv:1101.0593)
 Higgs production cross-section
(NLO): 0.1 - 50 pb
 Gluon fusion dominates at LHC
gg enhances the signal,
whereas the irreducible backgrounds
come from qqbar
At low mass (M < 2MZ)
-Dominant bb; huge QCD background
-H→tt accessible through VBF
-H→WW(*) accessible via gluon-gluon
fusion and VBF
-H→ gg low BR but good detector resolution
-H →ZZ*→4l also accessible
For higher masses
-H→WW and H→ZZ→4l final-states
A. Djouadi arXiv:hep-ph/0503172v2
H → gg
Analysis Features:
 two isolated photons required
 looking for a γγ mass peak
 large QCD background, estimated
from sidebands
A fermio-phobic Higgs with
mfph < 110 GeV would be excluded,
as for this mass range, the yield
σ(pp→hfph)xBR(hfph→gg) is ~ 4
The limit on the anomalous production is expected to be about 4 times
σ(pp→HSM)xBR(HSM→gg).
The projected exclusion reach is comparable to the current limits from LEP and Tevatron
H→WW→ll
Signature:
 Two high pT leptons
 MET from neutrinos
 Very clean events with little hadronic
activity
‣ Most sensitive channel for early studies
‣ Two parallel techniques: Cut-based and
boosted decision tree (BDT)
Electron-muon WW candidate
Event selection:
•Exactly two isolated, well identified leptons (e/µ), pT > 20 GeV
•Projected MET > 35 (20) GeV for ee/µµ (eµ) channels
•Z veto on invariant mass of same flavor leptons
•Jet and b-tag veto against events with top quarks
Excellent knowledge of backgrounds mandatory: control regions & data- driven methods
H→WW: Backgrounds
Signal selection mostly relying on opening angle between
leptons - ΔΦll
-Leptons from H tend to have small angles
Background estimations from data:
SM WW contribution from mll control regions
with low signal contamination
Z+jets estimated using inside/outside ratio
from MC applied on data
W+jets and QCD fakes estimated using
a fake-ratio method
Remaining ttbar estimated from additional
soft leptons and soft b-tagged jets
BDT takes input from kinematic relations between leptons
and MET in addition to cut based variables
H→WW→ll
 Results from BDT and cut-based analysis
virtually the same
 Spin correlation used for a Higgs analysis
 Search for H→WW sets limits to
Higgs production with a fourth generation
 Not yet sensitive to SM Higgs
-Need factor 3 more data at mH = 160 GeV
 However sensitive to some models with
4th fermion family assuming high masses
-Exclude them for mH ~ 144 -207 GeV @
95% C.L.
Phys. Lett. B 699 (2011) 25
arXiv:1102.5429
H → ZZ*→ 4l
CMS-PAS-HIG-08-003
Signal Signature:
Isolated high pT 4e, 4m, 2e2m leptons
(Clear Higgs mass peak)
Dominant Backgrounds:
ZZ, Zbb, tt+jets, Z+jets, W+jets, QCD
Preselection strategy
Single & double lepton triggers
4 loose isolated leptons, oppositely charge pairs
mll>12 GeV, m4l>100 GeV
Main discrimanting observables:
Tight isolation (against tt, Zbb)
Impact parameter (against Zbb and tt)
50 < mZ< 100 GeV, 20 < mZ*< 100 GeV
Baseline cut-based approach, mH-independent
Exclusion is out of reach across the whole mH range, Should a fourth generation of quarks exist, the
Higgs boson could be excluded in the range mH < ~ 420 GeV
Combination of channels
By combining the results for the three channels shown before
& assuming twice amount of Data (~ ATLAS + CMS)
 Expected exclusion range for SM Higgs: 140 < mH < 200 GeV
 The Higgs boson with a mass mH< 500 GeV would be excluded,
should a fourth generation of heavy quarks exist
Neutral Higgs Boson → TauTau
Search for a MSSM Higgs decaying to
a pair of taus (Φ = h/H/A)
-BR to ττ is about 10 %
-Consider ττ decays to e-µ, µ-had, e-had
Novel technique for τ pair mass
reconstruction:
-Likelihood fit to τ momenta
-Use all available kinematic information
and probability density for τ pT spectra
-Improvement in resolution compared to
visible mass
Limits on Neutral Higgs Boson
No signal excess observed
Set upper limits on σ(pp ➝ ΦX) x BR(Φ ➝ ττ), as a function of the Higgs boson
mass yield stringent new bounds in the MSSM parameter space mA versus tan β
extending as low as tan β = 23 for mA= 130 GeV.
Significantly extend previous limits
CMS-PAS-HIG-10-002
Charged Higgs in ttbar decays
Charged MSSM
Higgs
may contribute to
ttbar decays
tt~(Hb)(Wb)(tau_h v b)(lvb)
Event Selection as for ttbar cross section measurement:




One electron (muon) with pT > 30 (20) GeV
At least two jets ET > 30 GeV
[CMS PAS-TOP-10-002]
MET > 40 GeV
Hadronic τ pT > 20 GeV
Backgrounds in two categories:
① Fake hadronic τ: use fake rate method to estimate from data
② Real hadronic τ: use simulation to estimate background
H+ Limits & Exclusion
 No signal observed
 Set 95% C.L. on BR (t➔bH+) assuming BR(H+➔ τ+v)=1
 Limit ~0.25-0.28 for 80 GeV < mH+ < 140 GeV
CMS-PAS-HIG-11-002
Doubly Charged Higgs




Extend Standard Model adding scalar triplet: Φ±±, Φ± and Φ0
Triplet responsible for neutrino masses
Consider model where BR(Φ±± ➔ll)=100%
Final states with three/four isolated leptons (earlier multi-lepton search)
 Look for resonance peaks in dilepton mass distributions.
CMS-PAS-HIG-11-001
No peak observed ➔ set limit
extending reach of previous experiments
SM Higgs Projections @ 7 TeV
Several Higgs production and decay channels used for projections:
-mass range 114 – 600 GeV covered
-based upon the cut-and-count methodology
(in some cases, cut on the MVA-output)
Higher-order cross sections included:
-signal gg-fusion at NNLO+NNLL, VBF and VH at NLO, ttH at LO
-all backgrounds at NLO
Significance and Exclusion
Projected CMS sensitivity to SM Higgs Boson searches at √s = 7 TeV, 5 fb-1
-Statistical Exclusions (95%CL): Bayesian limits with a flat prior
-Significance: Profile Likelihood method
At √s = 7 TeV with L=5 fb-1, CMS is expected to reach an exclusion sensitivity in the mass
range from the LEP limits (114 GeV) to 600 GeV
Summary & Outlook
 With ∫L ~ few pb-1 at √s = 7 TeV CMS experiment have started
exploring Higgs Physics.
 Low mass SM Higgs searches require higher ∫L and √s.
 Search for SM H→WW alone sets limits to Higgs production with a
fourth generation.
 BSM Higgs searches from CMS
➔ Unfortunately no observations ➔ set limits
 Combined projections at √s = 7 TeV with luminosity ~ 5 fb-1, CMS
may exclude SM Higgs mass up to 600 GeV.