Transcript Diapositiva 1

ITEP Winter School – February 2012 Moscow
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Higgs searches before LHC
LEP  95% CL MH < 114 GeV/c2
TEVATRON:  95% CL 158 GeV/c2 < MH < 175 GeV/c2;
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CMS analysis on full 2011 data
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The Main Channel: H → WW → 2l 2n
μ: 32 GeV
Signature:
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e: 34 GeV
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MET:
47 GeV
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2 opposite charged leptons
(leptons only e, μ)
2 neutrinos  missing
transverse energy (MET)
No Higgs mass peak
Basically a counting analysis
Enhance sensitivity by
subdividing into (0,1,2) jets
Analysis challenges:
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Understand backgrounds
Normalize to control regions
Backgrounds: WW, W+jets,
top, DY
Conservative Cut and Count Analysis
Observations:
 Exclude Higgs masses from 132 GeV < mH < 238 GeV
 Expected exclusion
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129 GeV < mH < 236 GeV
Full MVA Shape Analysis
Observations:
 Exclude Higgs masses from 129 GeV < mH < 270 GeV
 Expected exclusion
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127 GeV < mH < 270 GeV
The Golden Mode: H → ZZ → 4l
Analysis overview:
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4 isolated high pT leptons
Consistent with Z decays
From same vertex
Fit mass peak with
resolution: 2-4 GeV
Little background, main
comes from non-resonant
ZZ production, irreducible
Also Zbb and top (2l2n2b)
Background removal
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Leptons from b-decays are non-isolated and displaced
Require isolation and small impact parameter
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The Golden Mode: H → ZZ → 4l
Observed events consistent with expectations:
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Excluded regions: 134 < mH < 158 GeV/c2, 180 <mH < 305 GeV/c2 and 340 < mH <465 GeV/c2
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High Mass Special: H → ZZ → 2l2τ
Analysis overviews:
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First time that there is a Higgs search with this final state.
Second Z going to tlth , thth and tetm  other final states already accounted for H ZZ
Cross sections excluded between a factor 4 and 7 larger than that expected for the SM
9 < 500 GeV/c2
mH in the range 190 < m
H
High Mass Special: H → ZZ → 2l2n
Analysis overview:
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Same final state as H→WW but with smaller production rate.
Starts to have sensitivity at about mH = 300 GeV
Limits based on cut based and shape of the MT distribution.
Excluding mass range between 270 -440 GeV/c2 @ 95% C. L
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High Mass: H → ZZ → 2l 2jets (or 2b-jets)
Analysis overview:
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Highest rate of H→ZZ analyses (20 times higher than
H → ZZ→ 4l)
Analysis optimized separetely for two mass ranges
Intermediate mass range 2mW < mH < 2mZ has low
sensitivity due to top background.
Signal discrimination: Great contribution of heavy
flavour quarks and absence of gluons.
Angular Likelihood Discriminator.
Not yet excluding but getting there
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Low Mass Specialist: H → γγ
86 GeV
56 GeV
Signature and background:
● Two high momentum photons
● Clean final-state topology with a
mass peak that can be reconstructed
with precision
● Low mass Higgs
● Background estimated from a fit to
the data.
● Large (irreducible) background from
prompt two photons production.
● Smaller fake photon background 
pp  jet + g , pp  jet + jet
Key analysis features:
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Clean final-state topology
Energy resolution
is almost everything: calibrate and optimize
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Low Mass Specialist: H → γγ
2.34 s
123.5 GeV/c2
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Limits evaluated using the two photon invariant mass distribution as observable.
The expected exclusion limit at 95% CL is between 1.5 and 2.0 times the Standard Model
cross section in the mass range between 110 and 140 GeV/c2.
The observed limit disfavours at 95% CL a Standard Model Higgs boson decaying into
two photons in the mass range 127 to 132 GeV/c2.
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Low Mass Special: H → ττ
Analysis overview:
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VBF category
Backgrounds
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DY  tt background in the mass
region is huge for gluon-fusion.
3 categories: incl. / VBF / boosted
VBF style most sensitive
Require 2 taus (at least one decaying
leptonically)
•e-μ, μ-τh and e-τh
top, EWK, DY (irreducible)
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Low Mass Special: H → ττ
Observations
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Data exclude at 95% CL a Higgs boson with a production cross
section between 2.9 and 6.3 times that in the standard model.
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Low Mass Special: VH → Vbb
Analysis overview:
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Backgrounds
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V+jets,VV, top
Result
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Upper limits calculated for 110 < mH < 135
Limit is around 3 at low mass
Further improvements possible
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Enormous background in H → bb due
to overwhelming QCD dijet production
Use VH with leptonic V decays
Also require high momentum: ‘boosted’
analysis
Limits based on M(jj) distribution or
BDT output.
CMS SM Higgs Combination
Observations:
– Exclude (95%): 127-600 GeV (exp. 117-583 GeV)
– Exclude (99%): 129-525 GeV (exp. 128-500 GeV)
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CMS SM Higgs Combination
– Some excess around ~124 GeV
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Conclusions
Higgs search in 2011 with CMS: ~5 fb-1 data
 Excluded region : 127 GeV < mH < 600 GeV
 Expected
: 117 GeV < mH < 543 GeV
 Small window left: 114.4 GeV < mH < 127 GeV
Looking beyond 95% CL → 99% CL:
 Search will not stop at 95% CL exclusion
 99% CL exclusion: 129 GeV < mH < 525 GeV
Comments on low mass region:
 Excluded less than expected
 Small excess, but inconclusive at this point
Need more data to come to a conclusion
2012 will be a decisive year !
Thank you!
большое спасибо!
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Backup
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CMS + ATLAS COMBINATION
The region from 146 to 443 GeV/c2 is excluded at 99\% C.L., with the exception of
three small regions between 220 and 320 GeV/c2.
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