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Search for the Higgs Boson
Rencontres de Physique de ParticulesMontpellier
May 14, 2012Dirk ZerwasLAL Orsay
• Standard Model Higgs (low mass)• Digression: Statistics• Couplings• Beyond the Standard Model Higgs• Conclusions
The LHC
Great Startup in 2012:Gain 1TeV
2012: conditions becoming more difficulty
2011:5.6fb-1 delivered,4.6fb-1 to 4.9fb-1 for analysis
The Standard Model Higgs at the LHC
• Signal dominated by gluon fusion• VBF (qqH) next candidate• VH smaller with large backgrounds• ttH for higher energies
• low mass: tau (but background)• photons (rare but pure)• VH (difficult)• ZZ, WW with low rates
H ττ
• usually 1 hadronic tau• transverse mass (W+misID-jet)• tau mass via LL technique• cat 1:VBF• cat 2: boosted (1j > 150GeV)• cat 3: 0/1 j >30GeV• Z decays dominate (resolution)
• VBF channel (2jets) as example• better separation (recoil)• MMC • Z decays dominate
• 5x signal• currently essentially inconclusive
W/Z+H WZ+bb
• 5x signal (ATLAS) • difficult without subjet (Gavin Salam)
• BDT used (jj, separation….)• order 100-150GeV pT• 2 b-tagged jets• missing ET/Z-mass• dominant BG: top, Z/Wbb
• similar approach• order by V pT• S/B 1% (lowest pT) - 10% (highest pT)• BG normalization from data (sidebands)
H WW lνlν
• mass information: weak (neutrinos)• Spin correlations (lepton acoplanarity) • up to 2 jets• separate by jet-multiplicity and flavour (e/μ)• at least 20GeV proj ETmiss• BDT used!• WW dominates….• no significant excess
H WW lνlν
• good description of ETmiss necessary• jet categories 0,1,2• separated by flavor• transverse mass final discriminant
• S/B order of 1/10• compatible with bg only
HZZ 4l
• good lepton ID down to low pT• 7/5GeV (electron/muon)• ZZ main background• Z+jets secondary background• clean channel
HZZ 4l
Low mass: width is detector resolutionHigh mass: width is width
Excellent description of BG:• on-shell Z ok (m12)• off/on-shell Z ok
Hγγ
Excellent description of BG necessary• vertex reco (83%) for mass resolution• side-bands (power-law)• different categories• use of BDT based on the reconstructed photons
Similar results with cut-based analysis
Irreducible BG > Reducible BG
Hγγ
Understanding of BG:• ABCD method for background decomposition• estimation from sideband• search for deviation (bump hunting)
Digression: Statistics
The frequentist approach (A can be repeated n times):
BAYES approach: subjective probability which includes a prior encoding a degree of belief (more useful for an ensemble view)
H0: background hypothesisH1: a signal hypothesis
Define a test statistic in variable tCut defines whether the background hypothesis is accepted or not
p-value: probability, under assumption of H0, to observe data withequal or lesser compatibility with H0 relative to the data(does not mean that H0 is true)
Significance related to n-sigma Gaussian interpretation
A simple counting experiment
Counting experiment: • n observed events• s expected signal events• b expected background events
Background free experiment: • b=0• exclude at 95% CL• equivalent to one-sided Gaussian –∞ to +1.64σ (95% of total area)
• observe 0: • P(0,s,0) = exp(-s) • deduce s = 3 exp(-3)=0.0497
• observe 1: • P(1,s,0) = s exp(-s)• deduce s = 4.75 4.75*exp(-4.75) = 0.05
• translate limit into excluded signal cross section:• σ = s/(ε * L)
And with background?
Measure n events • determine a limit on S+B?• subtract background
• Gaussian regime (extreme example): • b = 990 (known perfectly: theoretical calculation)• n = 900 • n-b = -90• limit = -90+1.64*30 = -41
• would exclude all signals, but also the background model • Define likelihood when n events are observed:
• Same thing for S+B• Test statistic Q:
• In practice: large fluctuations of the background decrease the significance
More complexity
Real life: need to extend the simple Likelihood • fit signal form: N bins• introduce a signal strength parameter μ• systematic errors: e.g. background is measured via M control measurements• nuisance parameters: θ
• Test statistic:
CLs
• use the same test statistic for μ=1 (S+B) and μ=0 (B)• find μ for which CLs = 0.05 (95%CL)
• In practice default method (PCL abandoned)• Viewed critically by true statisticians:
• not a true confidence level• over coverage
• systematic errors usually conservative
In practice:n=900b=990CLs+b small (exclusion)CLb also small (3σ)CLs increases (no exclusion)
Application (ATLAS)
The complete picture
• LEE!• about 2 sigma
Higgs couplings at the LHC
Define couplings as deviations from SM:
Restrict total width (LHC blindness):
• allow only tree-level deviations• tree-level transported to loops• no genuine deviations in loops
hep-ph/1205.2699
Future Higgs couplings
3000 fb-1
Near future (2012, <2020) 125GeV:• 14TeV: major improvement• loop couplings testable• typically 20%• portal order of 10%
Far future HL-LHC:• portal: 5% (saturation)• 7%-20% precision• no luminosity scaling
Supersymmetry: neutral Higgs bosonsHiggs sector: mass of A, tanβ (vev ratio)tanβ ↑: g(Hτ,b) ↑
D0:• final states with τ and bbbATLAS and CMS:• tau pair final states• mA ↑ cross section ↓• large exclusion with 4.6fb-1
mA up to 500GeV, tanβ down to 10
SM-like h
Supersymmetry: charged Higgs bosonSignature for m(H±) <m(top)• top pair production• increase decays of top to tau• larger transverse mass • no excess • exclude as function of BR
Interpretation in the MSSM:
Exclude down to 2%
Conclusions
• Higgs: not discovered yet• interesting indications• end of 2012 the SM Higgs case will be settled• being optimistic: couplings will be measured
Statistics part based on lectures by Glen Cowan