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