Search for the Higgs Boson Rencontres de Physique de Particules Montpellier May 14, 2012 Dirk Zerwas...

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