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

H  ZZ  4l good lepton ID down to low pT 7/5GeV (electron/muon) ZZ main background Z+jets secondary background clean channel

H  ZZ  4l Low mass: width is detector resolution High mass: width is width Excellent description of BG: on-shell Z ok (m12) off/on-shell Z ok

H  γγ 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 H  γγ 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 hypothesis H1: a signal hypothesis Define a test statistic in variable t Cut defines whether the background hypothesis is accepted or not p-value: probability, under assumption of H0, to observe data with equal 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)= observe 1: P(1,s,0) = s exp(-s) deduce s = *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 = *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=900 b=990 CLs+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/

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 bosons Higgs sector: mass of A, tanβ (vev ratio) tanβ ↑: g(Hτ,b) ↑ D0: final states with τ and bbb ATLAS 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 boson Signature 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