Higgs Boson Searches at CMS What Have We Found So Far? Patrick - PowerPoint PPT Presentation

Higgs Boson Searches at CMS What Have We Found So Far? Patrick Dunne Patrick Dunne 1 / 11 Higgs Boson Searches at CMS

Outline ◮ A Higgs-like boson was discovered in 2012 at the LHC. ◮ How did we decide we’d discovered something? ◮ How do we answer the question: “Is it the Higgs?” Patrick Dunne 2 / 11 Higgs Boson Searches at CMS

The Standard Model and the Higgs Boson 10 BR [pb] LHC HIGGS XS WG 2012 s = 8TeV - τ + τ ◮ Higgs boson is a consequence of ± WW → l ν q q 1 × + - VBF H → τ τ the Higgs mechanism which gives σ + - WW l l → ν ν mass to the weak vector bosons -1 10 - + ZZ → l l q q ◮ Higgs mechanism also gives rise to + - ZZ → l l ν ν -2 10 the fermion masses - - + + ZZ → l l l l -3 10 ± γ γ WH → l ν b b l = e, µ ◮ Standard Model couplings are well ttH → ttb b ν = ν , ν , ν e µ τ + - ZH l l b b q = udscb → -4 10 100 200 300 400 predicted M [GeV] H Patrick Dunne 3 / 11 Higgs Boson Searches at CMS

Combinations ◮ To decide if we have found the Higgs boson we need to understand its properties ◮ This requires a combination of all the search channels ◮ The combination has three aims: Patrick Dunne 4 / 11 Higgs Boson Searches at CMS

Combinations ◮ To decide if we have found the Higgs boson we need to understand its properties ◮ This requires a combination of all the search channels ◮ The combination has three aims: ◮ Setting exclusion limits on the SM Higgs Boson Patrick Dunne 4 / 11 Higgs Boson Searches at CMS

Combinations ◮ To decide if we have found the Higgs boson we need to understand its properties ◮ This requires a combination of all the search channels ◮ The combination has three aims: ◮ Setting exclusion limits on the SM Higgs Boson ◮ Characterising excesses over the background Patrick Dunne 4 / 11 Higgs Boson Searches at CMS

Combinations ◮ To decide if we have found the Higgs boson we need to understand its properties ◮ This requires a combination of all the search channels ◮ The combination has three aims: ◮ Setting exclusion limits on the SM Higgs Boson ◮ Characterising excesses over the background ◮ Extracting signal model parameters from the data Patrick Dunne 4 / 11 Higgs Boson Searches at CMS

Setting Exclusion Limits ◮ The CL s statistic is used, which is the number of times more likely the signal hypothesis is than the background hypothesis. ◮ It is defined as: CL s = P ( q µ � q obs µ | µ · s + b ) P ( q µ � q obs µ | b ) ◮ µ is a signal strength modifier ◮ q µ is a profile likelihood ratio defined as: q µ = − 2 ln L ( obs | µ · s + b , ˆ θ µ ) . µ · s + b , ˆ L ( obs | ˆ θ ) Patrick Dunne 5 / 11 Higgs Boson Searches at CMS

2011 Exclusion Patrick Dunne 6 / 11 Higgs Boson Searches at CMS

Discovery Exclusion Patrick Dunne 6 / 11 Higgs Boson Searches at CMS

HCP Exclusion Patrick Dunne 6 / 11 Higgs Boson Searches at CMS

Characterising Excesses ◮ Higgs analyses use the p value, defined as: p 0 = P ( q 0 � q obs | b ) , 0 ◮ q 0 is the profile likelihood from above with µ set to zero ◮ i.e. the p value is the probability of observing a background fluctuation as likely or less likely than that observed in the absence of signal. ◮ 1-p does not tell you P(signal)! Patrick Dunne 7 / 11 Higgs Boson Searches at CMS

2011 Significance Patrick Dunne 8 / 11 Higgs Boson Searches at CMS

Discovery Significance Patrick Dunne 8 / 11 Higgs Boson Searches at CMS

HCP Significance Patrick Dunne 8 / 11 Higgs Boson Searches at CMS

Signal Parameter Determination ◮ Most channels give their results in terms of σ xBR ◮ We want model parameters so another, slightly different, profile likelihood ratio is used q ( a ) = − 2 ln L ( obs | s ( a ) + b , ˆ θ a ) a ) + b , ˆ L ( obs | s (ˆ θ ) ◮ a is the parameter of interest and hatted values are the values which maximise L ◮ Basically a ∆ log likelihood method so 1 σ etc. contours can be plotted. Patrick Dunne 9 / 11 Higgs Boson Searches at CMS

Mass Patrick Dunne 10 / 11 Higgs Boson Searches at CMS

Signal Strength Patrick Dunne 10 / 11 Higgs Boson Searches at CMS

Couplings Patrick Dunne 10 / 11 Higgs Boson Searches at CMS

What Next? CMS Preliminary -1 -1 s = 7 TeV, L = 5.1 fb s = 8 TeV, L = 12.2 fb Generated experiments 3000 SM, 0+ 2500 ◮ Finish analysing the 2012 0- 2000 dataset CMS data ◮ Analyse parked data 1500 ◮ Determine spin and parity 1000 ◮ Better coupling 500 determination 0 -30 -20 -10 0 10 20 30 × -2 ln(L /L ) 0+ 0- Patrick Dunne 11 / 11 Higgs Boson Searches at CMS