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

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Higgs Boson Searches at CMS

What Have We Found So Far? Patrick Dunne

Patrick Dunne Higgs Boson Searches at CMS 1 / 11

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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?”

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The Standard Model and the Higgs Boson

◮ Higgs boson is a consequence of

the Higgs mechanism which gives mass to the weak vector bosons

◮ Higgs mechanism also gives rise to

the fermion masses

◮ Standard Model couplings are well

predicted

[GeV]

H

M 100 200 300 400 BR [pb] × σ

• 4

10

• 3

10

• 2

10

• 1

10 1 10

LHC HIGGS XS WG 2012

= 8TeV s

µ l = e,

τ

ν ,

µ

ν ,

e

ν = ν q = udscb b b ν

±

l → WH b b

• l

+

l → ZH b ttb → ttH

• τ

+

τ → VBF H

• τ

+

τ γ γ q q ν

±

l → WW ν

• l

ν

+

l → WW q q

• l

+

l → ZZ ν ν

• l

+

l → ZZ

• l

+

l

• l

+

l → ZZ

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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:

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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 Higgs Boson Searches at CMS 4 / 11

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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 Higgs Boson Searches at CMS 4 / 11

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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 Higgs Boson Searches at CMS 4 / 11

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Setting Exclusion Limits

◮ The CLs statistic is used, which is the number of times more

likely the signal hypothesis is than the background hypothesis.

◮ It is defined as:

CLs = P(qµ qobs

µ |µ · s + b)

P(qµ qobs

µ |b) ◮ µ is a signal strength modifier ◮ qµ is a profile likelihood ratio defined as:

qµ = −2 ln L(obs|µ · s + b, ˆ θµ) L(obs|ˆ µ · s + b, ˆ θ) .

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2011 Exclusion

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Discovery Exclusion

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HCP Exclusion

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Characterising Excesses

◮ Higgs analyses use the p value, defined as:

p0 = P(q0 qobs |b),

◮ q0 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)!

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2011 Significance

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Discovery Significance

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HCP Significance

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Signal Parameter Determination

◮ Most channels give their results in terms of σxBR ◮ We want model parameters so another, slightly different,

profile likelihood ratio is used q(a) = −2 ln L(obs|s(a) + b, ˆ θa) L(obs|s(ˆ a) + b, 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.

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Mass

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Signal Strength

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Couplings

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What Next?

◮ Finish analysing the 2012

dataset

◮ Analyse parked data ◮ Determine spin and parity ◮ Better coupling

determination

)

0+

/L

0-

ln(L ×

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• 30
• 20
• 10

10 20 30

Generated experiments

500 1000 1500 2000 2500 3000

SM, 0+ 0- CMS data CMS Preliminary

• 1

= 8 TeV, L = 12.2 fb s

• 1

= 7 TeV, L = 5.1 fb s

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