Combined results of SM Higgs searches at CMS

Nhan TranFermi National Accelerator Laboratory

on behalf of the CMS Collaboration

MCTP Higgs Symposium, 04.19.2012

introduction and outline

• We are at a critical and very exciting time in the search for the SM Higgs boson!

• Combination of most recent CMS SM Higgs searches

• Reference CMS-PAS-HIG-12-008

• Many informative and detailed presentations on searches in particular final states already!

• Presentation of full list of inputs of those analyses into CMS search and results from the combination

• Brief summary of each channel contributing to combination

• Exclusion limits and p-values

• Some additional channels have been added since the results of December 2011 combination (will be noted)

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production and decay

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[GeV]HM100 120 140 160 180 200

Higg

s BR

+ T

otal

Unc

ert

-310

-210

-110

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LHC

HIG

GS

XS W

G 2

011

bb

oo

cc

gg

aa aZ

WW

ZZ

[GeV] HM100 200 300 400 500 1000

H+X

) [pb

]

A(p

p m

-210

-110

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10= 7 TeVs

LHC

HIG

GS

XS W

G 2

010

H (NNLO+NNLL QCD + NLO EW)

App

qqH (NNLO QCD + NLO EW)

App

WH (NNLO QCD + NLO EW)

App

ZH (NNLO QCD +NLO EW)

App

ttH (NLO QCD)

App

values and theoretical uncertainties provided by

the LHC XSWG

Comprehensive search program to combine searches in as many unique production and decay channels

Higgs production

16-20 April 2012 2 J. Anderson - MCTP2012

gg Fusion

VBF

Higgs production

16-20 April 2012 2 J. Anderson - MCTP2012

gg Fusion

VBF

Higgs production

16-20 April 2012 2 J. Anderson - MCTP2012

gg Fusion

VBF

ggF VBF VH

combination channels

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Search for the Higgs boson in several independent channels in the mass range from 110-600 GeV

Channel (main production) mH range sub-channels mH resolution

H → γγ (ggH) 110-150 1 1-2%

H → γγ (VBF) 110-150 1 1-2%

H → ττ (ggH) 110-140/145 8 20%

H → ττ (VBF) 110-140/145 4 20%

H → ττ (VΗ) 100-140 2 20%

H → bb (VΗ) 110-135 5 10%

H → WW → lνlν (ggH) 110-600 4 20%

H → WW → lνlν (VBF) 110-600 1 20%

H → WW → lνlν (WH) 110-200 1 20%

H → ZZ → 4l (ggH) 110-600 3 1-2%

H → ZZ → llνν (ggH) 250-600 2 7%

H → ZZ → llqq (ggH) 130-164, 200-600 6 3%

H → ZZ → llττ (ggH) 190-600 8 10-15%

Each channel is characterized by its production and decay. mH range and resolution and number of sub-channels contributing to combination are listed

searches with 4.5-4.8 fb-1 of data

input to combination: H → ZZ → 2l + 2ν/2τ/2q

• 2l2ν: look for excess in transverse mass, mT, distribution from ETmiss and dilepton mass

• 2 sub-channels; Z → ee,μμ

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Channels contributing in the high mass Higgs region

)2 (GeV/cHm200 300 400 500 600

SMσ/

95%

CLs

σ

1

5

10

15

20

Observed σ 1±Expected σ 2±Expected

ττ ll→ ZZ →H

-1=7 TeV, 4.7 fbsCMS Preliminary,

• 2l2τ: look for broad excess in the 2l2τ mass

• 8 sub-channels; Z → ee,μμ; Z→ ττ( eμ, μτh, eτh, τhτh)

Observed 95% exclusion: 270-440 GeV

input to combination: H → ZZ → 2l + 2ν/2τ/2q

• 2l2q: unbinned 1-d shape analysis in m2l2j

• 6 sub-channels; Z → ee,μμ; Z → jj in 0/1/2 b-tag categories

• High mass region with two real Z’s and in the low mass region with Z → jj and Z* → ee,μμ

6significant contribution to combination in high mass region;

non-trivial contribution in low mass region

input to combination: H → WW → 2l2ν

• Broad search channel spanning the full search range from 110-600 GeV

• 5 sub-channels; 0-jet and 1-jet split into ee,μμ and eμ, 2-jet optimized for VBF

• Binned MVA classifiers trained for different Higgs mass hypotheses

• Added search for WH → WWW → 3l3ν (new!)

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Details, see talk by J. Anderson

6 7 Summary

Higgs mass [GeV]120 130 140 150 160 170 180 190 200

SMσ/

σ95

% C

L lim

it on

5

10

15

20

25CMS preliminary

ν 3l3→WH -1L = 4.6 fb

median expectedσ 1± expected σ 2± expected

observed

Figure 2: Upper limits at 95% CL for 4.6 fb�1of data.

Higgs mass [GeV]120 130 140 150 160 170 180 190 200

FPσ/

σ95

% C

L lim

it on

5

10

CMS preliminaryν 3l3→WH -1L = 4.6 fb

median expectedσ 1± expected σ 2± expected

observed

Figure 3: Upper limits at 95% CL for 4.6 fb�1of data in the fermiophobic Higgs boson scenario.

estimation techniques are presented, along with their expected systematic uncertainties. Thedescribed methodology is applied to the currently available dataset containing 4.6 ± 0.2 fb�1

of integrated luminosity. The observed (expected) upper limits at the 95% confidence level are10.4 (12.1) times larger than the SM Higgs boson expectation for mH = 120 GeV.

of particular interest for fermiophobic case(see R. Volpe talk)

Observed 95% exclusion: 129-270 GeV

input to combination: H → ττ

• search for a broad excess in the ττ mass distribution

• 12 categories as input to combination

• 4 Ηiggs to ττ final states: eμ, μτh, eτh, μμ (new!)

• 3 sub-categories: VBF signature, boosted (events with 1 high pT jet) , un-boosted (signatures with no jets or a jet with low ET)

• associated production (new!): search for WH → Wττ → eμτh/μμτh

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Details, see talk by M. Bachtis

sensitivity of ~2-3 times SM

input to combination: H → bb

• Most sensitive to a Higgs produced in association with weak vector boson

• Search in a boosted phase space

• 5 sub-channels: W → eν/μν or Z → ee/μμ/νν

• Input to combination from BDT output trained at each mass point

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_Details, see talk by S.M. Wang

Limits with BDT analysisbroad excess over full mass range

cross-check with mbb similar

input to combination: H → ZZ → 4l

• “golden channel”, high resolution decaymode, low statistics

• 3 sub-channels: 2e2μ, 4μ, 4e

• input to combination 1-d unbinned shape analysis in m4l distribution

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Details, see talk by M. Snowball

zoom

Observed 95% exclusion: [134-158],[180-305],[340-465] GeV

input to combination: H → γγ

• High resolution channel in the low mass region

• Updated analysis using MVA approach

• 2 sub-channels, VBF and non-VBF

• Categorize events according to BDT output for non-VBF case

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Details, see talk by O. Bondu

0123 0

2

1

3

input to combination: H → γγ

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Details, see talk by O. Bondu

• Comparison to Dec. 2011 result using 4 categories based on photon ID shows good agreement

• MVA approach shows ~20% improvement in expected UL

• Upper limit shows excess in the region near 125 GeV

• global significance = 1.6σ

VBF category

combination

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combination methodology

• Combination of 46 exclusive sub-channels with 156-222 systematic sources depending on Higgs mass

• Combination for 183 Higgs mass points with 0.5 GeV steps in low mass region to 20 GeV steps in high mass region

• Upper limits are computed using the CLs method with given test statistic

• Cross-check performed with asymptotic CLs and Bayesian methods

• Cross-checks with independently written frameworks

• To quantify an excess a trial factor is calculated to compute global significance

• Trial factor for 110-600 GeV and 110-145 GeV evaluated via pseudo-datasets

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4.3 Quantifying the absence of a signal 9

The test statistic q0 has one degree of freedom (µ) and, in the limit of a large number of events,its distribution under the background-only hypothesis converges to a half of the c2 distribu-tion for one degree of freedom plus 0.5 · d(q0) [84]. The term with the delta function d(q0)corresponds to the 50% probability not to observe an excess under the background-only hy-pothesis. This asymptotic property allows the significance to be evaluated directly from theobserved test statistic qobs

0 as [84]:

Z =q

qobs0 (5)

When the observed event yield lies below the expectation under the background-only hypothe-sis, µ̂ is at its minimum allowed value of µ̂ = 0, the test statistic qobs

µ = 0 (Eq. (2)), the asymptoticsignificance Z = 0 (Eq. (5)), and the corresponding p-value p0 = 0.5 (Eq. (4)).

The local p-value p0 characterises the probability of a background fluctuation resembling asignal-like excess for a given value of the Higgs boson mass. The probability for a backgroundfluctuation to be at least as large as the observed maximum excess anywhere in a specified massrange is given by the global probability or global p-value. This probability can be evaluatedby generating pseudo-datasets incorporating all correlations between analyses optimized fordifferent Higgs boson masses. It can also be estimated from the data by counting the numberof transitions from deficit to excess in a specified Higgs boson mass range [83, 85]. The globalsignificance is computed from the global p-value using Eq. (4).

4.3 Quantifying the absence of a signal

In order to set exclusion limits on a Higgs boson hypothesis, we define a test statistic qµ, whichdepends on the hypothesised signal rate µ. The definition of qµ makes use of a likelihood ratiosimilar to the one for q0, but uses instead the signal+background model in the numerator:

qµ = �2 lnL(data | µ·s(q̂µ) + b(q̂µ) )

L(data | µ̂·s(q̂) + b(q̂) ), 0 µ̂ < µ, (6)

where the subscript µ in q̂µ indicates that, in this case, the maximisation of the likelihood in thenumerator is done under the hypothesis of a signal of strength µ. In order to force one-sidedlimits on the Higgs boson production rate, we constrain µ̂ < µ.

This definition of the test statistic differs slightly from the one used in searches at LEP andthe Tevatron, where the background-only hypothesis was used in the denominator. With thedefinition of the test statistic given in Eq. (6), in the asymptotic limit of a large number ofbackground events, the expected distributions of qµ under the signal+background and underthe background-only hypotheses are known analytically [84].

For the calculation of the exclusion limit, we adopt the modified frequentist construction CLs [86,87]. We define two tail probabilities associated with the observed data; namely, the probabil-ity to obtain a value for the test statistic qµ larger than the observed value qobs

µ for the sig-nal+background (µ·s + b) and for the background-only (b) hypotheses:

CLs+b = P⇣

qµ � qobsµ | µ·s + b

⌘, (7)

CLb = P⇣

qµ � qobsµ | b

⌘, (8)

and obtain the CLs value from the ratio

CLs =CLs+bCLb

. (9)

LHC test statistic

combined CLs

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Full mass region Low mass region

Expected 95% exclusion from 114.5 - 543 GeVObserved 95% exclusion from 127.5 - 600 GeV

Observed 99% exclusion from 129 - 525 GeV

contributions of channels

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• WW, ZZ contribute across full search region• high mass region contributions from WW, ZZ → 4l, 2l2j, 2l2ν • γγ dominates at low mass with contributions from WW,ZZ,ττ,bb

Expected 95% exclusion for all contributing channels

statistical validation

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agreement to within 10% among various methods

Comparison of CLs method versus two other statistical methods: asymptotic CLs and Bayesian

combined p-value

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In low mass region, quantify excess with p-value

Minimum p-value corresponds to local significance 2.8σ and is compatible with SM Higgs signal

p-value driven by Hγγ channel at ~125 GeV while p-values at ~119 GeV and ~136 GeV not consistent between channels (more to come)

comparison to earlier results

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Comparison to Dec 2011Change dominated by update to

H → γγ MVA analysis

local significance is 2.8σ (3.1σ) for current (Dec. 2011) combination

global significance for:110-145 GeV is 2.1σ (2.1σ) for current (Dec. 2011) combination110-600 GeV is 0.8σ (1.6σ) for current (Dec. 2011) combination

γγ only

combination by channel resolution: γγ + ZZ vs. WW + bb + ττ

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hig

h resolu

tion c

hannels

γγ +

ZZ

low

resolu

tion c

hannels

ττ +

bb +

WW

broad excess in low resolution channels

over whole low mass region

small p-value at 119 GeV from ZZ4l

disappears due to deficit in the γγ

channel

signal strength fit

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Fit for the signal strength modifier to test for compatibility of excess with SM Higgs signal

ZZ+γγ

WW+bb+ττ

channel compatibility

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For few mass points, fit for signal strength in each individual channel.A measure of the statistical compatibility of the combination channels

At 125 GeV, compatible with a SM signal but error bars large, require more statistics draw conclusions

125 GeV119 GeV 136 GeV

summary and outlook

• Search for the SM Higgs boson combining several unique production+decay channels

• Expected 95% exclusion from 114.5 - 543 GeV

• Observed 95% exclusion from 127.5 - 600 GeV

• Observed 99% exclusion from 129 - 525 GeV

• Observed excess near 125 GeV

• Local significance of 2.8σ, global significance of 2.1σ (0.8σ) in the [110-145] GeV range (full search range)

• More data is needed to investigate the excess

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CMS 2012 integrated luminosity!