ATLAS Conference by Fabiola Giannotti

ATL

AS-

CO

NF-

2011

-163

13D

ecem

ber

2011

ATLAS NOTE

ATLAS-CONF-2011-163

December 12, 2011

Combination of Higgs Boson Searches with up to 4.9 fb−1 of pp Collision

Data Taken at√

s = 7 TeV with the ATLAS Experiment at the LHC

The ATLAS collaboration

Abstract

A preliminary combination of Standard Model Higgs searches with the ATLAS exper-

iment, in a dataset corresponding to an integrated luminosity of up to 4.9 fb−1 of pp col-

lisions collected at√

s = 7 TeV at the LHC, is presented. The Higgs boson mass ranges

from 112.7 GeV to 115.5 GeV, 131 GeV to 237 GeV and 251 GeV to 453 GeV are excluded

at the 95% confidence level (C.L.), while the expected Higgs boson mass exclusion in the

absence of a signal ranges from 124.6 GeV to 520 GeV. An excess of events is observed for

a Higgs boson mass hypothesis close to mH=126 GeV. The maximum local significance of

this excess is 3.6σ above the expected SM background, while the global probability of such

a fluctuation to happen anywhere in the full explored Higgs mass domain is estimated to be

approximately 1%, corresponding to a global significance of 2.3σ . The three most sensitive

channels in this mass range, H → γγ , H → ZZ(∗) → ℓ+ℓ−ℓ+ℓ− and H →WW (∗) → ℓ+νℓ−ν ,

contribute individual local significances of 2.8σ , 2.1σ and 1.4σ , respectively, to the excess.

1 Introduction

In 2011, the LHC delivered an integrated luminosity of more than 5 fb−1 of pp collisions at 7 TeV. This

outstanding performance allowed ATLAS to collect and analyse up to 4.9 fb−1 of useful data to update its

searches for the Higgs boson [1–6]. Reaching such a high integrated luminosity was done at the expense

of challenging conditions in terms of pile-up, which has reached an unprecedented level of 24 events

during short periods and an average of approximately 12 in the data taken since the last Higgs boson

search analyses and ATLAS combination updates [7].

This note presents the combination of searches for the Higgs boson by the ATLAS experiment

with data taken in 2011. Two channels, the H → ZZ(∗) → ℓ+ℓ−ℓ+ℓ− and H → γγ , are updated us-

ing the full 2011 dataset. The H → WW (∗) → ℓ+νℓ−ν , H → ZZ → ℓ+ℓ−νν and H → ZZ → ℓ+ℓ−qq

analyses have been updated with 2.1 fb−1 [8–10]. The analysis of the H → WW → ℓνqq′ channel

is unchanged with respect to the combination of Ref. [11] and the separation of cross-section uncer-

tainties is updated to follow the agreed procedure described in Ref. [12]. The very low sensitivity

ZH →ℓ+ℓ−bb̄ and WH →ℓνbb̄ [13] channels are not included in this combination. The other chan-

nels H → ττ → ℓτhad3ν [14] and H → ττ → ℓ+ℓ−4ν [15] are unchanged with respect to Ref. [11].

2 Analysis Updates

The two analyses with the largest changes since the combination of Ref. [7] are those of the H → γγ and

H → ZZ(∗) → ℓ+ℓ−ℓ+ℓ− channels. Their features are briefly discussed below.

• H → ZZ(∗) → ℓ+ℓ−ℓ+ℓ− update: The analysis is updated to include 4.8 fb−1 of data [16]. An

improved electron track fitting procedure is used, which better accounts for the bremsstrahlung of

electrons in the material of the inner detector. The muon momentum resolution is improved by

refining the alignment of the inner detector (ID) and the muon spectrometer (MS). Both the muon

and electron reconstruction efficiencies were thoroughly checked in-situ, including effects due to

the high pile-up conditions. Particular attention was given in the low mass range to the data-driven

estimates of the reducible backgrounds Zbb̄ and Zqq̄.

• H → γγ update: The analysis is updated to 4.9 fb−1 of data and complemented with a new catego-

rization inspired by the fermiophobic Higgs boson search analysis of Ref. [17], where instead of a

separation in transverse momentum categories, events are categorized in terms of their momentum

component transverse to the thrust axis in the transverse plane (pTt) [18]. The pTt

component of

the transverse momentum is less sensitive to resolution effects on the transverse momentum of the

diphoton system. Altogether nine categories using pTt, the pseudorapidity of the two photons and

their conversion status are used. Detailed studies of the photon reconstruction and identification

efficiencies have been carried out, in particular to check their robustness against pile-up.

The integrated luminosities used in each channel are reported in Table 1. The reconstructed invari-

ant and transverse mass distributions, used as final discriminants, are illustrated in Fig. 1 and Fig. 2.

The numbers of observed events and expected signal and background events in an interval containing

∼ 90% of the signal around the most probable value of the invariant or transverse mass distributions,

for all channels except the event-counting channels H →WW (∗) → ℓ+νℓ−ν and H → ττ → ℓ+ℓ−4ν , are

summarized in Table 2. An overall deficit is observed in the H → ττ → ℓτhad3ν channel; the number of

events observed is however compatible with the expected background within systematic uncertainties.

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Figure 1: The invariant or transverse mass distributions for the selected candidate events, the total back-

ground and the signal expected in the H → γγ (a), the H → ZZ(∗) → ℓ+ℓ−ℓ+ℓ− in the low mass region

(b), H → ZZ(∗) → ℓ+ℓ−ℓ+ℓ− in the entire mass range (c), and the H →WW (∗) → ℓ+νℓ−ν (d) channels.

3

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Figure 2: The invariant or transverse mass distributions for the selected candidate events, the total

background and the signal expected for the given value of mH in the H → ττ → ℓ+ℓ−4ν (a) and

H → ττ → ℓτhad3ν (b), the H → WW → ℓνqq′ (c), the H → ZZ → ℓ+ℓ−νν (d) channels and the

H → ZZ → ℓ+ℓ−qq channel for events selected in the untagged (e) and the tagged (f) categories [10]. The

signal distributions are displayed in a lighter red colour where they have been scaled up by an arbitrary

factor for illustration purposes.

4

Table 1: Summary of the updates compared to previous combination of Ref. [7], including the integrated

luminosities used, analysis optimization (Analysis Opt.) and references for each channel included in this

combination.

H → τ+τ−

H → γγH →WW (∗) H → ZZ(∗)

ℓτhad3ν τℓτℓ+ jet ℓνℓν ℓνqq ℓℓℓℓ ℓℓνν ℓℓqq

L (fb−1) Ref. [7] 1.1 1.1 1.1 1.7 - 2.0-2.3 1.0 1.0

L (fb−1) 1.1 1.1 4.9 2.1 1.1 4.8 2.1 2.1

Analysis Opt. No No Yes No No Yes No No

Reference [14] [15] [18] [8] [19] [16] [9] [10]

3 Systematic Uncertainties

The systematic uncertainties are mostly unchanged with respect to the combination of Ref. [7]. The main

detector-related correlated systematic uncertainties are the electron/photon-related and muon-related sys-

tematic uncertainties including identification, energy scale and energy resolution; the jet energy scale

(JES) and jet energy resolution (JER) with a specific treatment of the b-jet energy scale; the Missing

Transverse Energy (MET) related systematic uncertainty which is in a large part correlated to the JES

uncertainty; and the systematic uncertainties related to the b-tagging and associated veto. These sources

of systematic uncertainty are considered as 100% correlated among channels. They are discussed in

more detail in Ref. [11] and the references of each individual channel [8–10, 14–16, 18, 19].

The energy scale systematic uncertainty on the reconstructed invariant mass distribution for the H →γγ and H → ZZ(∗) → ℓ+ℓ−ℓ+ℓ− channels was neglected in previous combinations due to its small impact

on the exclusion results. This uncertainty is estimated for muons, electrons, and photons, mainly based

on studies of the Z boson lineshape. Contrary to electrons and muons whose energy scale is more

directly constrained by the Z dielectron and dimuon events, the energy scale uncertainty for photons is

in general at the per mille level, but for certain categories of the analysis it can be larger although always

within 1%. It is estimated using Monte Carlo simulation, to account for detailed detector effects such

as the presampler calorimeter energy scale or potentially unmodeled additional material upstream of the

calorimeter. A simplified model attempting to account for the intricate correlations between electron and

photon energy scales is implemented. In the case of the observation of an excess, the estimate of its

significance is sensitive to this systematic uncertainty.

The Higgs boson production cross-sections are computed up to next-to-next-to-leading order (NNLO)

in QCD for the gluon fusion (gg → H), vector boson fusion (qq′ → qq′H) and associated WH/ZH pro-

duction processes (qq̄ → WH/ZH) and to next-to-leading order (NLO) for the associated production

with a tt̄ pair (qq̄/gg → tt̄H). These cross-sections and decay branching ratios and their related uncer-

tainties are compiled in Ref. [20]. The QCD scale uncertainties amount to +12−7 % for the gg → H process,

±1% for the qq′ → qq′H and associated WH/ZH processes, and +4−1% for the qq̄/gg → tt̄H process. The

uncertainties related to the parton distribution functions (PDF) amount to ±8% for the predominantly

gluon-initiated processes gg → H and qq̄/gg → tt̄H, and ±4% for the predominantly quark-initiated

qq′ → qq′H and WH/ZH processes [21]. The PDF uncertainties are assumed to be 100% correlated

among processes with identical initial states, regardless of these being signal or background. The theo-

retical uncertainty associated with the exclusive Higgs boson production process with one additional jet

in the H →WW (∗) → ℓ+νℓ−ν channel amounts to ±20% and is treated according to the prescription of

Ref. [12], as is the uncertainty at high masses due to interference effects.

The Monte Carlo generators used in the updated and additional channels are the same as those used

in Ref. [11] and the treatment of correlations between Monte Carlo background normalizations, scale

5

Table 2: Numbers of observed events (Nobs) and the expected numbers of signal (s) and background (b)

events in the channels used in the combination. For all channels except H → WW (∗) → ℓ+νℓ−ν and

H → ττ → ℓ+ℓ−4ν these numbers are estimated in an interval containing ∼ 90% of the signal around

the most probable value of the invariant or transverse mass distributions of the signal at the specified

Higgs boson mass hypotheses. These numbers are for information only as the analyses typically fit the

distributions. Despite the deficit observed in the H → ττ → ℓτhad3ν channel, the number of events

observed is compatible with the expected background within systematic uncertainties.

H → τ+τ−

H → γγH →WW (∗) H → ZZ(∗)

ℓτhad3ν τℓτℓ+ jetℓνℓν ℓνqq

ℓℓℓℓ ℓℓνν ℓℓqq ℓℓbb0-jet 1-jet 0-jet 1-jet

mH=120 GeV

s 8.0 0.8 63.8 4.7 1.6 - - 0.7 - - -

b 1218 47.1 2943 43.3 15.3 - - 1.3 - - -

Nobs 1072 46 2935 54 19 - - 0 - - -

mH=130 GeV

s 5.9 0.7 57.3 14.3 4.9 - - 2.4 - - -

b 1166 47.1 2438 56.2 19.6 - - 1.7 - - -

Nobs 880 46 2475 67 27 - - 3 - - -

mH=150 GeV

s - - 36.7 40.4 14.3 - - 5.6 - - -

b - - 1662 63.7 27.9 - - 1.5 - - -

Nobs - - 1645 81 29 - - 1 - - -

mH=200 GeV

s - - - 13.9 16.6 - - 14.3 8.9 59.9 3.9

b - - - 39.6 47.4 - - 14.5 119.1 11393 37.4

Nobs - - - 36 44 - - 12 111 10820 38

mH=300 GeV

s - - - 13.5 8.6 19.6 22.0 9.0 17.9 12.7 1.1

b - - - 144 92.2 3981 3795 12.1 68.1 386 3.8

Nobs - - - 158 104 4493 4316 11 56 357 1

mH=400 GeV

s - - - - - 18.8 23.7 6.1 17.6 19.1 1.9

b - - - - - 1823 2485 9.5 57.1 457 4.0

Nobs - - - - - 2005 2790 8 47 416 2

6

factors and shape estimates is unchanged.

The effects of the major sources of systematic uncertainty on the signal and background yields are

summarized in Tables 3 and 4 respectively.

4 Exclusion Limit Results

The combination procedure of Refs. [7, 11, 12, 22], based on the profile likelihood test statistic [23],

is applied. The 95% C.L. cross-section limits in units of the Standard Model expectation set by the

individual channels using the CLs prescription [24,25] are shown in Fig. 3. The combination, in terms of

the observed and the expected upper limit at the 95% C.L. on the Higgs boson production cross-section,

normalized to the Standard Model value, of all channels is shown in Fig. 4(a) and Fig. 4(b). The limits

shown are made using the asymptotic approximation [23] which has been verified using an ensemble

test and a Bayesian calculation which agrees with these results to within a few percent. The expected

exclusion region covers the Standard Model Higgs boson mass range from 124.6 GeV to 520 GeV. The

observed 95% C.L. exclusion regions are from 131 GeV to 237 GeV and 251 GeV to 453 GeV. In addition

a very small mass range between 112.7 GeV and 115.5 GeV is excluded at the 95% C.L., corresponding

to a local deficit of events in the diphoton mass spectrum.

The deficit of events observed in the excluded mass range, and in particular between 300 GeV and

400 GeV as reported in Refs. [7, 11], is unchanged and still mainly due to the concordance of various

small deficits in several high mass channels. The observed exclusion covers a large part of the expected

exclusion range, except at low and high Higgs boson mass hypotheses where excesses of events are

observed, and at around 245 GeV where the excess mostly seen in the H → ZZ(∗) → ℓ+ℓ−ℓ+ℓ− channel

in Refs. [7, 11] is still present, as seen in Fig. 1(b) and Fig. 1(c). A similar excess of events was not seen

by the CMS experiment, and the unexcluded small mass range at around 245 GeV was excluded by the

combination of the ATLAS and CMS experiments [26].

The confidence level with which the Standard Model Higgs boson is excluded is shown in Fig. 5(a)

and Fig. 5(b). It should be noted that a signal of the Standard Model strength is excluded at high con-

fidence for 360 GeV, while an exclusion Confidence Level (CLs) in excess of 99% is observed in the

regions between 133 GeV and 230 GeV and between 260 GeV and 437 GeV. The strongest exclusions

have false exclusion rates at a level of one per million. When the best-fit value of the strength parameter

exceeds the tested signal hypothesis, which in this case is the Standard Model Higgs boson cross section,

the observed CLs is bound to be equal to 50 % by construction 1.

5 Observation of an Excess

The local significance of an excess is estimated using a consistency test of the observation with the

background-only hypothesis. It is estimated by the p0 probability that a background-only experiment

is more signal-like than the observed one. The probability p0 is constructed to be equal to 50% for

downward fluctuations of the background and smaller than 50% when more events are observed than

expected. This probability is displayed as a function of the Higgs boson mass hypothesis in Fig. 6(a)

and Fig. 6(b). Essentially identical results are derived using ensemble tests instead of the asymptotic

approximation (see Appendix).

An excess of events is observed near mH=126 GeV. This excess appears simultaneously in the high

resolution H → γγ and the H → ZZ(∗) → ℓ+ℓ−ℓ+ℓ− channels and is also observed in the H →WW (∗) →ℓ+νℓ−ν channel, which has a very low mass resolution. Its local significance is 3.6σ . The local sig-

nificance of the excess seen in the H → γγ channel is 2.8σ , in the H → ZZ(∗) → ℓ+ℓ−ℓ+ℓ− it amounts

1This feature of the CLs construction is related to the choice of the profile likelihood ratio test statistic.

7

Table 3: Main correlated signal systematic uncertainties used in the analysis. These relative uncertainties

(%) correspond to the overall effect on the signal yield of the ±1σ variation of the source of systematic

uncertainty for a Higgs boson mass hypothesis of 120 GeV for the H → γγ , H → ττ → ℓτhad3ν , H →ττ → ℓ+ℓ−4ν , H → WW (∗) → ℓ+νℓ−ν and H → ZZ(∗) → ℓ+ℓ−ℓ+ℓ− analyses and 300 GeV for the

H → ZZ → ℓ+ℓ−qq, H → ZZ → ℓ+ℓ−νν and H →WW → ℓνqq′ analyses.

H → τ+τ−H → γγ

H →WW (∗) H → ZZ(∗)


Luminosity +3.8−3.6

+3.8−3.6

+4.0−3.8

+3.8−3.6

+3.8−3.6

+3.9−3.8

+3.8−3.6

+3.8−3.6

e/γ eff. ±3.5 ±2.0 +13.5−11.9 ±2.0 ±0.9 ±2.9 ±1.2 ±1.2

e/γ E. scale +1.3−0.1 ±0.3 - ±0.4 - - ±0.7 ±0.4

e/γ res. - +0.2−0.5 - +0.20

−0.05 - - ±0.25 ±0.1

µ eff. ±1.0 ±2.0 - - ±0.3 ±0.16 ±0.7 ±0.5

µ res. Id. - +0.2−0.5 - +0.02

−0.04 - - ±1.1 ±1.1

µ res. MS. - - - +0.04+0.08 - - +1.1

−1.0 ±1.1

Jet/τ/MET E. scale +18.9−16.4

+3.4−10.0 - +4.46

−6.47+18.4−15.5 - ±1.6 ±15.0

JER - ±2.0 - +1.8−1.7

+9.0−8.2 - +0.3

−0.0+4.0−0.0

MET - +4.4−5.3 - +1.8

−1.7 - - - -

b-tag eff. - - - ±0.5 - - ±0.3 ±3.7

τ eff. ±9.1 - - - - - -

Table 4: Main correlated background systematic uncertainties used in the analysis. These relative un-

certainties (%) correspond to the overall effect on the background yield of the ±1σ variation of the

source of systematic uncertainty for a Higgs boson mass hypothesis of 120 GeV for the H → γγ ,

H → ττ → ℓτhad3ν , H → ττ → ℓ+ℓ−4ν , H →WW (∗) → ℓ+νℓ−ν and H → ZZ(∗) → ℓ+ℓ−ℓ+ℓ− analyses

and 300 GeV for the H → ZZ → ℓ+ℓ−qq, H → ZZ → ℓ+ℓ−νν and H →WW → ℓνqq′ analyses.

H → τ+τ−H → γγ

H →WW (∗) H → ZZ(∗)


Luminosity +3.0−2.9

+3.8−3.6 - ±0.2 - +3.7

−3.6+2.4−2.3

+0.3−0.2

e/γ eff. ±2.4 +0.5−1.6 - ±2.3 ±0.8 ±1.6 ±0.8 ±0.1

e/γ E. scale +0.9−0.3 ±0.8 - +0.2

−0.1 - - +1.7−1.6 ±0.1

e/γ res. - +0.3−2.6 - +0.1

−0.0 - - ±0.6 ±0.2

µ eff. ±1.4 +0.5−1.6 - - ±0.3 ±0.1 ±0.5 ±0.03

µ res. Id. - +0.3−2.6 - −0.03

−0.06 - - +1.7−1.6 ±0.2

µ res. MS. - - - +0.00−0.02 - - +1.7

−1.6 ±0.2

Jet/τ/MET E. scale +10.0−8.9

+7.0−9.8 - +8.5

−10.4 - - +6.9−5.2 ±1.0

JER - ±2.5 - +3.3−3.0 - - +1.8

−0.0+0.3−0.0

MET - +0.4−2.7 - +0.6

−0.5 - - - -

b-tag eff. - - - ±1.8 - - +7.0−5.5 ±0.2

τ eff. ±7.2 - - - - - -

8

to 2.1σ and in the H → WW (∗) → ℓ+νℓ−ν channel to 1.4σ . For each of these three channels, the ex-

pected local significance is approximately 1.4 σ for a 126 GeV Higgs boson. It should be noted that the

H → WW (∗) → ℓ+νℓ−ν analysis uses an integrated luminosity of 2.1 fb−1, corresponding to less than

half of the accumulated data. The two main components of this excess appear in the two channels with

high reconstructed invariant mass resolution, the H → γγ and H → ZZ(∗) → ℓ+ℓ−ℓ+ℓ− channels. The

local significance of the excess when combining these two channels alone is 3.4 σ .

The excess of events at around 126 GeV is visible in all three channels in the invariant mass (in

the H → γγ and H → ZZ(∗) → ℓ+ℓ−ℓ+ℓ− cases) and transverse mass (in the H → WW (∗) → ℓ+νℓ−νcase) distributions as seen in Fig. 1. The best fit values of the signal strength parameter for the combi-

nation and for these three channels are illustrated in Fig. 7 and Fig. 8 as a function of the Higgs boson

mass hypothesis. The requirement that the probability density function used to model the signal-plus-

background reconstructed mass distributions in each channel should never be negative, imposes a lower

limit on negative values of the best fit values of the signal strength. This can be observed in Fig. 7 and

Fig. 8(b), where in the low mass region the lower bound on the best fit value of the strength parameter is

constrained by the H → ZZ(∗) → ℓ+ℓ−ℓ+ℓ− channel. The excess observed is not incompatible with the

production of a Standard Model Higgs boson with a mass of around 126 GeV. These best fit values do

not account for energy scale systematic uncertainties. The expected and observed p0 as a function of the

Higgs boson mass hypothesis, in the low mass region, for individual channels and the combination are

illustrated in Fig. 9.

The observed combined significance of the excess taking into account the simplified model of energy

scale systematic uncertainties for photons described in Section 3 and neglecting the impact of energy

scale systematic uncertainties on electrons and muons on the reconstructed invariant mass shapes, is

∼ 0.01σ higher than the combined significance without any energy scale systematic uncertainties taken

into account on the invariant mass shapes.

[GeV]HM

100 200 300 400 500 600

SM

σ/σ

95

% C

L lim

it o

n

110

1

10

) 1 (4.9 fbγγ→H) 1 (2.1 fbνlν l→ WW→H

) 1 (1.1 fbττ→H

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) 1

(1.1 fbττ→H

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)1 (2.1 fbνν ll→ ZZ→H)1qq (1.1 fbν l→ WW →H

) 1

llll (4.8 fb→ ZZ→H

) 1

llqq (2.1 fb→ ZZ→H

)1

(2.1 fbνν ll→ ZZ→H

)1

qq (1.1 fbν l→ WW →H

Exp. Obs. Exp. Obs.

=7 TeVs, 1

L dt ~ 1.04.9 fb∫ CLs limitsATLAS Preliminary

Figure 3: The expected (dashed) and observed (solid) cross-section limits for the individual search chan-

nels, normalized to the Standard Model Higgs boson production cross-section, as functions of the Higgs

boson mass. These results use the profile likelihood technique with 95% C.L. limits using the CLS pre-

scription.

9

The local significance of the excess observed at Higgs boson mass hypotheses around 126 GeV,

corresponds to a local p-value for rejecting the background hypothesis of 1.9× 10−4. The prescription

described in Refs. [12, 27] is used to estimate a global p-value pglobal which is corrected for the fact that

the local excess could have appeared anywhere in the mass region in which the Higgs boson has been

searched for; this is known as the look-elsewhere effect. The global p-value is estimated by pglobal =

plocal + Ne−(Z2−Z20)/2, where N is the average number of times plocal(mH) is expected to cross, in a

single direction, a specified low significance Z0 in the relevant search region, and Z is the observed

local significance. In the absence of a Monte Carlo-based simulation, the data have been used to make

an approximate estimate of N. Examination of Fig. 7 shows that the number of upward crossings of

µ̂(mH) = Z = 0 is N = 6 across the full search range (from 110 GeV to 600 GeV) and N = 3 in the range

(from 110 GeV to 146 GeV) not excluded at the 99% confidence level by the recent LHC combined

Higgs boson search results [26]. Global probabilities of an excess of 0.6% (2.5σ ) to 1.4% (2.2σ ) are

found for the low mass unexcluded region and the full mass range respectively.

Taking the look-elsewhere effect into account the global probability of an excess of 2.8σ in the

H → γγ channel in its search mass domain is approximately 7% and the global probability to observe an

excess of 2.1σ in the H → ZZ(∗) → ℓ+ℓ−ℓ+ℓ− channel is approximately 1/3.

6 Conclusion

More than 5 fb−1 of integrated luminosity has been accumulated in 2011 by the ATLAS experiment, and

up to 4.9 fb−1 has been used to update the searches for the Higgs boson. At present, not all channels

use the full integrated luminosity, but nevertheless the sensitivity of the analysis allows searches for the

Standard Model Higgs boson in a significantly greater range than has been possible up to now.

With this dataset, Higgs boson masses between 124.6 GeV and 520 GeV are expected to be excluded

at the 95% C.L. or considerably higher. The observed Higgs boson mass exclusion at the 95% C.L.

ranges from 112.7 GeV to 115.5 GeV, 131 GeV to 237 GeV and 251 GeV to 453 GeV. An exclusion

of the Standard Model Higgs boson production cross-section at the 99% C.L. is reached in the regions

between 133 GeV and 230 GeV and between 260 GeV and 437 GeV.

An excess of events is observed in the H → γγ and H → ZZ(∗) → ℓ+ℓ−ℓ+ℓ− channels, at Higgs

mass hypotheses close to 126 GeV, which is also supported by a broad low-significance excess in the

H →WW (∗) → ℓ+νℓ−ν channel. The expected sensitivities in terms of local significance for a 126 GeV

Higgs boson for each of these three channels are approximately 1.4σ . The observed local significances

of the individual excesses are 2.8σ , 2.1σ and 1.4σ , respectively. The combined local significance of

these excesses is 3.6σ . Taking the look-elsewhere effect into account the global probability of such an

excess to occur in the full search range is approximately 1%, corresponding to 2.3σ .

10

[GeV]HM100 200 300 400 500 600

SM

σ/σ

95%

CL L

imit o

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1

10

ObservedExpected

σ1 ±σ2 ±

= 7 TeVs

1 Ldt = 1.04.9 fb∫

ATLAS Preliminary 2011 Data

CLs Limits

(a)

[GeV]HM110 115 120 125 130 135 140 145 150

SM

σ/σ

95%

CL L

imit o

n

1

10ObservedExpected

σ1 ±σ2 ± = 7 TeVs

1 Ldt = 1.04.9 fb∫


CLs Limits

(b)

Figure 4: The combined upper limit on the Standard Model Higgs boson production cross-section divided

by the Standard Model expectation as a function of mH is indicated by the solid curve. This is a 95% C.L.

limit using the CLs method in the full mass range of this analysis (a) and in the low mass range (b). The

dotted curve shows the median expected limit in the absence of a signal and the green and yellow bands

indicate the corresponding 68% and 95% expected regions.

11

[GeV]HM100 200 300 400 500 600

CLs

610

510

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1

Observed

Expected

95%

99%

= 7 TeVs

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(a)

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CLs

510

410

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1

Observed

Expected

95%

99%

= 7 TeVs

1 Ldt = 1.04.9 fb∫


(b)

Figure 5: The value of the combined CLs for µ = 1 (testing the Standard Model Higgs boson hypothesis)

as a function of mH in the full mass range of this analysis (a) and in the low mass range (b). By definition,

the regions with CLs < α are considered excluded at the (1−α) C.L. or stronger. When the best-fit value

of the strength parameter exceeds the tested signal hypothesis (µ = 1) the observed CLs is bound to be

equal to 50 % by construction.

12

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Local P

Valu

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1

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Expected

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σ3

σ4

σ5 = 7 TeVs

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(a)

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Local P

Valu

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610

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1

Observed

Expected

σ2

σ3

σ4

σ5 = 7 TeVs

1 Ldt = 1.04.9 fb∫


(b)

Figure 6: The consistency of the observed results with the background-only hypothesis is shown in the

full mass range of this analysis (a) and in the low mass range (b). The dashed curve show the median

expected significance in the hypothesis of a Standard Model Higgs boson production signal. The four

horizontal dashed lines indicate the p-values corresponding to significances of 2σ , 3σ , 4σ and 5σ .

13

[GeV]HM100 200 300 400 500 600

Sig

nal str

ength

2

1

0

1

2

3

4

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σ1 ± = 7 TeVs

1 Ldt = 1.04.9 fb∫


(a)

[GeV]HM110 115 120 125 130 135 140 145 150

Sig

nal str

ength

2

1.5

1

0.5

0

0.5

1

1.5

2

2.5

Best fit

σ1 ± = 7 TeVs

1 Ldt = 1.04.9 fb∫


(b)

Figure 7: The best-fit signal strength µ = σ/σSM as a function of the Higgs boson mass hypothesis is

shown in the full mass range of this analysis (a) and in the low mass range (b). The µ value indicates by

what factor the SM Higgs boson cross-section would have to be scaled to best match the observed data.

The light-blue band shows the approximate ±1σ range.

14

[GeV]HM110 115 120 125 130 135 140 145 150

Sig

na

l str

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gth

3

2

1

0

1

2

3

Best fit

σ1 ± = 7 TeVs

1 Ldt = 4.9 fb∫

ATLAS Preliminary γγ→H

2011 Data

(a)

[GeV]HM110 115 120 125 130 135 140 145 150

Sig

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2

1

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6

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σ1 ± = 7 TeVs

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ATLAS Preliminary llll→ZZ→H

2011 Data

(b)

[GeV]HM110 115 120 125 130 135 140 145 150

Sig

na

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gth

2

0

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4

6

8

10

12

Best fit

σ1 ± = 7 TeVs

1 Ldt = 2.05 fb∫

ATLAS Preliminary νlνl→WW→H

2011 Data

(c)

Figure 8: The best-fit signal strength µ = σ/σSM as a function of the Higgs boson mass hypothesis for

the H → γγ (a), the H → ZZ(∗) → ℓ+ℓ−ℓ+ℓ− (b) and H → WW (∗) → ℓ+νℓ−ν (c) individual channels.

The µ value indicates by what factor the SM Higgs boson cross-section would have to be scaled to best

match the observed data. The light-blue band shows the approximate ±1σ range.

15

[GeV]HM

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Exp. Comb.

Obs. Comb.

4l→Exp. H

4l→Obs. H

γγ →Exp. H

γγ →Obs. H

νlν l→Exp. H

νlν l→Obs. H

ATLAS Preliminary

σ2

σ3

σ4

1 L dt ~ 2.054.9 fb∫

2011 Data

Figure 9: The consistency of the observed results with the background-only hypothesis for the three

strongest channels and the combination in the low mass region. The dashed curves show the median

expected significance in the hypothesis of a Standard Model Higgs boson production signal, which is

about equal for all three of these channels near 125 GeV.

16

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18

Appendix - Comparison of Methods

The asymptotic method used to derive the results shown in this note, following the prescription described

in Ref. [23], relies on the assumption of large numbers of events, which is not necessarily the case,

even when combining various channels. To validate the use of the asymptotic formulae in the context

of the observation of an excess, the combined probability of a background fluctuation p0, derived using

asymptotic formulae is verified using an ensemble of pseudo-experiments. To reach a sufficient statistical

precision 120,000 pseudo-experiments per mass hypotheses are used in the region of the excess. Only

5,000 pseudo-experiments were used in the Higgs boson mass region where p0 is much larger. As

shown in Fig. 10, good agreement is observed between p0(mH) calculated with pseudo-experiments

and the asymptotic expressions used for the primary results over most of the mass range. At 126 GeV,

the ensemble of pseudo-experiments approach yields p0 = (2.2± 0.4)× 10−4 in agreement with p0 =1.9×10−4 obtained using the asymptotic formulae.

[GeV]Hm

110 115 120 125 130 135 140 145 150

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Combined observed ensemble

Combined observed asymptotic

Combined expected asymptotic

σ2

σ3

σ4

1Ldt = 1.04.9 fb∫ = 7 TeVs

ATLAS Preliminary 2011 data

Figure 10: The observed (red dots) and expected (red dashed line) local p0 values in the presence of a

signal using the asymptotic approximation. The results of the asymptotic approach are compared with

those obtained using an ensemble of pseudo-experiments (black dots) as a function of mH in the low

mass region.

19

ATLAS Conference by Fabiola Giannotti

Technology

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