1. 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
ATLAS-CONF-2011-163
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
13 December 2011
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+ ν ℓ− ν ,
contribute individual local significances of 2.8σ , 2.1σ and 1.4σ , respectively, to the excess.

2. 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+ ν ℓ− 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 → ℓν qq′ 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− bb and W H →ℓν bb [13] channels are not included in this combination. The other chan-
¯ ¯
nels H → ττ → ℓτhad 3ν [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 Zbb and Zqq. ¯
• 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+ ν ℓ− ν and H → ττ → ℓ+ ℓ− 4ν , are
summarized in Table 2. An overall deficit is observed in the H → ττ → ℓτhad 3ν channel; the number of
events observed is however compatible with the expected background within systematic uncertainties.
2

3. 800
Events / 1 GeV
Events / 5 GeV
12
700
ATLAS Preliminary
Data Data 2011, s = 7 TeV, ∫ Ldt = 4.8 fb -1
10 Data
ATLAS Preliminary
600 MC mH=130 GeV, 1xSM
mH=130 GeV, 1xSM
500 Total background (Fit) 8
Total background
400 (*)
H→γ γ 6 H→ZZ →4l
300
4
200
100 Data 2011, s = 7 TeV, ∫ Ldt = 4.9 fb -1
2
0 0
100 110 120 130 140 150 160 100 120 140 160 180 200 220 240
mγ γ [GeV] m4l [GeV]
(a) (b)
Events / 5 GeV
Events / 10 GeV
50
∫ Ldt = 2.05 fb
12 ATLAS Preliminary -1
Data 2011, s = 7 TeV,
10
Data 2011, s = 7 TeV, ∫ -1
Ldt = 4.8 fb
40
ATLAS Preliminary
Data
Data
mH=130 GeV, 1xSM
8 mH=130 GeV, 1xSM
30 Total background
Total background
6
H→WW→lνlν+0/1jet
(*)
H→ZZ →4l 20
4
10
2
0 0
100 150 200 250 300 350 400 450 500 550 600 60 80 100 120 140 160 180 200 220 240
m4l [GeV] mT [GeV]
(c) (d)
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+ ν ℓ− ν (d) channels.
3

4. Events / 20 GeV
Events / 10 GeV
50 Data 2011, s = 7 TeV, ∫ Ldt = 1.06 fb -1 350 Data 2011, s = 7 TeV, ∫ Ldt = 1.06 fb -1
ATLAS Preliminary 300 ATLAS Preliminary
40
Data
250 Data
30 mH=120 GeV, 12xSM 200 mH=120 GeV, 30xSM
Total background 150 Total background
20
H→τlτl+1j 100 H→τlτh
10
50
0 0
0 50 100 150 200 250 300 350 400 0 50 100 150 200 250 300 350 400
mττ [GeV] mττ [GeV]
(a) (b)
4
10 100
Events / 25 GeV
Events / 100 GeV
Data 2011, s = 7 TeV, ∫ Ldt = 1.04 fb
-1
90
80
ATLAS Preliminary
Data 2011, s = 7 TeV, ∫ Ldt = 2.05 fb -1
103 ATLAS Preliminary
70
Data
Data
60
102 mH=400 GeV, 50xSM
50 mH=500 GeV, 2xSM
Total background 40 Total background
10
30
H→WW→lνqq
20 H→ZZ→ llνν
1 10
0
0 200 400 600 800 1000 1200 1400 1600 1800 2000 100 200 300 400 500 600 700 800 900
mlνqq [GeV] mT [GeV]
(c) (d)
6
Events / 25 GeV
Events / 25 GeV
140 ATLAS Preliminary ATLAS Preliminary
120 ∫
Data 2011, s = 7 TeV, Ldt = 2.05 fb
-1
5 Data 2011, s = 7 TeV, ∫ Ldt = 2.05 fb -1
Data Data
100 4
mH=500 GeV, 5xSM mH=500 GeV, 1xSM
80 3
Total background Total background
60
H→ZZ→ llqq 2
H→ZZ→ llbb
40
1
20
0 0
200 300 400 500 600 700 800 900 200 300 400 500 600 700 800 900
mlljj [GeV] mlljj [GeV]
(e) (f)
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 → ℓτhad 3ν (b), the H → WW → ℓν qq′ (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

5. 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 → WW (∗) H → ZZ (∗)
ℓτhad 3ν τℓ τℓ + jet H → γγ ℓν ℓν ℓν qq 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 W H/ZH pro-
duction processes (qq → W H/ZH) and to next-to-leading order (NLO) for the associated production
¯
¯ ¯
with a t t pair (qq/gg → t t H). These cross-sections and decay branching ratios and their related uncer-
¯
tainties are compiled in Ref. [20]. The QCD scale uncertainties amount to +12 % for the gg → H process,
−7
±1% for the qq′ → qq′ H and associated W H/ZH processes, and +4 % for the qq/gg → t t H process. The
−1 ¯ ¯
uncertainties related to the parton distribution functions (PDF) amount to ±8% for the predominantly
¯
gluon-initiated processes gg → H and qq/gg → t t H, and ±4% for the predominantly quark-initiated
¯
qq ′ → qq′ H and W H/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+ ν ℓ− 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

6. 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+ ν ℓ− 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 → ℓτhad 3ν channel, the number of events
observed is compatible with the expected background within systematic uncertainties.
H → τ +τ − H → WW (∗) H → ZZ (∗)
H → γγ ℓν ℓν ℓν qq
ℓτhad 3ν τℓ τℓ + jet ℓℓℓℓ ℓℓνν ℓℓqq ℓℓbb
0-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

7. 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+ ν ℓ− 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
1 This feature of the CLs construction is related to the choice of the profile likelihood ratio test statistic.
7

8. 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 → ℓτhad 3ν , H →
ττ → ℓ+ ℓ− 4ν , H → WW (∗) → ℓ+ ν ℓ− 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 → ℓν qq′ analyses.
H → τ +τ − H → WW (∗) H → ZZ (∗)
H → γγ
ℓτhad 3ν τℓ τℓ + jet ℓν ℓν ℓν qq ℓℓℓℓ ℓℓνν ℓℓqq
+3.8 +3.8 +4.0 +3.8 +3.8 +3.9 +3.8 +3.8
Luminosity −3.6 −3.6 −3.8 −3.6 −3.6 −3.8 −3.6 −3.6
+13.5
e/γ eff. ±3.5 ±2.0 −11.9 ±2.0 ±0.9 ±2.9 ±1.2 ±1.2
+1.3
e/γ E. scale −0.1 ±0.3 - ±0.4 - - ±0.7 ±0.4
+0.2 +0.20
e/γ res. - −0.5 - −0.05 - - ±0.25 ±0.1
µ eff. ±1.0 ±2.0 - - ±0.3 ±0.16 ±0.7 ±0.5
+0.2 +0.02
µ res. Id. - −0.5 - −0.04 - - ±1.1 ±1.1
+0.04 +1.1
µ res. MS. - - - +0.08 - - −1.0 ±1.1
+18.9 +3.4 +4.46 +18.4
Jet/τ /MET E. scale −16.4 −10.0 - −6.47 −15.5 - ±1.6 ±15.0
+1.8 +9.0 +0.3 +4.0
JER - ±2.0 - −1.7 −8.2 - −0.0 −0.0
+4.4 +1.8
MET - −5.3 - −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 → ℓτhad 3ν , H → ττ → ℓ+ ℓ− 4ν , H → WW (∗) → ℓ+ ν ℓ− 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 → ℓν qq′ analyses.
H → τ +τ − H → WW (∗) H → ZZ (∗)
H → γγ
ℓτhad 3ν τℓ τℓ + jet ℓν ℓν ℓν qq ℓℓℓℓ ℓℓνν ℓℓqq
+3.0 +3.8 +3.7 +2.4 +0.3
Luminosity −2.9 −3.6 - ±0.2 - −3.6 −2.3 −0.2
+0.5
e/γ eff. ±2.4 −1.6 - ±2.3 ±0.8 ±1.6 ±0.8 ±0.1
+0.9 +0.2 +1.7
e/γ E. scale −0.3 ±0.8 - −0.1 - - −1.6 ±0.1
+0.3 +0.1
e/γ res. - −2.6 - −0.0 - - ±0.6 ±0.2
+0.5
µ eff. ±1.4 −1.6 - - ±0.3 ±0.1 ±0.5 ±0.03
+0.3 +1.7
µ res. Id. - −2.6 - −0.03
−0.06 - - −1.6 ±0.2
+0.00 +1.7
µ res. MS. - - - −0.02 - - −1.6 ±0.2
+10.0 +7.0 +8.5 +6.9
Jet/τ /MET E. scale −8.9 −9.8 - −10.4 - - −5.2 ±1.0
+3.3 +1.8 +0.3
JER - ±2.5 - −3.0 - - −0.0 −0.0
+0.4 +0.6
MET - −2.7 - −0.5 - - - -
+7.0
b-tag eff. - - - ±1.8 - - −5.5 ±0.2
τ eff. ±7.2 - - - - - -
8

9. to 2.1σ and in the H → WW (∗) → ℓ+ ν ℓ− 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+ ν ℓ− 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+ ν ℓ− ν
case) 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.
95% CL limit on σ/ σSM
Exp. Obs. Exp. Obs.
-1 -1
H→γ γH(4.9 fb-1) fb )
→γ γ (4.9 H→ ZZ→ ZZ→(4.8 fb -1)fb )
H→ llll llll (4.8
-1 -1
H→ WW→ (2.1 fb-1
H→ WW → lν lν lν lν (2.1 )fb ) H→ llqq (2.1 fb-1
H→ ZZ→ ZZ→ llqq (2.1)fb )
-1
H→ττH→τ τ fb-1) fb )
(1.1 (1.1 H→ llν ν (2.1 fb-1 -1
H→ ZZ→ ZZ→ llν ν (2.1)fb )
H→ WW qq qq fb-1 -1
H→ WW → lν → lν(1.1 (1.1 )fb )
10
1
-1
ATLAS Preliminary ∫ L dt ~ 1.0-4.9 fb ,
-1
s=7 TeV CLs limits
10
100 200 300 400 500 600
MH [GeV]
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

10. 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 =
2 2
plocal + Ne−(Z −Z0 )/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
µ (mH ) = Z = 0 is N = 6 across the full search range (from 110 GeV to 600 GeV) and N = 3 in the range
ˆ
(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+ ν ℓ− 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

11. 10
95% CL Limit on σ/σSM
ATLAS Preliminary 2011 Data
Observed
Expected
±1 σ
∫ Ldt = 1.0-4.9 fb -1
s = 7 TeV
±2 σ
1
CLs Limits
10-1 100 200 300 400 500 600
MH [GeV]
(a)
95% CL Limit on σ/σSM
ATLAS Preliminary 2011 Data
Observed
10 Expected
±1 σ
∫ Ldt = 1.0-4.9 fb-1
±2 σ s = 7 TeV
1
CLs Limits
110 115 120 125 130 135 140 145 150
MH [GeV]
(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

12. CLs
1 ATLAS Preliminary 2011 Data
10-1 95%
10-2 99%
10-3 Observed
Expected
10-4
10-5 ∫ Ldt = 1.0-4.9 fb -1
s = 7 TeV
-6
10
100 200 300 400 500 600
MH [GeV]
(a)
CLs
1 ATLAS Preliminary 2011 Data
10-1
95%
10-2 99%
Observed
10-3 Expected
10-4 ∫ Ldt = 1.0-4.9 fb-1
s = 7 TeV
-5
10
110 115 120 125 130 135 140 145 150
MH [GeV]
(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

13. Local P-Value ATLAS Preliminary 2011 Data
1
10-1
2σ
10-2
3σ
10-3
Observed
10-4 Expected
4σ
∫ Ldt = 1.0-4.9 fb
-5
10 -1
10-6 s = 7 TeV
5σ
-7
10 100 200 300 400 500 600
MH [GeV]
(a)
Local P-Value
ATLAS Preliminary 2011 Data
1
10-1
2σ
10-2
3σ
10-3
Observed
10-4 Expected
4σ
∫ Ldt = 1.0-4.9 fb
-5
10 -1
10-6 s = 7 TeV
5σ
-7
10 110 115 120 125 130 135 140 145 150
MH [GeV]
(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

14. 4
Signal strength ATLAS Preliminary 2011 Data
3 Best fit
±1 σ
∫ Ldt = 1.0-4.9 fb
-1
s = 7 TeV
2
1
0
-1
-2
100 200 300 400 500 600
MH [GeV]
(a)
2.5
Signal strength
ATLAS Preliminary 2011 Data
2
1.5
Best fit
±1 σ ∫ Ldt = 1.0-4.9 fb
-1
s = 7 TeV
1
0.5
0
-0.5
-1
-1.5
-2
110 115 120 125 130 135 140 145 150
MH [GeV]
(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

15. Signal strength
3 ATLAS Preliminary H→γ γ
2
Best fit
±1 σ
∫ Ldt = 4.9 fb
-1
s = 7 TeV
1
0
-1
-2
-3 2011 Data
110 115 120 125 130 135 140 145 150
(a) MH [GeV]
Signal strength
6 ATLAS Preliminary H→ZZ→llll
∫ Ldt = 4.8 fb
5 Best fit -1
4 ±1 σ
s = 7 TeV
3
2
1
0
-1
-2
2011 Data
-3
110 115 120 125 130 135 140 145 150
(b) MH [GeV]
Signal strength
12
ATLAS Preliminary H→WW→lνlν
∫ Ldt = 2.05 fb
10 Best fit -1
8 ±1 σ
s = 7 TeV
6
4
2
0
-2
2011 Data
110 115 120 125 130 135 140 145 150
(c) MH [GeV]
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+ ν ℓ− 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

16. 1
Local P-Value
10-1
2σ
10-2
3σ
-3
10
10-4 4σ ATLAS Preliminary
Exp. Comb. Exp. H → γ γ
10-5 Obs. Comb. Obs. H → γ γ
Exp. H → 4l Exp. H → lν lν 2011 Data
10-6 ∫ L dt ~ 2.05-4.9 fb
Obs. H → 4l Obs. H → lν lν -1
110 115 120 125 130 135 140 145 150
MH [GeV]
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

19. 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.
Local P-Value
10-1
2σ
-2
10
3σ
10-3
10-4
4σ
10-5 ATLAS Preliminary 2011 data
∫ Ldt = 1.0-4.9 fb
Combined observed ensemble -1
Combined observed asymptotic s = 7 TeV
Combined expected asymptotic
10-6
110 115 120 125 130 135 140 145 150
mH [GeV]
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