Measurements of the total cross sections for Higgs boson production combining the H → γγ and H → Z Z

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ATLAS-CONF-2015-069 15December2015

ATLAS NOTE

ATLAS-CONF-2015-069

December 14, 2015

Measurements of the total cross sections for Higgs boson production combining the H → γγ and H → Z Z

^∗

→ 4` decay

channels at 7, 8 and 13 TeV center-of-mass energies with the ATLAS detector

The ATLAS Collaboration

Abstract

Measurements of total cross sections are presented for Higgs boson production in proton–

proton collisions recorded by the ATLAS detector at the Large Hadron Collider. The data correspond to 4.5 fb⁻¹ collected at the center-of-mass energy of 7 TeV, 20.3 fb⁻¹at 8 TeV, and most recently 3.2 fb⁻¹at 13 TeV. Cross sections are obtained from measured H → γγ andH →ZZ^∗ →4`event yields, which are combined accounting for detector effects, fiducial acceptances and branching fractions. The total Higgs boson production cross sections are measured to be 34⁺₋₁₀¹¹pb, 33.3⁺_−5.5^5.8pb, and 24⁺₋₁₈²¹pb for center-of-mass energies of 7, 8 and 13 TeV, respectively. These measurements are compared to state-of-the-art Standard Model theoretical predictions.

c

2015 CERN for the benefit of the ATLAS Collaboration.

Reproduction of this article or parts of it is allowed as specified in the CC-BY-3.0 license.

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1 Introduction and methods

This note presents measurements of total cross sections of inclusive Higgs boson production usingH → γγandH →ZZ^∗→4`final states in proton–proton (pp) collisions recorded by the ATLAS detector [1]

at the Large Hadron Collider (LHC). Results are presented for center-of-mass energies of 7, 8 and 13 TeV, corresponding to integrated luminosities of 4.5 fb⁻¹, 20.3 fb⁻¹ and 3.2 fb⁻¹, respectively. The measured cross sections probe the properties of the Higgs boson and can be compared to state-of-the-art theoretical calculations at the three different center-of-mass energies.

The analysis uses event yields measured in H → γγ andH → ZZ^∗ → 4`decays corrected for detector effects, fiducial acceptances and branching fractions, assuming a value of the Higgs boson mass of 125.09 GeV. This value is assumed for all the cross-section measurements presented in this note, updat- ing the published measurements at the center-of-mass energy of 8 TeV [2–4]. The Standard Model (SM) values of the Higgs boson branching fraction and acceptances are assumed in deriving the total cross sections. A full description of the individual measurements using data from center-of-mass energies of 7 and 13 TeV is given in Refs. [5,6]. In this note we report their combination.

Measurements of Higgs boson cross sections at center-of-mass energies of 7 and 8 TeV have also been reported by the CMS collaboration in Refs. [7,8].

As described in Refs. [2,3], the signal yields are measured inclusively in both channels, and no attempt is made to disentangle the different Higgs boson production modes. The corrections for acceptance and detector effects are computed assuming that the ratio between the different production modes follows the SM prediction. Uncertainties on these ratios have been taken into account following the procedure described in Refs. [2–4], based on the measurements reported in Ref. [9].

For all center-of-mass energies, the combination is performed using the same statistical method described in Ref. [4], which is based on a likelihood combination of the two decay channels. The correlation between the experimental and theoretical uncertainties affecting the two channels is taken into account using common nuisance parameters with Gaussian constraints. The uncertainties on the integrated luminosities, the Higgs boson mass measurement, the contributions of the different production modes, and the theoretical uncertainties due to higher-order QCD corrections and parton density functions (PDFs), are treated in the fit as fully correlated between the two decay channels. All other experimental uncertainties are treated as uncorrelated. The asymptotic approximation [10] is used when deriving the uncertainties on the cross- section measurements. In theH →ZZ^∗ → 4`decay channel, characterized by low event statistics, this approximation is verified by performing pseudo-experiments. Since the measurement uncertainties are statistically dominated, and both decay channels have comparable sensitivities, the precision of the cross- section measurements is improved at all center-of-mass energies by combining the two decay channels.

The uncertainty in the integrated luminosity measurement for 13 TeV is 5.0 %. It is derived, following a methodology similar to that detailed in Ref. [11], from a preliminary calibration of the luminosity scale using a van der Meer beam-separation scan performed in August 2015.

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QCD, and the associated production with bottom quarks is accurate to NNLO (NLO) in QCD for the five-flavor (four-flavor) scheme. The theoretical uncertainties on the predictions are taken from Ref. [12]

and are dominated by the QCD scale and PDF variations, which also include the uncertainty of the strong- coupling value.

2 Results and conclusions

A summary of the measured total pp → Hcross sections is shown in Table 1for H → γγ andH → ZZ^∗ →4`decays at center-of-mass energies of 7, 8 and 13 TeV. The results of the two decay channels are compatible within the quoted uncertainties. For the combined measurements, the breakdown into the statistical and systematic uncertainty components is also given. Statistical uncertainties are derived by fixing all nuisance parameters for the systematic uncertainties to their best-fit values. The systematic uncertainties are smaller than the statistical uncertainties for the measurements at all three center-of- mass energies. The total Higgs boson production cross sections are found to be 34⁺¹¹₋₁₀pb, 33.3^+5.8_−5.5pb, and 24⁺₋₁₈²¹pb for center-of-mass energies of 7, 8 and 13 TeV, respectively. The large asymmetry of the systematic uncertainties on the 13 TeV measurement is a result of the asymmetric uncertainty in the photon energy resolution for theH→γγchannel, as described in Ref. [6].

The cross-section measurements at 7, 8 and 13 TeV are compared with the SM predictions in Figure1.

In particular, the compatibility of the combined measurement at 13 TeV and the SM prediction is 1.3σ, computed under the asymptotic approximation. Under the same approximation, the compatibility of the observed (expected) cross-section measurement at 13 TeV and the no-signal hypothesis is 1.4σ(3.4σ).

Using the 13 TeV data set, a combined upper limit of 68 pb at 95% confidence level is obtained on the total Higgs boson cross section. This limit is derived using the asymptotic approximation, corresponding to the value of the cross section at which the profiled log-likelihood difference crosses the value of 3.84.

Total cross section [pb] 7 TeV 8 TeV 13 TeV

H→γγ 35⁺¹³₋₁₂ 30.5^+7.5₋_7.4 40⁺³¹₋₂₈

H→ZZ^∗→4` 33⁺₋₁₆²¹ 37⁺₋₈⁹ 12⁺₋₁₆²⁵

Combination 34±10 (stat.)⁺⁴₋₂(syst.) 33.3^+5.5_−5.3(stat.)^+1.7₋_1.3(syst.) 24⁺²⁰₋₁₇(stat.)⁺⁷₋₃(syst.)

LHC-XS 17.5±1.6 22.3±2.0 50.9⁺_−4.4^4.5

Table 1: Totalpp→Hcross sections measured usingH→γγandH→ZZ^∗→4`decays, and their combination, for center-of-mass energies of 7, 8 and 13 TeV. The cross sections are derived assuming the SM values of the branching fractions for the two decay channels. For the combined values the breakdown into the statistical and systematic uncertainty components is also given. The LHC-XS theory predictions [12] are computed for a Higgs boson mass of 125.09 GeV.

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[TeV]

s

7 8 9 10 11 12 13

[pb] H→ppσ

−10 0 10 20 30 40 50 60 70 80 90

ATLAS Preliminary σ_pp_→_H m_H = 125.09 GeV QCD scale uncertainty

s) α PDF+

⊕ (scale

Tot. uncert.

γ γ

→

H H→ZZ*→4l comb. data syst. unc.

= 7 TeV, 4.5 fb-1

s

= 8 TeV, 20.3 fb-1

s

= 13 TeV, 3.2 fb-1

s

Figure 1: Totalpp→Hcross sections measured at different center-of-mass energies compared to LHC-XS theory

predictions at up to NNLO in QCD. The red markers are the measurements from theH → γγchannel, the green

markers are the measurements from theH → ZZ^∗ → 4`channel, and the black markers are the combinations of these two channels. The gray bands on the combined measurements represent the systematic uncertainty, while the black lines are the total uncertainty. Theory predictions refer to a Higgs boson mass of 125.09 GeV, with the light (dark) blue band representing the QCD scale (total) uncertainty. The smooth curve of the SM prediction is obtained by applying a third-order polynomial fit to the values available in Ref. [12]. The theoretical uncertainties, described by the blue band, are partially correlated between different values of the center-of-mass energy.

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Appendix

The total cross sections for Higgs boson production are derived from the fiducial cross sections measured in theH →γγandH→ZZ^∗→4`decay channels [5,6]. The values of the fiducial acceptance factors, the fiducial cross sections, and the total cross sections measured at center-of-mass energies of 7, 8 and 13 TeV are summarized in Table2.

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7TeV8TeV13TeV Acceptancefactor H→γγ0.620±0.0070.611±0.0120.570±0.006 H→ZZ∗ →4`0.467±0.0100.460±0.0100.427±0.006 Fiducialcrosssection[fb] H→γγ49±1843±1052+40 −37 H→ZZ∗ →4`1.9+1.2 −0.92.1±0.50.6+1.3 −0.9 Totalcrosssection[pb] H→γγ35+13 −1230.5+7.5 −7.440+31 −28 H→ZZ∗ →4`33+21 −1637+9 −812+25 −16 Combination34±10(stat.)+4 −2(syst.)33.3+5.5 −5.3(stat.)+1.7 −1.3(syst.)24+20 −17(stat.)+7 −3(syst.) LHC-XS17.5±1.622.3±2.050.9+4.5 −4.4 factors,fiducialcrosssectionsandtotalpp→HproductioncrosssectionsmeasuredusingH→γγandH→ZZ∗ →4`decays, forcenter-of-massenergies7,8and13TeV.ThedetailsonthedefinitionofthefiducialphasespacecanbefoundinRefs.[5,6].The asthecenter-of-massenergyincreasesduetoabroadeningoftheHiggsbosonrapiditydistributionforhighercenter-of-massenergies. arederivedassumingtheSMvaluesofthebranchingfractionsforthetwodecaychannels.Forthecombinedvalues,thestatisticaland arealsogiven.Thetotaluncertaintiesontheacceptancefactorsaregiven.TheLHC-XStheorypredictionsarecomputedforaHiggs GeV.

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