Property measurement of the Higgs boson in the γγ final state with the ATLAS detector at the LHC

With pp collision data sample recorded by the ATLAS experiment at the CERN Large Hadron Collider at centerof-mass energies of 7 TeV and 8 TeV, corresponding to an integrated luminosity of 25 fb−1, an improved measurement of the mass of the Higgs boson is derived from a combined fit to the decay channels H → γγ and H → ZZ∗ → 4l with improved energy-scale calibrations for photons, electrons and muons as well as other analysis improvements. It is mH = 125.36 ± 0.37(stat) ± 0.18(syst) GeV. Furthermore, measurement of fiducial and differential cross sections are presented in the H → γγ decay channel using only the 8 TeV data sample with a luminosity of 20.3fb−1. The observed spectra are statistically limited but broadly in line with the theoretical expectations.


Introduction
In 2012, the ATLAS and CMS collaborations announced a new particle [1,2] in the search for the Standard Model (SM) Higgs boson [3,4] at the CERN Large Hadron Collider. With the increasing dataset, the emphasis has shifted to determining the properties of the new particle and testing the consistensy of the standard Model against the data. The Higgs mass is not predicted in the SM, which is important for precise calculations of electroweak observables including the production and decay properties of the Higgs boson, as well as the coupling structure of the SM Higgs boson, etc. Based on the measured Higgs mass, the fiducial and differential cross sections of H → γγ are presented, which allows a diverse range of physical phenomena to be probed.

Calibration and Particle Identification (PID) efficiency of photons
The ATLAS experiment is a general purpose particle physics detector with a forward-backward symmetric cylindrical geometry and near 4π coverage in solid angle as described in [5]. The latest major improvement includes energy-scale calibrations and photon PID efficiency measurement. The energy reconstruction of electrons and photons is optimised using multivariate algorithms. After correcting for modifications of the data taking conditions with time, the stability of the LAr calorimeter response is at the level of 0.05% and the residual non-uniformal is at the level of 0.7% or better. The response of the calorimeter layers are equalised in data and simulation, and the longitudinal profile of the electromagnetic showers is exploited to estimate the passive material in front of the calorimeter and reoptimise the detector simulation. After all corrections, the Z resonance is used to set the absolute energy scale. The achieved calibration accuracy for electrons from Z decays is typically 0.05% in most of the detector acceptance, rising to 0.2% in regions with large amounts of passive material; 0.2% to 1% for electrons with a transverse energy of 10 GeV, and on average of 0.3% for photons. The energy scale is verified using J/ψ → ee and Z → llγ(l = e, μ) decays as shown in Fig.1 [6]. The detector resolution is determined with a relative accuracy better than 10% for electrons and photons up to 60 GeV Available online at www.sciencedirect.com  transverse energy, rising to 40% for transverse energies above 500 GeV. There is a combination of three different methods of measurements of Z → llγ decays, extrapolation from electron in Z → ee decays, and Matrix methods based on photon purity, the PID uncertainty is at the level of 1% as shown in Fig. 2 [7].

Mass measurement
A model-independent approach has been chosen to measure the Higgs boson mass based on fitting of the reconstructed invariant masses spectra of the decay modes H → γγ and H → ZZ * → 4l [6] with a narrow mass peak of a typical experimental resolution of 1.6 GeV to 2 GeV over a smooth background, from which the mass can be extracted without assumptions on the signal production and decay yields. The H → γγ channel profits from an improved calibration of the energy measurements of electron and photon candidates, which results in a sizable reduction of the systematic uncertainties on their energy scales. In the H → ZZ * → 4l channel both the expected statistical uncertainty and the systematic uncertainty on the mass measurement have been reduced with respect to the previous publication. The improvement of the statistical uncertainty arises primarily from the use of a multivariate discriminate that is designed to increase the separation of the signal from background. The systematic uncertainty reduction comes from both the improved electromagnetic energy calibration and a reduction in the muon momentum scale uncertainty, which was obtained by studying large samples of Z → μ + μ − and J/ψ → μ + μ − decays.
The combined mass measurement is m H = 125.36 ± 0.37(stat) ± 0.18(sys) GeV [6], with measured signal strength of μ = 1.29 ± 0.30 which is set to be free in the fit are shown in Fig.3. No significant correlation between the two fitted variables is observed, confirming the model independence of the mass measurement.

Measurement of Fiducial and Differential cross sections
The H → γγ cross section is measured in a fiducial region (baseline) defined by two isolated photons that have absolute pseudorapidity in the interval |η| < 2.37, with the leading (subleading) photon satisfying p T /m γγ > 0.35(0.25), where p T is the transverse momentum of the photon and m γγ is the diphoton invariant mass. Four additional cross sections contains at least one jet, at least two jets at least three jets and VBF enriched case are studied in ficucial regions, as well as cross-section limits on two VH regions including the single-lepton region and large E miss T region. The definition is described in [8]. For the differential cross sections, they are measured in the baseline fiducial region for four categories of kinematic variables [8]. For each fiducial region (or bin of a differential distribution), the signal yield is extracted using a signal plus background fit to the diphoton invariant mass spectrum with the Higgs mass is fixed to 125.36 GeV. The cross sections are determined by correcting these yields for detector inefficiency and resolution, and by accounting for the integrated luminosity of the dataset and is defined by σ i = v sig i c i Ldt in a given region. production mechanisms. The HRES calculation is normalised to the LHC-XS prediction using a K ggF = 1.15. The shapes of the distributions are satisfactorily described by the SM prediction, with an overall offset that is consistent with baseline fiducial cross section. Furthermore, the agreement between data and theoretical prediction is quantified with the first and second moment, as well as the χ 2 test. The increased jet activity and harder jet transverse momentum spectra suggest that there is more quark and gluon radiation in the data than in the theoretical prediction. In general, the event generator redictions are in good agreement with the data.

Conclusion
With improved energy-scale calibrations for photons, electrons and muons, as well as other analysis improvements, the Higgs mass is measured to be m H = 125.36± 0.37(stat) ± 0.18(syst) GeV. The fiducial and differential cross section of pp → H → γγ are presented. The observed spectra are statistically limited but broadly in line with the theoretical expectations.