Measurements of Higgs boson properties in thediphoton decay channel with 36 fb −1 of p p collision data at √ s = 13 TeV with the ATLAS detectorThe ATLAS CollaborationProperties of the Higgs boson are measured in the two-photon final state using 36.1 fb −1 of proton-proton collision data recorded at √ s = 13 TeV by the ATLAS experiment at the Large Hadron Collider. Cross-section measurements for the production of a Higgs boson through gluon-gluon fusion, vector-boson fusion, and in association with a vector boson or a top-quark pair are reported. The signal strength, defined as the ratio of the observed to the expected signal yield, is measured for each of these production processes as well as inclusively. The global signal strength measurement of 0.99 ± 0.14 improves on the precision of the ATLAS measurement at √ s = 7 and 8 TeV by a factor of two. Measurements of gluon-gluon fusion and vector-boson fusion productions yield signal strengths compatible with the Standard Model prediction. Measurements of simplified template cross sections, designed to quantify the different Higgs boson production processes in specific regions of phase space, are reported. The cross section for the production of the Higgs boson decaying to two isolated photons in a fiducial region closely matching the experimental selection of the photons is measured to be 55±10 fb, which is in good agreement with the Standard Model prediction of 64 ± 2 fb. Furthermore, cross sections in fiducial regions enriched in Higgs boson production in vector-boson fusion or in association with large missing transverse momentum, leptons or top-quark pairs are reported. Differential and double-differential measurements are performed for several variables related to the diphoton kinematics as well as the kinematics and multiplicity of the jets produced in association with a Higgs boson. These differential cross sections are sensitive to higher order QCD corrections and properties of the Higgs boson, such as its spin and CP quantum numbers. No significant deviations from a wide array of Standard Model predictions are observed. Finally, the strength and tensor structure of the Higgs boson interactions are investigated using an effective Lagrangian, which introduces additional CP-even and CP-odd interactions. No significant new physics contributions are observed. 9 Measurement of fiducial integrated and differential cross sections 53 9.1 Particle-level fiducial definition of the Higgs boson diphoton cross sections 53 9.2 Fiducial integrated and differential cross sections 54 9.3 Measurements of cross sections of fiducial integrated regions 56 9.4 Measurements of cross sections of inclusive and exclusive jet multiplicities 62 9.5 Measurements of differential and double-differential cross sections 63 9.5.1 Measurements of cross sections probing the Higgs boson production kinematics 63 9.5.2 Measurements of cross sections probing the jet kinematics 65 9.5.3 Measurements of cross sections probing spin and CP 66 9.5.4 Cross sections probing the VBF production mode 68 9....

Searches for new resonances decaying into two photons in the ATLAS experiment at the CERN Large Hadron Collider are described. The analysis is based on protonproton collision data corresponding to an integrated luminosity of 3.2 fb −1 at √ s = 13 TeV recorded in 2015. Two searches are performed, one targeted at a spin-2 particle of mass larger than 500 GeV, using Randall-Sundrum graviton states as a benchmark model, and one optimized for a spin-0 particle of mass larger than 200 GeV. Varying both the mass and the decay width, the most significant deviation from the background-only hypothesis is observed at a diphoton invariant mass around 750 GeV with local significances of 3.8 and 3.9 standard deviations in the searches optimized for a spin-2 and spin-0 particle, respectively. The global significances are estimated to be 2.1 standard deviations for both analyses. The consistency between the data collected at 13 TeV and 8 TeV is also evaluated. Limits on the production cross section times branching ratio to two photons for the two resonance types are reported. Conclusion 26The ATLAS collaboration 33 IntroductionNew high-mass states decaying into two photons are predicted in many extensions of the Standard Model (SM). The diphoton final state provides a clean experimental signature with excellent invariant mass resolution and moderate backgrounds. Searches for new high-mass resonances decaying into two photons are described, using CERN Large Hadron Collider (LHC) [1] proton-proton (pp) collision data at √ s= 13 TeV recorded in 2015 by the ATLAS detector. The data correspond to an integrated luminosity of 3.2 fb −1 .The decay photons would have different kinematic properties depending on whether the hypothetical particle has spin-0 or spin-2. These are exploited by applying two different -1 - JHEP09(2016)001selections, with looser kinematic selection requirements for the spin-2 resonance search. The photon identification criteria and the event pre-selection are common to both searches.The search for a spin-2 γγ resonance uses the Randall-Sundrum (RS) model [2] graviton as a benchmark. This entails a lightest spin-2 graviton excitation (G * ) with a dimensionless coupling k/M Pl , where M Pl = M Pl / √ 8π is the reduced Planck scale and k the curvature scale of the extra dimension. The lightest graviton excitation is expected to be a fairly narrow resonance for k/M Pl < 0.3 [4], with the width given by 1.44(k/M Pl ) 2 m G * , where m G * is the mass of the lightest graviton state. For k/M Pl = 0.1, the natural width increases from 11 GeV at m G * = 800 GeV to 30 GeV at m G * = 2200 GeV. For m G * = 800 GeV, the contributions of the natural width and of the experimental mass resolution to the width of the resonance are comparable. The shape of the invariant mass distribution of the main background from the production of prompt photon pairs is estimated from theoretical computations, and the contribution from the reducible background of events where at least one jet is misidentified as a photon is added f...