The large rate of multiple simultaneous proton–proton interactions, or pile-up, generated by the Large Hadron Collider in Run 1 required the development of many new techniques to mitigate the adverse...

The reconstruction of the signal from hadrons and jets emerging from the proton–proton collisions at the Large Hadron Collider (LHC) and entering the ATLAS calorimeters is based on a three-dimensional topological clustering of individual calorimeter cell signals. The cluster formation follows cell signal-significance patterns generated by electromagnetic and hadronic showers. In this, the clustering algorithm implicitly performs a topological noise suppression by removing cells with insignificant signals which are not in close proximity to cells with significant signals. The resulting topological cell clusters have shape and location information, which is exploited to apply a local energy calibration and corrections depending on the nature of the cluster. Topological cell clustering is established as a well-performing calorimeter signal definition for jet and missing transverse momentum reconstruction in ATLAS.

The luminosity calibration for the ATLAS detector at the LHC during pp collisions at [equation]in 2010 and 2011 is presented. Evaluation of the luminosity scale is performed using several...

ATLAS has measured two-particle correlations as a function of the relative azimuthal angle, $\mathrm{\Delta }\varphi$, and pseudorapidity, $\mathrm{\Delta }\eta$, in $\sqrt{s}=13$ and 2.76 TeV $pp$ collisions at the LHC using charged particles measured in the pseudorapidity interval $|\eta |<2.5$. The correlation functions evaluated in different intervals of measured charged-particle multiplicity show a multiplicity-dependent enhancement at $\mathrm{\Delta }\varphi \sim 0$ that extends over a wide range of $\mathrm{\Delta }\eta$, which has been referred to as the “ridge.” Per-trigger-particle yields, $Y(\mathrm{\Delta }\varphi )$, are measured over $2<|\mathrm{\Delta }\eta |<5$. For both collision energies, the $Y(\mathrm{\Delta }\varphi )$ distribution in all multiplicity intervals is found to be consistent with a linear combination of the per-trigger-particle yields measured in collisions with less than 20 reconstructed tracks, and a constant combinatoric contribution modulated by $\cos (2\mathrm{\Delta }\varphi )$. The fitted Fourier coefficient, $v_{2,2}$, exhibits factorization, suggesting that the ridge results from per-event $\cos (2\varphi )$ modulation of the single-particle distribution with Fourier coefficients $v_2$. The $v_2$ values are presented as a function of multiplicity and transverse momentum. They are found to be approximately constant as a function of multiplicity and to have a $p_{\mathrm{T}}$ dependence similar to that measured in $p+\mathrm{Pb}$ and $\mathrm{Pb}+\mathrm{Pb}$ collisions. The $v_2$ values in the 13 and 2.76 TeV data are consistent within uncertainties. These results suggest that the ridge in $pp$ collisions arises from the same or similar underlying physics as observed in $p+\mathrm{Pb}$ collisions, and that the dynamics responsible for the ridge has no strong $\sqrt{s}$ dependence.

The measurement of missing transverse momentum in the ATLAS detector, described in this paper, makes use of the full event reconstruction and a calibration based on reconstructed physics objects. The...

A measurement of the production processes of the recently discovered Higgs boson is performed in the two-photon final state using $4.5{\mathrm{fb}}^{-1}$ of proton-proton collisions data at $\sqrt{s}=7\mathrm{TeV}$ and $20.3{\mathrm{fb}}^{-1}$ at $\sqrt{s}=8\mathrm{TeV}$ collected by the ATLAS detector at the Large Hadron Collider. The number of observed Higgs boson decays to diphotons divided by the corresponding Standard Model prediction, called the signal strength, is found to be $\mu =1.17\pm 0.27$ at the value of the Higgs boson mass measured by ATLAS, $m_H=125.4\mathrm{GeV}$. The analysis is optimized to measure the signal strengths for individual Higgs boson production processes at this value of $m_H$. They are found to be ${\mu }_{\mathrm{ggF}}=1.32\pm 0.38$, ${\mu }_{\mathrm{VBF}}=0.8\pm 0.7$, ${\mu }_{WH}=1.0\pm 1.6$, ${\mu }_{ZH}={0.1}_{-0.1}^{+3.7}$, and ${\mu }_{t\overline{t}H}={1.6}_{-1.8}^{+2.7}$, for Higgs boson production through gluon fusion, vector-boson fusion, and in association with a $W$ or $Z$ boson or a top-quark pair, respectively. Compared with the previously published ATLAS analysis, the results reported here also benefit from a new energy calibration procedure for photons and the subsequent reduction of the systematic uncertainty on the diphoton mass resolution. No significant deviations from the predictions of the Standard Model are found.