Abstract
Results of a search for physics beyond the Standard Model in events containing an energetic photon and large missing transverse momentum with the ATLAS detector at the Large Hadron Collider are reported. As the number of events observed in data, corresponding to an integrated luminosity of 36.1 fb^{-1} of proton–proton collisions at a centre-of-mass energy of 13~mathrm{TeV}, is in agreement with the Standard Model expectations, model-independent limits are set on the fiducial cross section for the production of events in this final state. Exclusion limits are also placed in models where dark-matter candidates are pair-produced. For dark-matter production via an axial-vector or a vector mediator in the s-channel, this search excludes mediator masses below 750–1200~mathrm{GeV} for dark-matter candidate masses below 230–480~mathrm{GeV} at 95% confidence level, depending on the couplings. In an effective theory of dark-matter production, the limits restrict the value of the suppression scale M_{*} to be above 790~mathrm{GeV} at 95% confidence level. A limit is also reported on the production of a high-mass scalar resonance by processes beyond the Standard Model, in which the resonance decays to Zgamma and the Z boson subsequently decays into neutrinos.
Highlights
Background estimationTheSM background to the γ +final state is due to events containing either a true photon or an object misidentified as a photon
Two different configurations are used for the fit: the background-only inclusive fit, which determines the normalisations for W γ, Z γ and γ +jets backgrounds for each inclusive SR independently and the background-only multiple-bin fit, which determines the normalisations for the three exclusive SRs simultaneously
6.5.1 Background-only inclusive-SR fit Background estimates in each inclusive SR are derived from a simultaneous fit to the respective four control regions (1muCR, 2muCR, 2eleCR and PhJetCR)
Summary
The ATLAS detector [25] is a multipurpose particle physics apparatus with a forward–backward symmetric cylindrical geometry and near 4π coverage in solid angle. The inner tracking detector (ID), covering the pseudorapidity range |η| < 2.5, consists of a silicon pixel detector including the insertable B-layer [26,27], which was added around a new, smaller-radius beam-pipe before the start of Run 2; a silicon microstrip detector; and, for |η| < 2.0, a straw-tube transi-. The ID is surrounded by a thin superconducting solenoid which provides a 2 T magnetic field. A high-granularity lead/liquid-argon sampling electromagnetic calorimeter (EM) covers the region |η| < 3.2. It is segmented longitudinally in shower depth. The first layer has a high granularity in the η direction in order to provide an efficient discrimination between single-photon showers and two overlapping photons originating from a π 0 decay. A muon spectrometer (MS) surrounds the calorimeters It consists of three large air-core superconducting toroidal magnet systems, precision tracking chambers providing accurate muon tracking out to |η| = 2.7, and fast detectors for triggering in the region |η| < 2.4. A two-level trigger system is used to select events for offline analysis [28]
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