Measurements of $Z\gamma$ and $Z\gamma \gamma$ production in $pp$ collisions at $\sqrt{s}=$ 8 TeV with the ATLAS detector

Abstract

The production of $Z$ bosons with one or two isolated high-energy photons is studied using $pp$ collisions at $\sqrt{s}$ = 8 TeV. The analyses use a data sample with an integrated luminosity of 20.3 fb${}^{-1}$ collected by the ATLAS detector during the 2012 LHC data taking. The $Z\gamma$ and $Z\gamma \gamma$ production cross sections are measured with leptonic ($e^{+}{e}^{-}$, ${\mu }^{+}{\mu }^{-}$, $\nu \bar{\nu }$) decays of the $Z$ boson, in extended fiducial regions defined in terms of the lepton and photon acceptance. They are then compared to cross-section predictions from the Standard Model, where the sources of the photons are radiation off initial-state quarks and radiative $Z$-boson decay to charged leptons, and from fragmentation of final-state quarks and gluons into photons. The yields of events with photon transverse energy $E_T>$ 250 GeV from ${\ell }^{+}{\ell }^{-}\gamma$ events and with $E_T>$ 400 GeV from $\nu \bar{\nu }\gamma$ events are used to search for anomalous triple gauge-boson couplings $ZZ\gamma$ and $Z\gamma \gamma$. The yields of events with diphoton invariant mass $m_{\gamma \gamma }>$ 200 GeV from ${\ell }^{+}{\ell }^{-}\gamma \gamma$ events and with $m_{\gamma \gamma }>$ 300 GeV from $\nu \bar{\nu }\gamma \gamma$ events are used to search for anomalous quartic gauge-boson couplings $ZZ\gamma \gamma$ and $Z\gamma \gamma \gamma$. No deviations from Standard Model predictions are observed and limits are placed on parameters used to describe anomalous triple and quartic gauge-boson couplings.

41 pages plus author list + cover pages (59 pages total), 17 figures, 13 tables, submitted to Physical Review D, All figures including auxiliary figures are available at https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/STDM-2014-01/

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• Model-independent study for the tau-neutrino electromagnetic dipole moments in $e^{+}{e}^{-}\to {\nu }_{\tau }{\bar{\nu }}_{\tau }\gamma$ at the CLIC
• Bounds on anomalous quartic $WWZ\gamma$ couplings in $e^{-}p$ collisions at the FCC-he
• Non-diagonal contributions to $Z\gamma V^{\ast }$ vertex, polarizations and bounds on $Z\overline{t}q$ couplings
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