Abstract
This paper presents a measurement of the electroweak production of two jets in association with a Zgamma pair, with the Z boson decaying into two neutrinos. It also presents a search for invisible or partially invisible decays of a Higgs boson with a mass of 125 text {GeV} produced through vector-boson fusion with a photon in the final state. These results use data from LHC proton–proton collisions at sqrt{s} = 13 text {TeV} collected with the ATLAS detector and corresponding to an integrated luminosity of 139 hbox {fb}^{-1}. The event signature, shared by all benchmark processes considered for the measurements and searches, is characterized by a significant amount of unbalanced transverse momentum and a photon in the final state, in addition to a pair of forward jets. Electroweak Zgamma production in association with two jets is observed in this final state with a significance of 5.2 (5.1 expected) standard deviations. The measured fiducial cross-section for this process is 1.31pm 0.29 fb. An observed (expected) upper limit of 0.37 (0.34^{+0.15}_{-0.10}) at 95% confidence level is set on the branching ratio of a 125 text {GeV} Higgs boson to invisible particles, assuming the Standard Model production cross-section. The signature is also interpreted in the context of decays of a Higgs boson into a photon and a dark photon. An observed (expected) 95% CL upper limit on the branching ratio for this decay is set at 0.018 (0.017^{+0.007}_{-0.005}), assuming the Standard Model production cross-section for a 125 text {GeV} Higgs boson.
Highlights
Background contribution estimationThe dominant contributions entering the analysis signal regions (SRs) are Z (→ νν)γ + jets and W (→ ν)γ + jets events in which thePage 13 of 41 105 lepton from the W decay is lost mostly because it falls outside of the pT or η acceptance
Profiting from the full 139 fb−1 dataset collected with the ATLAS detector during Run 2 of the Large Hadron Collider (LHC), the analysis presented in this paper reports the observation of EW Z (→ νν)γ jj production at the LHC
In order to correct the normalization of the γ + jet background process and validate the modelling of its kinematic features, an orthogonal sample of events characterized by a small amount of ETmiss in the final state is considered
Summary
The ATLAS experiment [29,30,31] at the LHC is a multipurpose particle detector with a forward–backward symmetric cylindrical geometry and near 4π coverage in solid angle.. The ATLAS experiment [29,30,31] at the LHC is a multipurpose particle detector with a forward–backward symmetric cylindrical geometry and near 4π coverage in solid angle.3 It consists of an inner tracking detector surrounded by a thin superconducting solenoid providing a 2 T axial magnetic field, electromagnetic and hadron calorimeters, and a muon spectrometer. The inner tracking detector (ID) covers the pseudorapidity range |η| < 2.5. It consists of silicon pixel, silicon microstrip, and transition radiation tracking detectors. An extensive software suite [34] is used in the reconstruction and analysis of real and simulated data, in detector operations, and in the trigger and data acquisition systems of the experiment
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