Abstract: The ATLAS experiment at the Large Hadron Collider (LHC) employs a trigger system consisting of a first-level hardware trigger (L1) and a software-based high-level trigger. The L1 muon trigger system selects muon candidates, assigns them to the correct LHC bunch crossing and classifies them into one of six transverse-momentum threshold classes. The L1 muon trigger system uses resistive-plate chambers (RPCs) to generate the muon-induced trigger signals in the central (barrel) region of the ATLAS detector. The ATLAS RPCs are arranged in six concentric layers and operate in a toroidal magnetic field with a bending power of 1.5 to 5.5 Tm. The RPC detector consists of about 3700 gas volumes with a total surface area of more than 4000 m(2). This paper reports on the performance of the RPC detector and L1 muon barrel trigger using 60.8 fb(-1) of proton-proton collision data recorded by the ATLAS experiment in 2018 at a centre-of-mass energy of 13 TeV. Detector and trigger performance are studied using Z boson decays into a muon pair. Measurements of the RPC detector response, efficiency, and time resolution are reported. Measurements of the L1 muon barrel trigger efficiencies and rates are presented, along with measurements of the properties of the selected sample of muon candidates. Measurements of the RPC currents, counting rates and mean avalanche charge are performed using zero-bias collisions. Finally, RPC detector response and efficiency are studied at different high voltage and front-end discriminator threshold settings in order to extrapolate detector response to the higher luminosity expected for the High Luminosity LHC.

Abstract: The results of a search for gluino and squark pair production with the pairs decaying via the lightest charginos into a final state consisting of two W bosons, the lightest neutralinos ((chi) over tilde (0)(1)), and quarks, are presented: the signal is characterised by the presence of a single charged lepton (e(+/-) or mu(+/-)) from a W boson decay, jets, and missing transverse momentum. The analysis is performed using 139 fb(-1) of proton-proton collision data taken at a centre-of-mass energy root s = 13 delivered by the Large Hadron Collider and recorded by the ATLAS experiment. No statistically significant excess of events above the Standard Model expectation is found. Limits are set on the direct production of squarks and gluinos in simplified models. Masses of gluino (squark) up to 2.2 (1.4 ) are excluded at 95% confidence level for a light (chi) over tilde (0)(1).

Abstract: Exclusive dimuon production in ultraperipheral collisions (UPC), resulting from photon-photon interactions in the strong electromagnetic fields of colliding high-energy lead nuclei, PbPb(gamma gamma) -> mu(+) mu(-) (Pb-(*Pb-)(()*())), is studied using L-int = 0.48 nb(-1) of root S-NN = 5.02 TeV lead-lead collision data at the LHC with the ATLAS detector. Dimuon pairs are measured in the fiducial region p(T,mu) > 4 GeV, vertical bar eta(mu)vertical bar < 2.4, invariant m(mu mu) > 10 GeV, and p(T,mu mu) <2 GeV. The primary background from single-dissociative processes is extracted from the data using a template fitting technique. Differential cross sections are presented as a function of m(mu mu), absolute pair rapidity (vertical bar y(mu mu)vertical bar), scattering angle in the dimuon rest frame (vertical bar cos v*(mu mu)vertical bar), and the colliding photon energies. The total cross section of the UPC gamma gamma -> mu(+) mu(-) process in the fiducial volume is measured to be sigma(mu mu)(fid) = 34.1 +0.3(stat.)+0.7(syst.) μb. Generally good agreement is found with calculations from STARlight, which incorporate the leading-order Breit-Wheeler process with no final-state effects, albeit differences between the measurements and theoretical expectations are observed. In particular, the measured cross sections at larger vertical bar y(mu mu)vertical bar are found to be about 10-20% larger in data than in the calculations, suggesting the presence of larger fluxes of photons in the initial state. Modification of the dimuon cross sections in the presence of forward and/or backward neutron production is also studied and is found to be associated with a harder incoming photon spectrum, consistent with expectations.