Charged-particle distributions at low transverse momentum and a search for low-mass resonances decaying into two jets at ATLAS

The charged-particle multiplicity distributions are measured in pp collisions at a center-of-mass energy of 13TeV, with the ATLAS detector. The data are collected using a dedicated trigger for minimum bias data during 2015. The events are selected by requiring at least two stable charged particles with transverse momentum above 100 MeV and an absolute value of the pseudorapidity of |η| < 2.5. This region of phase space is interesting to explore as it is poorly described by models of soft QCD. Thus the charged particle multiplicity per event, the dependence of the number of charged particles on their transverse momentum (p T ) and pseudorapidity (η), and the average p T are measured and compared with several Monte Carlo QCD- like models to better constrain their parameters. In addition, a search is performed for localized excesses in the dijet mass distributions of low-dijet-mass events produced in association with a high transverse energy photon. The search uses up to 79.8f b -1 , at a center-of- mass energy of 13TeV with data recorded during 2015–2017. Two variants are presented: one which makes no jet flavor requirements and one which requires both jets to be tagged as b-jets. The observed mass distributions are consistent with multi-jet processes in the Standard Model. Thus the data was used to set upper limits on the production cross-section for a benchmark Z' model and, separately, on generic Gaussian-shaped contributions to the mass distributions, extending the current ATLAS constraints on dijet resonances to the mass range between 225 and 1100 GeV.