ATLAS Collaboration(Aad, G. et al), Amos, K. R., Aparisi Pozo, J. A., Bailey, A. J., Bouchhar, N., Cabrera Urban, S., et al. (2023). A search for new resonances in multiple final states with a high transverse momentum Z boson in root s = 13 TeV pp collisions with the ATLAS detector. J. High Energy Phys., 06(6), 036–56pp.
Abstract: A generic search for resonances is performed with events containing a Z boson with transverse momentum greater than 100 GeV, decaying into e+e− or μ+μ−. The analysed data collected with the ATLAS detector in proton-proton collisions at a centre-of-mass energy of 13 TeV at the Large Hadron Collider correspond to an integrated luminosity of 139 fb−1. Two invariant mass distributions are examined for a localised excess relative to the expected Standard Model background in six independent event categories (and their inclusive sum) to increase the sensitivity. No significant excess is observed. Exclusion limits at 95% confidence level are derived for two cases: a model-independent interpretation of Gaussian-shaped resonances with the mass width between 3% and 10% of the resonance mass, and a specific heavy vector triplet model with the decay mode W′ → ZW → ℓℓqq.
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ATLAS Collaboration(Aad, G. et al), Amos, K. R., Aparisi Pozo, J. A., Bailey, A. J., Bouchhar, N., Cabrera Urban, S., et al. (2023). Measurement of the Sensitivity of Two-Particle Correlations in pp Collisions to the Presence of Hard Scatterings. Phys. Rev. Lett., 131(16), 162301–21pp.
Abstract: A key open question in the study of multiparticle production in high-energy pp collisions is the relationship between the “ridge”-i.e., the observed azimuthal correlations between particles in the underlying event that extend over all rapidities-and hard or semihard scattering processes. In particular, it is not known whether jets or their soft fragments are correlated with particles in the underlying event. To address this question, two-particle correlations are measured in pp collisions at collected by the ATLAS experiment at the LHC, with an integrated luminosity of 15.8 pb-1, in two different configurations. In the first case, charged particles associated with jets are excluded from the correlation analysis, while in the second case, correlations are measured between particles within jets and charged particles from the underlying event. Second-order flow coefficients, v2, are presented as a function of event multiplicity and transverse momentum. These measurements show that excluding particles associated with jets does not affect the measured correlations. Moreover, particles associated with jets do not exhibit any significant azimuthal correlations with the underlying event, ruling out hard processes contributing to the ridge. p= 13 TeV using data ffiffi s
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ATLAS Collaboration(Aad, G. et al), Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Fassi, F., Ferrer, A., et al. (2014). Light-quark and gluon jet discrimination in collisions at root s=7 TeV with the ATLAS detector. Eur. Phys. J. C, 74(8), 3023–29pp.
Abstract: A likelihood-based discriminant for the identification of quark- and gluon-initiated jets is built and validated using 4.7 fb of proton-proton collision data at collected with the ATLAS detector at the LHC. Data samples with enriched quark or gluon content are used in the construction and validation of templates of jet properties that are the input to the likelihood-based discriminant. The discriminating power of the jet tagger is established in both data and Monte Carlo samples within a systematic uncertainty of 10-20 %. In data, light-quark jets can be tagged with an efficiency of while achieving a gluon-jet mis-tag rate of in a range between and for jets in the acceptance of the tracker. The rejection of gluon-jets found in the data is significantly below what is attainable using a Pythia 6 Monte Carlo simulation, where gluon-jet mis-tag rates of 10 % can be reached for a 50 % selection efficiency of light-quark jets using the same jet properties.
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ATLAS Collaboration(Aad, G. et al), Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Ferrer, A., Fiorini, L., et al. (2015). Search for anomalous production of prompt same-sign lepton pairs and pair-produced doubly charged Higgs bosons with root s=8 TeV pp collisions using the ATLAS detector. J. High Energy Phys., 03(3), 041–48pp.
Abstract: A low-background inclusive search for new physics in events with same-sign dileptons is presented. The search uses proton-proton collisions corresponding to 20.3 fb(-1) of integrated luminosity taken in 2012 at a centre-of-mass energy of 8TeV with the ATLAS detector at the LHC. Pairs of isolated leptons with the same electric charge and large transverse momenta of the type e(+/-) e(+/-), e(+/-) mu(+/-), and mu(+/-) mu(+/-) are selected and their invariant mass distribution is examined. No excess of events above the expected level of Standard Model background is found. The results are used to set upper limits on the cross-sections for processes beyond the Standard Model. Limits are placed as a function of the dilepton invariant mass within a fiducial region corresponding to the signal event selection criteria. Exclusion limits are also derived for a specific model of doubly charged Higgs boson production.
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ATLAS Collaboration(Aad, G. et al), Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Fernandez Martinez, P., Ferrer, A., et al. (2015). Measurement of the top pair production cross section in 8 TeV proton-proton collisions using kinematic information in the lepton plus jets final state with ATLAS. Phys. Rev. D, 91(11), 112013–25pp.
Abstract: A measurement is presented of the (tt) over bar inclusive production cross section in pp collisions at a center-ofmass energy of pffisffiffi root s = 8 TeV using data collected by the ATLAS detector at the CERN Large Hadron Collider. The measurement was performed in the lepton + jets final state using a data set corresponding to an integrated luminosity of 20.3 fb(-1). The cross section was obtained using a likelihood discriminant fit and b-jet identification was used to improve the signal-to-background ratio. The inclusive (tt) over bar production cross section was measured to be 260 +/- 1(stat)(-23)(+22)(stat) +/- 8(lumi) +/- 4(beam) pb assuming a top-quark mass of 172.5 GeV, in good agreement with the theoretical prediction of 253(-15)(+13) pb. The (tt) over bar -> (e, mu) + jets production cross section in the fiducial region determined by the detector acceptance is also reported.
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