ATLAS Collaboration(Aaboud, M. et al), Alvarez Piqueras, D., Aparisi Pozo, J. A., Bailey, A. J., Barranco Navarro, L., Cabrera Urban, S., et al. (2019). Measurements of inclusive and differential fiducial cross-sections of t(t)over-bar production with additional heavy-flavour jets in proton-proton collisions at root s=13 TeV with the ATLAS detector. J. High Energy Phys., 04(4), 046–66pp.
Abstract: This paper presents measurements of tt (t) over bar production in association with additional b-jets in pp collisions at the LHC at a centre-of-mass energy of 13 TeV. The data were recorded with the ATLAS detector and correspond to an integrated luminosity of 36.1 fb(-1). Fiducial cross-section measurements are performed in the dilepton and lepton-plus-jets tt (t) over bar decay channels. Results are presented at particle level in the form of inclusive cross-sections of tt final states with three and four b-jets as well as differential cross-sections as a function of global event properties and properties of b-jet pairs. The measured inclusive fiducial cross-sections generally exceed the t (t) over barb (b) over bar predictions from various next-to-leading-order matrix element calculations matched to a parton shower but are compatible within the total uncertainties. The experimental uncertainties are smaller than the uncertainties in the predictions. Comparisons of state-of-the-art theoretical predictions with the differential measurements are shown and good agreement with data is found for most of them.
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ATLAS Collaboration(Aad, G. et al), Alvarez Piqueras, D., Aparisi Pozo, J. A., Bailey, A. J., Cabrera Urban, S., Castillo, F. L., et al. (2020). Measurement of isolated-photon plus two-jet production in pp collisions at root s=13 TeV with the ATLAS detector. J. High Energy Phys., 03(3), 179–49pp.
Abstract: The dynamics of isolated-photon plus two-jet production in pp collisions at a centre-of-mass energy of 13 TeV are studied with the ATLAS detector at the LHC using a dataset corresponding to an integrated luminosity of 36.1 fb(-1). Cross sections are measured as functions of a variety of observables, including angular correlations and invariant masses of the objects in the final state, gamma + jet + jet. Measurements are also performed in phase-space regions enriched in each of the two underlying physical mechanisms, namely direct and fragmentation processes. The measurements cover the range of photon (jet) transverse momenta from 150 GeV (100 GeV) to 2 TeV. The tree-level plus parton-shower predictions from Sherpa and Pythia as well as the next-to-leading-order QCD predictions from Sherpa are compared with the measurements. The next-to-leading-order QCD predictions describe the data adequately in shape and normalisation except for regions of phase space such as those with high values of the invariant mass or rapidity separation of the two jets, where the predictions overestimate the data.
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ATLAS Collaboration(Aaboud, M. et al), Alvarez Piqueras, D., Bailey, A. J., Barranco Navarro, L., Cabrera Urban, S., Castillo, F. L., et al. (2018). Measurements of b-jet tagging efficiency with the ATLAS detector using t(t)over-bar events at root s=13 TeV. J. High Energy Phys., 08(8), 089–47pp.
Abstract: The efficiency to identify jets containing b-hadrons (b-jets) is measured using a high purity sample of dileptonic top quark-antiquark pairs (t (t) over bar) selected from the 36.1 fb(-1) of data collected by the ATLAS detector in 2015 and 2016 from proton-proton collisions produced by the Large Hadron Collider at a centre-of-mass energy root s = 13 TeV. Two methods are used to extract the efficiency from t (t) over bar events, a combinatorial likelihood approach and a tag-and-probe method. A boosted decision tree, not using b-tagging information, is used to select events in which two b-jets are present, which reduces the dominant uncertainty in the modelling of the flavour of the jets. The efficiency is extracted for jets in a transverse momentum range from 20 to 300 GeV, with data-to-simulation scale factors calculated by comparing the efficiency measured using collision data to that predicted by the simulation. The two methods give compatible results, and achieve a similar level of precision, measuring data-to-simulation scale factors close to unity with uncertainties ranging from 2% to 12% depending on the jet transverse momentum.
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ATLAS Collaboration(Aaboud, M. et al), Alvarez Piqueras, D., Bailey, A. J., Barranco Navarro, L., Cabrera Urban, S., Castillo Gimenez, V., et al. (2018). Search for supersymmetry in final states with charm jets and missing transverse momentum in 13 TeV pp collisions with the ATLAS detector. J. High Energy Phys., 09(9), 050–44pp.
Abstract: A search for supersymmetric partners of top quarks decaying as (t) over tilde (1) -> c (chi) over tilde (0)(1)and supersymmetric partners of charm quarks decaying as (c) over tilde (1) -> c (chi) over tilde (0 )(1)where (chi) over tilde (0)(1) is the lightest neutralino, is presented. The search uses 36.1 fb(-1) pp collision data at a centre-of-mass energy of 13 TeV collected by the ATLAS experiment at the Large Hadron Collider and is performed in final states with jets identified as containing charm hadrons. Assuming a 100% branching ratio to c (chi) over tilde (0)(1), top and charm squarks with masses up to 850 GeV are excluded at 95% confidence level for a massless lightest neutralino. For m (t) over tilde (1,(c) over tilde1) – m((chi) over tilde 10)< 100 GeV, top and charm squark masses up to 500 GeV are excluded.
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ATLAS Collaboration(Aaboud, M. et al), Alvarez Piqueras, D., Aparisi Pozo, J. A., Bailey, A. J., Barranco Navarro, L., Cabrera Urban, S., et al. (2018). Search for new phenomena in events with same-charge leptons and b-jets in pp collisions at >=13 TeV with the ATLAS detector. J. High Energy Phys., 12(12), 039–56pp.
Abstract: A search for new phenomena in events with two same- charge leptons or three leptons and jets identi fi ed as originating from b – quarks in a data sample of 36.1 fb of pp collisions at ps = 13TeV recorded by the ATLAS detector at the Large Hadron Collider is reported. No signi fi cant excess is found and limits are set on vector- like quark, fourtop- quark, and same- sign top- quark pair production. The observed ( expected) 95% CL mass limits for a vector- like T – and B – quark singlet are mT > 0 : 98 ( 0 : 99) TeV and mB > 1 : 00 ( 1 : 01) TeV respectively. Limits on the production of the vector- like T5=3 – quark are also derived considering both pair and single production; in the former case the lower limit on the mass of the T5=3 – quark is ( expected to be) 1.19 ( 1.21) TeV. The Standard Model fourtop- quark production cross- section upper limit is ( expected to be) 69 ( 29) fb. Constraints are also set on exotic four- top- quark production models. Finally, limits are set on samesign top- quark pair production. The upper limit on uu ! tt production is ( expected to be) 89 ( 59) fb for a mediator mass of 1TeV, and a dark- matter interpretation is also derived, excluding a mediator of 3TeV with a dark- sector coupling of 1.0 and a coupling to ordinary matter above 0.31.
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