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ATLAS Collaboration(Aad, G. et al), Amos, K. R., Aparisi Pozo, J. A., Bailey, A. J., Cabrera Urban, S., Cardillo, F., et al. (2023). Search for the charged-lepton-flavor-violating decay Z → eμ in pp collisions at √s=13 TeV with the ATLAS detector. Phys. Rev. D, 108(3), 032015–22pp.
Abstract: A search for the charged-lepton-flavor-violating process Z -> e μis presented, using 139 fb(-1) of root s = 13 TeV pp collision data collected by the ATLAS experiment at the LHC. An excess in the e μinvariant mass spectrum near the Z boson mass would be a striking signature of new physics. No excess is observed, and an upper limit B(Z -> e mu) < 2.62 x 10(-7) is placed on the branching fraction at 95% confidence level, which is the most stringent limit to date.
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ATLAS Collaboration(Aad, G. et al), Amos, K. R., Aparisi Pozo, J. A., Bailey, A. J., Cabrera Urban, S., Cardillo, F., et al. (2023). Search for flavor-changing neutral-current couplings between the top quark and the Z boson with proton-proton collisions at √s=13 TeV with the ATLAS detector. Phys. Rev. D, 108(3), 032019–34pp.
Abstract: A search for flavor-changing neutral-current couplings between a top quark, an up or charm quark, and a Z boson is presented, using proton-proton collision data at root s = 13 TeV collected by the ATLAS detector at the Large Hadron Collider. The analyzed data set corresponds to an integrated luminosity of 139 fb(-1). The search targets both single-top- quark events produced as gq -> tZ (with q = u, c) and top-quark-pair events, with one top quark decaying through the t -> Zq channel. The analysis considers events with three leptons (electrons or muons), a b-tagged jet, possible additional jets, and missing transverse momentum. The data are found to be consistent with the background-only hypothesis and 95% confidence-level limits on the t -> Zq branching ratios, assuming only tensor operators of the Standard Model effective field theory framework contribute to the tZq vertices. These are 6.2 x 10(-5) (13 x 10(-5)) for t -> Zu (t -> Zc) for a left-handed tZq coupling, and 6.6 x 10(-5) (12 x 10(-5)) in the case of a right-handed coupling. These results are interpreted as 95% CL upper limits on the strength of the corresponding couplings, yielding limits for |C-uW((13))*| and |C-uB((13))*| (|C-uW((31))| and |C-uB((31))|) of 0.15 (0.16), and limits for |C-uW((23))*| and |C-uB((23))*| (|C-uW((32))| and |C-uB((32))|) of 0.22 (0.21), assuming a new-physics energy scale Lambda(NP) of 1 TeV.
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ATLAS Collaboration(Aad, G. et al), Aparisi Pozo, J. A., Bailey, A. J., Cabrera Urban, S., Castillo, F. L., Castillo Gimenez, V., et al. (2024). Search for quantum black hole production in lepton plus jet final states using proton-proton collisions at √s=13 TeV with the ATLAS detector. Phys. Rev. D, 109(3), 032010–28pp.
Abstract: A search for quantum black holes in electron + jet and muon + jet invariant mass spectra is performed with 140 fb(-1) of data collected by the ATLAS detector in proton-proton collisions at root s = 13 TeV at the Large Hadron Collider. The observed invariant mass spectrum of lepton + jet pairs is consistent with Standard Model expectations. Upper limits are set at 95% confidence level on the production cross section times branching fractions for quantum black holes decaying into a lepton and a quark in a search region with invariant mass above 2.0 TeV. The resulting quantum black hole lower mass threshold limit is 9.2 TeV in the Arkani-Hamed-Dimopoulos-Dvali model, and 6.8 TeV in the Randall-Sundrum model.
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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). Measurement of jet fragmentation in Pb plus Pb and pp collisions at root S-NN=5.02 TeV with the ATLAS detector. Phys. Rev. C, 98(2), 024908–34pp.
Abstract: This paper presents a measurement of jet fragmentation functions in 0.49 nb(-1) of Pb +Pb collisions and 25 pb(-1) of pp collisions at root S-NN = 5.02 TeV collected in 2015 with the ATLAS detector at the LHC. These measurements provide insight into the jet quenching process in the quark-gluon plasma created in the aftermath of ultrarelativistic collisions between two nuclei. The modifications to the jet fragmentation functions are quantified by dividing the measurements in Pb+Pb collisions by baseline measurements in pp collisions. This ratio is studied as a function of the transverse momentum of the jet, the jet rapidity, and the centrality of the collision. In both collision systems, the jet fragmentation functions are measured for jets with transverse momentum between 126 and 398 GeV and with an absolute value of jet rapidity less than 2.1. An enhancement of particles carrying a small fraction of the jet momentum is observed, which increases with centrality and with increasing jet transverse momentum. Yields of particles carrying a very large fraction of the jet momentum are also observed to be enhanced. Between these two enhancements of the fragmentation functions a suppression of particles carrying an intermediate fraction of the jet momentum is observed in Pb+Pb collisions. A small dependence of the modifications on jet rapidity is observed.
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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). Measurement of the suppression and azimuthal anisotropy of muons from heavy-flavor decays in Pb plus Pb collisions at root s(NN)=2.76 TeV with the ATLAS detector. Phys. Rev. C, 98(4), 044905–34pp.
Abstract: ATLAS measurements of the production of muons from heavy-flavor decays in root s(NN) = 2.76 TeV Pb+Pb collisions and root s = 2.76 TeV pp collisions at the LHC are presented. Integrated luminosities of 0.14 nb(-1) and 570 nb(-1) are used for the Pb+Pb and pp measurements, respectively, which are performed over the muon transverse momentum range 4 < pT < 14 GeV and for five Pb+Pb centrality intervals. Backgrounds arising from in-flight pion and kaon decays, hadronic showers, and misreconstructed muons are statistically removed using a template-fitting procedure. The heavy-flavor muon differential cross sections and per-event yields are measured in pp and Pb+Pb collisions, respectively. The nuclear modification factor R-AA obtained from these is observed to be independent of pT, within uncertainties, and to be less than unity, which indicates suppressed production of heavy-flavor muons in Pb+Pb collisions. For the 10% most central Pb+Pb events, the measured R-AA is approximately 0.35. The azimuthal modulation of the heavy-flavor muon yields is also measured and the associated Fourier coefficients v(n) for n = 2, 3, and 4 are given as a function of pT and centrality. They vary slowly with pT and show a systematic variation with centrality which is characteristic of other anisotropy measurements, such as that observed for inclusive hadrons. The measured R-AA and v(n) values are also compared with theoretical calculations.
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