ATLAS Collaboration(Aad, G. et al), Alvarez Piqueras, D., Barranco Navarro, L., Cabrera Urban, S., Castillo Gimenez, V., Cerda Alberich, L., et al. (2016). Measurement of the double-differential high-mass Drell-Yan cross section in pp collisions at root s=8 TeV with the ATLAS detector. J. High Energy Phys., 08(8), 009–61pp.
Abstract: This paper presents a measurement of the double-differential cross section for the Drell-Yan Z/gamma* -> l(+)l(-) and photon-induced gamma gamma -> l(+)l(-) processes where l is an electron or muon. The measurement is performed for invariant masses of the lepton pairs, m(ll), between 116 GeV and 1500 GeV using a sample of 20.3 fb(-1) of pp collisions data at centre-of-mass energy of root s = 8TeV collected by the ATLAS detector at the LHC in 2012. The data are presented double differentially in invariant mass and absolute dilepton rapidity as well as in invariant mass and absolute pseudorapidity separation of the lepton pair. The single-differential cross section as a function of m(ll) is also reported. The electron and muon channel measurements are combined and a total experimental precision of better than 1% is achieved at low m(ll). A comparison to next-to-next-to-leading order perturbative QCD predictions using several recent parton distribution functions and including next-to-leading order electroweak effects indicates the potential of the data to constrain parton distribution functions. In particular, a large impact of the data on the photon PDF is demonstrated.
|
ATLAS Collaboration(Aad, G. et al), Alvarez Piqueras, D., Aparisi Pozo, J. A., Bailey, A. J., Barranco Navarro, L., Cabrera Urban, S., et al. (2019). Properties of jet fragmentation using charged particles measured with the ATLAS detector in pp collisions at root s=13 TeV. Phys. Rev. D, 100(5), 052011–38pp.
Abstract: This paper presents a measurement of quantities related to the formation of jets from high-energy quarks and gluons (fragmentation). Jets with transverse momentum 100 GeV < p(T) < 2.5 TeV and pseudorapidity vertical bar eta vertical bar < 2.1 from an integrated luminosity of 33 fb(-1) of root s = 13 TeV proton-proton collisions are reconstructed with the ATLAS detector at the Large Hadron Collider. Charged-particle tracks with p(T) > 500 MeV and vertical bar eta vertical bar < 2.5 are used to probe the detailed structure of the jet. The fragmentation properties of the more forward and the more central of the two leading jets from each event are studied. The data are unfolded to correct for detector resolution and acceptance effects. Comparisons with parton shower Monte Carlo generators indicate that existing models provide a reasonable description of the data across a wide range of phase space, but there are also significant differences. Furthermore, the data are interpreted in the context of quark- and gluon-initiated jets by exploiting the rapidity dependence of the jet flavor fraction. A first measurement of the charged-particle multiplicity using model-independent jet labels (topic modeling) provides a promising alternative to traditional quark and gluon extractions using input from simulation. The simulations provide a reasonable description of the quark-like data across the jet p(T) range presented in-this measurement, but the gluon-like data have systematically fewer charged particles than the simulation.
|
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.
|
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). Dijet azimuthal correlations and conditional yields in pp and p plus Pb collisions at root S-NN=5.02 TeV with the ATLAS detector. Phys. Rev. C, 100(3), 034903–24pp.
Abstract: This paper presents a measurement of forward-forward and forward-central dijet azimuthal angular correlations and conditional yields in proton-proton (pp) and proton-lead (p + Pb) collisions as a probe of the nuclear gluon density in regions where the fraction of the average momentum per nucleon carried by the parton entering the hard scattering is low. In these regions, gluon saturation can modify the rapidly increasing parton distribution function of the gluon. The analysis utilizes 25 pb(-1) of pp data and 360 μb(-1) of p + Pb data, both at root S-NN = 5.02 TeV, collected in 2015 and 2016, respectively, with the ATLAS detector at the Large Hadron Collider. The measurement is performed in the center-of-mass frame of the nucleon-nucleon system in the rapidity range between -4.0 and 4.0 using the two highest transverse-momentum jets in each event, with the highest transverse-momentum jet restricted to the forward rapidity range. No significant broadening of azimuthal angular correlations is observed for forward-forward or forward-central dijets in p + Pb compared to pp collisions. For forward-forward jet pairs in the proton-going direction, the ratio of conditional yields in p + Pb collisions to those in pp collisions is suppressed by approximately 20%, with no significant dependence on the transverse momentum of the dijet system. No modification of conditional yields is observed for forward-central dijets.
|
ATLAS Collaboration(Aaboud, M. et al), Alvarez Piqueras, D., Barranco Navarro, L., Cabrera Urban, S., Castillo Gimenez, V., Cerda Alberich, L., et al. (2018). Direct top-quark decay width measurement in the t(t)over-bar lepton+jets channel at root s=8 TeV with the ATLAS experiment. Eur. Phys. J. C, 78(2), 129–30pp.
Abstract: This paper presents a direct measurement of the decay width of the top quark using t (t) over bar events in the lepton+jets final state. The data sample was collected by the ATLAS detector at the LHC in proton-proton collisions at a centre-of-mass energy of 8 TeV and corresponds to an integrated luminosity of 20.2 fb(-1). The decay width of the top quark is measured using a template fit to distributions of kinematic observables associated with the hadronically and semileptonically decaying top quarks. The result, Gamma(t) = 1.76 +/- 0.33 (stat.) (+0.79)(-0.68) (syst.) GeV for a top-quark mass of 172.5 GeV, is consistent with the prediction of the Standard Model.
|
