This paper presents a measurement of the production cross-section of a Z boson in association with b-jets, in proton–proton collisions at √ s = 13 TeV with the ATLAS experiment at the Large Hadron Collider using data corresponding to an integrated luminosity of 35.6 fb−1. Inclusive and differential cross-sections are measured for events containing a Z boson decaying into electrons or muons and produced in association with at least one or at least two b-jets with transverse momentum pT > 20 GeV and rapidity |y | < 2.5. Predictions from several Monte Carlo generators based on leading-order (LO) or next-to-leading-order (NLO) matrix elements interfaced with a parton-shower simulation and testing different flavour schemes for the choice of initial-state partons are compared with measured cross-sections. The 5-flavour number scheme predictions at NLO accuracy agree better with data than 4-flavour number scheme ones. The 4-flavour number scheme predictions underestimate data in events with at least one b-jet.

We have studied the dependence of azimuthal anisotropy nu(2) for inclusive and identified charged hadrons in Au + Au and Cu + Cu collisions on collision energy, species, and centrality. The values of nu(2) as a function of transverse momentum pT and centrality in Au + Au collisions at root s(NN) = 200 and 62.4 GeV are the same within uncertainties. However, in Cu + Cu collisions we observe a decrease in nu(2) values as the collision energy is reduced from 200 to 62.4 GeV. The decrease is larger in the more peripheral collisions. By examining both Au + Au and Cu + Cu collisions we find that nu(2) depends both on eccentricity and the number of participants, N-part. We observe that nu(2) divided by eccentricity (epsilon) monotonically increases with N-part and scales as N-part(1/3). The Cu + Cu data at 62.4 GeV falls below the other scaled nu(2) data. For identified hadrons, nu(2) divided by the number of constituent quarks n(q) is independent of hadron species as a function of transverse kinetic energy K E-T = m(T) - m between 0.1 < K E-T / n(q) < 1 GeV. Combining all of the above scaling and normalizations, we observe a near-universal scaling, with the exception of the Cu + Cu data at 62.4 GeV, of nu(2)/(nq center dot e center dot N-part(1/3)) vs K E-T / n(q) for all measured particles.

Measurements of the midrapidity transverse-energy distribution, dE(T)/d eta, are presented for p + p, d+Au, and Au+Au collisions atv root s(NN) = 200 GeV and additionally for Au+Au collisions atv root s(NN) = 62.4 and 130 GeV. The dE(T)/d eta distributions are first compared with the number of nucleon participants N-part, number of binary collisions N-coll, and number of constituent-quark participants N-qp calculated from a Glauber model based on the nuclear geometry. For Au+Au, < dE(T)/d eta >/N-part increases with N-part, while < dE(T)/d eta >/N-qp is approximately constant for all three energies. This indicates that the two-component ansatz, dE(T)/d eta alpha (1 - x)N-part/2 + xN(coll), which was used to represent E-T distributions, is simply a proxy for N-qp, and that the N-coll term does not represent a hard-scattering component in E-T distributions. The dE(T)/d eta distributions of Au+Au and d+Au are then calculated from the measured p + p E-T distribution using two models that both reproduce the Au+Au data. However, while the number-of-constituent-quark-participant model agrees well with the d+Au data, the additive-quark model does not.

Aad, G.; et al., [Unknown]; Aben, R.; Beemster, L.J.; Bentvelsen, S.; Berglund, E.; Bobbink, G.J.; Bos, K.; Boterenbrood, H.; Colijn, A.P.; de Jong, P.; de Nooij, L.; Deluca, C.; Deviveiros, P.O.; Doxiadis, A.D.; Ferrari, P.; Gadatsch, S.; Geerts, D.A.A.; Gosselink, M.; Hartjes, F.; Hessey, N.P.; Igonkina, O.; Klous, S.; Kluit, P.; Koffeman, E.; Lee, H.; Lenz, T.; Linde, F.; Luijckx, G.; Mahlstedt, J.; Massaro, G.; Mechnich, J.; Mussche, I.; Ottersbach, J.P.; Pani, P.; Ruckstuhl, N.; Ta, D.; Tsiakiris, M.; van der Deijl, P.C.; van der Geer, R.; van der Graaf, H.; van der Leeuw, R.; van der Poel, E.; van Vulpen, I.; Verkerke, W.; Vermeulen, J.C.; Vranjes Milosavljevic, M.; Vreeswijk, M.

All of the experimental data points presented in the original paper are correct and unchanged (including statistical and systematic uncertainties). However, herein we correct a comparison between the experimental data and a theoretical picture, because we discovered a mistake in the code used. All of the most probable sigma_breakup values differ by less than 0.4 mb from those originally presented. However, the one standard deviation uncertainties (that include contributions from both the statistical and systematic uncertainties on the experimental data points) are approximately 30-60% larger than originally reported. We give a table of the new comparison results and corrected versions of Figs. 8-11 of the original paper and we note that no correction is needed for results from the data-driven method in Fig. 13.

All of the experimental data points presented in the original paper are correct and unchanged (including statistical and systematic uncertainties). However, herein we correct a comparison between the experimental data and a theoretical picture, because we discovered a mistake in the code used. All of the most probable sigma_breakup values differ by less than 0.4 mb from those originally presented. However, the one standard deviation uncertainties (that include contributions from both the statistical and systematic uncertainties on the experimental data points) are approximately 30-60% larger than originally reported. We give a table of the new comparison results and corrected versions of Figs. 8-11 of the original paper and we note that no correction is needed for results from the data-driven method in Fig. 13.

We report the observation at the Relativistic Heavy Ion Collider (RHIC) of suppression of back-to-back correlations in the direct photon+jet channel in Au+Au relative to p+p collisions. Two-particle correlations of direct photon triggers with associated hadrons are obtained by statistical subtraction of the decay photon-hadron background. The initial momentum of the away-side parton is tightly constrained, because the parton-photon pair exactly balance in momentum at leading order in perturbative quantum chromodynamics (pQCD), making such correlations a powerful probe of the in-medium parton energy loss. The away-side nuclear suppression factor, I_AA, in central Au+Au collisions, is 0.32 +/- 0.12(stat) +/- 0.09(syst) for hadrons of 3 < p_T < 5 in coincidence with photons of 5 < p_T < 15 GeV/c. The suppression is comparable to that observed for high-p_T single hadrons and dihadrons. The direct photon associated yields in p+p collisions scale approximately with the momentum balance, z_T = p_T^hadron/p_T^photon, as expected for a measure of the away-side parton fragmentation function. We compare to Au+Au collisions for which the momentum balance dependence of the nuclear modification should be sensitive to the path-length dependence of parton energy loss.

Bose-Einstein correlations of charged kaons are used to probe Au+Au collisions at sqrt[S(NN)]=200 GeV and are compared to charged pion probes, which have a larger hadronic scattering cross section. Three-dimensional Gaussian source radii are extracted, along with a one-dimensional kaon emission source function. The centrality dependences of the three Gaussian radii are well described by a single linear function of N(part)1/3 with a zero intercept. Imaging analysis shows a deviation from a Gaussian tail at r greater than or approximately equal to 10 fm, although the bulk emission at lower radius is well described by a Gaussian. The presence of a non-Gaussian tail in the kaon source reaffirms that the particle emission region in a heavy-ion collision is extended, and that similar measurements with pions are not solely due to the decay of long-lived resonances.

We present the first measurement of photoproduction of J/ψ and of two-photon production of high-mass e+e− pairs in electromagnetic (or ultra-peripheral) nucleus–nucleus interactions, using Au+Au data at sNN=200 GeV. The events are tagged with forward neutrons emitted following Coulomb excitation of one or both Au⋆ nuclei. The event sample consists of 28 events with me+e−>2 GeV/c2 with zero like-sign background. The measured cross sections at midrapidity of dσ/dy(J/ψ+Xn,y=0)=76±33(stat)±11(syst) μb and d2σ/dmdy(e+e−+Xn,y=0)=86±23(stat)±16(syst) μb/(GeV/c2) for me+e−∈[2.0,2.8] GeV/c2 have been compared and found to be consistent with models for photoproduction of J/ψ and QED based calculations of two-photon production of e+e− pairs.

We present inclusive charged hadron elliptic flow (v(2)) measured over the pseudorapidity range vertical bar eta vertical bar < 0.35 in Au+Au collisions at s(NN)=200 GeV. Results for v(2) are presented over a broad range of transverse momentum (p(T)=0.2-8.0 GeV/c) and centrality (0-60%). To study nonflow effects that are correlations other than collective flow, as well as the fluctuations of v(2), we compare two different analysis methods: (1) the event-plane method from two independent subdetectors at forward (vertical bar eta vertical bar=3.1-3.9) and beam (vertical bar eta vertical bar>6.5) pseudorapidities and (2) the two-particle cumulant method extracted using correlations between particles detected at midrapidity. The two event-plane results are consistent within systematic uncertainties over the measured p(T) and in centrality 0-40%. There is at most a 20% difference in the v(2) between the two event-plane methods in peripheral (40-60%) collisions. The comparisons between the two-particle cumulant results and the standard event-plane measurements are discussed.

In 2004, a combined system test was performed in the H8 beam line at the CERN SPS with a setup reproducing the geometry of sectors of the ATLAS Muon Spectrometer, formed by three stations of Monitored Drift Tubes (MDT). The full ATLAS analysis chain was used to obtain the results presented in this paper. The basic design performances of the Muon Spectrometer were verified. The stability of MDT calibration constants, the alignment system using optical devices and high energy tracks, as well as the intrinsic sagitta resolution of the Muon Spectrometer were studied and found to agree with expectations. The reconstruction of muon tracks using the combined information from both the Inner Detector and the Muon Spectrometer are also presented.

Measurements of the azimuthal anisotropy of high-p(T) neutral pion (pi(0)) production in Au+Au collisions at s(NN)=200 GeV by the PHENIX experiment are presented. The data included in this article were collected during the 2004 Relativistic Heavy Ion Collider running period and represent approximately an order of magnitude increase in the number of analyzed events relative to previously published results. Azimuthal angle distributions of pi(0) mesons detected in the PHENIX electromagnetic calorimeters are measured relative to the reaction plane determined event-by-event using the forward and backward beam-beam counters. Amplitudes of the second Fourier component (v(2)) of the angular distributions are presented as a function of pi(0) transverse momentum (p(T)) for different bins in collision centrality. Measured reaction plane dependent pi(0) yields are used to determine the azimuthal dependence of the pi(0) suppression as a function of p(T), R-AA(Delta phi,p(T)). A jet-quenching motivated geometric analysis is presented that attempts to simultaneously describe the centrality dependence and reaction plane angle dependence of the pi(0) suppression in terms of the path lengths of hypothetical parent partons in the medium. This set of results allows for a detailed examination of the influence of geometry in the collision region and of the interplay between collective flow and jet-quenching effects along the azimuthal axis.

We present a new analysis of J/psi production yields in deuteron-gold collisions at root s(NN) =200 GeV using data taken from the PHENIX experiment in 2003 and previously published in S. S. Adler [Phys. Rev. Lett 96, 012304 (2006)]. The high statistics proton-proton J/psi data taken in 2005 are used to improve the baseline measurement and thus construct updated cold nuclear matter modification factors (R-dAu). A suppression of J/psi in cold nuclear matter is observed as one goes forward in rapidity (in the deuteron-going direction), corresponding to a region more sensitive to initial-state low-x gluons in the gold nucleus. The measured nuclear modification factors are compared to theoretical calculations of nuclear shadowing to which a J/psi (or precursor) breakup cross section is added. Breakup cross sections of sigma(breakup)=2.8(-1.4)(+1.7) (2.2(-1.5)(+1.6)) mb are obtained by fitting these calculations to the data using two different models of nuclear shadowing. These breakup cross-section values are consistent within large uncertainties with the 4.2 +/- 0.5 mb determined at lower collision energies. Projecting this range of cold nuclear matter effects to copper-copper and gold-gold collisions reveals that the current constraints are not sufficient to firmly quantify the additional hot nuclear matter effect.