Binary Collision Scaling Research Articles

Measurement of electroweak boson production in pp, p+Pb and Pb+Pb collisions with the ATLAS detector

Measurement of electroweak boson production in different collision systems are of great interest to understanding the partonic structure of heavy nuclei, and serve as a constraint on the initial state in larger collision systems. Their production yields in Pb+Pb with respect to pp collisions provide direct tests of both binary collision scaling and possible modification of parton distribution functions (nPDF) due to nuclear effects. Further, the p+Pb collisions provide a relatively clean environment to study nPDFs in detail. The ATLAS detector has a broad acceptance with excellent performance even in the high occupancy environment of central heavy-ion collisions. In this proceedings the latest ATLAS results on W and Z boson production at the centre-of-mass energy of 5.02 TeV are presented, including updated precise result production in pp collisions. Also photon yields are reported in 8.16 TeV in different collision systems are presented.

Measurement of W and Z Boson Production in 5 TeV pp, p+Pb and Pb+Pb Collisions with the ATLAS Detector

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Z Boson Production in 5.02 TeV pp, p + Pb and Pb + Pb Collisions with ATLAS

Z bosons are short lived and do not participate in the strong interaction. Thus their production yields, observed via dilepton decay channels in proton-lead and lead-lead collisions, provide direct tests of both binary collision scaling and the nuclear modification of parton distribution functions (nPDF). Proton-lead collisions further provide a relatively clean environment for benchmarking nuclear parton distribution functions (PDF). The ATLAS detector has a broad acceptance in the muon and electron channels, with excellent performance even in the high occupancy environment of central heavy-ion collisions. In this talk, Z yields, are presented differentially in rapidity and as a function of centrality in lead-lead and proton-lead collisions.

Open heavy-flavour and electroweak boson measurements via the (di-)muonic decay channel with ALICE at the LHC

Heavy flavours (charm and beauty) and electroweak bosons (W and Z) are produced in initial hard partonic scatterings. The former interact strongly with the medium formed in ultra-relativistic heavy-ion collisions throughout its evolution, thus making them well suited to investigate its properties. Furthermore, heavy-flavour measurements in proton-nucleus collisions can be used to investigate initial-state effects whereas in proton-proton (pp) collisions they are considered an important test for perturbative Quantum ChromoDynamics (pQCD) predictions. In addition, open heavy-flavour measurements in pp collisions are used as a reference for proton-lead (p–Pb) and lead-lead (Pb–Pb) collisions. On the other hand, electroweak bosons and their leptonic decay products only interact weakly with the QCD matter and thus are suitable probes to test the validity of binary-collision scaling of hard processes. Moreover, their measurements in p–Pb collisions could help to constrain nuclear parton distribution functions. The ALICE muon spectrometer allows the measurement of open heavy flavour, W- and Z-boson production at forward rapidity (−4.0 < η < −2.5) exploiting their (di)muonic decay channel. In this talk the results obtained with the LHC Run I data in pp, p–Pb and Pb–Pb collisions will be discussed and compared with theoretical predictions.

Measurement of W- and Z-boson production in p—Pb collisions at √sNN = 5.02 TeV with ALICE at the LHC

In hadronic collisions, electroweak bosons (W and Z) are produced in initial hard scattering processes and they are not affected by the strong interaction. In proton-proton collisions, they have been suggested as standard candles for luminosity monitoring and their measurement can improve the evaluation of detector performances. In nucleus-nucleus and proton-nucleus collisions, electroweak bosons provide a check at first order of the validity of binary collision scaling, while small deviations allow studying the nuclear modifications of parton distribution functions. In ALICE, the production of W and Z bosons is measured via the contribution of W-boson decays to the inclusive pT-differential muon yield and the invariant mass of unlike-sign muon pairs from Z-boson decays, respectively, at large rapidities. This paper reports on the production cross sections of muons from W muonic decay and of Z boson in p- Pb collisions at √sNN = 5.02 TeV, which are compared to the theoretical calculations, and on the W-boson yields normalized to the average number of binary nucleon-nucleon collisions as a function of the event activity.

Vector-boson production in p + Pb collisions measured with ATLAS at the LHC

Electroweak–boson production processes (W, Z and photon) provide access to the earliest moments of heavy–ion collisions. Furthermore, because they do not undergo strong interactions, they are sensitive to the initial–state geometry of the collision and potentially the details of the nuclear parton distribution functions. ATLAS results on vector–boson yields have demonstrated binary collision scaling in Pb+Pb collisions. In p + Pb collisions, the measurement of vector bosons provides possible constraints on the nuclear parton distribution functions and insights into the details of the initial collision geometry. We report on the latest results of vector–boson production in p + Pb collisions at sNN=5.02 TeV. Production yields of Z and W bosons and lepton charge asymmetry of W bosons are presented as a function of pseudo-rapidity and centrality. The vector–boson yields are compared to calculations incorporating different parton distribution functions, as well as different centrality calculations.

ForwardJ/ψproduction in U + U collisions atsNN=193GeV

The invariant yields for $J/\psi$ production at forward rapidity $(1.2<|y|<2.2)$ in U$+$U collisions at $\sqrt{s_{_{NN}}}$=193 GeV have been measured as a function of collision centrality. The invariant yields and nuclear-modification factor $R_{AA}$ are presented and compared with those from Au$+$Au collisions in the same rapidity range. Additionally, the direct ratio of the invariant yields from U$+$U and Au$+$Au collisions within the same centrality class is presented, and used to investigate the role of $c\bar{c}$ coalescence. Two different parameterizations of the deformed Woods-Saxon distribution were used in Glauber calculations to determine the values of the number of nucleon-nucleon collisions in each centrality class, $N_{\rm coll}$, and these were found to give significantly different $N_{\rm coll}$ values. Results using $N_{\rm coll}$ values from both deformed Woods-Saxon distributions are presented. The measured ratios show that the $J/\psi$ suppression, relative to binary collision scaling, is similar in U$+$U and Au$+$Au for peripheral and midcentral collisions, but that $J/\psi$ show less suppression for the most central U$+$U collisions. The results are consistent with a picture in which, for central collisions, increase in the $J/\psi$ yield due to $c\bar{c}$ coalescence becomes more important than the decrease in yield due to increased energy density. For midcentral collisions, the conclusions about the balance between $c\bar{c}$ coalescence and suppression depend on which deformed Woods-Saxon distribution is used to determine $N_{\rm coll}$.

Alternative ansatz to wounded nucleon and binary collision scaling in high-energy nuclear collisions

We introduce TRENTO, a new parametric initial condition model for high-energy nuclear collisions based on eikonal entropy deposition via a "reduced thickness" function. The model simultaneously describes experimental proton-proton, proton-nucleus, and nucleus-nucleus multiplicity distributions, and generates nucleus-nucleus eccentricity harmonics consistent with experimental flow constraints. In addition, the model is compatible with ultra-central uranium-uranium data unlike existing models that include binary collision terms.

Measurement of W-boson production in p–Pb collisions at = 5.02 TeV with ALICE at the LHC

In hadronic collisions, electroweak bosons are produced in initial hard scattering processes and they are not affected by the strong interaction. In proton-proton collisions, they have been suggested as standard candles for luminosity monitoring and their measurement can improve the evaluation of detector performances. In nucleus-nucleus and proton-nucleus collisions, W-bosons allow one to check at first order the validity of binary collision scaling, while small deviations allow to study the nuclear modifications of parton distribution functions. The W-boson production in p-Pb collisions at = 5.02 TeV is measured via the contribution of W-boson decays to the inclusive pT-differential muon yield reconstructed with the ALICE muon spectrometer at forward (2.03 < yμcms < 3.53) and backward (-4.46 < yμcms < -2.96) rapidity. This paper reports the production cross section of muons from W-boson decays for pμT > 10 GeV/c and the yields normalised to the average number of binary nucleon-nucleon collisions as a function of the event activity.

Study of Z production in PbPb and pp collisions at s N N = 2.76 $$ \sqrt{s_{\mathrm{NN}}}=2.76 $$ TeV in the dimuon and dielectron decay channels

The production of Z bosons is studied in the dimuon and dielectron decay channels in PbPb and pp collisions at sqrt(s[NN]) = 2.76 TeV, using data collected by the CMS experiment at the LHC. The PbPb data sample corresponds to an integrated luminosity of about 150 inverse microbarns, while the pp data sample collected in 2013 at the same nucleon-nucleon centre-of-mass energy has an integrated luminosity of 5.4 inverse picobarns. The Z boson yield is measured as a function of rapidity, transverse momentum, and collision centrality. The ratio of PbPb to pp yields, scaled by the number of inelastic nucleon-nucleon collisions, is found to be 1.06 +/- 0.05 (stat) +/- 0.08 (syst) in the dimuon channel and 1.02 +/- 0.08 (stat) +/- 0.15 (syst) in the dielectron channel, for centrality-integrated Z boson production. This binary collision scaling is seen to hold in the entire kinematic region studied, as expected for a colourless probe that is unaffected by the hot and dense QCD medium produced in heavy ion collisions.

Z boson production in p+Pb collisions measured by the ATLAS experiment

The measurement of Z bosons in heavy ion collisions is an excellent tool for probing the nucleus at the partonic level. The inclusive yield provides a stringent test of binary collision scaling. Recent ATLAS results demonstrate such scaling in Pb + Pb collisions. The 2013 p + Pb physics run at the LHC providing more than 30 nb −1 of collisions at an energy of s NN = 5.02 TeV has made a detailed study of Z boson production in proton + nucleus interactions possible for the first time. These data may confirm binary scaling in the p + Pb system and help to clarify the present ambiguity of centrality classifications in proton + nucleus collisions. Furthermore, the momentum and rapidity differential yields of Z bosons allow the detection of modification of the initial state present in such collisions.

Transverse momentum dependence of inclusive primary charged-particle production in p–Pb collisions at $$\sqrt{s_\mathrm{{NN}}}=5.02~\text {TeV}$$ s NN = 5.02 TeV

The transverse momentum ( $$p_{\mathrm T}$$ ) distribution of primary charged particles is measured at midrapidity in minimum-bias p–Pb collisions at $$\sqrt{s_{\mathrm {NN}}}=5.02$$ TeV with the ALICE detector at the LHC in the range $$0.15<p_{\mathrm T}<50$$ GeV/ $$c$$ . The spectra are compared to the expectation based on binary collision scaling of particle production in pp collisions, leading to a nuclear modification factor consistent with unity for $$p_{\mathrm T}$$ larger than 2 GeV/ $$c$$ , with a weak indication of a Cronin-like enhancement for $$p_\mathrm {T}$$ around 4 $$\mathrm {GeV}\!/c$$ . The measurement is compared to theoretical calculations and to data in Pb–Pb collisions at $$\sqrt{s_{\mathrm {NN}}}=2.76$$ TeV.

ATLAS high-pT measurements in lead-lead collisions

Measurements of high-pT processes in lead-lead collisions can provide insight into the physics responsible for the quenching of jets in the quark gluon plasma. Results are presented from ATLAS measurements of single jet suppression, both inclusively and as a function of the azimuthal angle of the jets with respect to the elliptic flow plane, single jet fragmentation, and gamma-jet correlations. Implications of the results are discussed. Results are also presented for measurements of photon and Z boson production that provide essential tests on calculations of hard scattering rates using binary collision scaling.

ATLAS results from lead-lead collisions at sNN=2.76TeV

Abstract An overview of the recent results on particle production in lead–lead collisions at s NN = 2.76 TeV obtained with the ATLAS detector is presented. Two samples of collision events recorded in 2010 and 2011 LHC heavy-ion runs, with integrated luminosities of approximately 9 μ b − 1 and 0.15 nb − 1 respectively, were used. The measurements of collective flow phenomena in lead–lead collisions, related to the initial geometry and its fluctuations, are shown providing new constraints on the initial geometry models and on hydrodynamic evolution of the system. They are complemented with studies of high-transverse momentum probes. The latter include measurements of electro-weakly interacting probes such as Z bosons and prompt photons as well as strongly interacting probes like charged hadrons, heavy quarks and jets. Yields of electro-weak probes are found to be consistent with binary collision scaling for all collision centralities. On the contrary, charged hadrons, heavy quarks and jets are suppressed in central collisions relative to peripheral events. The azimuthal angle dependence of jet yields is consistent with the expected dependence on the path length traversed by partons through the medium, while the measured jet fragmentation function shows some centrality-dependent modifications. These measurements, supplemented with the results on correlations between electro-weak probes and jets, provide new insights into the mechanism of in-medium parton energy loss.

Anomalous centrality evolution of two-particle angular correlations from Au-Au collisions atsNN=62and 200 GeV

We present two-dimensional (2D) two-particle angular correlations measured with the STAR detector on relative pseudorapidity η and azimuth ϕ for charged particles from Au-Au collisions at sNN=62 and 200 GeV with transverse momentum pt≥0.15 GeV/c, |η|≤1, and 2π in azimuth. Observed correlations include a same-side (relative azimuth <π/2) 2D peak, a closely related away-side azimuth dipole, and an azimuth quadrupole conventionally associated with elliptic flow. The same-side 2D peak and away-side dipole are explained by semihard parton scattering and fragmentation (minijets) in proton-proton and peripheral nucleus-nucleus collisions. Those structures follow N-N binary-collision scaling in Au-Au collisions until midcentrality, where a transition to a qualitatively different centrality trend occurs within one 10% centrality bin. Above the transition point the number of same-side and away-side correlated pairs increases rapidly relative to binary-collision scaling, the η width of the same-side 2D peak also increases rapidly (η elongation), and the ϕ width actually decreases significantly. Those centrality trends are in marked contrast with conventional expectations for jet quenching in a dense medium. The observed centrality trends are compared to perturbative QCD predictions computed in hijing, which serve as a theoretical baseline, and to the expected trends for semihard parton scattering and fragmentation in a thermalized opaque medium predicted by theoretical calculations and phenomenological models. We are unable to reconcile a semihard parton scattering and fragmentation origin for the observed correlation structure and centrality trends with heavy-ion collision scenarios that invoke rapid parton thermalization. If the collision system turns out to be effectively opaque to few-GeV partons the present observations would be inconsistent with the minijet picture discussed here.4 MoreReceived 17 September 2011DOI:https://doi.org/10.1103/PhysRevC.86.064902©2012 American Physical Society

Nuclear-modification factor for open-heavy-flavor production at forward rapidity in Cu+Cu collisions atsNN=200GeV

Background: Heavy-flavor production in p+p collisions tests perturbative-quantum-chromodynamics (pQCD) calculations. Modification of heavy-flavor production in heavy-ion collisions relative to binary-collision scaling from p+p results, quantified with the nuclear-modification factor (R_AA), provides information on both cold- and hot-nuclear-matter effects. Purpose: Determine transverse-momentum, pt, spectra and the corresponding R_AA for muons from heavy-flavor mesons decay in p+p and Cu+Cu collisions at sqrt(s_NN)=200 GeV and y=1.65. Method: Results are obtained using the semi-leptonic decay of heavy-flavor mesons into negative muons. The PHENIX muon-arm spectrometers measure the p_T spectra of inclusive muon candidates. Backgrounds, primarily due to light hadrons, are determined with a Monte-Carlo calculation using a set of input hadron distributions tuned to match measured-hadron distributions in the same detector and statistically subtracted. Results: The charm-production cross section in p+p collisions at sqrt{s}=200 GeV, integrated over pt and in the rapidity range 1.4<y<1.9 is found to be dsigma_ccbar/dy = 0.139 +/- 0.029 (stat) ^{+0.051}_{-0.058} (syst) mb. This result is consistent with calculations and with expectations based on the corresponding midrapidity charm-production cross section measured earlier by PHENIX. The R_AA for heavy-flavor muons in Cu+Cu collisions is measured in three centrality intervals for 1<pt<4 GeV/c. Suppression relative to binary-collision scaling (R_AA<1) increases with centrality. Conclusions: Within experimental and theoretical uncertainties, the measured heavy-flavor yield in p+p collisions is consistent with state-of-the-art pQCD calculations. Suppression in central Cu+Cu collisions suggests the presence of significant cold-nuclear-matter effects and final-state energy loss.

Recent heavy-ion results with the ATLAS detector at the LHC

Results are presented from the ATLAS collaboration from the 2010 LHC heavy-ion run, during which nearly ten inverse microbarns of luminosity were delivered. Soft physics results include charged particle multiplicities and collective flow. The charged particle multiplicity, which tracks initial state entropy production, increases by a factor of 2 relative to the top RHIC energy, with a centrality dependence very similar to that already measured at the RHIC. Measurements of elliptic flow out to large transverse momentum also show similar results to what was measured at the RHIC, but no significant pseudorapidity dependence. Extensions of these measurements to higher harmonics have also been made and can be used to explain structures in the two-particle correlation functions that had long been attributed to jet–medium interactions. New hard probe measurements include single muons, jets and high pT hadrons. Single muons at high momentum are used to extract the yield of W± bosons and are found to be consistent within statistical uncertainties with binary collision scaling. Conversely, jets are found to be suppressed in central events by a factor of 2 relative to peripheral events, with no significant dependence on the jet energy. Fragmentation functions are also found to be the same in central and peripheral events. Finally, charged hadrons have been measured out to 30 GeV, and their centrality dependence relative to peripheral events is similar to that found for jets.

Anomalous centrality variation of minijet angular correlations in Au–Au collisions at 62 and 200 GeV from STAR

We have measured 2D autocorrelations for all charged hadrons in STAR with pt> 0.15 GeV/c and |η|< 1 from Au+Au collisions at 62 and 200 GeV. The correlation structure is dominated by a peak centered at zero relative opening angles on η and ϕ which we hypothesize is caused by minimum-bias jets (minijets). We observe a large excess of minijet correlations in more-central Au–Au collisions relative to binary-collision scaling (more correlated pairs than expected from surface emission or even volume emission). We also observe a sudden increase of the minijet peak amplitude and η width relative to binary-collision scaling of scattered partons which occurs at an energy-dependent centrality point. There is a possible scaling of the transition point with transverse particle density. The large minijet correlations bring into question the degree of thermalization in RHIC collisions.

J/ψProduction versus Centrality, Transverse Momentum, andRapidity inAu+AuCollisions atsNN=200 GeV

The PHENIX experiment at the BNL Relativistic Heavy Ion Collider (RHIC) has measured J/psi production for rapidities -2.2<y<2.2 in Au+Au collisions at square root sNN=200 GeV. The J/psi invariant yield and nuclear modification factor RAA as a function of centrality, transverse momentum, and rapidity are reported. A suppression of J/psi relative to binary collision scaling of proton-proton reaction yields is observed. Models which describe the lower energy J/psi data at the CERN Super Proton Synchrotron invoking only J/psi destruction based on the local medium density predict a significantly larger suppression at RHIC and more suppression at midrapidity than at forward rapidity. Both trends are contradicted by our data.

Measurement of single muon yields from charm semileptonic decay at $\sqrt{s_{NN}}$ =200 GeV Au + Au collisions at STAR

We report on the first measurement of single muon from charm semileptonic decays at low transverse momentum (p(T)) in root s(NN) = 200 GeV Au + Au collisions. Muon identification was obtained using the STAR time projection chamber in conjunction with a time-of-flight detector. The p(T) spectra of electron and muon from charm semileptonic decays are presented. The measured D -> mu + X at p(T) < 0.25 GeV / c greatly constrains the charm total cross section. The charm differential cross section d sigma(cc)/dy is found to be consistent with the number of binary collision scaling.