Abstract Cross sections for the coherent photoproduction of ρ 0 vector mesons in ultra-peripheral Pb-Pb collisions at s NN $$ \sqrt{{\mathrm{s}}_{\mathrm{NN}}} $$ = 5.02 TeV are reported. The ...measurements, which rely on the π + π − decay channel, are presented in three regions of rapidity covering the range |y| < 0.8. For each rapidity interval, cross sections are shown for different nuclear-breakup classes defined according to the presence of neutrons measured in the zero-degree calorimeters. The results are compared with predictions based on different models of nuclear shadowing. Finally, the observation of a coherently produced resonance-like structure with a mass around 1.7 GeV/c 2 and a width of about 140 MeV/c 2 is reported and compared with similar observations from other experiments.
Abstract This article reports measurements characterizing the Underlying Event (UE) associated with hard scatterings at midrapidity (|η| < 0.8) in pp collisions at s $$ \sqrt{s} $$ = 13 TeV. The hard ...scatterings are identified by the leading particle, the charged particle with the highest transverse momentum ( p T leading $$ {p}_{\mathrm{T}}^{\mathrm{leading}} $$ ) in the event. Charged-particle number-densities and summed transverse-momentum densities are measured in different azimuthal regions defined with respect to the leading particle direction: Toward, Transverse, and Away. The Toward and Away regions contain the fragmentation products of the hard scatterings in addition to the UE contribution, whereas particles in the Transverse region are expected to originate predominantly from the UE. The study is performed as a function of p T leading $$ {p}_{\mathrm{T}}^{\mathrm{leading}} $$ with three different p T thresholds for the associated particles, p T track $$ {p}_{\mathrm{T}}^{\mathrm{track}} $$ > 0.15, 0.5, and 1.0 GeV/c. The charged-particle density in the Transverse region rises steeply for low values of p T leading $$ {p}_{\mathrm{T}}^{\mathrm{leading}} $$ and reaches a plateau. The results confirm the trend that the charged-particle density in the Transverse region shows a stronger increase with s $$ \sqrt{s} $$ than the inclusive charged-particle density at midrapidity. The UE activity is increased by approximately 20% when going from 7 TeV to 13 TeV pp collisions. The plateau in the Transverse region (5 < p T leading $$ {p}_{\mathrm{T}}^{\mathrm{leading}} $$ < 40 GeV/c) is further characterized by the probability distribution of its charged-particle multiplicity normalized to its average value (relative transverse activity, R T) and the mean transverse momentum as a function of R T. Experimental results are compared to model calculations using PYTHIA 8 and EPOS LHC. The overall agreement between models and data is within 30%. These measurements provide new insights on the interplay between hard scatterings and the associated UE in pp collisions.
Abstract The multiplicity dependence of electron production from heavy-flavour hadron decays as a function of transverse momentum was measured in p-Pb collisions at s NN $$ \sqrt{s_{\mathrm{NN}}} $$ ...= 5.02 TeV using the ALICE detector at the LHC. The measurement was performed in the centre-of-mass rapidity interval −1.07 < y cms < 0.14 and transverse momentum interval 2 < p T < 16 GeV/c. The multiplicity dependence of the production of electrons from heavy-flavour hadron decays was studied by comparing the p T spectra measured for different multiplicity classes with those measured in pp collisions (Q pPb) and in peripheral p-Pb collisions (Q cp). The Q pPb results obtained are consistent with unity within uncertainties in the measured p T interval and event classes. This indicates that heavy-flavour decay electron production is consistent with binary scaling and independent of the geometry of the collision system. Additionally, the results suggest that cold nuclear matter effects are negligible within uncertainties, in the production of heavy-flavour decay electrons at midrapidity in p-Pb collisions.
Abstract The inclusive J/ψ production in Pb–Pb collisions at the center-of-mass energy per nucleon pair s NN $$ \sqrt{s_{\mathrm{NN}}} $$ = 5.02 TeV, measured with the ALICE detector at the CERN LHC, ...is reported. The J/ψ meson is reconstructed via the dimuon decay channel at forward rapidity (2.5 < y < 4) down to zero transverse momentum. The suppression of the J/ψ yield in Pb–Pb collisions with respect to binary-scaled pp collisions is quantified by the nuclear modification factor (R AA). The R AA at s NN $$ \sqrt{s_{\mathrm{NN}}} $$ = 5.02 TeV is presented and compared with previous measurements at s NN $$ \sqrt{s_{\mathrm{NN}}} $$ = 2.76 TeV as a function of the centrality of the collision, and of the J/ψ transverse momentum and rapidity. The inclusive J/ψ RAA shows a suppression increasing toward higher transverse momentum, with a steeper dependence for central collisions. The modification of the J/ψ average transverse momentum and average squared transverse momentum is also studied. Comparisons with the results of models based on a transport equation and on statistical hadronization are carried out.
Abstract The measurement of the production of prompt D0, D+, D*+, and D S + $$ {\mathrm{D}}_{\mathrm{S}}^{+} $$ mesons in proton–lead (p–Pb) collisions at the centre-of-mass energy per nucleon pair ...of s NN $$ \sqrt{s_{\mathrm{NN}}} $$ = 5.02 TeV, with an integrated luminosity of 292 ± 11 μb −1, are reported. Differential production cross sections are measured at mid-rapidity (−0.96 < y cms < 0.04) as a function of transverse momentum (p T) in the intervals 0 < p T < 36 GeV/c for D0, 1 < p T < 36 GeV/c for D+ and D*+, and 2 < p T < 24 GeV/c for D+ mesons. For each species, the nuclear modification factor R pPb is calculated as a function of p T using a proton-proton (pp) ref- erence measured at the same collision energy. The results are compatible with unity in the whole p T range. The average of the non-strange D mesons R pPb is compared with theoretical model predictions that include initial-state effects and parton transport model predictions. The p T dependence of the D0, D+, and D*+ nuclear modification factors is also reported in the interval 1 < p T < 36 GeV/c as a function of the collision centrality, and the central-to-peripheral ratios are computed from the D-meson yields measured in different centrality classes. The results are further compared with charged-particle measurements and a similar trend is observed in all the centrality classes. The ratios of the p T-differential cross sections of D0, D+, D*+, and D S + $$ {\mathrm{D}}_{\mathrm{S}}^{+} $$ mesons are also reported. The D S + $$ {\mathrm{D}}_{\mathrm{S}}^{+} $$ and D+ yields are compared as a function of the charged-particle multiplicity for several p T intervals. No modification in the relative abundances of the four species is observed with respect to pp collisions within the statistical and systematic uncertainties.
Abstract Inclusive J/ψ production is studied in minimum-bias proton-proton collisions at a centre-of-mass energy of s $$ \sqrt{s} $$ = 5.02 TeV by ALICE at the CERN LHC. The measurement is performed ...at mid-rapidity (|y| < 0.9) in the dielectron decay channel down to zero transverse momentum p T, using a data sample corresponding to an integrated luminosity of L int = 19.4 ± 0.4 nb −1. The measured p T-integrated inclusive J/ψ production cross sec- tion is dσ/dy = 5.64 ± 0.22(stat.) ± 0.33(syst.) ± 0.12(lumi.) μb. The p T-differential cross section d2 σ/dp Tdy is measured in the p T range 0–10 GeV/c and compared with state-of- the-art QCD calculations. The J/ψ 〈p T〉 and p T 2 $$ \left\langle {p}_{\mathrm{T}}^2\right\rangle $$ are extracted and compared with results obtained at other collision energies.
Abstract Two-particle correlations in high-energy collision experiments enable the extraction of particle source radii by using the Bose-Einstein enhancement of pion production at low relative ...momentum q ∝ 1/R. It was previously observed that in pp collisions at s $$ \sqrt{s} $$ = 7TeV the average pair transverse momentum k T range of such analyses is limited due to large background correlations which were attributed to mini-jet phenomena. To investigate this further, an event-shape dependent analysis of Bose-Einstein correlations for pion pairs is performed in this work. By categorizing the events by their transverse sphericity S T into spherical (S T > 0:7) and jet-like (S T < 0:3) events a method was developed that allows for the determination of source radii for much larger values of k T for the first time. Spherical events demonstrate little or no background correlations while jet-like events are dominated by them. This observation agrees with the hypothesis of a mini-jet origin of the non-femtoscopic background correlations and gives new insight into the physics interpretation of the k T dependence of the radii. The emission source size in spherical events shows a substantially diminished k T dependence, while jet-like events show indications of a negative trend with respect to k T in the highest multiplicity events. Regarding the emission source shape, the correlation functions for both event sphericity classes show good agreement with an exponential shape, rather than a Gaussian one.
Abstract The production of charm jets in proton-proton collisions at a center-of-mass energy of s = 7 $$ \sqrt{s}=7 $$ TeV was measured with the ALICE detector at the CERN Large Hadron Collider. The ...measurement is based on a data sample corresponding to a total integrated luminosity of 6.23 nb−1, collected using a minimum-bias trigger. Charm jets are identified by the presence of a D0 meson among their constituents. The D0 mesons are reconstructed from their hadronic decay D0 →K−π+. The D0-meson tagged jets are reconstructed using tracks of charged particles (track-based jets) with the anti-k T algorithm in the jet transverse momentum range 5 < p T , jet ch $$ 5<{p}_{{\mathrm{T}}_{,\mathrm{jet}}}^{\mathrm{ch}} $$ < 30 GeV/c and pseudorapidity |η jet| < 0.5. The fraction of charged jets containing a D0-meson increases with p T , jet ch $$ {p}_{{\mathrm{T}}_{,\mathrm{jet}}}^{\mathrm{ch}} $$ from 0.042 ± 0.004 (stat) ± 0.006 (syst) to 0.080 ± 0.009 (stat) ± 0.008 (syst). The distribution of D0-meson tagged jets as a function of the jet momentum fraction carried by the D0 meson in the direction of the jet axis z ∥ ch $$ \left({z}_{\parallel}^{\mathrm{ch}}\right) $$ is reported for two ranges of jet transverse momenta, 5 < p T , jet ch $$ 5<{p}_{{\mathrm{T}}_{,\mathrm{jet}}}^{\mathrm{ch}} $$ < 15 GeV/c and 15 < p T , jet ch $$ 15<{p}_{{\mathrm{T}}_{,\mathrm{jet}}}^{\mathrm{ch}} $$ < 30 GeV/c in the intervals 0.2 < z ‖ ch < 1.0 $$ 0.2<{z}_{\Big\Vert}^{\mathrm{ch}}<1.0 $$ and 0.4 < z ‖ ch < 1.0 $$ 0.4<{z}_{\Big\Vert}^{\mathrm{ch}}<1.0 $$ , respectively. The data are compared with results from Monte Carlo event generators (PYTHIA 6, PYTHIA 8 and Herwig 7) and with a Next-to-Leading-Order perturbative Quantum Chromodynamics calculation, obtained with the POWHEG method and interfaced with PYTHIA 6 for the generation of the parton shower, fragmentation, hadronisation and underlying event.
Abstract The transverse structure of jets was studied via jet fragmentation transverse momentum (j T) distributions, obtained using two-particle correlations in proton-proton and proton-lead ...collisions, measured with the ALICE experiment at the LHC. The highest transverse momentum particle in each event is used as the trigger particle and the region 3 < p Tt < 15GeV/c is explored in this study. The measured distributions show a clear narrow Gaussian component and a wide non-Gaussian one. Based on Pythia simulations, the narrow component can be related to non-perturbative hadronization and the wide component to quantum chromodynamical splitting. The width of the narrow component shows a weak dependence on the transverse momentum of the trigger particle, in agreement with the expectation of universality of the hadronization process. On the other hand, the width of the wide component shows a rising trend suggesting increased branching for higher transverse momentum. The results obtained in pp collisions at s = 7 $$ \sqrt{s}=7 $$ TeV and in p–Pb collisions at s N N = 5.02 $$ \sqrt{s_{\mathrm{NN}}}=5.02 $$ TeV are compatible within uncertainties and hence no significant cold nuclear matter effects are observed. The results are compared to previous measurements from CCOR and PHENIX as well as to Pythia 8 and Herwig 7 simulations.
Abstract The second (v 2) and third (v 3) flow harmonic coefficients of J/ψ mesons are measured at forward rapidity (2.5 < y < 4.0) in Pb-Pb collisions at s N N = 5.02 $$ \sqrt{s_{\mathrm{NN}}}=5.02 ...$$ TeV with the ALICE detector at the LHC. Results are obtained with the scalar product method and reported as a function of transverse momentum, p T, for various collision centralities. A positive value of J/ψ v 3 is observed with 3.7σ significance. The measurements, compared to those of prompt D0 mesons and charged particles at mid-rapidity, indicate an ordering with v n(J/ψ) < v n(D0) < v n(h±) (n = 2, 3) at low and intermediate p T up to 6 GeV/c and a convergence with v 2(J/ψ) ≈ v 2(D0) ≈ v 2(h±) at high p T above 6–8 GeV/c. In semi-central collisions (5–40% and 10–50% centrality intervals) at intermediate p T between 2 and 6 GeV/c, the ratio v 3/v 2 of J/ψ mesons is found to be significantly lower (4.6σ) with respect to that of charged particles. In addition, the comparison to the prompt D0-meson ratio in the same p T interval suggests an ordering similar to that of the v 2 and v 3 coefficients. The J/ψ v 2 coefficient is further studied using the Event Shape Engineering technique. The obtained results are found to be compatible with the expected variations of the eccentricity of the initial-state geometry.
PDF