12 Search Results

Dihadron azimuthal correlations containing a high transverse momentum ($$ p_{T} $$) trigger particle are sensitive to the properties of the nuclear medium created at RHIC through the strong interactions occurring between the traversing parton and the medium, i.e. jet-quenching. Previous measurements revealed a strong modification to dihadron azimuthal correlations in Au+Au collisions with respect to p+p and d+Au collisions. The modification increases with the collision centrality, suggesting a path-length or energy density dependence to the jet-quenching effect. This paper reports STAR measurements of dihadron azimuthal correlations in mid-central (20%-60%) Au+Au collisions at $$ \sqrt{s_{\rm{NN}}} = 200 $$ GeV as amore » function of the trigger particle's azimuthal angle relative to the event plane, $$ \phi_{s} = | \phi_{t}- \psi_{{\rm{EP}}}| $$. The azimuthal correlation is studied as a function of both the trigger and associated particle $$ p_{T} $$. The subtractions of the combinatorial background and anisotropic flow, assuming Zero Yield At Minimum (ZYAM), are described. The correlation results are first discussed with subtraction of the even harmonic (elliptic and quadrangular) flow backgrounds. The away-side correlation is strongly modified, and the modification varies with $$ \phi_{s} $$, with a double-peak structure for out-of-plane trigger particles. The near-side ridge (long range pseudo-rapidity $$ \Delta\eta $$ correlation) appears to drop with increasing $$ \phi_{s} $$ while the jet-like component remains approximately constant. The correlation functions are further studied with the subtraction of odd harmonic triangular flow background arising from fluctuations. It is found that the triangular flow, while responsible for the majority of the amplitudes, is not sufficient to explain the $$ \phi_{s} $$-dependence of the ridge or the away-side double-peak structure. The dropping ridge with $$ \phi_{s} $$ could be attributed to a $$ \phi_{s} $$-dependent elliptic anisotropy; however, the physics mechanism of the ridge remains an open question. Even with a $$ \phi_{s} $$-dependent elliptic flow, the away-side correlation structure is robust. These results, with extensive systematic studies of the dihadron correlations as a function of $$ \phi_{s} $$, trigger and associated particle $$ p_{T} $$, and the pseudo-rapidity range $$ \Delta\eta $$, should provide stringent inputs to help understand the underlying physics mechanisms of jet-medium interactions in high energy nuclear collisions.« less

We report the measurement of a new observable of jet quenching in central Pb-Pb collisions at $$ \sqrt{s_{\mathrm{NN}}}=2.76 $$ TeV, based on the semi-inclusive rate of charged jets recoiling from a high transverse momentum (high-p T) charged hadron trigger. Jets are measured using collinear-safe jet reconstruction with infrared cutoff for jet constituents of 0.15 GeV, for jet resolution parameters R = 0.2, 0.4 and 0.5. Underlying event background is corrected at the event-ensemble level, without imposing bias on the jet population. Recoil jet spectra are reported in the range 20 < p T,jet ch < 100 GeV. Reference distributions formore » pp collisions at $$ \sqrt{s}=2.76 $$ TeV are calculated using Monte Carlo and NLO pQCD methods, which are validated by comparing with measurements in pp collisions at $$ \sqrt{s}=7 $$ TeV. The recoil jet yield in central Pb-Pb collisions is found to be suppressed relative to that in pp collisions. No significant medium-induced broadening of the intra-jet energy profile is observed within 0.5 radians relative to the recoil jet axis. Additionally, the angular distribution of the recoil jet yield relative to the trigger axis is found to be similar in central Pb-Pb and pp collisions, with no significant medium-induced acoplanarity observed. Large-angle jet deflection, which may provide a direct probe of the nature of the quasi-particles in hot QCD matter, is explored.« less

Prompt D meson and non-prompt J/ψ yields are studied as a function of the multiplicity of charged particles produced in inelastic proton-proton collisions at a centre-of-mass energy of √s = 7 TeV. The results are reported as a ratio between yields in a given multiplicity interval normalised to the multiplicity-integrated ones (relative yields). They are shown as a function of the multiplicity of charged particles normalised to the average value for inelastic collisions (relative charged-particle multiplicity). D⁰, D⁺ and D^*+ mesons are measured in five p T intervals from 1 GeV/c to 20 GeV/c and for |y| < 0.5 viamore » their hadronic decays. The D-meson relative yield is found to increase with increasing charged-particle multiplicity. For events with multiplicity six times higher than the average multiplicity of inelastic collisions, a yield enhancement of a factor about 15 relative to the multiplicity-integrated yield in inelastic collisions is observed. The yield enhancement is independent of transverse momentum within the uncertainties of the measurement. The D0-meson relative yield is also measured as a function of the relative multiplicity at forward pseudo-rapidity. The non-prompt J/ψ, i.e. the B hadron, contribution to the inclusive J/ψ production is measured in the di-electron decay channel at central rapidity. It is evaluated for p_T > 1.3 GeV/c and |y| < 0.9, and extrapolated to p_T > 0. The fraction of non-prompt J/ψ in the inclusive J/ψ yields shows no dependence on the charged-particle multiplicity at central rapidity. Charm and beauty hadron relative yields exhibit a similar increase with increasing charged-particle multiplicity. The measurements are compared to PYTHIA 8, EPOS 3 and percolation calculations.« less

The transverse momentum (p_T) dependence of the nuclear modification factor RAA and the centrality dependence of the average transverse momentum < p_T > for inclusive J/ψ have been measured with ALICE for Pb-Pb collisions at $$\sqrt{s_{_\text {NN}}}$$ = 2.76 TeV in the e+e– decay channel at mid-rapidity (|y| < 0.8). The < p_T > is significantly smaller than the one observed for pp collisions at the same centre-of-mass energy. Consistently, an increase of R_AA is observed towards low p_T. These observations might be indicative of a sizable contribution of charm quark coalescence to the J/ψ production. Additionally, the fraction ofmore » non-prompt J/ψ from beauty hadron decays, f_B, has been determined in the region 1.5 < p_T< 10 GeV/c in three centrality intervals. No significant centrality dependence of fB is observed. Finally, the R_AA of non-prompt J/ψ is discussed and compared with model predictions. The nuclear modification in the region 4.5 < p_T< 10 GeV/c is found to be stronger than predicted by most models.« less

We have studied the transverse-momentum (p _T) dependence of the inclusive J/ψ production in p-Pb collisions at $$\sqrt{^sNN}$$ = 5.02 TeV, in three center-of-mass rapidity (y _cms) regions, down to zero p _T. Results in the forward and backward rapidity ranges (2.03 < y _cms < 3.53 and –4.46 < y _cms < –2.96) are obtained by studying the J/ψ decay to μ ⁺ μ ^–, while the mid-rapidity region (–1.37 < y _cms < 0.43) is investigated by measuring the e⁺e^– decay channel. The p _T dependence of the J/ψ production cross section and nuclear modification factor are presentedmore » for each of the rapidity intervals, as well as the J/ψ mean p _T values. Forward and mid-rapidity results show a suppression of the J/ψ yield, with respect to pp collisions, which decreases with increasing p _T. At backward rapidity no significant J/ψ suppression is observed. Theoretical models including a combination of cold nuclear matter effects such as shadowing and partonic energy loss, are in fair agreement with the data, except at forward rapidity and low transverse momentum. The implications of the p-Pb results for the evaluation of cold nuclear matter effects on J/ψ production in Pb-Pb collisions are also discussed.« less

Differential cross sections of charged particles in inelastic pp collisions as a function of p_T have been measured at $$\sqrt{s}$$ = 0.9, 2.76 and 7 TeV at the LHC. The p_T spectra are compared to NLO-pQCD calculations. Though the differential cross section for an individual √s cannot be described by NLO-pQCD, the relative increase of cross section with √s is in agreement with NLO-pQCD. Based on these measurements and observations, procedures are discussed to construct pp reference spectra at $$\sqrt{s}$$ = 2.76 and 5.02 TeV up to p_T=50 GeV/c as required for the calculation of the nuclear modification factor inmore » nucleus–nucleus and proton–nucleus collisions.« less

The ALICE Collaboration at the LHC has measured the J/ψ and ψ' photoproduction at mid-rapidity in ultra-peripheral Pb-Pb collisions at $$\sqrt{s_{\mathrm{NN}}} $$ = 2.76 Tev. The charmonium is identified via its leptonic decay for events where the hadronic activity is required to be minimal. The analysis is based on an event sample corresponding to an integrated luminosity of about 23 μb^-1. The cross section for coherent and incoherent J/ψ production in the rapidity interval -0.9 < y < 0.9, are dσ^coh_J/ψ/dy = 2.38^+0.34_–0.24(sta + sys) mb and dσ^inc_J/ψ/dy = 0.98^+0.19_–0.17(sta + sys) mb, respectively. The results are compared to theoreticalmore » models for J/ψ production and the coherent cross section is found to be in good agreement with those models incorporating moderate nuclear gluon shadowing at Bjorken-x around 10^–3, such as EPS09 parametrization. In addition the cross section for the process γγ→e ⁺ e ^– has been measured and found to be in agreement with models implementing QED at leading order.« less

The ratios of yields of anti-baryons to baryons probes the mechanisms of baryon-number transport. Results for \(\overline{\mathrm {p}}/{\mathrm{p}}\), \(\overline{\Lambda}/{\Lambda}\), \({{\overline{\Xi}}^{+}}/{\Xi ^{-}}\) and \({{\overline{\Omega}}^{+}}/{\Omega^{-}}\) in pp collisions at \(\sqrt{s} = 0.9,\ 2.76\mbox{ and }7\mbox{ TeV}\), measured with the ALICE detector at the LHC, are reported. Within the experimental uncertainties and ranges covered by our measurement, these ratios are independent of rapidity, transverse momentum and multiplicity for all measured energies. The results are compared to expectations from event generators, such as PYTHIA and HIJING/B, that are used to model the particle production in pp collisions. The energy dependence of \(\overline{\mathrm{p}}/{\mathrm{p}}\), \(\overline {\Lambda}/{\Lambda}\),more » \({{\overline{\Xi}}^{+}}/{\Xi^{-}}\) and \({{\overline{\Omega}}^{+}}/{\Omega^{-}}\), reaching values compatible with unity for \(\sqrt{s} = 7\mbox{ TeV}\), complement the earlier \(\overline{\mathrm{p}}/{\mathrm{p}}\) measurement of ALICE. These dependencies can be described by exchanges with the Regge-trajectory intercept of \(\alpha_{\rm{J}} \approx0.5\), which are suppressed with increasing rapidity interval Δy. Any significant contribution of an exchange not suppressed at large Δy (reached at LHC energies) is disfavoured.« less

Measurements of cross sections of inelastic and diffractive processes in proton–proton collisions at LHC energies were carried out with the ALICE detector. The fractions of diffractive processes in inelastic collisions were determined from a study of gaps in charged particle pseudorapidity distributions: for single diffraction (diffractive mass M _X<200 GeV/c ²) \(\sigma_{\mathrm{SD}}/\sigma_{\mathrm {INEL}} = 0.21 \pm 0.03, 0.20^{+0.07}_{-0.08}\), and \(0.20^{+0.04}_{-0.07}\), respectively at centre-of-mass energies \(\sqrt{s} = 0.9, 2.76\mbox{, and }7~\mbox{TeV}\); for double diffraction (for a pseudorapidity gap Δη>3) σ _DD/σ _INEL=0.11±0.03,0.12±0.05, and \(0.12^{+0.05}_{-0.04}\), respectively at \(\sqrt{s} = 0.9, 2.76\mbox{, and }7~\mbox{TeV}\). To measure the inelastic cross section, beam properties weremore » determined with van der Meer scans, and, using a simulation of diffraction adjusted to data, the following values were obtained: \(\sigma_{\mathrm{INEL}} = 62.8^{+2.4}_{-4.0} (\mathrm{model}) \pm1.2 (\mathrm{lumi})\) mb at \(\sqrt{s} = 2.76\ \mbox{TeV}\) and \(73.2^{+2.0}_{-4.6} (\mathrm{model}) \pm2.6 (\mathrm{lumi})~\mbox{mb}\) at \(\sqrt{s} = 7\ \mbox{TeV}\). The single- and double-diffractive cross sections were calculated combining relative rates of diffraction with inelastic cross sections. The results are compared to previous measurements at proton–antiproton and proton–proton colliders at lower energies, to measurements by other experiments at the LHC, and to theoretical models.« less

Here we present results on strange and multistrange particle production in Au + Au collisions at $$\sqrt{s_{NN}}$$ = 62.4 GeV as measured with the STAR detector at RHIC. Midrapidity transverse momentum spectra and integrated yields of $$K^{0}_{S}$$, Λ, Ξ, and Ω and their antiparticles are presented for different centrality classes. The particle yields and ratios follow a smooth energy dependence. Chemical freeze-out parameters, temperature, baryon chemical potential, and strangeness saturation factor obtained from the particle yields are presented. Intermediate transverse momentum ( p_T) phenomena are discussed based on the ratio of the measured baryon-to-meson spectra and nuclear modification factor. Themore » centrality dependence of various measurements presented show a similar behavior as seen in Au + Au collisions at $$\sqrt{s_{NN}}$$ = 200 GeV.« less