arXiv subjects
Publications and source records attributed to I. Danchev.
We present measured J/psi production rates in d+Au collisions at sqrt(s_NN) = 200 GeV over a broad range of transverse momentum (p_T=0-14 GeV/c) and rapidity (-2.2 1) for p_T>2 GeV/c. The observed enhancement at negative rapidity has implications for the observed modification in heavy-ion collisions at high p_T.
Measurements of the anisotropy parameter v_2 of identified hadrons (pions, kaons, and protons) as a function of centrality, transverse momentum p_T, and transverse kinetic energy KE_T at midrapidity (|η|<0.35) in Au+Au collisions at sqrt(s_NN) = 200 GeV are presented. Pions and protons are identified up to p_T = 6 GeV/c, and kaons up to p_T = 4 GeV/c, by combining information from time-of-flight and aerogel Cherenkov detectors in the PHENIX Experiment. The scaling of v_2 with the number of valence quarks (n_q) has been studied in different centrality bins as a function of transverse momentum and transverse kinetic energy. A deviation from previously observed quark-number scaling is observed at large values of KE_T/n_q in noncentral Au+Au collisions (20--60%), but this scaling remains valid in central collisions (0--10%).
Back-to-back hadron pair yields in d+Au and p+p collisions at sqrt(s_NN)=200 GeV were measured with the PHENIX detector at the Relativistic Heavy Ion Collider. Rapidity separated hadron pairs were detected with the trigger hadron at pseudorapidity |eta|<0.35 and the associated hadron at forward rapidity (deuteron direction, 3.0<eta<3.8). Pairs were also detected with both hadrons measured at forward rapidity; in this case the yield of back-to-back hadron pairs in d+Au collisions with small impact parameters is observed to be suppressed by a factor of 10 relative to p+p collisions. The kinematics of these pairs is expected to probe partons in the Au nucleus with low fraction x of the nucleon momenta, where the gluon densities rise sharply. The observed suppression as a function of nuclear thickness, p_T, and eta points to cold nuclear matter effects arising at high parton densities.
The second Fourier component v_2 of the azimuthal anisotropy with respect to the reaction plane was measured for direct photons at midrapidity and transverse momentum (p_T) of 1--13 GeV/c in Au+Au collisions at sqr(s_NN)=200 GeV. Previous measurements of this quantity for hadrons with p_T < 6 GeV/c indicate that the medium behaves like a nearly perfect fluid, while for p_T > 6 GeV/c a reduced anisotropy is interpreted in terms of a path-length dependence for parton energy loss. In this measurement with the PHENIX detector at the Relativistic Heavy Ion Collider we find that for p_T > 4 GeV/c the anisotropy for direct photons is consistent with zero, as expected if the dominant source of direct photons is initial hard scattering. However, in the p_T < 4 GeV/c region dominated by thermal photons, we find a substantial direct photon v_2 comparable to that of hadrons, whereas model calculations for thermal photons in this kinematic region significantly underpredict the observed v_2.
The PHENIX experiment at the Relativistic Heavy Ion Collider (RHIC) has measured omega meson production via leptonic and hadronic decay channels in p+p, d+Au, Cu+Cu, and Au+Au collisions at sqrt(s_NN) = 200 GeV. The invariant transverse momentum spectra measured in different decay modes give consistent results. Measurements in the hadronic decay channel in Cu+Cu and Au+Au collisions show that omega production has a suppression pattern at high transverse momentum, similar to that of pi^0 and eta in central collisions, but no suppression is observed in peripheral collisions. The nuclear modification factors, R_AA, are consistent in Cu+Cu and Au+Au collisions at similar numbers of participant nucleons.
We report on charmonium measurements [J/psi(1S), psi'(2S), and chi_c(1P)] in p+p collisions at sqrt(s)=200 GeV. We find that the fraction of J/psi coming from the feed-down decay of psi' and chi_c in the midrapidity region ($|η|<0.35$) is 9.6+/-2.4% and 32+/-9%, respectively. We also report new, higher statistics p_T and rapidity dependencies of the J/psi yield via dielectron decay in the same midrapidity range and at forward rapidity (1.2<|eta|<2.4) via dimuon decay. These results are compared with measurements from other experiments and discussed in the context of current charmonium production models.
Heavy quarkonia are observed to be suppressed in relativistic heavy ion collisions relative to their production in p+p collisions scaled by the number of binary collisions. In order to determine if this suppression is related to color screening of these states in the produced medium, one needs to account for other nuclear modifications including those in cold nuclear matter. In this paper, we present new measurements from the PHENIX 2007 data set of J/psi yields at forward rapidity (1.2<|y|<2.2) in Au+Au collisions at sqrt(s_NN)=200 GeV. The data confirm the earlier finding that the suppression of J/psi at forward rapidity is stronger than at midrapidity, while also extending the measurement to finer bins in collision centrality and higher transverse momentum (pT). We compare the experimental data to the most recent theoretical calculations that incorporate a variety of physics mechanisms including gluon saturation, gluon shadowing, initial-state parton energy loss, cold nuclear matter breakup, color screening, and charm recombination. We find J/psi suppression beyond cold-nuclear-matter effects. However, the current level of disagreement between models and d+Au data precludes using these models to quantify the hot-nuclear-matter suppression.
Measurements of electrons from the decay of open-heavy-flavor mesons have shown that the yields are suppressed in Au+Au collisions compared to expectations from binary scaled p+p collisions. These measurements indicate that charm and bottom quarks interact with the hot-dense matter produced in heavy-ion collisions much more than expected. Here we extend these studies to two-particle correlations where one particle is an electron from the decay of a heavy-flavor meson and the other is a charged hadron from either the decay of the heavy meson or from jet fragmentation. These measurements provide more detailed information about the interactions between heavy quarks and the matter, such as whether the modification of the away-side-jet shape seen in hadron-hadron correlations is present when the trigger particle is from heavy-meson decay and whether the overall level of away-side jet suppression is consistent. We statistically subtract correlations of electrons arising from background sources from the inclusive electron-hadron correlations and obtain two-particle azimuthal correlations at sqrt(s_NN)=$200 GeV between electrons from heavy-flavor decay with charged hadrons in p+p and also first results in Au+Au collisions. We find the away-side-jet shape and yield to be modified in Au+Au collisions compared to p+p collisions.
Pair correlations between large transverse momentum neutral pion triggers (p_T=4--7 GeV/c) and charged hadron partners (p_T=3--7 GeV/c) in central (0--20%) and midcentral (20--60%) Au+Au collisions are presented as a function of trigger orientation with respect to the reaction plane. The particles are at larger momentum than where jet shape modifications have been observed, and the correlations are sensitive to the energy loss of partons traveling through hot dense matter. An out-of-plane trigger particle produces only 26+/-20% of the away-side pairs that are observed opposite of an in-plane trigger particle. In contrast, near-side jet fragments are consistent with no suppression or dependence on trigger orientation with respect to the reaction plane. These observations are qualitatively consistent with a picture of little near-side parton energy loss either due to surface bias or fluctuations and increased away-side parton energy loss due to a long path through the medium. The away-side suppression as a function of reaction-plane angle is shown to be sensitive to both the energy loss mechanism in and the space-time evolution of heavy-ion collisions.
We present measurements of J/psi yields in d+Au collisions at sqrt(s_NN) = 200 GeV recorded by the PHENIX experiment and compare with yields in p+p collisions at the same energy per nucleon-nucleon collision. The measurements cover a large kinematic range in J/psi rapidity (-2.2 < y < 2.4) with high statistical precision and are compared with two theoretical models: one with nuclear shadowing combined with final state breakup and one with coherent gluon saturation effects. To remove model dependent systematic uncertainties we also compare the data to a simple geometric model. We find that calculations where the nuclear modification is linear or exponential in the density weighted longitudinal thickness are difficult to reconcile with the forward rapidity data.
We report the first measurement of transverse single-spin asymmetries in $J/ψ$ production from transversely polarized $p+p$ collisions at $\sqrt{s} = 200$ GeV with data taken by the PHENIX experiment in 2006 and 2008. The measurement was performed over the rapidity ranges $1.2 < |y| < 2.2$ and $ |y| < 0.35$ for transverse momenta up to 6 GeV/$c$. $J/ψ$ production at RHIC is dominated by processes involving initial-state gluons, and transverse single-spin asymmetries of the $J/ψ$ can provide access to gluon dynamics within the nucleon. Such asymmetries may also shed light on the long-standing question in QCD of the $J/ψ$ production mechanism. Asymmetries were obtained as a function of $J/ψ$ transverse momentum and Feynman-$x$, with a value of $-0.086 \pm 0.026^{\rm stat} \pm 0.003^{\rm syst}$ in the forward region. This result suggests possible nonzero trigluon correlation functions in transversely polarized protons and, if well defined in this reaction, a nonzero gluon Sivers distribution function.
New measurements by the PHENIX experiment at RHIC for eta production at midrapidity as a function of transverse momentum (p_T) and collision centrality in sqrt(s_NN) = 200 GeV Au+Au and p+p collisions are presented. They indicate nuclear modification factors (R_AA) that are similar both in magnitude and trend to those found in earlier pi^0 measurements. Linear fits to R_AA in the 5--20 GeV/c p_T region show that the slope is consistent with zero within two standard deviations at all centralities although a slow rise cannot be excluded. Having different statistical and systematic uncertainties the pi^0 and eta measurements are complementary at high p_T; thus, along with the extended p_T range of these data they can provide additional constraints for theoretical modeling and the extraction of transport properties.
Differential measurements of the elliptic (v_2) and hexadecapole (v_4) Fourier flow coefficients are reported for charged hadrons as a function of transverse momentum (p_T) and collision centrality or the number of participant nucleons (N_part) for Au+Au collisions at sqrt(s_NN)=200 GeV. The v_{2,4} measurements at pseudorapidity |η|<=0.35 obtained with four separate reaction plane detectors positioned in the range 1.0<|η|<3.9 show good agreement, indicating the absence of significant η-dependent nonflow perturbations. Sizable values for v_4(p_T) are observed with a ratio v_4(p_T,N_part)/v_2^2(p_T,N_part)~0.8 for 50 200 this ratio increases up to 1.7 in the most central collisions.
The PHENIX experiment at the Relativistic Heavy Ion Collider (RHIC) has performed systematic measurements of phi meson production in the K+K- decay channel at midrapidity in p+p, d+Au, Cu+Cu and Au+Au collisions at sqrt(S_NN)=200 GeV. Results are presented on the phi invariant yield and the nuclear modification factor R_AA for Au+Au and Cu+Cu, and R_dA for d+Au collisions, studied as a function of transverse momentum (1<p_T<7 GeV/c) and centrality. In central and mid-central Au+Au collisions, the R_AA of phi exhibits a suppression relative to expectations from binary scaled p+p results. The amount of suppression is smaller than that of the neutral pion and the eta meson in the intermediate p_T range (2--5 GeV/c); whereas at higher p_T the phi, pi^0, and eta show similar suppression. The baryon (protons and anti-protons) excess observed in central Au+Au collisions at intermediate p_T is not observed for the phi meson despite the similar mass of the proton and the phi. This suggests that the excess is linked to the number of constituent quarks rather than the hadron mass. The difference gradually disappears with decreasing centrality and for peripheral collisions the R_AA values for both particles are consistent with binary scaling. Cu+Cu collisions show the same yield and suppression as Au+Au collisions for the same number of N_part. The R_dA of phi shows no evidence for cold nuclear effects within uncertainties.
Fast parton probes produced by hard scattering and embedded within collisions of large nuclei have shown that partons suffer large energy loss and that the produced medium may respond collectively to the lost energy. We present measurements of neutral pion trigger particles at transverse momenta p^t_T = 4-12 GeV/c and associated charged hadrons (p^a_T = 0.5-7 GeV/c) as a function of relative azimuthal angle Delta Phi at midrapidity in Au+Au and p+p collisions at sqrt(s_NN) = 200 GeV. These data lead to two major observations. First, the relative angular distribution of low momentum hadrons, whose shape modification has been interpreted as a medium response to parton energy loss, is found to be modified only for p^t_T < 7 GeV/c. At higher p^t_T, the data are consistent with unmodified or very weakly modified shapes, even for the lowest measured p^a_T. This observation presents a quantitative challenge to medium response scenarios. Second, the associated yield of hadrons opposite to the trigger particle in Au+Au relative to that in p+p (I_AA) is found to be suppressed at large momentum (IAA ~ 0.35-0.5), but less than the single particle nuclear modification factor (R_AA ~0.2).
Previously, the near-threhold $pp\to ppπ^0$ reaction was studied with the use of transition operators derived from chiral perturbation theory ($χ$PT) and the nuclear wave functions generated by high-precision phenomenological potentials. A conceptual problem in that approach was that the transition amplitude receives contributions from very high momentum components (above the cutoff scale of $χ$PT) in the nuclear wave functions. In the present work, we avoid this problem by replacing the "bare" phenomenological potentials with V_{low-k}, which is an effective potential derived from a bare potential by integrating out momentum components higher than a specified cutoff scale. The use of V_{low-k} is found to give an enhancement of the $pp\to ppπ^0$ cross sections over the values obtained with bare potentials. Although this enhancement brings the calculated cross sections closer to the experimental values, the incident-energy dependence of the cross section is not well reproduced, a problem that seems to indicate the necessity of including higher chiral order terms than considered in the present work.