Search Results (59 total)

Muon production at forward rapidity (1.5≤|η|≤1.8) has been measured by the PHENIX experiment over the transverse momentum range 1≤p_T≤3  GeV/c in sqrt[s]=200  GeV p+p collisions at the Relativistic Heavy Ion Collider. After statistically subtracting contributions from light hadron decays an excess remains which is attributed to the semileptonic decays of hadrons carrying heavy flavor, i.e. charm quarks or, at high p_T, bottom quarks. The resulting muon spectrum from heavy flavor decays is compared to PYTHIA and a next-to-leading-order perturbative QCD calculation. PYTHIA is used to determine the charm quark spectrum that would produce the observed muon excess. The corresponding differential cross section for charm quark production at forward rapidity is determined to be dσ_cc̅ /dy|_y=1.6=0.243±0.013(stat.)±0.105(data syst.)+0.049 / -0.087(PYTHIA syst.)  mb.

Measurements of neutral pion (π⁰) production at midrapidity in sqrt[s_NN]=200 GeV Au+Au collisions as a function of transverse momentum, p_T, collision centrality, and angle with respect to reaction plane are presented. The data represent the final π⁰ results from the PHENIX experiment for the first RHIC Au+Au run at design center-of-mass energy. They include additional data obtained using the PHENIX Level-2 trigger with more than a factor of 3 increase in statistics over previously published results for p_T>6 GeV/c. We evaluate the suppression in the yield of high-p_T π⁰'s relative to pointlike scaling expectations using the nuclear modification factor R_AA. We present the p_T dependence of R_AA for nine bins in collision centrality. We separately integrate R_AA over larger p_T bins to show more precisely the centrality dependence of the high-p_T suppression. We then evaluate the dependence of the high-p_T suppression on the emission angle Δϕ of the pions with respect to event reaction plane for seven bins in collision centrality. We show that the yields of high-p_T π⁰'s vary strongly with Δϕ, consistent with prior measurements . We show that this variation persists in the most peripheral bin accessible in this analysis. For the peripheral bins we observe no suppression for neutral pions produced aligned with the reaction plane, whereas the yield of π⁰'s produced perpendicular to the reaction plane is suppressed by a factor of ~2. We analyze the combined centrality and Δϕ dependence of the π⁰ suppression in different p_T bins using different possible descriptions of parton energy loss dependence on jet path-length averages to determine whether a single geometric picture can explain the observed suppression pattern.

Longitudinal density correlations of produced matter in Au+Au collisions at sqrt[s_NN]=200 GeV have been measured from the inclusive charged particle distributions as a function of pseudorapidity window sizes. The extracted αξ parameter, related to the susceptibility of the density fluctuations in the long-wavelength limit, exhibits a nonmonotonic behavior as a function of the number of participant nucleons, N_part. A local maximum is seen at N_part~90, with corresponding energy density based on the Bjorken picture of ε_Bjτ~2.4 GeV/(fm²c) with a transverse area size of 60 fm². This behavior may suggest a critical phase boundary based on the Ginzburg-Landau framework.

Emission source functions are extracted from correlation functions constructed from charged pions produced at midrapidity in Au+Au collisions at sqrt[s_NN]=200  GeV. The source parameters extracted from these functions at low k_T give first indications of a long tail for the pion emission source. The source extension cannot be explained solely by simple kinematic considerations. The possible role of a halo of secondary pions from resonance emissions is explored.

Inclusive transverse momentum spectra of η mesons in the range p_T≈2–12 GeV/c have been measured at midrapidity (|η|<0.35) by the PHENIX experiment at RHIC in p+p,d+Au, and Au+Au collisions at sqrt[s_NN]=200 GeV. The η mesons are reconstructed through their η→γ γ channel for the three colliding systems as well as through the η→π⁰π⁺π^- decay mode in p+p and d+Au collisions. The nuclear modification factor in d+Au collisions, R_dAu(p_T)≈1.0–1.1, suggests at most only modest p_T broadening (“Cronin enhancement”). In central Au+Au reactions, the η yields are significantly suppressed, with R_AuAu(p_T)≈0.2. The ratio of η to π⁰ yields is approximately constant as a function of p_T for the three colliding systems in agreement with the high-p_T world average of R_η/π⁰≈0.5 in hadron-hadron, hadron-nucleus, and nucleus-nucleus collisions for a wide range of center-of-mass energies (sqrt[s_NN]≈3–1800 GeV) as well as, for high scaled momentum x_p, in e⁺e^- annihilations at sqrt[s]=91.2 GeV. These results are consistent with a scenario where high-p_T η production in nuclear collisions at the Relativistic Heavy Ion Collider is largely unaffected by initial-state effects but where light-quark mesons (π⁰,η) are equally suppressed due to final-state interactions of the parent partons in the dense medium produced in Au+Au reactions.

Azimuthal correlations of jet-induced high-p_T charged hadron pairs are studied at midrapidity in Au+Au collisions at sqrt[s_NN]=200  GeV. The distribution of jet-associated partner hadrons (1.0<p_T<2.5  GeV/c) per trigger hadron (2.5<p_T<4.0  GeV/c) is found to vary with collision centrality, in both shape and yield, indicating a significant effect of the nuclear collision medium on the jet fragmentation process.

Inclusive transverse momentum spectra of η mesons have been measured within p_T=2–10  GeV/c at midrapidity by the PHENIX experiment in Au+Au collisions at sqrt[s_NN]=200  GeV. In central Au+Au the η yields are significantly suppressed compared to peripheral Au+Au, d+Au, and p+p yields scaled by the corresponding number of nucleon-nucleon collisions. The magnitude, centrality, and p_T dependence of the suppression is common, within errors, for η and π⁰. The ratio of η to π⁰ spectra at high p_T amounts to 0.40<R_η/π⁰<0.48 for the three systems, in agreement with the world average measured in hadronic and nuclear reactions and, at large scaled momentum, in e⁺e^- collisions.

The azimuthal distribution of identified π⁰ and inclusive photons has been measured in sqrt[s_NN]=200  GeV Au+Au collisions with the PHENIX experiment at the Relativistic Heavy-Ion Collider (RHIC). The second-harmonic parameter (v₂) was measured to describe the observed anisotropy of the azimuthal distribution. The measured inclusive photon v₂ is consistent with the value expected for the photons from hadron decay and is also consistent with the lack of direct photon signal over the measured p_T range 1–6  GeV/c. An attempt is made to extract v₂ of direct photons.

The PHENIX experiment has measured midrapidity (|η|<0.35) transverse momentum spectra (0.4<p_T<5.0  GeV/c) of electrons as a function of centrality in Au+Au collisions at sqrt[s_NN]=200  GeV. Contributions from photon conversions and from light hadron decays, mainly Dalitz decays of π⁰ and η mesons, were removed. The resulting nonphotonic electron spectra are primarily due to the semileptonic decays of hadrons carrying heavy quarks. Nuclear modification factors were determined by comparison to nonphotonic electrons in p+p collisions. A significant suppression of electrons at high p_T is observed in central Au+Au collisions, indicating substantial energy loss of heavy quarks.

The invariant differential cross section for inclusive electron production in p+p collisions at sqrt[s]=200  GeV has been measured by the PHENIX experiment at the BNL Relativistic Heavy Ion Collider over the transverse momentum range 0.4≤p_T≤5.0  GeV/c in the central rapidity region (|η|≤0.35). The contribution to the inclusive electron spectrum from semileptonic decays of hadrons carrying heavy flavor, i.e., charm quarks or, at high p_T, bottom quarks, is determined via three independent methods. The resulting electron spectrum from heavy-flavor decays is compared to recent leading and next-to-leading order perturbative QCD calculations. The total cross section of charm quark-antiquark pair production is determined to be σ_cc̅ =0.92±0.15(stat)±0.54(syst)  mb.

Transverse single-spin asymmetries to probe the transverse-spin structure of the proton have been measured for neutral pions and nonidentified charged hadrons from polarized proton-proton collisions at midrapidity and sqrt[s]=200  GeV. The data cover a transverse momentum (p_T) range 1.0–5.0  GeV/c for neutral pions and 0.5–5.0  GeV/c for charged hadrons, at a Feynman-x value of approximately zero. The asymmetries seen in this previously unexplored kinematic region are consistent with zero within errors of a few percent. In addition, the inclusive charged hadron cross section at midrapidity from 0.5<p_T<7.0  GeV/c is presented and compared to next-to-leading order perturbative QCD (pQCD) calculations. Successful description of the unpolarized cross section above ∼2  GeV/c suggests that pQCD is applicable in the interpretation of the asymmetry results in the relevant kinematic range.

The transverse momentum dependence of the azimuthal anisotropy parameter v₂, the second harmonic of the azimuthal distribution, for electrons at midrapidity (|η|<0.35) has been measured with the PHENIX detector in Au+Au collisions at sqrt[s_NN]=200 GeV. The measurement was made with respect to the reaction plane defined at high rapidities (|η|=3.1-3.9). From the result we have measured the v₂ of electrons from heavy flavor decay after subtraction of the v₂ of electrons from other sources such as photon conversions and Dalitz decay from light neutral mesons. We observe a nonzero single electron v₂ with a 90% confidence level in the intermediate-p_T region. The precision of the present data set does not permit us to conclude definitively that heavy quarks exhibit thermalization with the transverse flow of the bulk matter.

We present the results of ϕ meson production in the K⁺K^- decay channel from Au+Au collisions at sqrt[s_NN]=200  GeV as measured at midrapidity by the PHENIX detector at Brookhaven National Laboratory's Relativistic Heavy Ion Collider. Precision resonance centroid and width values are extracted as a function of collision centrality. No significant variation from the Particle Data Group accepted values is observed, contrary to some model predictions. The ϕ transverse mass spectra are fitted with a linear exponential function for which the derived inverse slope parameter is seen to be constant as a function of centrality. However, when these data are fitted by a hydrodynamic model the result is that the centrality-dependent freeze-out temperature and the expansion velocity values are consistent with the values previously derived from fitting identified charged hadron data. As a function of transverse momentum the collisions scaled peripheral-to-central yield ratio R_CP for the ϕ is comparable to that of pions rather than that of protons. This result lends support to theoretical models that distinguish between baryons and mesons instead of particle mass for explaining the anomalous (anti) proton yield.

This paper was published online on 20 May 2005 without several of the authors’ corrections incorporated. Equation (13) has been replaced. The captions of Figs. 16–18 have also been replaced. Typographical errors on pages 4, 6, 14, 15, 18, 19, 22, and 24 have all been corrected. The paper has been corrected as of 8 June 2005. The text is correct in the printed version of the journal.

New measurements are presented for charged hadron azimuthal correlations at midrapidity in Au+Au collisions at sqrt[s_NN]=62.4 and 200 GeV. They are compared to earlier measurements obtained at sqrt[s_NN]=130  GeV and in Pb+Pb collisions at sqrt[s_NN]=17.2  GeV. Sizeable anisotropies are observed with centrality and transverse momentum (p_T) dependence characteristic of elliptic flow (v₂). For a broad range of centralities, the observed magnitudes and trends of the differential anisotropy, v₂(p_T), change very little over the collision energy range sqrt[s_NN]=62–200  GeV, indicating saturation of the excitation function for v₂ at these energies. Such a saturation may be indicative of the dominance of a very soft equation of state for sqrt[s_NN]∼60–200  GeV.

The first measurement of direct photons in Au+Au collisions at sqrt[s_NN]=200  GeV is presented. The direct photon signal is extracted as a function of the Au+Au collision centrality and compared to next-to-leading order perturbative quantum chromodynamics calculations. The direct photon yield is shown to scale with the number of nucleon-nucleon collisions for all centralities.

Two particle correlations between identified meson and baryon trigger particles with 2.5<p_T<4.0  GeV/c and lower p_T charged hadrons have been measured at midrapidity by the PHENIX experiment at RHIC in p+p,d+Au, and Au+Au collisions at sqrt[s_NN]=200  GeV. In noncentral Au+Au collisions, the probability of finding a hadron near in azimuthal angle to the trigger particles is almost identical for mesons and baryons and significantly higher than in p+p collisions. The associated yields for trigger baryons decrease in the most central collisions, consistent with some baryon production by thermal recombination in addition to hard scattering.

The ratio of the proton elastic electromagnetic form factors, G_Ep/G_Mp, was obtained by measuring P_t and P_ℓ, the transverse and longitudinal recoil proton polarization components, respectively, for the elastic e→p→ep→reaction in the four-momentum transfer squared range of 0.5 to 3.5  GeV². In the single-photon exchange approximation, G_Ep/G_Mp is directly proportional to P_t/P_ℓ. The simultaneous measurement of P_t and P_ℓ in a polarimeter reduces systematic uncertainties. The results for G_Ep/G_Mp show a systematic decrease with increasing Q², indicating for the first time a definite difference in the distribution of charge and magnetization in the proton. The data have been reanalyzed and their systematic uncertainties have become significantly smaller than those reported previously.

A measurement of direct photons in p+p collisions at sqrt[s]=200  GeV is presented. A photon excess above background from π⁰→γ+γ, η→γ+γ and other decays is observed in the transverse momentum range 5.5<p_T<7  GeV/c. The result is compared to a next-to-leading-order perturbative QCD calculation. Within errors, good agreement is found between the QCD calculation and the measured result.

The production of deuterons and antideuterons in the transverse momentum range 1.1<p_T<4.3  GeV/c at midrapidity in Au+Au collisions at sqrt[s_NN]=200  GeV has been studied by the PHENIX experiment at RHIC. A coalescence analysis, comparing the deuteron and antideuteron spectra with that of proton and antiproton, has been performed. The coalescence probability is equal for both deuterons and antideuterons and it increases as a function of p_T, which is consistent with an expanding collision zone. Comparing (anti)proton yields, p̅ /p=0.73±0.01, with (anti)deuteron yields, d̅ /d=0.47±0.03, we estimate that n̅ /n=0.64±0.04. The nucleon phase space density is estimated from the coalescence measurement.

The PHENIX experiment at the relativistic heavy ion collider (RHIC) has measured transverse energy and charged particle multiplicity at midrapidity in Au+Au collisions at center-of-mass energies sqrt[s_NN]=19.6,130, and 200  GeV as a function of centrality. The presented results are compared to measurements from other RHIC experiments and experiments at lower energies. The sqrt[s_NN] dependence of dE_T/dη and dN_ch/dη per pair of participants is consistent with logarithmic scaling for the most central events. The centrality dependence of dE_T/dη and dN_ch/dη is similar at all measured incident energies. At RHIC energies, the ratio of transverse energy per charged particle was found to be independent of centrality and growing slowly with sqrt[s_NN]. A survey of comparisons between the data and available theoretical models is also presented.

The PHENIX experiment has measured midrapidity transverse momentum spectra (0.4<p_T<4.0  GeV/c) of single electrons as a function of centrality in Au+Au collisions at sqrt[s_NN]=200  GeV. Contributions from photon conversions and Dalitz decays of light neutral mesons are measured by introducing a thin (1.7% X₀) converter into the PHENIX acceptance and are statistically removed. The subtracted nonphotonic electron spectra are primarily due to the semileptonic decays of hadrons containing heavy quarks, mainly charm at lower p_T. For all centralities, the charm production cross section is found to scale with the nuclear overlap function, T_AA. For minimum-bias collisions the charm cross section per binary collision is N_cc̅ /T_AA=622±57(stat)±160(syst)   μb.

Bose-Einstein correlations of identically charged pion pairs were measured by the PHENIX experiment at midrapidity in Au+Au collisions at sqrt[s_NN]=200  GeV. The Bertsch-Pratt radius parameters were determined as a function of the transverse momentum of the pair and as a function of the centrality of the collision. Using the standard core-halo partial Coulomb fits, and a new parametrization which constrains the Coulomb fraction as determined from the unlike-sign pion correlation, the ratio R_out/R_side is within 0.8–1.1 for 0.25<〈k_T〉<1.2  GeV/c. The centrality dependence of all radii is well described by a linear scaling in N_part^1/3, and R_out/R_side for 〈k_T〉∼0.45  GeV/c is approximately constant at unity as a function of centrality.

Electroproduction of the ω meson was investigated in the ¹H(e,e^′p)ω reaction. The measurement was performed at a four-momentum transfer Q²≈0.5  GeV². Angular distributions of the virtual photon-proton center-of-momentum cross sections have been extracted over the full angular range. These distributions exhibit a strong enhancement over t-channel parity exchange processes in the backward direction. According to a newly developed electroproduction model, this enhancement provides significant evidence of resonance formation in the γ*p→ωp reaction channel.