Search Results (75 total)

We present a new analysis of J/ψ production yields in deuteron-gold collisions at sqrt[s_NN]=200 GeV using data taken from the PHENIX experiment in 2003 and previously published in S. S. Adler [Phys. Rev. Lett 96, 012304 (2006)]. The high statistics proton-proton J/ψ data taken in 2005 are used to improve the baseline measurement and thus construct updated cold nuclear matter modification factors (R_dAu). A suppression of J/ψ in cold nuclear matter is observed as one goes forward in rapidity (in the deuteron-going direction), corresponding to a region more sensitive to initial-state low-x gluons in the gold nucleus. The measured nuclear modification factors are compared to theoretical calculations of nuclear shadowing to which a J/ψ (or precursor) breakup cross section is added. Breakup cross sections of σ_breakup=2.8_-1.4^+1.7 (2.2_-1.5^+1.6) mb are obtained by fitting these calculations to the data using two different models of nuclear shadowing. These breakup cross-section values are consistent within large uncertainties with the 4.2±0.5 mb determined at lower collision energies. Projecting this range of cold nuclear matter effects to copper-copper and gold-gold collisions reveals that the current constraints are not sufficient to firmly quantify the additional hot nuclear matter effect.

We present transverse momentum (p_T) spectra of charged hadrons measured in deuteron-gold and nucleon-gold collisions at sqrt[s_NN]=200 GeV for four centrality classes. Nucleon-gold collisions were selected by tagging events in which a spectator nucleon was observed in one of two forward rapidity detectors. The spectra and yields were investigated as a function of the number of binary nucleon-nucleon collisions, ν, suffered by deuteron nucleons. A comparison of charged particle yields to those in p+p collisions show that yield per nucleon-nucleon collision saturates with ν for high momentum particles. We also present the charged hadron to neutral pion ratios as a function of p_T.

The HERMES experiment has measured the transverse polarization of Λ and Λ̅ hyperons produced inclusively in quasireal photoproduction at a positron beam energy of 27.6 GeV. The transverse polarization P_n^Λ of the Λ hyperon is found to be positive while the observed Λ̅ polarization is compatible with zero. The values averaged over the kinematic acceptance of HERMES are P_n^Λ=0.078±0.006(stat)±0.012(syst) and P_n^Λ̅ =-0.025±0.015(stat)±0.018(syst) for Λ and Λ̅ , respectively. The dependences of P_n^Λ and P_n^Λ̅ on the fraction ζ of the beam’s light-cone momentum carried by the hyperon and on the hyperon’s transverse momentum p_T were investigated. The measured Λ polarization rises linearly with p_T and exhibits a different behavior for low and high values of ζ, which approximately correspond to the backward and forward regions in the center-of-mass frame of the γ^*N reaction.

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.

A search for TeV-PeV muon neutrinos from unresolved sources was performed on AMANDA-II data collected between 2000 and 2003 with an equivalent live time of 807 days. This diffuse analysis sought to find an extraterrestrial neutrino flux from sources with nonthermal components. The signal is expected to have a harder spectrum than the atmospheric muon and neutrino backgrounds. Since no excess of events was seen in the data over the expected background, an upper limit of E²Φ_90%C.L.<7.4×10^-8  GeV cm^-2 s^-1 sr^-1 is placed on the diffuse flux of muon neutrinos with a Φ∝E^-2 spectrum in the energy range 16 TeV to 2.5 PeV. This is currently the most sensitive Φ∝E^-2 diffuse astrophysical neutrino limit. We also set upper limits for astrophysical and prompt neutrino models, all of which have spectra different from Φ∝E^-2.

The IceCube neutrino detector is a cubic kilometer TeV to PeV neutrino detector under construction at the geographic South Pole. The dominant population of neutrinos detected in IceCube is due to meson decay in cosmic-ray air showers. These atmospheric neutrinos are relatively well understood and serve as a calibration and verification tool for the new detector. In 2006, the detector was approximately 10% completed, and we report on data acquired from the detector in this configuration. We observe an atmospheric neutrino signal consistent with expectations, demonstrating that the IceCube detector is capable of identifying neutrino events. In the first 137.4 days of live time, 234 neutrino candidates were selected with an expectation of 211±76.1(syst)±14.5(stat) events from atmospheric neutrinos.

We present azimuthal angle correlations of intermediate transverse momentum (1–4  GeV/c) hadrons from dijets in Cu+Cu and Au+Au collisions at sqrt[s_NN]=62.4 and 200 GeV. The away-side dijet induced azimuthal correlation is broadened, non-Gaussian, and peaked away from Δϕ=π in central and semicentral collisions in all the systems. The broadening and peak location are found to depend upon the number of participants in the collision, but not on the collision energy or beam nuclei. These results are consistent with sound or shock wave models, but pose challenges to Cherenkov gluon radiation models.

We report the results of a five-year survey of the northern sky to search for point sources of high energy neutrinos. The search was performed on the data collected with the AMANDA-II neutrino telescope in the years 2000 to 2004, with a live time of 1001 days. The sample of selected events consists of 4282 upward going muon tracks with high reconstruction quality and an energy larger than about 100 GeV. We found no indication of point sources of neutrinos and set 90% confidence level flux upper limits for an all-sky search and also for a catalog of 32 selected sources. For the all-sky search, our average (over declination and right ascension) experimentally observed upper limit Φ⁰=(E / 1  TeV)^γ·dΦ / dE to a point source flux of muon and tau neutrino (detected as muons arising from taus) is Φ_{νμ+ν̅ μ}⁰+Φ_{ντ+ν̅ τ}⁰=11.1×  10^-11  TeV^-1 cm^-2 s^-1, in the energy range between 1.6 TeV and 2.5 PeV for a flavor ratio Φ_{νμ+ν̅ μ}⁰/Φ_{ντ+ν̅ τ}⁰=1 and assuming a spectral index γ=2. It should be noticed that this is the first time we set upper limits to the flux of muon and tau neutrinos. In previous papers we provided muon neutrino upper limits only neglecting the sensitivity to a signal from tau neutrinos, which improves the limits by 10% to 16%. The value of the average upper limit presented in this work corresponds to twice the limit on the muon neutrino flux Φ_{νμ+ν̅ μ}⁰=5.5×10^-11  TeV^-1 cm^-2 s^-1. A stacking analysis for preselected active galactic nuclei and a search based on the angular separation of the events were also performed. We report the most stringent flux upper limits to date, including the results of a detailed assessment of systematic uncertainties.

The PHENIX experiment at RHIC has measured the invariant cross section for ω-meson production at midrapidity in the transverse momentum range 2.5<p_T<9.25 GeV/c in p+p and d+Au collisions at sqrt[s_NN]=200 GeV. Measurements in two decay channels (ω→π⁰π⁺π^- and ω→π⁰γ) yield consistent results, and the reconstructed ω mass agrees with the accepted value within the p_T range of the measurements. The ω/π⁰ ratio is found to be 0.85±0.05^stat±0.09^sys in p+p and 0.94±0.08^stat±0.12^sys in d+Au collisions, independent of p_T. The nuclear modification factor R_dA^ω is 1.03±0.12^stat±0.21^sys and 0.83±0.21^stat±0.17^sys in minimum bias and central (0–20%) d+Au collisions, respectively.

The dependence of transverse momentum spectra of neutral pions and η mesons with p_T<16  GeV/c and p_T<12  GeV/c, respectively, on the centrality of the collision has been measured at midrapidity by the PHENIX experiment at the BNL Relativistic Heavy Ion Collider (RHIC) in d+Au collisions at sqrt[s_NN]=200  GeV. The measured yields are compared to those in p+p collisions at the same sqrt[s_NN] scaled by the number of underlying nucleon-nucleon collisions in d+Au. At all centralities, the yield ratios show no suppression, in contrast to the strong suppression seen for central Au+Au collisions at RHIC. Only a weak p_T and centrality dependence can be observed.

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.

Cross sections for midrapidity production of direct photons in p+p collisions at the Relativistic Heavy Ion Collider (RHIC) are reported for transverse momenta of 3<p_T<16  GeV/c. Next-to-leading order perturbative QCD (pQCD) describes the data well for p_T>5  GeV/c, where the uncertainties of the measurement and theory are comparable. We also report on the effect of requiring the photons to be isolated from parton jet energy. The observed fraction of isolated photons is well described by pQCD for p_T>7  GeV/c.

On 27 December 2004, a giant γ flare from the Soft Gamma-Ray Repeater 1806-20 saturated many satellite gamma-ray detectors, being the brightest transient event ever observed in the Galaxy. AMANDA-II was used to search for down-going muons indicative of high-energy gammas and/or neutrinos from this object. The data revealed no significant signal, so upper limits (at 90% C.L.) on the normalization constant were set: 0.05(0.5)  TeV^-1 m^-2 s^-1 for γ=-1.47 (-2) in the gamma flux and 0.4(6.1)  TeV^-1 m^-2 s^-1 for γ=-1.47 (-2) in the high-energy neutrino flux.

Measurements of the production of forward π⁰ mesons from p+p and d+Au collisions at sqrt[s_NN]=200  GeV are reported. The p+p yield generally agrees with next-to-leading order perturbative QCD calculations. The d+Au yield per binary collision is suppressed as η increases, decreasing to ∼30% of the p+p yield at ⟨η⟩=4.00, well below shadowing expectations. Exploratory measurements of azimuthal correlations of the forward π⁰ with charged hadrons at η≈0 show a recoil peak in p+p that is suppressed in d+Au at low pion energy. These observations are qualitatively consistent with a saturation picture of the low-x gluon structure of heavy nuclei.

The properties of jets produced in p+p collisions at sqrt[s]=200  GeV are measured using the method of two-particle correlations. The trigger particle is a leading particle from a large transverse momentum jet while the associated particle comes from either the same jet or the away-side jet. Analysis of the angular width of the near-side peak in the correlation function determines the jet-fragmentation transverse momentum j_T. The extracted value, sqrt[⟨j_T²⟩]=585±6(stat)±15(sys)  MeV/c, is constant with respect to the trigger particle transverse momentum, and comparable to the previous lower sqrt[s] measurements. The width of the away-side peak is shown to be a convolution of j_T with the fragmentation variable, z, and the partonic transverse momentum, k_T. The ⟨z⟩ is determined through a combined analysis of the measured π⁰ inclusive and associated spectra using jet-fragmentation functions measured in e⁺e^- collisions. The final extracted values of k_T are then determined to also be independent of the trigger particle transverse momentum, over the range measured, with value of sqrt[⟨k_T²⟩]=2.68±0.07(stat)±0.34(sys)  GeV/c.

PHENIX has measured the centrality dependence of midrapidity pion, kaon, and proton transverse momentum distributions in d+Au and p+p collisions at sqrt[s_NN]=200 GeV. The p+p data provide a reference for nuclear effects in d+Au and previously measured Au+Au collisions. Hadron production is enhanced in d+Au, relative to independent nucleon-nucleon scattering, as was observed in lower energy collisions. The nuclear modification factor for (anti)protons is larger than that for pions. The difference increases with centrality but is not sufficient to account for the abundance of baryon production observed in central Au+Au collisions at the BNL Relativistic Heavy Ion Collider (RHIC). The centrality dependence in d+Au shows that the nuclear modification factor increases gradually with the number of collisions encountered by each participant nucleon. We also present comparisons with lower energy data as well as with parton recombination and other theoretical models of nuclear effects on particle production.

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.

Deuteron-gold (d+Au) collisions at the Relativistic Heavy Ion Collider provide ideal platforms for testing QCD theories in dense nuclear matter at high energy. In particular, models suggesting strong saturation effects for partons carrying small nucleon momentum fraction (x) predict modifications to jet production at forward rapidity (deuteron-going direction) in d+Au collisions. We report on two-particle azimuthal angle correlations between charged hadrons at forward/backward (deuteron/gold going direction) rapidity and charged hadrons at midrapidity in d+Au and p+p collisions at sqrt[s_NN]=200  GeV. Jet structures observed in the correlations are quantified in terms of the conditional yield and angular width of away-side partners. The kinematic region studied here samples partons in the gold nucleus with x∼0.1 to ∼0.01. Within this range, we find no x dependence of the jet structure in d+Au collisions.

We present an improved measurement of the double helicity asymmetry for π⁰ production in polarized proton-proton scattering at sqrt[s]=200  GeV employing the PHENIX detector at the Relativistic Heavy Ion Collider (RHIC). The improvements to our previous measurement come from two main factors: Inclusion of a new data set from the 2004 RHIC run with higher beam polarizations than the earlier run and a recalibration of the beam polarization measurements for the earlier run, which resulted in reduced uncertainties and increased beam polarizations. The results are compared to a Next to Leading Order (NLO) perturbative Quantum Chromodynamics (pQCD) calculation with a range of polarized gluon distributions.

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.

Dihadron correlations at high transverse momentum p_T in d+Au collisions at sqrt[s_NN]=200 GeV at midrapidity are measured by the PHENIX experiment at the Relativistic Heavy Ion Collider. From these correlations, we extract several structural characteristics of jets: the root-mean-squared transverse momentum of fragmenting hadrons with respect to the jet sqrt[〈j_T²〉], the mean sine-squared of the azimuthal angle between the jet axes 〈sin²ϕ_jj〉, and the number of particles produced within the dijet that are associated with a high-p_T particle (dN/dx_E distributions). We observe that the fragmentation characteristics of jets in d+Au collisions are very similar to those in p+p collisions and that there is little dependence on the centrality of the d+Au collision. This is consistent with the nuclear medium having little influence on the fragmentation process. Furthermore, there is no statistically significant increase in the value of 〈sin²ϕ_jj〉 from p+p to d+Au collisions. This constrains the effect of multiple scattering that partons undergo in the cold nuclear medium before and after a hard collision.