Search Results (108 total)

We report a first study of the depolarization behavior of spin polarized ³He in a mixture of ³He-⁴He at a temperature below the ⁴He λ point in a deuterated tetraphenyl butadiene-doped deuterated polystyrene (dTPB-dPS) coated acrylic cell. In our experiment the measured ³He relaxation time is due to the convolution of the ³He longitudinal relaxation time, T₁, and the diffusion time constant of ³He in superfluid ⁴He since depolarization takes place on the walls. We have obtained a ³He relaxation time of ∼3000 s at a temperature around 1.9 K. We have shown that it is possible to achieve values of wall depolarization probability on the order of (1–2)×10⁻⁷ for polarized ³He in the superfluid ⁴He from a dTPB-dPS coated acrylic surface.

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.

We have measured the nuclear transparency of the A(e,e^′π⁺) process in ²H, ¹²C, ²⁷Al, ⁶³Cu, and ¹⁹⁷Au targets. These measurements were performed at the Jefferson Laboratory over a four momentum transfer squared range Q²=1.1 to 4.7  (GeV/c)². The nuclear transparency was extracted as the super-ratio of (σ_A/σ_H) from data to a model of pion-electroproduction from nuclei without π-N final-state interactions. The Q² and atomic number dependence of the nuclear transparency both show deviations from traditional nuclear physics expectations and are consistent with calculations that include the quantum chromodynamical phenomenon of color transparency.

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.

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.

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.

We measured the angular dependence of the three recoil-proton polarization components in two-body photodisintegration of the deuteron at a photon energy of 2 GeV. These new data provide a benchmark for calculations based on quantum chromodynamics. Two of the five existing models have made predictions of polarization observables. Both explain the longitudinal polarization transfer satisfactorily. Transverse polarizations are not well described, but suggest isovector dominance.

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.

Cross-section values for Compton scattering on the proton were measured at 25 kinematic settings over the range s=5–11 and -t=2–7  GeV² with a statistical accuracy of a few percent. The scaling power for the s dependence of the cross section at fixed center-of-mass angle was found to be 8.0±0.2, strongly inconsistent with the prediction of perturbative QCD. The observed cross-section values are in fair agreement with the calculations using the handbag mechanism, in which the external photons couple to a single quark.

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.

We have measured the transverse asymmetry A_T^' in the quasielastic ³He→(e→,e^') process with high precision at Q² values from 0.1 to 0.6  (GeV/c)². The neutron magnetic form factor G_Mⁿ was extracted at Q² values of 0.1 and 0.2  (GeV/c)² using a nonrelativistic Faddeev calculation which includes both final-state interactions (FSI) and meson-exchange currents (MEC). Theoretical uncertainties due to the FSI and MEC effects were constrained with a precision measurement of the spin-dependent asymmetry in the threshold region of ³He→(e→,e^'). We also extracted the neutron magnetic form factor G_Mⁿ at Q² values of 0.3 to 0.6  (GeV/c)² based on plane wave impulse approximation calculations.

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.

We report the first precision measurement of the proton electric to magnetic form factor ratio from spin-dependent elastic scattering of longitudinally polarized electrons from a polarized hydrogen internal gas target. The measurement was performed at the MIT-Bates South Hall Ring over a range of four-momentum transfer squared Q² from 0.15 to 0.65  (GeV/c)². Significantly improved results on the proton electric and magnetic form factors are obtained in combination with existing cross-section data on elastic electron-proton scattering in the same Q² region.

A large data set of charged-pion (π^±) electroproduction from both hydrogen and deuterium targets has been obtained spanning the low-energy residual-mass region. These data conclusively show the onset of the quark-hadron duality phenomenon, as predicted for high-energy hadron electroproduction. We construct several ratios from these data to exhibit the relation of this phenomenon to the high-energy factorization ansatz of electron-quark scattering and subsequent quark→pion production mechanisms.

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.

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.

We report the performance of a laser-driven polarized internal hydrogen gas target (LDT) in a configuration similar to that used in scattering experiments. This target used the technique of spin-exchange optical pumping to produce nuclear spin polarized hydrogen gas that was fed into a cylindrical storage (target) cell. We present in this paper the performance of the target, methods that were tried to improve the figure-of-merit (FOM) of the target, and a Monte Carlo simulation of spin-exchange optical pumping. The dimensions of the apparatus were optimized using the simulation and the experimental results were in good agreement with the results from the simulation. The best experimental result achieved was at a hydrogen flow rate of 1.1×10¹⁸  atoms∕s, where the sample beam exiting the storage cell had 58.2% degree of dissociation and 50.5% polarization. Based on this measurement, the atomic fraction in the storage cell was 49.6% and the density averaged nuclear polarization was 25.0%. This represents the highest FOM for hydrogen from an LDT and is higher than the best FOM reported by atomic beam sources that used storage cells.

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.