Search Results (127 total)

Measurements of the azimuthal anisotropy of high-p_T neutral pion (π⁰) production in Au+Au collisions at sqrt[s_NN]=200 GeV by the PHENIX experiment are presented. The data included in this article were collected during the 2004 Relativistic Heavy Ion Collider running period and represent approximately an order of magnitude increase in the number of analyzed events relative to previously published results. Azimuthal angle distributions of π⁰ mesons detected in the PHENIX electromagnetic calorimeters are measured relative to the reaction plane determined event-by-event using the forward and backward beam-beam counters. Amplitudes of the second Fourier component (v₂) of the angular distributions are presented as a function of π⁰ transverse momentum (p_T) for different bins in collision centrality. Measured reaction plane dependent π⁰ yields are used to determine the azimuthal dependence of the π⁰ suppression as a function of p_T, R_AA(Δϕ,p_T). A jet-quenching motivated geometric analysis is presented that attempts to simultaneously describe the centrality dependence and reaction plane angle dependence of the π⁰ suppression in terms of the path lengths of hypothetical parent partons in the medium. This set of results allows for a detailed examination of the influence of geometry in the collision region and of the interplay between collective flow and jet-quenching effects along the azimuthal axis.

Bose-Einstein correlations of charged kaons are used to probe Au+Au collisions at sqrt[s_NN]=200  GeV and are compared to charged pion probes, which have a larger hadronic scattering cross section. Three-dimensional Gaussian source radii are extracted, along with a one-dimensional kaon emission source function. The centrality dependences of the three Gaussian radii are well described by a single linear function of N_part^1/3 with a zero intercept. Imaging analysis shows a deviation from a Gaussian tail at r≳10  fm, although the bulk emission at lower radius is well described by a Gaussian. The presence of a non-Gaussian tail in the kaon source reaffirms that the particle emission region in a heavy-ion collision is extended, and that similar measurements with pions are not solely due to the decay of long-lived resonances.

We present inclusive charged hadron elliptic flow (v₂) measured over the pseudorapidity range |η|<0.35 in Au+Au collisions at sqrt[s_NN]=200 GeV. Results for v₂ are presented over a broad range of transverse momentum (p_T=0.2-8.0 GeV/c) and centrality (0–60%). To study nonflow effects that are correlations other than collective flow, as well as the fluctuations of v₂, we compare two different analysis methods: (1) the event-plane method from two independent subdetectors at forward (|η|=3.1-3.9) and beam (|η|>6.5) pseudorapidities and (2) the two-particle cumulant method extracted using correlations between particles detected at midrapidity. The two event-plane results are consistent within systematic uncertainties over the measured p_T and in centrality 0–40%. There is at most a 20% difference in the v₂ between the two event-plane methods in peripheral (40–60%) collisions. The comparisons between the two-particle cumulant results and the standard event-plane measurements are discussed.

We report the observation at the Relativistic Heavy Ion Collider of suppression of back-to-back correlations in the direct photon+jet channel in Au+Au relative to p+p collisions. Two-particle correlations of direct photon triggers with associated hadrons are obtained by statistical subtraction of the decay photon-hadron (γ-h) background. The initial momentum of the away-side parton is tightly constrained, because the parton-photon pair exactly balance in momentum at leading order in perturbative quantum chromodynamics, making such correlations a powerful probe of the in-medium parton energy loss. The away-side nuclear suppression factor, I_AA, in central Au+Au collisions, is 0.32±0.12^stat±0.09^syst for hadrons of 3<p_T^h<5 in coincidence with photons of 5<p_T^γ<15 GeV/c. The suppression is comparable to that observed for high-p_T single hadrons and dihadrons. The direct photon associated yields in p+p collisions scale approximately with the momentum balance, z_T≡p_T^h/p_T^γ, as expected for a measurement of the away-side parton fragmentation function. We compare to Au+Au collisions for which the momentum balance dependence of the nuclear modification should be sensitive to the path-length dependence of parton energy loss.

The momentum distribution of electrons from semileptonic decays of charm and bottom quarks for midrapidity |y|<0.35 in p+p collisions at sqrt[s]=200  GeV is measured by the PHENIX experiment at the Relativistic Heavy Ion Collider over the transverse momentum range 2<p_T<7  GeV/c. The ratio of the yield of electrons from bottom to that from charm is presented. The ratio is determined using partial D/D̅ →e^±K^∓X (K unidentified) reconstruction. It is found that the yield of electrons from bottom becomes significant above 4  GeV/c in p_T. A fixed-order-plus-next-to-leading-log perturbative quantum chromodynamics calculation agrees with the data within the theoretical and experimental uncertainties. The extracted total bottom production cross section at this energy is σ_bb̅ =3.2_-1.1^+1.2(stat)_-1.3^+1.4(syst)μb.

The double helicity asymmetry in neutral pion production for p_T=1 to 12  GeV/c was measured with the PHENIX experiment to access the gluon-spin contribution, ΔG, to the proton spin. Measured asymmetries are consistent with zero, and at a theory scale of μ²=4  GeV² a next to leading order QCD analysis gives ΔG^[0.02,0.3]=0.2, with a constraint of -0.7<ΔG^[0.02,0.3]<0.5 at Δχ²=9 (∼3σ) for the sampled gluon momentum fraction (x) range, 0.02 to 0.3. The results are obtained using predictions for the measured asymmetries generated from four representative fits to polarized deep inelastic scattering data. We also consider the dependence of the ΔG constraint on the choice of the theoretical scale, a dominant uncertainty in these predictions.

The PHENIX experiment presents results from the RHIC 2006 run with polarized p+p collisions at sqrt[s]=62.4  GeV, for inclusive π⁰ production at midrapidity. Unpolarized cross section results are measured for transverse momenta p_T=0.5 to 7  GeV/c. Next-to-leading order perturbative quantum chromodynamics calculations are compared with the data, and while the calculations are consistent with the measurements, next-to-leading logarithmic corrections improve the agreement. Double helicity asymmetries A_LL are presented for p_T=1 to 4  GeV/c and probe the higher range of Bjorken x of the gluon (x_g) with better statistical precision than our previous measurements at sqrt[s]=200  GeV. These measurements are sensitive to the gluon polarization in the proton for 0.06<x_g<0.4.

For Au+Au collisions at 200 GeV, we measure neutral pion production with good statistics for transverse momentum, p_T, up to 20  GeV/c. A fivefold suppression is found, which is essentially constant for 5<p_T<20  GeV/c. Experimental uncertainties are small enough to constrain any model-dependent parametrization for the transport coefficient of the medium, e.g., ⟨q-^ ⟩ in the parton quenching model. The spectral shape is similar for all collision classes, and the suppression does not saturate in Au+Au collisions.

Neutral pion transverse momentum (p_T) spectra at midrapidity (|y|≲0.35) were measured in Cu+Cu collisions at sqrt[s_NN]=22.4, 62.4, and 200 GeV. Relative to π⁰ yields in p+p collisions scaled by the number of inelastic nucleon-nucleon collisions (N_coll) the π⁰ yields for p_T≳2  GeV/c in central Cu+Cu collisions are suppressed at 62.4 and 200 GeV whereas an enhancement is observed at 22.4 GeV. A comparison with a jet-quenching model suggests that final state parton energy loss dominates in central Cu+Cu collisions at 62.4 and 200 GeV, while the enhancement at 22.4 GeV is consistent with nuclear modifications in the initial state alone.

A comprehensive survey of event-by-event fluctuations of charged hadron multiplicity in relativistic heavy ions is presented. The survey covers Au+Au collisions at sqrt[s_NN]=62.4 and 200 GeV, and Cu+Cu collisions at sqrt[s_NN]=22.5,62.4, and 200 GeV. Fluctuations are measured as a function of collision centrality, transverse momentum range, and charge sign. After correcting for nondynamical fluctuations due to fluctuations in the collision geometry within a centrality bin, the remaining dynamical fluctuations expressed as the variance normalized by the mean tend to decrease with increasing centrality. The dynamical fluctuations are consistent with or below the expectation from a superposition of participant nucleon-nucleon collisions based upon p+p data, indicating that this dataset does not exhibit evidence of critical behavior in terms of the compressibility of the system. A comparison of the data with a model where hadrons are independently emitted from a number of hadron clusters suggests that the mean number of hadrons per cluster is small in heavy ion collisions.

Yields for J/ψ production in Cu+Cu collisions at sqrt[s_NN]=200  GeV have been measured over the rapidity range |y|<2.2 and compared with results in p+p and Au+Au collisions at the same energy. The Cu+Cu data offer greatly improved precision over existing Au+Au data for J/ψ production in collisions with small to intermediate numbers of participants, in the range where the quark-gluon plasma transition threshold is predicted to lie. Cold nuclear matter estimates based on ad hoc fits to d+Au data describe the Cu+Cu data up to N_part∼50, corresponding to a Bjorken energy density of at least 1.5  GeV/fm³.

One of the most surprising results is to find that a consistent description of all the experimental results on particle multiplicities and ratios obtained from the lowest Alternating Gradient Synchrotron to the highest Relativistic Heavy Ion Collider energies is possible within the framework of a thermal statistical model. We explore here the utility of a thermodynamically consistent excluded volume model recently proposed by us in explaining the above experimental results and we further compare our results with those obtained from an ideal gas model and other excluded-volume model that are often used in describing a grand canonical statistical system consisting of hot, dense hadron gas. We find that the energy dependence of the total multiplicities of strange and nonstrange hadrons in general shows close agreement with the experimental data, although slight deviation is observed for some multistrange hadrons, e.g., Ω+Ω̅ ,Ξ, and Φ. The difference observed in these cases does not clearly support our assumption of the same freeze-out volume of the fireball that homogeneously emit all kinds of particles. Similarly we have calculated the ratios for particles and antiparticles such as K^-/K⁺,  p̅ /p,  Λ̅ /Λ,  Ξ̅ /Ξ, and Ω̅ /Ω, as well as the ratios of the unequal mass particles 〈K⁺〉/〈π⁺〉,〈K^-〉/〈π^-〉, 〈Λ〉/〈π〉,  〈Ξ^-〉/〈π〉,  〈Ω+Ω̅ 〉/〈π〉, and 〈Φ〉/〈π〉 and studied their variations with respect to the center-of-mass energy in the excluded-volume models and, finally, the results are compared with the experimental data. We find that in some cases, although the calculated results show close agreements with the experimental data, the deviations between theory and experiment in cases of unequal mass and multistrange particle ratios, like 〈Λ〉/〈π〉,  〈Ξ^-〉/〈π〉,  〈Ω+Ω̅ 〉/〈π〉,〈Φ〉/〈π〉, etc., appear to be quite large and thus warrant further investigations on the suitability of thermal hadron gas models.

Measurements in Au+Au collisions at sqrt[s_NN]=200  GeV of jet correlations for a trigger hadron at intermediate transverse momentum (p_T,trig) with associated mesons or baryons at lower p_T,assoc indicate strong modification of the away-side jet. The ratio of jet-associated baryons to mesons increases with centrality and p_T,assoc. For the most central collisions, the ratio is similar to that for inclusive measurements. This trend is incompatible with in-vacuum fragmentation but could be due to jetlike contributions from correlated soft partons, which recombine upon hadronization.

Azimuthal angle (Δϕ) correlations are presented for a broad range of transverse momentum (0.4<p_T<10 GeV/c) and centrality (0–92%) selections for charged hadrons from dijets in Au+Au collisions at sqrt[s_NN]=200 GeV. With increasing p_T, the away-side Δϕ distribution evolves from a broad and relatively flat shape to a concave shape, then to a convex shape. Comparisons with p+p data suggest that the away-side distribution can be divided into a partially suppressed “head” region centered at Δϕ~π, and an enhanced “shoulder” region centered at Δϕ~π±1.1. The p_T spectrum for the associated hadrons in the head region softens toward central collisions. The spectral slope for the shoulder region is independent of centrality and trigger p_T. The properties of the near-side distributions are also modified relative to those in p+p collisions, reflected by the broadening of the jet shape in Δϕ and Δη, and an enhancement of the per-trigger yield. However, these modifications seem to be limited to p_T≲4 GeV/c, above which both the hadron pair shape and per-trigger yield become similar to p+p collisions. These observations suggest that both the away- and near-side distributions contain a jet fragmentation component which dominates for p_T≳5 GeV/c and a medium-induced component which is important for p_T≲4 GeV/c. We also quantify the role of jets at intermediate and low p_T through the yield of jet-induced pairs in comparison with binary scaled p+p pair yield. The yield of jet-induced pairs is suppressed at high pair proxy energy (sum of the p_T magnitudes of the two hadrons) and is enhanced at low pair proxy energy. The former is consistent with jet quenching; the latter is consistent with the enhancement of soft hadron pairs due to transport of lost energy to lower p_T.

The PHENIX experiment has measured the suppression of semi-inclusive single high-transverse-momentum π⁰'s in Au+Au collisions at sqrt[s_NN]=200 GeV. The present understanding of this suppression is in terms of energy loss of the parent (fragmenting) parton in a dense color-charge medium. We have performed a quantitative comparison between various parton energy-loss models and our experimental data. The statistical point-to-point uncorrelated as well as correlated systematic uncertainties are taken into account in the comparison. We detail this methodology and the resulting constraint on the model parameters, such as the initial color-charge density dN^g/dy, the medium transport coefficient 〈q-^ 〉, or the initial energy-loss parameter ε₀. We find that high-transverse-momentum π⁰ suppression in Au+Au collisions has sufficient precision to constrain these model-dependent parameters at the ±20–25% (one standard deviation) level. These constraints include only the experimental uncertainties, and further studies are needed to compute the corresponding theoretical uncertainties.

A three-dimensional correlation function obtained from midrapidity, low p_T, pion pairs in central Au+Au collisions at sqrt[s_NN]=200  GeV is studied. The extracted model-independent source function indicates a long range tail in the directions of the pion pair transverse momentum (out) and the beam (long). A proper breakup time τ₀∼9  fm/c and a mean proper emission duration Δτ∼2  fm/c, leading to sizable emission time differences (⟨|Δt_LCM|⟩≈12  fm/c), are required to allow models to be successfully matched to these tails. The model comparisons also suggest an outside-in “burning” of the emission source reminiscent of many hydrodynamical models.

In this paper, we explore the use of isomorphic problem pairs (IPPs) to assess introductory physics students’ ability to solve and successfully transfer problem-solving knowledge from one context to another in mechanics. We call the paired problems “isomorphic” because they require the same physics principle to solve them. We analyze written responses and individual discussions for a range of isomorphic problems. We examine potential factors that may help or hinder transfer of problem-solving skills from one problem in a pair to the other. For some paired isomorphic problems, one context often turned out to be easier for students in that it was more often correctly solved than the other. When quantitative and conceptual questions were paired and given back to back, students who answered both questions in the IPP often performed better on the conceptual questions than those who answered the corresponding conceptual questions only. Although students often took advantage of the quantitative counterpart to answer a conceptual question of an IPP correctly, when only given the conceptual question, students seldom tried to convert it into a quantitative question, solve it, and then reason about the solution conceptually. Even in individual interviews when students who were given only conceptual questions had difficulty and the interviewer explicitly encouraged them to convert the conceptual question into the corresponding quantitative problem by choosing appropriate variables, a majority of students were reluctant and preferred to guess the answer to the conceptual question based upon their gut feeling. Misconceptions associated with friction in some problems were so robust that pairing them with isomorphic problems not involving friction did not help students discern their underlying similarities. Alternatively, from the knowledge-in-pieces perspective, the activation of the knowledge resource related to friction was so strongly and automatically triggered by the context, which is outside the conscious control of the student, that students did not look for analogies with paired problems or other aids that may be present.

Investigations related to expertise in problem solving and ability to transfer learning from one context to another are important for developing strategies to help students perform more expertlike tasks. Here we analyze written responses to a pair of nonintuitive isomorphic problems given to introductory physics students and discussions with a subset of students about them. Students were asked to explain their reasoning for their written responses. We call the paired problems isomorphic because they require the same physics principle to solve them. However, the initial conditions are different, and the frictional force is responsible for increasing the linear speed of an object in one of the problems while it is responsible for decreasing the linear speed in the other problem. We categorize student responses and evaluate student performance within the context of their evolving expertise. We compare and contrast the patterns of student categorization for the two isomorphic problems. We discuss why certain incorrect responses were better than others and shed light on the evolution of students’ expertise. We compare the performance of students who worked on both isomorphic problems with those who worked only on one of the problems to understand whether students recognized their underlying similarity and whether isomorphic pairs gave students additional insight into solving each problem.

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.

Azimuthal angle (Δϕ) correlations are presented for charged hadrons from dijets for 0.4<p_T<10 GeV/c in Au+Au collisions at sqrt[s_NN]=200 GeV. With increasing p_T, the away-side distribution evolves from a broad and relatively flat shape to a concave shape, then to a convex shape. Comparisons to p+p data suggest that the away-side can be divided into a partially suppressed “head” region centered at Δϕ~π and an enhanced “shoulder” region centered at Δϕ~π±1.1. The p_T spectrum for the head region softens toward central collisions, consistent with the onset of jet quenching. The spectral slope for the shoulder region is independent of centrality and trigger p_T, which offers constraints on energy transport mechanisms and suggests that it contains the medium response to energetic jets.

We perform atomistic and mesoscale simulations to explain the origin of experimentally observed stripelike patterns formed by immiscible ligands coadsorbed on the surfaces of gold and silver nanoparticles. We show that when the conformational entropy gained via this morphology is sufficient, microphase-separated stripelike patterns form. When the entropic gain is not sufficient, we instead predict bulk phase-separated Janus particles. We also show corroborating experimental results that confirm our simulational predictions that stripes form on flat surfaces as well as on curved nanoparticle surfaces.

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.

The PHENIX experiment presents results from the Relativistic Heavy Ion Collider 2005 run with polarized proton collisions at sqrt[s]=200  GeV, for inclusive π⁰ production at midrapidity. Unpolarized cross section results are given for transverse momenta p_T=0.5 to 20  GeV/c, extending the range of published data to both lower and higher p_T. The cross section is described well for p_T<1  GeV/c by an exponential in p_T, and, for p_T>2  GeV/c, by perturbative QCD. Double helicity asymmetries A_LL are presented based on a factor of 5 improvement in uncertainties as compared to previously published results, due to both an improved beam polarization of 50%, and to higher integrated luminosity. These measurements are sensitive to the gluon polarization in the proton. Using one representative model of gluon polarization it is demonstrated that the gluon spin contribution to the proton spin is significantly constrained.

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.