Search Results (136 total)

The STAR Collaboration at the Relativistic Heavy Ion Collider presents measurements of J/ψ→e⁺e^- at midrapidity and high transverse momentum (p_T>5 GeV/c) in p+p and central Cu+Cu collisions at sqrt[s_NN]=200 GeV. The inclusive J/ψ production cross section for Cu+Cu collisions is found to be consistent at high p_T with the binary collision-scaled cross section for p+p collisions. At a confidence level of 97%, this is in contrast to a suppression of J/ψ production observed at lower p_T. Azimuthal correlations of J/ψ with charged hadrons in p+p collisions provide an estimate of the contribution of B-hadron decays to J/ψ production of 13%±5%.

The results of midrapidity (0<y<0.8) neutral pion spectra over an extended transverse momentum range (1<p_T<12 GeV/c) in sqrt[s_NN]=200 GeV Au+Au collisions, measured by the STAR experiment, are presented. The neutral pions are reconstructed from photons measured either by the STAR Barrel Electro-Magnetic Calorimeter or by the Time Projection Chamber via tracking of conversion electron-positron pairs. Our measurements are compared to previously published π^± and π⁰ results. The nuclear modification factors R_CP and R_AA of π⁰ are also presented as a function of p_T. In the most central Au+Au collisions, the binary collision scaled π⁰ yield at high p_T is suppressed by a factor of about 5 compared to the expectation from the yield of p+p collisions. Such a large suppression is in agreement with previous observations for light quark mesons and is consistent with the scenario that partons suffer considerable energy loss in the dense medium formed in central nucleus-nucleus collisions at the Relativistic Heavy Ion Collider.

Forward-backward multiplicity correlation strengths have been measured with the STAR detector for Au+Au and p+p collisions at sqrt[s_NN]=200  GeV. Strong short- and long-range correlations (LRC) are seen in central Au+Au collisions. The magnitude of these correlations decrease with decreasing centrality until only short-range correlations are observed in peripheral Au+Au collisions. Both the dual parton model (DPM) and the color glass condensate (CGC) predict the existence of the long-range correlations. In the DPM, the fluctuation in the number of elementary (parton) inelastic collisions produces the LRC. In the CGC, longitudinal color flux tubes generate the LRC. The data are in qualitative agreement with the predictions of the DPM and indicate the presence of multiple parton interactions.

We report measurements of the beam spin asymmetry in deeply virtual Compton scattering (DVCS) at an electron beam energy of 4.8 GeV using the CLAS detector at the Thomas Jefferson National Accelerator Facility. The DVCS beam spin asymmetry has been measured in a wide range of kinematics, 1.0<Q²<2.8 (GeV/c)², 0.12<x_B<0.48, and 0.1<-t<0.8 (GeV/c)², using the reaction e→p→e^'pX. The number of H(e,e^'γp) and H(e,e^'π⁰p) events are separated in each (Q²,x_B,t) bin by a fit to the line shape of the H(e,e^'p)X M_x² distribution. The validity of the method was studied in detail using experimental and simulated data. It was shown that with the achieved missing mass squared resolution and the available statistics, the separation of DVCS–Bethe-Heitler and π⁰ events can reliably be done with less than 5% uncertainty. Also, the Q² and t dependences of the sinϕ moments of the asymmetry are extracted and compared with theoretical calculations.

We report K/π fluctuations from Au+Au collisions at sqrt[s_NN]=19.6, 62.4, 130, and 200 GeV using the STAR detector at the Relativistic Heavy Ion Collider. K/π fluctuations in central collisions show little dependence on incident energy and are on the same order as those from NA49 at the Super Proton Synchrotron in central Pb+Pb collisions at sqrt[s_NN]=12.3 and 17.3 GeV. We report results for the collision centrality dependence of K/π fluctuations and results for charge-separated fluctuations. We observe that the K/π fluctuations scale with the charged particle multiplicity density.

We present a systematic analysis of two-pion interferometry in Au+Au collisions at sqrt[s_NN]=62.4 GeV and Cu+Cu collisions at sqrt[s_NN]=62.4 and 200 GeV using the STAR detector at the Relativistic Heavy Ion Collider (RHIC). The multiplicity and transverse momentum dependences of the extracted correlation lengths (radii) are studied. The scaling with charged particle multiplicity of the apparent system volume at final interaction is studied for the RHIC energy domain. The multiplicity scaling of the measured correlation radii is found to be independent of colliding system and collision energy.

Semi-inclusive π⁺ electroproduction on protons has been measured with the CLAS detector at Jefferson Lab. The measurement was performed on a liquid-hydrogen target using a 5.75 GeV electron beam. The complete five-fold differential cross sections were measured over a wide kinematic range including the complete range of azimuthal angles between hadronic and leptonic planes, ϕ, enabling us to separate the ϕ-dependent terms. Our measurements of the ϕ-independent term of the cross section at low Bjorken x were found to be in fairly good agreement with pQCD calculations. Indeed, the conventional current fragmentation calculation can account for almost all of the observed cross section, even at small π⁺ momentum. The measured center-of-momentum spectra are in qualitative agreement with high-energy data, which suggests a surprising numerical similarity between the spectator diquark fragmentation in the present reaction and the antiquark fragmentation measured in e⁺e^- collisions. We have observed that the two ϕ-dependent terms of the cross section are small. Within our precision the cos⁡2ϕ term is compatible with zero, except for the low-z region, and the measured cos⁡ϕ term is much smaller in magnitude than the sum of the Cahn and Berger effects.

We present results for the measurement of ϕ meson production via its charged kaon decay channel ϕ→K⁺K^- in Au+Au collisions at sqrt[s_NN]=62.4,130, and 200 GeV, and in p+p and d+Au collisions at sqrt[s_NN]=200 GeV from the STAR experiment at the BNL Relativistic Heavy Ion Collider (RHIC). The midrapidity (|y|<0.5) ϕ meson transverse momentum (p_T) spectra in central Au+Au collisions are found to be well described by a single exponential distribution. On the other hand, the p_T spectra from p+p, d+Au, and peripheral Au+Au collisions show power-law tails at intermediate and high p_T and are described better by Levy distributions. The constant ϕ/K^- yield ratio vs beam species, collision centrality, and colliding energy is in contradiction with expectations from models having kaon coalescence as the dominant mechanism for ϕ production at RHIC. The Ω/ϕ yield ratio as a function of p_T is consistent with a model based on the recombination of thermal s quarks up to p_T~4 GeV/c, but disagrees at higher transverse momenta. The measured nuclear modification factor, R_dAu, for the ϕ meson increases above unity at intermediate p_T, similar to that for pions and protons, while R_AA is suppressed due to the energy loss effect in central Au+Au collisions. Number of constituent quark scaling of both R_cp and v₂ for the ϕ meson with respect to other hadrons in Au+Au collisions at sqrt[s_NN]=200 GeV at intermediate p_T is observed. These observations support quark coalescence as being the dominant mechanism of hadronization in the intermediate p_T region at RHIC.

We report the measurement of charged D^* mesons in inclusive jets produced in proton-proton collisions at a center-of-mass energy sqrt[s]=200  GeV with the STAR experiment at the Relativistic Heavy Ion Collider. For D^* mesons with fractional momenta 0.2<z<0.5 in inclusive jets with 11.5 GeV mean transverse energy, the production rate is found to be N(D^*++D^*-)/N(jet)=0.015±0.008(stat)±0.007(sys). This rate is consistent with perturbative QCD evaluation of gluon splitting into a pair of charm quarks and subsequent hadronization.

The neutron elastic magnetic form factor was extracted from quasielastic electron scattering on deuterium over the range Q²=1.0–4.8  GeV² with the CLAS detector at Jefferson Lab. High precision was achieved with a ratio technique and a simultaneous in situ calibration of the neutron detection efficiency. Neutrons were detected with electromagnetic calorimeters and time-of-flight scintillators at two beam energies. The dipole parametrization gives a good description of the data.

Identified charged-particle spectra of π^±, K^±, p, and p̅ at midrapidity (|y|<0.1) measured by the dE/dx method in the STAR (solenoidal tracker at the BNL Relativistic Heavy Ion Collider) time projection chamber are reported for pp and d+Au collisions at sqrt[s_NN]=200 GeV and for Au+Au collisions at 62.4, 130, and 200 GeV. Average transverse momenta, total particle production, particle yield ratios, strangeness, and baryon production rates are investigated as a function of the collision system and centrality. The transverse momentum spectra are found to be flatter for heavy particles than for light particles in all collision systems; the effect is more prominent for more central collisions. The extracted average transverse momentum of each particle species follows a trend determined by the total charged-particle multiplicity density. The Bjorken energy density estimate is at least several GeV/fm³ for a formation time less than 1 fm/c. A significantly larger net-baryon density and a stronger increase of the net-baryon density with centrality are found in Au+Au collisions at 62.4 GeV than at the two higher energies. Antibaryon production relative to total particle multiplicity is found to be constant over centrality, but increases with the collision energy. Strangeness production relative to total particle multiplicity is similar at the three measured RHIC energies. Relative strangeness production increases quickly with centrality in peripheral Au+Au collisions, to a value about 50% above the pp value, and remains rather constant in more central collisions. Bulk freeze-out properties are extracted from thermal equilibrium model and hydrodynamics-motivated blast-wave model fits to the data. Resonance decays are found to have little effect on the extracted kinetic freeze-out parameters because of the transverse momentum range of our measurements. The extracted chemical freeze-out temperature is constant, independent of collision system or centrality; its value is close to the predicted phase-transition temperature, suggesting that chemical freeze-out happens in the vicinity of hadronization and the chemical freeze-out temperature is universal despite the vastly different initial conditions in the collision systems. The extracted kinetic freeze-out temperature, while similar to the chemical freeze-out temperature in pp, d+Au, and peripheral Au+Au collisions, drops significantly with centrality in Au+Au collisions, whereas the extracted transverse radial flow velocity increases rapidly with centrality. There appears to be a prolonged period of particle elastic scatterings from chemical to kinetic freeze-out in central Au+Au collisions. The bulk properties extracted at chemical and kinetic freeze-out are observed to evolve smoothly over the measured energy range, collision systems, and collision centralities.

In ultraperipheral relativistic heavy-ion collisions, a photon from the electromagnetic field of one nucleus can fluctuate to a quark-antiquark pair and scatter from the other nucleus, emerging as a ρ⁰. The ρ⁰ production occurs in two well-separated (median impact parameters of 20 and 40 F for the cases considered here) nuclei, so the system forms a two-source interferometer. At low transverse momenta, the two amplitudes interfere destructively, suppressing ρ⁰ production. Since the ρ⁰ decays before the production amplitudes from the two sources can overlap, the two-pion system can only be described with an entangled nonlocal wave function, and is thus an example of the Einstein-Podolsky-Rosen paradox. We observe this suppression in 200 GeV per nucleon-pair gold-gold collisions. The interference is 87%±5%(stat.)±8%(syst.) of the expected level. This translates into a limit on decoherence due to wave function collapse or other factors of 23% at the 90% confidence level.

A search for exotic mesons in the π⁺π⁺π^- system photoproduced by the charge exchange reaction γp→π⁺π⁺π^-(n) was carried out by the CLAS Collaboration at Jefferson Lab. A tagged-photon beam with energies in the 4.8 to 5.4 GeV range, produced through bremsstrahlung from a 5.744 GeV electron beam, was incident on a liquid-hydrogen target. A partial wave analysis was performed on a sample of 83 000 events, the highest such statistics to date in this reaction at these energies. The main objective of this study was to look for the photoproduction of an exotic J^PC=1^-+ resonant state in the 1 to 2 GeV mass range. Our partial wave analysis shows production of the a₂(1320) and the π₂(1670) mesons, but no evidence for the a₁(1260), nor the π₁(1600) exotic state at the expected levels. An upper limit of 13.5 nb is determined for the exotic π₁(1600) cross section, less than 2% of the a₂(1320) production.

We present measurements of net charge fluctuations in Au+Au collisions at sqrt[s_NN]=19.6, 62.4, 130, and 200 GeV, Cu+Cu collisions at sqrt[s_NN]=62.4 and 200 GeV, and p+p collisions at sqrt[s]=200 GeV using the dynamical net charge fluctuations measure ν_+-,dyn. We observe that the dynamical fluctuations are nonzero at all energies and exhibit a modest dependence on beam energy. A weak system size dependence is also observed. We examine the collision centrality dependence of the net charge fluctuations and find that dynamical net charge fluctuations violate 1/N_ch scaling but display approximate 1/N_part scaling. We also study the azimuthal and rapidity dependence of the net charge correlation strength and observe strong dependence on the azimuthal angular range and pseudorapidity widths integrated to measure the correlation.

Three-particle azimuthal correlation measurements with a high transverse momentum trigger particle are reported for pp, d+Au, and Au+Au collisions at sqrt[s_NN]=200  GeV by the STAR experiment. Dijet structures are observed in pp, d+Au and peripheral Au+Au collisions. An additional structure is observed in central Au+Au data, signaling conical emission of correlated charged hadrons. The conical emission angle is found to be θ=1.37±0.02(stat)_-0.07^+0.06(syst), independent of p_⊥.

This paper reports on the most comprehensive data set obtained on differential and fully integrated cross sections for the process ep→e^'pπ⁺π^-. The data were collected with the CLAS detector at Jefferson Laboratory. Measurements were carried out in the as yet unexplored kinematic region of photon virtuality 0.2<Q²<0.6 GeV² and invariant mass of the final hadron system W from 1.3 to 1.57 GeV. For the first time, nine independent one-fold differential cross sections were determined in each bin of W and Q² covered by the measurements. A phenomenological analysis of the data allowed us to establish the most significant mechanisms contributing to the reaction. The nonresonant mechanisms account for a major part of cross sections. However, we find sensitivity to s-channel excitations of low-mass nucleon resonances, especially to the N(1440)P₁₁ and N(1520)D₁₃ states in kinematic dependencies of the one-fold differential cross sections.

We measure directed flow (v₁) for charged particles in Au+Au and Cu+Cu collisions at sqrt[s_NN]=200 and 62.4 GeV, as a function of pseudorapidity (η), transverse momentum (p_t), and collision centrality, based on data from the STAR experiment. We find that the directed flow depends on the incident energy but, contrary to all available model implementations, not on the size of the colliding system at a given centrality. We extend the validity of the limiting fragmentation concept to v₁ in different collision systems, and investigate possible explanations for the observed sign change in v₁(p_t).

We report precision measurements of the Feynman x (x_F) dependence, and first measurements of the transverse momentum (p_T) dependence, of transverse single-spin asymmetries for the production of π⁰ mesons from polarized proton collisions at sqrt[s]=200  GeV. The x_F dependence of the results is in fair agreement with perturbative QCD model calculations that identify orbital motion of quarks and gluons within the proton as the origin of the spin effects. Results for the p_T dependence at fixed x_F are not consistent with these same perturbative QCD-based calculations.

The helicity amplitudes of the electroexcitation of the Roper resonance are extracted for 1.7<Q²<4.5  GeV² from recent high precision JLab-CLAS cross section and longitudinally polarized beam asymmetry data for π⁺ electroproduction on protons at W=1.15-1.69 GeV. The analysis is made using two approaches, dispersion relations and a unitary isobar model, which give consistent Q² behavior of the helicity amplitudes for the γ^*p→N(1440)P₁₁ transition. It is found that the transverse helicity amplitude A_1/2, which is large and negative at Q²=0, becomes large and positive at Q²≃2  GeV², and then drops slowly with Q². The longitudinal helicity amplitude S_1/2, which was previously found from CLAS e→p→epπ⁰,enπ⁺ data to be large and positive at Q²=0.4,0.65  GeV², drops with Q². Available model predictions for γ^*p→N(1440)P₁₁ allow us to conclude that these results provide strong evidence in favor of N(1440)P₁₁ as a first radial excitation of the 3q ground state. The results of the present paper also confirm the conclusion of our previous analysis for Q²<1 GeV² that the presentation of N(1440)P₁₁ as a q³G hybrid state is ruled out.

We present the first measurements of the ρ(770)⁰,K^*(892),Δ(1232)⁺⁺,Σ(1385), and Λ(1520) resonances in d+Au collisions at sqrt[s_NN]=200 GeV, reconstructed via their hadronic decay channels using the STAR detector (the solenoidal tracker at the BNL Relativistic Heavy Ion Collider). The masses and widths of these resonances are studied as a function of transverse momentum p_T. We observe that the resonance spectra follow a generalized scaling law with the transverse mass m_T. The 〈p_T〉 of resonances in minimum bias collisions are compared with the 〈p_T〉 of π,K, and p̅ . The ρ⁰/π^-,K^*/K^-,Δ⁺⁺/p,Σ(1385)/Λ, and Λ(1520)/Λ ratios in d+Au collisions are compared with the measurements in minimum bias p+p interactions, where we observe that both measurements are comparable. The nuclear modification factors (R_dAu) of the ρ⁰,K^*, and Σ^* scale with the number of binary collisions (N_bin) for p_T> 1.2 GeV/c.

Electroproduction of exclusive ϕ vector mesons has been studied with the CLAS detector in the kinematic range 1.4≤Q²≤3.8 GeV²,0.0≤t^'≤3.6 GeV², and 2.0≤W≤3.0 GeV. The scaling exponent for the total cross section as 1/(Q²+M_ϕ²)ⁿ was determined to be n=2.49±0.33. The slope of the four-momentum transfer t^' distribution is b_ϕ=0.98±0.17 GeV^-2. Under the assumption of s-channel helicity conservation, we determine the ratio of longitudinal to transverse cross sections to be R=0.86±0.24. A two-gluon exchange model is able to reproduce the main features of the data.

We report new measurements of the neutron charge form factor at low momentum transfer using quasielastic electrodisintegration of the deuteron. Longitudinally polarized electrons at an energy of 850 MeV were scattered from an isotopically pure, highly polarized deuterium gas target. The scattered electrons and coincident neutrons were measured by the Bates Large Acceptance Spectrometer Toroid (BLAST) detector. The neutron form factor ratio G_Eⁿ/G_Mⁿ was extracted from the beam-target vector asymmetry A_ed^V at four-momentum transfers Q²=0.14, 0.20, 0.29, and 0.42  (GeV/c)².

The (W,Q²) dependence of the ratio of inclusive electron scattering cross sections for ¹⁵N/¹²C was determined in the kinematic ranges 0.8<W<2 GeV and 0.2<Q²<1 GeV² using 2.285 GeV electrons and the CLAS detector at Jefferson Lab. The ratios exhibit only slight resonance structure, in agreement with a simple phenomenological model and an extrapolation of deep-inelastic scattering ratios to low Q². Ratios of ⁴He/¹²C using 1.6 to 2.5 GeV electrons were measured with very high statistical precision and were used to correct for He in the N and C targets. The (W,Q²) dependence of the ⁴He/¹²C ratios is in good agreement with that of the phenomenological model and exhibit significant resonance structure centered at W=0.94,1.23, and 1.5 GeV.

We report a new STAR measurement of the longitudinal double-spin asymmetry A_LL for inclusive jet production at midrapidity in polarized p+p collisions at a center-of-mass energy of sqrt[s]=200  GeV. The data, which cover jet transverse momenta 5<p_T<30  GeV/c, are substantially more precise than previous measurements. They provide significant new constraints on the gluon spin contribution to the nucleon spin through the comparison to predictions derived from one global fit to polarized deep-inelastic scattering measurements.

We present the first spin alignment measurements for the K^*0(892) and ϕ(1020) vector mesons produced at midrapidity with transverse momenta up to 5 GeV/c at sqrt[s_NN]=200 GeV at RHIC. The diagonal spin-density matrix elements with respect to the reaction plane in Au+Au collisions are ρ₀₀=0.32±0.04 (stat) ± 0.09 (syst) for the K^*0 (0.8<p_T<5.0 GeV/c) and ρ₀₀=0.34±0.02 (stat) ± 0.03 (syst) for the ϕ (0.4<p_T<5.0 GeV/c) and are constant with transverse momentum and collision centrality. The data are consistent with the unpolarized expectation of 1/3 and thus no evidence is found for the transfer of the orbital angular momentum of the colliding system to the vector-meson spins. Spin alignments for K^*0 and ϕ in Au+Au collisions were also measured with respect to the particle's production plane. The ϕ result, ρ₀₀=0.41±0.02 (stat) ± 0.04 (syst), is consistent with that in p+p collisions, ρ₀₀=0.39±0.03 (stat) ± 0.06 (syst), also measured in this work. The measurements thus constrain the possible size of polarization phenomena in the production dynamics of vector mesons.