Search Results (114 total)

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.

The first measurements of the polarized structure function σ_LT^' for the reaction ¹H(e→,e^'K⁺)Λ in the nucleon resonance region are reported. Measurements are included from threshold up to W=2.05 GeV for central values of Q² of 0.65 and 1.00 GeV², and nearly the entire kaon center-of-mass angular range. σ_LT^' is the imaginary part of the longitudinal-transverse response and is expected to be sensitive to interferences between competing intermediate s-channel resonances, as well as resonant and nonresonant processes. The results for σ_LT^' are comparable in magnitude to previously reported results from CLAS for σ_LT, the real part of the same response. An intriguing sign change in σ_LT^' is observed in the high Q² data at W≈1.9 GeV. Comparisons to several existing model predictions are shown.

We present STAR results on the elliptic flow v₂ of charged hadrons, strange and multistrange particles from sqrt[s_NN]=200 GeV Au+Au collisions at the BNL Relativistic Heavy Ion Collider (RHIC). The detailed study of the centrality dependence of v₂ over a broad transverse momentum range is presented. Comparisons of different analysis methods are made in order to estimate systematic uncertainties. To discuss the nonflow effect, we have performed the first analysis of v₂ with the Lee-Yang zero method for K_S⁰ and Λ. In the relatively low p_T region, p_T≤2 GeV/c, a scaling with m_T-m is observed for identified hadrons in each centrality bin studied. However, we do not observe v₂(p_T) scaled by the participant eccentricity to be independent of centrality. At higher p_T,2≤p_T≤6 GeV/c,v₂ scales with quark number for all hadrons studied. For the multistrange hadron Ω, which does not suffer appreciable hadronic interactions, the values of v₂ are consistent with both m_T-m scaling at low p_T and number-of-quark scaling at intermediate p_T. As a function of collision centrality, an increase of p_T-integrated v₂ scaled by the participant eccentricity has been observed, indicating a stronger collective flow in more central Au+Au collisions.

We report on the observed differences in production rates of strange and multistrange baryons in Au+Au collisions at sqrt[s_NN]=200 GeV compared to p+p interactions at the same energy. The strange baryon yields in Au+Au collisions, when scaled down by the number of participating nucleons, are enhanced relative to those measured in p+p reactions. The enhancement observed increases with the strangeness content of the baryon, and it increases for all strange baryons with collision centrality. The enhancement is qualitatively similar to that observed at the lower collision energy sqrt[s_NN]=17.3 GeV. The previous observations are for the bulk production, while at intermediate p_T,1<p_T<4 GeV/c, the strange baryons even exceed binary scaling from p+p yields.

Photoproduction reactions occur when the electromagnetic field of a relativistic heavy ion interacts with another heavy ion. The STAR Collaboration presents a measurement of ρ⁰ and direct π⁺π^- photoproduction in ultraperipheral relativistic heavy ion collisions at sqrt[s_NN]=200 GeV. We observe both exclusive photoproduction and photoproduction accompanied by mutual Coulomb excitation. We find a coherent cross section of σ(AuAu→Au^*Au^*ρ⁰)=530±19(stat.)±57(syst.) mb, in accord with theoretical calculations based on a Glauber approach, but considerably below the predictions of a color dipole model. The ρ⁰ transverse momentum spectrum (p_T²) is fit by a double exponential curve including both coherent and incoherent coupling to the target nucleus; we find σ_inc/σ_coh=0.29±0.03(stat.)±0.08(syst.). The ratio of direct π⁺π^- to ρ⁰ production is comparable to that observed in γp collisions at HERA and appears to be independent of photon energy. Finally, the measured ρ⁰ spin helicity matrix elements agree within errors with the expected s-channel helicity conservation.

The exclusive electroproduction process e→p→e^'nπ⁺ was measured in the range of the photon virtuality Q²=1.7-4.5 GeV², and the invariant mass range for the nπ⁺ system of W=1.15-1.7 GeV using the CEBAF Large Acceptance Spectrometer. For the first time, these kinematics are probed in exclusive π⁺ production from protons with nearly full coverage in the azimuthal and polar angles of the nπ⁺ center-of-mass system. The nπ⁺ channel has particular sensitivity to the isospin ½ excited nucleon states, and together with the pπ⁰ final state will serve to determine the transition form factors of a large number of resonances. The largest discrepancy between these results and present modes was seen in the σ_LT^' structure function. In this experiment, 31,295 cross section and 4,184 asymmetry data points were measured. Because of the large volume of data, only a reduced set of structure functions and Legendre polynomial moments can be presented that are obtained in model-independent fits to the differential cross sections.

We report the measurement of Λ and Λ̅ yields and inverse slope parameters in d+Au collisions at sqrt[s_NN]=200 GeV at forward and backward rapidities (y=±2.75), using data from the STAR forward time projection chambers. The contributions of different processes to baryon transport and particle production are probed exploiting the inherent asymmetry of the d+Au system. Comparisons to model calculations show that baryon transport on the deuteron side is consistent with multiple collisions of the deuteron nucleons with gold participants. On the gold side, HIJING-based models without a hadronic rescattering phase do not describe the measured particle yields, while models that include target remnants or hadronic rescattering do. The multichain model can provide a good description of the net baryon density in d+Au collisions at energies currently available at the BNL Relativistic Heavy Ion Collider, and the derived parameters of the model agree with those from nuclear collisions at lower energies.

We determine rapidity asymmetry in the production of charged pions, protons, and antiprotons for large transverse momentum (p_T) for d+Au collisions at sqrt[s_NN]=200 GeV. The rapidity asymmetry is defined as the ratio of particle yields at backward rapidity (Au beam direction) to those at forward rapidity (d beam direction). The identified hadrons are measured in the rapidity regions |y|<0.5 and 0.5<|y|<1.0 for the p_T range 2.5<p_T<10  GeV/c. We observe significant rapidity asymmetry for charged pion and proton+antiproton production in both the rapidity regions. The asymmetry is larger for 0.5<|y|<1.0 than for |y|<0.5 and is almost independent of particle type. The measurements are compared to various model predictions employing multiple scattering, energy loss, nuclear shadowing, saturation effects, and recombination and also to a phenomenological parton model. We find that asymmetries are sensitive to model parameters and show model preference. The rapidity dependence of π^-/π⁺ and p̅ /p ratios in peripheral d+Au and forward neutron-tagged events are used to study the contributions of valence quarks and gluons to particle production at high p_T.

We report the first measurement of the opening angle distribution between pairs of jets produced in high-energy collisions of transversely polarized protons. The measurement probes (Sivers) correlations between the transverse spin orientation of a proton and the transverse momentum directions of its partons. With both beams polarized, the wide pseudorapidity (-1≤η≤+2) coverage for jets permits separation of Sivers functions for the valence and sea regions. The resulting asymmetries are all consistent with zero and considerably smaller than Sivers effects observed in semi-inclusive deep inelastic scattering. We discuss theoretical attempts to reconcile the new results with the sizable transverse spin effects seen in semi-inclusive deep inelastic scattering and forward hadron production in pp collisions.

We present first measurements of the ϕ-meson elliptic flow (v₂(p_T)) and high-statistics p_T distributions for different centralities from sqrt[s_NN]=200  GeV Au+Au collisions at RHIC. In minimum bias collisions the v₂ of the ϕ meson is consistent with the trend observed for mesons. The ratio of the yields of the Ω to those of the ϕ as a function of transverse momentum is consistent with a model based on the recombination of thermal s quarks up to p_T∼4  GeV/c, but disagrees at higher momenta. The nuclear modification factor (R_CP) of ϕ follows the trend observed in the K_S⁰ mesons rather than in Λ baryons, supporting baryon-meson scaling. These data are consistent with ϕ mesons in central Au+Au collisions being created via coalescence of thermalized s quarks and the formation of a hot and dense matter with partonic collectivity at RHIC.

The system created in noncentral relativistic nucleus-nucleus collisions possesses large orbital angular momentum. Because of spin-orbit coupling, particles produced in such a system could become globally polarized along the direction of the system angular momentum. We present the results of Λ and Λ̅ hyperon global polarization measurements in Au+Au collisions at sqrt[s_NN]=62.4 and 200 GeV performed with the STAR detector at the BNL Relativistic Heavy Ion Collider (RHIC). The observed global polarization of Λ and Λ̅ hyperons in the STAR acceptance is consistent with zero within the precision of the measurements. The obtained upper limit, |P_Λ,Λ̅ |≤0.02, is compared with the theoretical values discussed recently in the literature.

Differential cross sections for the reaction γp→pπ⁰ have been measured with the CEBAF Large Acceptance Spectrometer (CLAS) and a tagged photon beam with energies from 0.675 to 2.875 GeV. The results reported here possess greater accuracy in the absolute normalization than previous measurements. They disagree with recent CB-ELSA measurements for the process at forward scattering angles. Agreement with the SAID and MAID fits is found below 1 GeV. The present set of cross sections has been incorporated into the SAID database, and exploratory fits have been extended to 3 GeV. Resonance couplings have been extracted and compared to previous determinations.

New cross sections for the reaction ep→e^'ηp are reported for total center-of-mass energy W=1.5-2.3 GeV and invariant squared momentum transfer Q²=0.13-3.3 GeV². This large kinematic range allows the extraction of new information about response functions, photocouplings, and ηN coupling strengths of baryon resonances. A sharp structure is seen at W~1.7 GeV. The shape of the differential cross section is indicative of the presence of a P-wave resonance that persists to high Q². Improved values are derived for the photocoupling amplitude for the S₁₁(1535) resonance. The new data greatly expand the Q² range covered, and an interpretation of all data with a consistent parametrization is provided.

We have searched for strangelets in a triggered sample of 61 million central (top 4%) Au+Au collisions at sqrt[s_NN]=200 GeV near beam rapidities at the STAR solenoidal tracker detector at the BNL Relativistic Heavy Ion Collider. We have sensitivity to metastable strangelets with lifetimes of order ≥0.1 ns, in contrast to limits over ten times longer in BNL Alternating Gradient Synchrotron (AGS) studies and longer still at the CERN Super Proton Synchrotron (SPS). Upper limits of a few 10^-6 to 10^-7 per central Au+Au collision are set for strangelets with mass ≳30 GeV/c².

The reaction ²H(e,e^′p)n has been studied with full kinematic coverage for photon virtuality 1.75<Q²<5.5  GeV². Comparisons of experimental data with theory indicate that for very low values of neutron recoil momentum (p_n<100  MeV/c) the neutron is primarily a spectator and the reaction can be described by the plane-wave impulse approximation. For 100<p_n<750  MeV/c, proton-neutron rescattering dominates the cross section, while Δ production followed by the NΔ→NN transition is the primary contribution at higher momenta.

We present strange particle spectra and yields measured at midrapidity in sqrt[s]=200 GeV proton-proton (p+p) collisions at the BNL Relativistic Heavy Ion Collider (RHIC). We find that the previously observed universal transverse mass (m_T≡sqrt[p_T²+m²]) scaling of hadron production in p+p collisions seems to break down at higher m_T and that there is a difference in the shape of the m_T spectrum between baryons and mesons. We observe midrapidity antibaryon to baryon ratios near unity for Λ and Ξ baryons and no dependence of the ratio on transverse momentum, indicating that our data do not yet reach the quark-jet dominated region. We show the dependence of the mean transverse momentum 〈p_T〉 on measured charged particle multiplicity and on particle mass and infer that these trends are consistent with gluon-jet dominated particle production. The data are compared with previous measurements made at the CERN Super Proton Synchrotron and Intersecting Storage Rings and in Fermilab experiments and with leading-order and next-to-leading-order string fragmentation model predictions. We infer from these comparisons that the spectral shapes and particle yields from p+p collisions at RHIC energies have large contributions from gluon jets rather than from quark jets.

The STAR collaboration at the BNL Relativistic Heavy-Ion Collider (RHIC) reports measurements of the inclusive yield of nonphotonic electrons, which arise dominantly from semileptonic decays of heavy flavor mesons, over a broad range of transverse momenta (1.2<p_T<10  GeV/c) in p+p, d+Au, and Au+Au collisions at sqrt[s_NN]=200  GeV. The nonphotonic electron yield exhibits an unexpectedly large suppression in central Au+Au collisions at high p_T, suggesting substantial heavy-quark energy loss at RHIC. The centrality and p_T dependences of the suppression provide constraints on theoretical models of suppression.

We present STAR measurements of the azimuthal anisotropy parameter v₂ for pions, kaons, protons, Λ,Λ̅ ,Ξ+Ξ̅ , and Ω+Ω̅ , along with v₄ for pions, kaons, protons, and Λ+Λ̅ at midrapidity for Au+Au collisions at sqrt[s_NN]=62.4 and 200 GeV. The v₂(p_T) values for all hadron species at 62.4 GeV are similar to those observed in 130 and 200 GeV collisions. For observed kinematic ranges, v₂ values at 62.4, 130, and 200 GeV are as little as 10–15% larger than those in Pb+Pb collisions at sqrt[s_NN]=17.3 GeV. At intermediate transverse momentum (p_T from 1.5–5 GeV/c), the 62.4 GeV v₂(p_T) and v₄(p_T) values are consistent with the quark-number scaling first observed at 200 GeV. A four-particle cumulant analysis is used to assess the nonflow contributions to pions and protons and some indications are found for a smaller nonflow contribution to protons than pions. Baryon v₂ is larger than antibaryon v₂ at 62.4 and 200 GeV, perhaps indicating either that the initial spatial net-baryon distribution is anisotropic, that the mechanism leading to transport of baryon number from beam- to midrapidity enhances v₂ or that antibaryon and baryon annihilation is larger in the in-plane direction.

We report measurements of the exclusive electroproduction of K⁺Λ and K⁺Σ⁰ final states from a proton target using the Continuous Electron Beam Accelerator Facility (CEBAF) large-acceptance spectrometer (CLAS) detector at the Thomas Jefferson National Accelerator Facility. The separated structure functions σ_T,σ_L,σ_TT, and σ_LT were extracted from the Φ- and ε-dependent differential cross sections taken with electron beam energies of 2.567, 4.056, and 4.247 GeV. This analysis represents the first σ_L/σ_T separation with the CLAS detector, and the first measurement of the kaon electroproduction structure functions away from parallel kinematics. The data span a broad range of momentum transfers from 0.5≤Q²≤2.8GeV² and invariant energy from 1.6≤W≤2.4 GeV, while spanning nearly the full center-of-mass angular range of the kaon. The separated structure functions reveal clear differences between the production dynamics for the Λ and Σ⁰ hyperons. These results provide an unprecedented data sample with which to constrain current and future models for the associated production of strangeness, which will allow for a better understanding of the underlying resonant and nonresonant contributions to hyperon production.

Spin transfer from circularly polarized real photons to recoiling hyperons has been measured for the reactions γ→+p→K++Λ→ and γ→+p→K++Σ→0. The data were obtained using the CEBAF Large Acceptance Spectrometer (CLAS) detector at the Jefferson Lab for center-of-mass energies W between 1.6 and 2.53 GeV, and for -0.85<cosθK+c.m.<+0.95. For the Λ, the polarization transfer coefficient along the photon momentum axis, Cz, was found to be near unity for a wide range of energy and kaon production angles. The associated transverse polarization coefficient Cx is smaller than Cz by a roughly constant difference of unity. Most significantly, the total Λ polarization vector, including the induced polarization P, has magnitude consistent with unity at all measured energies and production angles when the beam is fully polarized. For the Σ0 this simple phenomenology does not hold. All existing hadrodynamic models are in poor agreement with these results.

We report charged particle pair correlation analyses in the space of Δϕ (azimuth) and Δη (pseudorapidity), for central Au+Au collisions at sqrt[s_NN]=200 GeV in the STAR detector. The analysis involves unlike-sign charged pairs and like-sign charged pairs, which are transformed into charge-dependent (CD) signals and charge-independent (CI) signals. We present detailed parametrizations of the data. A model featuring dense gluonic hot spots as first proposed by Van Hove predicts that the observables under investigation would have sensitivity to such a substructure should it occur, and the model also motivates selection of transverse momenta in the range 0.8<p_t<2.0 GeV/c. Both CD and CI correlations of high statistical significance are observed, and possible interpretations are discussed.

New measurements of the spin structure functions of the proton and deuteron g₁^p(x,Q²) and g₁^d(x,Q²) in the nucleon resonance region are compared with extrapolations of target-mass-corrected next-to-leading-order (NLO) QCD fits to higher energy data. Averaged over the entire resonance region (W<2 GeV), the data and QCD fits are in good agreement in both magnitude and Q² dependence for Q²>1.7 GeV²/c². This “global” duality appears to result from cancellations among the prominent “local” resonance regions: in particular strong σ_3/2 contributions in the Δ(1232) region appear to be compensated by strong σ_1/2 contributions in the resonance region centered on 1.5 GeV. These results are encouraging for the extension of NLO QCD fits to lower W and Q² than have been used previously.

We present the scaling properties of Λ, Ξ, and Ω in midrapidity Au+Au collisions at the Brookhaven National Laboratory Relativistic Heavy Ion Collider at sqrt[s_NN]=200  GeV. The yield of multistrange baryons per participant nucleon increases from peripheral to central collisions more rapidly than that of Λ, indicating an increase of the strange-quark density of the matter produced. The strange phase-space occupancy factor γ_s approaches unity for the most central collisions. Moreover, the nuclear modification factors of p, Λ, and Ξ are consistent with each other for 2<p_T<5  GeV/c in agreement with a scenario of hadron formation from constituent quark degrees of freedom.