Search

A sample of 762 decays $K_L\to \pi +e+\nu$ in the 80-inch hydrogen bubble chamber have been identified using electron detection by shower production in a tantalum plate. The results are consistent with vector interaction and constant form factor.

We report a direct measurement of the final-state energy spectrum in the electromagnetic dissociation of ${}_{}{}^{28}{}_{}{}^{}\mathrm{Si}$ into p+ ${}_{}{}^{27}{}_{}{}^{}\mathrm{Al}$ at an energy of 14.6 GeV/nucleon. The final-state energy is obtained through a calculation of the p${\mathrm{-}}^{27}$Al invariant mass in kinematically reconstructed events. The final-state energy spectrum for all targets is peaked near the isovector giant-dipole resonance in ${}_{}{}^{28}{}_{}{}^{}\mathrm{Si}$ and the dependence of the magnitude of the cross section on target charge confirms that the excitation is largely electromagnetic. By exploiting the expected scaling behavior on target Z and A, the background from nuclear interactions is evaluated and subtracted, leaving a pure electromagnetic dissociation final-state energy distribution. This distribution is well reproduced by simulated events, in which the photon spectrum calculated in the Weiszäcker-Williams approximation is combined with experimental data on the photonuclear reaction ${}_{}{}^{28}{}_{}{}^{}\mathrm{Si}$(γ,p) ${}_{}{}^{27}{}_{}{}^{}\mathrm{Al}$, and slight differences are observed only at low final-state energy.

We present results from a measurement of the differential cross sections for ${\Sigma }^{-}p$, ${\Xi }^{-}p$, and ${\pi }^{-}p$ elastic scattering at 23 GeV/c. We have collected samples of 6200 ${\Sigma }^{-}p$ events, 67 ${\Xi }^{-}p$ events, and 30 000 ${\pi }^{-}p$ events in the interval $0.10<\mathrm{}\left|t\right|<0.23\mathrm{}$ ${(\mathrm{G}\mathrm{e}\mathrm{V}/\mathit{c})}^2$.

The reactions of 24.6-GeV/c ${\Sigma }^{-}$ and ${\Xi }^{-}$ incident on a scintillator target were studied in a counter-spark chamber experiment. Upper limits on the production cross sections of ${\Sigma }^{*-}$ resonances which decay into ${\Lambda }^0{\pi }^{-}$ and ${\Xi }^{*-}$ resonances which decay into ${\Lambda }^0{K}^{-}$ were obtained. A single ${\Omega }^{-}$ that was produced in the scintillator target has been observed, corresponding to a 115-μb/nucleon production cross section if produced by a ${\Xi }^{-}$. The ${\Lambda }^0{\pi }^{-}$ mass spectrum was found to be consistent with a Deck-type threshold enhancement.

Multiple-scattering distributions were measured at Fermilab energies with various targets from hydrogen to lead. Results are presented and compared with predictions of Moliere, Mayes et al., and Highland.

Collisions of ${}_{}{}^{28}{}_{}{}^{}\mathrm{Si}$+Al, Cu, and Pb at ${\mathit{E}}_{\mathrm{lab}}$=14.6 GeV/nucleon were studied at the Brookhaven National Laboratory Alternating Gradient Synchrotron. Charged particle multiplicity was measured over the pseudorapidity interval 0.875≤η≤3.86 with a silicon pad detector. A strong correlation is seen between the multiplicity and the transverse energy measured in the interval -0.5≤η≤0.8. Correlation with the energy going forward after the collision and comparison with calculations indicate that rescattering is required to explain the data. The data are compared under the assumption of Koba-Nielson-Olesen scaling. The measured multiplicity scales approximately with the total number of participant nucleons and less well with the available center-of-mass kinetic energy.

We present results of a search for strange quark matter (strangelets) in $11.5A\mathrm{GeV}/c\mathrm{Au}+\mathrm{Pb}$ collisions from the 1994 and 1995 runs of experiment E864 at Brookhaven's Alternating Gradient Synchrotron. We observe no strangelet candidates and set a 90% confidence level upper limit of approximately $3\times {10}^{-8\mathrm{}}$ per 10% central interaction for the production of $|Z|=1\mathrm{}$ and $|Z|=2\mathrm{}$ strangelets over a large mass range and with metastable lifetimes of about 50 ns or more. These results place constraints primarily on quark-gluon plasma based production models for strangelets.

We report first results from an analysis based on a new multi-hadron correlation technique, exploring jet-medium interactions and di-jet surface emission bias at the BNL Relativistic Heavy Ion Collider (RHIC). Pairs of back-to-back high-transverse-momentum hadrons are used for triggers to study associated hadron distributions. In contrast with two- and three-particle correlations with a single trigger with similar kinematic selections, the associated hadron distribution of both trigger sides reveals no modification in either relative pseudorapidity $\Delta \eta$ or relative azimuthal angle $\Delta \varphi$ from $\text{d}+\text{Au}$ to central $\text{Au}+\text{Au}$ collisions. We determine associated hadron yields and spectra as well as production rates for such correlated back-to-back triggers to gain additional insights on medium properties.

We discuss the problem of observing spontaneous parity and $\mathrm{CP}$ violation in collision systems. We discuss and propose observables which may be used in heavy-ion collisions to observe such violations, as well as event-by-event methods to analyze the data. Finally, we discuss simple Monte Carlo models of these $\mathrm{CP}$-violating effects which we have used to develop our techniques and from which we derive rough estimates of sensitivities to signals that may be seen at the relativistic heavy-ion collider.

We report STAR results on the azimuthal anisotropy parameter $v_2$ for strange particles $K_S^0$, $\Lambda$, and $\overline{\Lambda }$ at midrapidity in $\mathrm{A}\mathrm{u}+\mathrm{A}\mathrm{u}$ collisions at $\sqrt{s_{NN}}=130\mathrm{G}\mathrm{e}\mathrm{V}$ at the Relativistic Heavy Ion Collider. The value of $v_2$ as a function of transverse momentum, $p_t$, of the produced particle and collision centrality is presented for both particles up to $p_t\sim 3.0\mathrm{G}\mathrm{e}\mathrm{V}/c$. A strong $p_t$ dependence in $v_2$ is observed up to $2.0\mathrm{G}\mathrm{e}\mathrm{V}/c$. The $v_2$ measurement is compared with hydrodynamic model calculations. The physics implications of the $p_t$ integrated $v_2$ magnitude as a function of particle mass are also discussed.

We present measurements of the near side of triggered di-hadron correlations using neutral strange baryons $(\mathrm{\Lambda },\overline{\mathrm{\Lambda }})$ and mesons $\left(K_S^0\right)$ at intermediate transverse momentum $(3 < p_T <6 \mathrm{GeV}/c)$ to look for possible flavor and baryon-meson dependence. This study is performed in $d+\mathrm{Au}$, Cu+Cu, and Au+Au collisions at $\sqrt{s_{NN}}=200$ GeV measured by the STAR experiment at RHIC. The near-side di-hadron correlation contains two structures, a peak which is narrow in azimuth and pseudorapidity consistent with correlations from jet fragmentation, and a correlation in azimuth which is broad in pseudorapidity. The particle composition of the jet-like correlation is determined using identified associated particles. The dependence of the conditional yield of the jet-like correlation on the trigger particle momentum, associated particle momentum, and centrality for correlations with unidentified trigger particles are presented. The neutral strange particle composition in jet-like correlations with unidentified charged particle triggers is not well described by PYTHIA. However, the yield of unidentified particles in jet-like correlations with neutral strange particle triggers is described reasonably well by the same model.

Two-particle correlation measurements projected onto two-dimensional, transverse rapidity coordinates $(y_{T1},y_{T2})$ provide an independent, orthogonal view of the multidimensional correlation distribution that is most often studied via angular projections. As such, these independent transverse projections allow access to manifestations of dynamical fluctuations in relativistic heavy-ion collisions that angular-correlation measurements may not be sensitive to. We report nonidentified charged-particle correlations for $\text{Au}+\text{Au}$ minimum-bias collisions at $\sqrt{s_{\mathrm{NN}}}=200$ GeV taken by the STAR experiment at the Relativistic Heavy-Ion Collider (RHIC). Correlations are presented as two-dimensional functions of transverse rapidity for like-sign, unlike-sign, and all charged-particle pairs, as well as for particle pairs whose relative azimuthal angles lie on the near-side, the away-side, or at all relative azimuth. The correlations are constructed using charged particles with transverse momentum $p_T\ge 0.15$ $\mathrm{GeV}/c$, pseudorapidity from $-1$ to 1, and azimuthal angles from $-\pi$ to $\pi$. The significant correlation structures that are observed evolve smoothly with collision centrality. The major correlation features include a saddle shape plus a broad peak with maximum near $y_T\approx 3$, corresponding to $p_T\approx 1.5$ $\mathrm{GeV}/c$. The broad peak is observed in both like- and unlike-sign charge combinations and in near- and away-side relative azimuthal angles. The all-charge, all-azimuth correlation measurements are compared with the predictions of hijing and epos to provide theoretical context for these new measurements. The results indicate that the correlations for peripheral to mid-central collisions can be approximately described as a superposition of nucleon $+$ nucleon collisions with minimal effects from the quantum chromodynamics medium. Strong medium effects are indicated in mid- to most-central collisions.

We report a new measurement of the midrapidity inclusive jet longitudinal double-spin asymmetry, $A_{LL}$, in polarized $pp$ collisions at center-of-mass energy $\sqrt{s}=200\mathrm{GeV}$. The STAR data place stringent constraints on polarized parton distribution functions extracted at next-to-leading order from global analyses of inclusive deep-inelastic scattering (DIS), semi-inclusive DIS, and RHIC $pp$ data. The measured asymmetries provide evidence at the $3\sigma$ level for positive gluon polarization in the Bjorken-$x$ region $x>0.05$.

We present measurements of the differential cross sections of inclusive $J/\psi$ meson production as a function of transverse momentum ($p_T^{J/\psi }$) using the ${\mu }^{+}{\mu }^{-}$ and $e^{+}{e}^{-}$ decay channels in $\mathrm{proton}+\mathrm{proton}$ collisions at center-of-mass energies of 510 and 500 GeV, respectively, recorded by the STAR detector at the Relativistic Heavy Ion Collider. The measurement from the ${\mu }^{+}{\mu }^{-}$ channel is for $0<p_T^{J/\psi }<9\mathrm{GeV}/c$ and rapidity range $|y^{J/\psi }|<0.4$, and that from the $e^{+}{e}^{-}$ channel is for $4<p_T^{J/\psi }<20\mathrm{GeV}/c$ and $|y^{J/\psi }|<1.0$. The $\psi (2S)$ to $J/\psi$ ratio is also measured for $4<p_T^{\text{meson}}<12\mathrm{GeV}/c$ through the $e^{+}{e}^{-}$ decay channel. Model calculations, which incorporate different approaches toward the $J/\psi$ production mechanism, are compared with experimental results and show reasonable agreement within uncertainties.

We report the first measurement of the inclusive jet and the dijet longitudinal double-spin asymmetries, $A_{LL}$, at midrapidity in polarized $pp$ collisions at a center-of-mass energy $\sqrt{s}=510\mathrm{GeV}$. The inclusive jet $A_{LL}$ measurement is sensitive to the gluon helicity distribution down to a gluon momentum fraction of $x\approx 0.015$, while the dijet measurements, separated into four jet-pair topologies, provide constraints on the $x$ dependence of the gluon polarization. Both results are consistent with previous measurements made at $\sqrt{s}=200\mathrm{GeV}$ in the overlapping kinematic region, $x>0.05$, and show good agreement with predictions from recent next-to-leading order global analyses.

We present the first measurement of charge-dependent directed flow in $\mathrm{Cu}+\mathrm{Au}$ collisions at $\sqrt{s_{NN}}=200\mathrm{GeV}$. The results are presented as a function of the particle transverse momentum and pseudorapidity for different centralities. A finite difference between the directed flow of positive and negative charged particles is observed that qualitatively agrees with the expectations from the effects of the initial strong electric field between two colliding ions with different nuclear charges. The measured difference in directed flow is much smaller than that obtained from the parton-hadron-string-dynamics model, which suggests that most of the electric charges, i.e., quarks and antiquarks, have not yet been created during the lifetime of the strong electric field, which is of the order of, or less than, $1\mathrm{fm}/c$.