The contribution of B meson decays to non-photonic electrons, which are mainly produced by the semi-leptonic decays of heavy flavor mesons, in p+p collisions at $\sqrt{s} =$ 200 GeV has been measured ...using azimuthal correlations between non-photonic electrons and hadrons. The extracted B decay contribution is approximately 50% at a transverse momentum of $p_{T} \geq 5$ GeV/c. These measurements constrain the nuclear modification factor for electrons from B and D meson decays. The result indicates that B meson production in heavy ion collisions is also suppressed at high pT even under the extreme case for the ratio of B to D contributions to non-photonic electrons.
We present measurements of the charge balance function, from the charged particles, for diverse pseudorapidity and transverse momentum ranges in Au + Au collisions at 200 GeV using the STAR detector ...at RHIC. We observe that the balance function is boost-invariant within the pseudorapidity coverage -1.3, 1.3. The balance function properly scaled by the width of the observed pseudorapidity window does not depend on the position or size of the pseudorapidity window. This scaling property also holds for particles in different transverse momentum ranges. In addition, we find that the width of the balance function decreases monotonically with increasing transverse momentum for all centrality classes.
In ultra-peripheral 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 ...$\rho^0$. The $\rho^0$ production occurs in two well-separated (median impact parameters of 20 and 40 fermi for the cases considered here) nuclei, so the system forms a 2-source interferometer. At low transverse momenta, the two amplitudes interfere destructively, suppressing $\rho^0$ production. Since the $\rho^0$ decay before the production amplitudes from the two sources can overlap, the two-pion system can only be described with an entangled non-local 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 $87% \pm 5% {\rm (stat.)}\pm 8%$ (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.
Three-particle azimuthal correlation measurements with a high transverse momentum trigger particle are reported for pp, d+Au, and Au+Au collisions at 200 GeV by the STAR experiment. The ...acoplanarities in pp and d+Au indicate initial state kT broadening. Larger acoplanarity is observed in Au+Au collisions. The central Au+Au data show an additional effect signaling conical emission of correlated charged hadrons.
Forward-backward multiplicity correlation strengths have been measured for the first time with the STAR detector for Au+Au and $\textit{p+p}$ collisions at $\sqrt{s_{NN}}$ = 200 GeV. Strong short and ...long range correlations are seen in central (0-10%) Au+Au collisions. The magnitude of these correlations decrease with decreasing centrality until only short range correlations are observed in 40-50% Au+Au collisions. The results are in agreement with predictions from the Dual Parton and Color Glass Condensate models.
We report a measurement of the longitudinal double-spin asymmetry A{sub LL} and the differential cross section for inclusive midrapidity jet production in polarized proton collisions at ...{radical}(s)=200 GeV. The cross section data cover transverse momenta 5<p{sub T}<50 GeV/c and agree with next-to-leading order perturbative QCD evaluations. The A{sub LL} data cover 5<p{sub T}<17 GeV/c and disfavor at 98% C.L. maximal positive gluon polarization in the polarized nucleon.
Transverse momentum spectra of {pi}{sup {+-}}, p, and p up to 12 GeV/c at midrapidity in centrality selected Au+Au collisions at {radical}(s{sub NN})=200 GeV are presented. In central Au+Au ...collisions, both {pi}{sup {+-}} and p(p) show significant suppression with respect to binary scaling at p{sub T}(greater-or-similar sign)4 GeV/c. Protons and antiprotons are less suppressed than {pi}{sup {+-}}, in the range 1.5 < or approx. p{sub T} < or approx. 6 GeV/c. The {pi}{sup -}/{pi}{sup +} and p/p ratios show at most a weak p{sub T} dependence and no significant centrality dependence. The p/{pi} ratios in central Au+Au collisions approach the values in p+p and d+Au collisions at p{sub T} > or approx. 5 GeV/c. The results at high p{sub T} indicate that the partonic sources of {pi}{sup {+-}}, p, and p have similar energy loss when traversing the nuclear medium.
We report the measurements of {sigma}(1385) and {lambda}(1520) production in p+p and Au+Au collisions at {radical}(s{sub NN})=200 GeV from the STAR Collaboration. The yields and the p{sub T} spectra ...are presented and discussed in terms of chemical and thermal freeze-out conditions and compared to model predictions. Thermal and microscopic models do not adequately describe the yields of all the resonances produced in central Au+Au collisions. Our results indicate that there may be a time span between chemical and thermal freeze-out during which elastic hadronic interactions occur.
We report a new STAR measurement of the longitudinal double-spin asymmetry A_LL for inclusive jet production at mid-rapidity 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 of polarized deep-inelastic scattering measurements.
We measure directed flow ($v_1$) for charged particles in Au+Au and Cu+Cu collisions at $\sqrt{s_{NN}} =$ 200 GeV and 62.4 GeV, as a function of pseudorapidity ($\eta$), 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 existing models, not on the size of the colliding system at a given centrality. We extend the validity of the limiting fragmentation concept to different collision systems, and investigate possible explanations for the observed sign change in $v_1(p_t)$.
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