Here, an improved measurement of the 3ΛH lifetime is presented. In this paper, the mesonic decay modes 3ΛH → 3He + π– and 3ΛH → d + p + π– are used to reconstruct the 3ΛH from Au+Au collision data ...collected by the STAR collaboration at Relativistic Heavy Ion Collider (RHIC). A minimum χ2 estimation is used to determine the lifetime of τ = 142+24–21 (stat.) ± 29 (syst.) ps. This lifetime is about 50% shorter than the lifetime τ = 263 ± 2 ps of a free Λ, indicating strong hyperon-nucleon interaction in the hypernucleus system. The branching ratios of the mesonic decay channels are also determined to satisfy B.R.(3He+π–)/(B.R.(3He+π–) + B.R.(d+p+π–)) = 0.32 ± 0.05 (stat.) ± 0.08 (syst.). Our ratio result favors the assignment J(3ΛH) = 1/2 over J(3ΛH) = 3/2. These measurements will help to constrain models of hyperon-baryon interactions.
In this paper, we report the first measurements of transverse single-spin asymmetries for inclusive jet and jet+π± production at midrapidity from transversely polarized proton-proton collisions at √s ...= 500 GeV. The data were collected in 2011 with the STAR detector sampled from 23 pb-1 integrated luminosity with an average beam polarization of 53%. Asymmetries are reported for jets with transverse momenta 6 < pT < 55 GeV/c and pseudorapidity |η| < 1. Presented are measurements of the inclusive-jet azimuthal transverse single-spin asymmetry, sensitive to twist-3 initial-state quarkgluon correlators; the Collins asymmetry, sensitive to quark transversity coupled to the polarized Collins fragmentation function; and the first measurement of the “Collins-like” asymmetry, sensitive to linearly polarized gluons. Within the present statistical precision, inclusive-jet and Collins-like 3 asymmetries are small, with the latter allowing the first experimental constraints on gluon linear polarization in a polarized proton. At higher values of jet transverse momenta, we observe the first non-zero Collins asymmetries in polarized-proton collisions, with a statistical significance of greater than 5σ. The results span a range of x similar to results from SIDIS but at much higher Q2. Finally, the Collins results enable tests of universality and factorization-breaking in the transverse momentum-dependent formulation of perturbative quantum chromodynamics.
We report the first measurement of the elliptic anisotropy (v2) of the charm meson D0 at midrapidity (|y| < 1) in Au+Au collisions at sNN=200 GeV. The measurement was conducted by the STAR experiment ...at RHIC utilizing a new high-resolution silicon tracker. The measured D0 v2 in 0%-80% centrality Au+Au collisions can be described by a viscous hydrodynamic calculation for a transverse momentum (pT) of less than 4 GeV/c. The D0 v2 as a function of transverse kinetic energy (mT-m0, where mT=pT2+m02) is consistent with that of light mesons in 10%-40% centrality Au+Au collisions. These results suggest that charm quarks have achieved local thermal equilibrium with the medium created in such collisions. Several theoretical models, with the temperature-dependent, dimensionless charm spatial diffusion coefficient (2πTDs) in the range of ~2-12, are able to simultaneously reproduce our D0 v2 result and our previously published results for the D0 nuclear modification factor.
The inclusive J/ψ transverse momentum spectra and nuclear modification factors are reported at mid-rapidity (|y|<1.0) in Au + Au collisions at √sNN = 39, 62.4 and 200 GeV taken by the STAR ...experiment. A suppression of J/ψ production, with respect to the production in p+p scaled by the number of binary nucleon–nucleon collisions, is observed in central Au + Au collisions at these three energies. No significant energy dependence of nuclear modification factors is found within uncertainties. The measured nuclear modification factors can be described by model calculations that take into account both suppression of direct J/ψ production due to the color screening effect and J/ψ regeneration from recombination of uncorrelated charm–anticharm quark pairs.
We present the measurements of elliptic flow ($v_2$) of non-photonic electrons (NPE) by the STAR experiment using 2- and 4-particle correlations, $v_2${2} and $v_2${4}, and the event plane method in ...Au+Au collisions at $\sqrt{s_{NN}} = 200$ GeV, and $v_2${2} at 62.4 and 39 GeV. $v_2${2} and $v_2${4} are non-zero at low and intermediate transverse momentum ($p_T$) at 200 GeV, and $v_2${2} is consistent with zero at low $p_T$ at other energies. For Au+Au collisions at $p_T<1$ GeV/c, there is a statistically significant difference between $v_2${2} at 200 GeV and $v_2${2} at the two lower beam energies.
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