A precise measurement of the hypertriton lifetime is presented. In this letter, the mesonic decay modes $\mathrm{{^3_\Lambda}H \rightarrow ^3He + \pi^-}$ and $\mathrm{{^3_\Lambda}H \rightarrow d + p + \pi^-}$ are used to reconstruct the hypertriton from Au+Au collision data collected by the STAR collaboration at RHIC. A minimum $\chi^2$ estimation is used to determine the lifetime of $\tau = 142^{+24}_{-21}\,{\rm (stat.)} {\pm} 31\,{\rm (syst.)}$ ps. This lifetime is about 50\% shorter than the lifetime $\tau = 263\pm2$ ps of a free $\Lambda$, indicating strong hyperon-nucleon interaction in the hypernucleus system. The branching ratios of the mesonic decay channels are also determined to satisfy B.R.$_{(^3{\rm He}+\pi^-)}/$(B.R.$_{(^3{\rm He}+\pi^-)}+$B.R.$_{(d+p+\pi^-)})$ = $0.32\rm{\pm}0.05\,{\rm (stat.)}\pm 0.08\,{\rm (syst.)}$. Our ratio result favors the assignment $J(\mathrm{^{3}_{\Lambda}H})$ = $\frac{1}{2}$ over $J(\mathrm{^{3}_{\Lambda}H})$ = $\frac{3}{2}$. These measurements will help to constrain models of hyperon-baryon interactions.
We report new STAR measurements of the single-spin asymmetries $A_L$ for $W^+$ and $W^-$ bosons produced in polarized proton--proton collisions at $\sqrt{s}$ = 510 GeV as a function of the decay-positron and decay-electron pseudorapidity. The data were obtained in 2013 and correspond to an integrated luminosity of 250 pb$^{-1}$. The results are combined with previous results obtained with 86 pb$^{-1}$. A comparison with theoretical expectations based on polarized lepton-nucleon deep-inelastic scattering and prior polarized proton--proton data suggests a difference between the $\bar{u}$ and $\bar{d}$ quark helicity distributions for $0.05 < x < 0.25$. In addition, we report new results for the double-spin asymmetries $A_{LL}$ for $W^\pm$, as well as $A_L$ for $Z/\gamma^*$ production and subsequent decay into electron--positron pairs.
The $\Lambda$ ($\bar{\Lambda}$) hyperon polarization along the beam direction has been measured for the first time in Au+Au collisions at $\sqrt{s_{_{NN}}}$ = 200 GeV. The polarization dependence on the hyperons' emission angle relative to the second-order event plane exhibits a sine modulation, indicating a quadrupole pattern of the vorticity component along the beam direction. The polarization is found to increase in more peripheral collisions, and shows no strong transverse momentum ($p_T$) dependence at $p_T>1$ GeV/$c$. The magnitude of the signal is about five times smaller than those predicted by hydrodynamic and multiphase transport models; the observed phase of the emission angle dependence is also opposite to these model predictions. In contrast, blast-wave model calculations reproduce the modulation phase measured in the data and capture the centrality and transverse momentum dependence of the signal once the model is required to reproduce the azimuthal dependence of the Gaussian source radii measured via the Hanbury-Brown and Twiss intensity interferometry technique.
The longitudinal spin transfer $D_{LL}$ to $\Lambda$ and $\bar{\Lambda}$ hyperons produced in high-energy polarized proton--proton collisions is expected to be sensitive to the helicity distribution functions of strange quarks and anti-quarks of the proton, and to longitudinally polarized fragmentation functions. We report an improved measurement of $D_{LL}$ from data obtained at a center-of-mass energy of $\sqrt{s}$ = 200 GeV with the STAR detector at RHIC. The data have an approximately twelve times larger figure-of-merit than prior results and cover $|\eta|<$ 1.2 in pseudo-rapidity with transverse momenta $p_T$ up to 6 GeV/c. In the forward scattering hemisphere at largest $p_T$, the longitudinal spin transfer is found to be $D_{LL}$ = -0.036 $\pm$ 0.048 (stat) $\pm$ 0.013(sys) for $\Lambda$ hyperons and $D_{LL}$ = 0.032 $\pm$ 0.043\,(stat) $\pm$ 0.013\,(sys) for $\bar{\Lambda}$ anti-hyperons. The dependences on $\eta$ and $p_T$ are presented and compared with model evaluations.
We report the first measurement of rapidity-odd directed flow ($v_{1}$) for $D^{0}$ and $\overline{D^{0}}$ mesons at mid-rapidity ($|y| < 0.8$) in Au+Au collisions at $\sqrt{s_{\rm NN}}$ = 200\,GeV using the STAR detector at the Relativistic Heavy Ion Collider. In 10--80\% Au+Au collisions, the slope of the $v_{1}$ rapidity dependence ($dv_{1}/dy$), averaged over $D^{0}$ and $\overline{D^{0}}$ mesons, is -0.080 $\pm$ 0.017 (stat.) $\pm$ 0.016 (syst.) for transverse momentum $p_{\rm T}$ above 1.5~GeV/$c$. The absolute value of $D^0$-meson $dv_1/dy$ is about 25 times larger than that for charged kaons, with 3.4$\sigma$ significance. These data give a unique insight into the initial tilt of the produced matter, and offer constraints on the geometric and transport parameters of the hot QCD medium created in relativistic heavy-ion collisions.
Pion-kaon correlation functions are constructed from central Au+Au data taken at $\sqrt{s_{NN}} = 130$ GeV. The results suggest that pions and kaons are not emitted at the same average space-time point. Space-momentum correlations, i.e. transverse flow, lead to a space-time emission asymmetry of pions and kaons that is consistent with the data. This result provides new independent evidence that the system created at RHIC undergoes a collective transverse expansion.
We report on p-Lambda, p-Lambda bar, p bar-Lambda and p bar-Lambda bar correlation functions constructed in central Au-Au collisions at sqrt(s_NN)=200GeV by the STAR experiment at RHIC. The proton and lambda source size is inferred from the p-Lambda and p bar-Lambda bar correlation functions. They are found to be smaller than the pion source size also measured by the STAR detector. This could be a consequence of the collision fireball's collective expansion. The p-Lambda bar and p bar-Lambda correlations, which are measured for the first time, exhibit a large anti-correlation. Annihilation channels and/or a negative real part of the spin-averaged scattering length must be included in the final-state interactions calculation to reproduce the measured correlation function.
Vector mesons may be photoproduced in relativistic heavy-ion collisions when a virtual photon emitted by one nucleus scatters from the other nucleus, emerging as a vector meson. The STAR Collaboration has previously presented measurements of coherent $\rho^0$ photoproduction at center of mass energies of 130 GeV and 200 GeV in AuAu collisions. Here, we present a measurement of the cross section at 62.4 GeV; we find that the cross section for coherent $\rho^0$ photoproduction with nuclear breakup is $10.5\pm1.5\pm 1.6$ mb at 62.4 GeV. The cross-section ratio between 200 GeV and 62.4 GeV is $2.8\pm0.6$, less than is predicted by most theoretical models. It is, however, proportionally much larger than the previously observed $15\pm 55$% increase between 130 GeV and 200 GeV.
We report the first three-particle coincidence measurement in pseudorapidity ($\Delta\eta$) between a high transverse momentum ($p_{\perp}$) trigger particle and two lower $p_{\perp}$ associated particles within azimuth $\mid$$\Delta\phi$$\mid$$<$0.7 in $\sqrt{{\it s}_{NN}}$ = 200 GeV $d$+Au and Au+Au collisions. Charge ordering properties are exploited to separate the jet-like component and the ridge (long-range $\Delta\eta$ correlation). The results indicate that the particles from the ridge are uncorrelated in $\Delta\eta$ not only with the trigger particle but also between themselves event-by-event. In addition, the production of the ridge appears to be uncorrelated to the presence of the narrow jet-like component.
Measurements of the production of forward high-energy pi0 mesons from transversely polarized proton collisions at \sqrt{s}=200 GeV are reported. The cross section is generally consistent with next-to-leading order perturbative QCD calculations. The analyzing power is small at x_F below about 0.3, and becomes positive and large at higher x_F, similar to the trend in data at \sqrt{s}<=20 GeV. The analyzing power is in qualitative agreement with perturbative QCD model expectations. This is the first significant spin result seen for particles produced with p_T>1 GeV/c at a polarized proton collider.
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