The production rates and the transverse momentum distribution of strange hadrons at mid-rapidity ($\ |y\ | < 0.5$) are measured in proton-proton collisions at $\sqrt{s}$ = 13 TeV as a function of the charged particle multiplicity, using the ALICE detector at the LHC. The production rates of $\rm{K}^{0}_{S}$, $\Lambda$, $\Xi$, and $\Omega$ increase with the multiplicity faster than what is reported for inclusive charged particles. The increase is found to be more pronounced for hadrons with a larger strangeness content. Possible auto-correlations between the charged particles and the strange hadrons are evaluated by measuring the event-activity with charged particle multiplicity estimators covering different pseudorapidity regions. When comparing to lower energy results, the yields of strange hadrons are found to depend only on the mid-rapidity charged particle multiplicity. Several features of the data are reproduced qualitatively by general purpose QCD Monte Carlo models that take into account the effect of densely-packed QCD strings in high multiplicity collisions. However, none of the tested models reproduce the data quantitatively. This work corroborates and extends the ALICE findings on strangeness production in proton-proton collisions at 7 TeV.
$K^{0}_{S}$ transverse momentum spectrum - V0M multiplicity classes. Total systematic uncertainties include both correlated and uncorrelated uncertainties across multiplicity. Uncorrelated systematic originating from the multiplicity dependence of the efficiency (2%) is not included.
$K^{0}_{S}$ transverse momentum spectrum - V0M multiplicity classes. Total systematic uncertainties include both correlated and uncorrelated uncertainties across multiplicity. Uncorrelated systematic originating from the multiplicity dependence of the efficiency (2%) is not included.
$\Lambda+\bar{\Lambda}$ transverse momentum spectrum - V0M multiplicity classes. Total systematic uncertainties include both correlated and uncorrelated uncertainties across multiplicity. Uncorrelated systematic originating from the multiplicity dependence of the efficiency (2%) is not included.
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.
$\langle \cos\theta_p* \rangle$ of $\Lambda$ and $\bar{\Lambda}$ hyperons as a function of azimuthal angle $\phi$ relative to the second-order event plane $\Psi_2$ for 20%–60% centrality bin in Au+Au collisions at $\sqrt{s_{NN}}=200$ GeV.
The second Fourier sine coefficient $\langle P_Z \sin(2\phi-2\Psi_2) \rangle$ of the polarization of $\Lambda$ and $\bar{\Lambda}$ along the beam direction as a function of the collision centrality in Au+Au collisions at $\sqrt{s_{NN}}=200$ GeV.
The second Fourier sine coefficient $\langle P_Z \sin(2\phi-2\Psi_2) \rangle$ of the polarization of $\Lambda$ and $\bar{\Lambda}$ along the beam direction as a function of the collision centrality in Au+Au collisions at $\sqrt{s_{NN}}=200$ GeV. Results updated with $\alpha_{\Lambda} = -\alpha_{\bar{\Lambda}} = 0.732$.
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.
The invariant mass distribution for $\Lambda$ and $\bar{\Lambda}$ candidates with 3 < p_T < $ 4 GeV/c in this analysis
The raw spin transfer $D _{LL}^{raw}$ versus cos$\theta^*$ for a) $\Lambda$ and b) $\bar{\Lambda}$ hyperons and c) the spin asymmetry $\delta_{LL}$ for the control sample of $K_S^0$ mesons versus cos$\theta^*$ for $3<p_T<4$ GeV/c for JP1 triggered sample.
The raw spin transfer $D _{LL}^{raw}$ versus cos$\theta^*$ for a) $\Lambda$ and b) $\bar{\Lambda}$ hyperons and c) the spin asymmetry $\delta_{LL}$ for the control sample of $K_S^0$ mesons versus cos$\theta^*$ for $3<p_T<4$ GeV/c for JP1 triggered sample. Results updated with $\alpha_{\Lambda (\bar{\Lambda})} = 0.732$.
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