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Register a new userSpin alignment measurements of the $K^{*0}(892)$ and $phi(1020)$ vector mesons at RHIC
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We present the first spin alignment measurements for the $K^{*0}(892)$ and $phi(1020)$ vector mesons produced at mid-rapidity with transverse momenta up to 5 GeV/c at $sqrt{s_{NN}}$ = 200 GeV at RHIC. The diagonal spin density matrix elements with respect to the reaction plane in Au+Au collisions are $rho_{00}$ = 0.32 $pm$ 0.04 (stat) $pm$ 0.09 (syst) for the $K^{*0}$ ($0.8<p_T<5.0$ GeV/c) and $rho_{00}$ = 0.34 $pm$ 0.02 (stat) $pm$ 0.03 (syst) for the $phi$ ($0.4<p_T<5.0$ GeV/c), and are constant with transverse momentum and collision centrality. The data are consistent with the unpolarized expectation of 1/3 and thus no evidence is found for the transfer of the orbital angular momentum of the colliding system to the vector meson spins. Spin alignments for $K^{*0}$ and $phi$ in Au+Au collisions were also measured with respect to the particles production plane. The $phi$ result, $rho_{00}$ = 0.41 $pm$ 0.02 (stat) $pm$ 0.04 (syst), is consistent with that in p+p collisions, $rho_{00}$ = 0.39 $pm$ 0.03 (stat) $pm$ 0.06 (syst), also measured in this work. The measurements thus constrain the possible size of polarization phenomena in the production dynamics of vector mesons.
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We present new measurements related to spin alignment of $mathrm{K^{*0}}$ vector mesons at mid-rapidity for Pb-Pb collisions at $sqrt{s_{mathrm{NN}}}$ = 2.76 and 5.02 TeV. The spin alignment measurements are carried out with respect to production plane and second order event plane. At low $p_{mathrm{T}}$ the spin density matrix element $rho_{00}$ for $mathrm{K^{*0}}$ is found to have values slightly below 1/3, while it is consistent with 1/3, i.e. no spin alignment, at high $p_{mathrm{T}}$. Similar values of $rho_{00}$ are observed w.r.t. both production plane and event plane. Within statistical and systematic uncertainties, $rho_{00}$ values are also found to be independent of $sqrt{s_{mathrm{NN}}}$. $rho_{00}$ also shows centrality dependence with maximum deviation from 1/3 for mid-central collisions w.r.t. both the kinematic planes. The measurements for $mathrm{K^{*0}}$ in pp collisions at $sqrt{s}$ = 13 TeV and for $mathrm{K^{0}_{S}}$ (a spin 0 hadron) in 20-40% central Pb-Pb collisions at $sqrt{s_{mathrm{NN}}}$ = 2.76 TeV are consistent with no spin alignment.
An angular analysis of the decay $B^0 to phi K^*(892)^0$ is reported based on a $pp$ collision data sample, corresponding to an integrated luminosity of 1.0 fb$^{-1}$, collected at a centre-of-mass energy of $sqrt{s} = 7$ TeV with the LHCb detector. The P-wave amplitudes and phases are measured with a greater precision than by previous experiments, and confirm about equal amounts of longitudinal and transverse polarization. The S-wave $K^+ pi^-$ and $K^+K^-$ contributions are taken into account and found to be significant. A comparison of the $B^0 to phi K^*(892)^0$ and $bar{B}^0 to phi bar{K}^*(892)^0$ results shows no evidence for direct CP violation in the rate asymmetry, in the triple-product asymmetries or in the polarization amplitudes and phases.
The cross sections of the processes $e^+e^- to K^+K^-$, $e^+e^- to K_SK_L$ and $e^+e^- to pi^+pi^-pi^0$ were measured in the SND experiment at the VEPP-2M collider in the energy region near the $phi(1020)$ meson. These measurements were based on about $10^6$ $K^+K^-$, $0.5 times 10^6$ $K_SK_L$ and $0.4 times 10^6$ $pi^+pi^-pi^0$ selected events. Themeasured cross sections have been analyzed in the framework of the vector meson dominance model and the main parameters of the $phi$-resonance were obtained, such as its mass, width, the production cross section and branching ratios of the main decay modes. The measured value of the $phi$ meson total width, $Gamma_{phi} = 4.21 pm 0.04$ is lower than the present world average of $4.458 pm 0.032$ MeV. Contributions in addition to the conventional vector meson dominance model were found in the $e^+e^- to pi^+pi^-pi^0$ reaction cross section.
The STAR Collaboration reports on the photoproduction of $pi^+pi^-$ pairs in gold-gold collisions at a center-of-mass energy of 200 GeV/nucleon-pair. These pion pairs are produced when a nearly-real photon emitted by one ion scatters from the other ion. We fit the $pi^+pi^-$ invariant mass spectrum with a combination of $rho$ and $omega$ resonances and a direct $pi^+pi^-$ continuum. This is the first observation of the $omega$ in ultra-peripheral collisions, and the first measurement of $rho-omega$ interference at energies where photoproduction is dominated by Pomeron exchange. The $omega$ amplitude is consistent with the measured $gamma prightarrow omega p$ cross section, a classical Glauber calculation and the $omegarightarrowpi^+pi^-$ branching ratio. The $omega$ phase angle is similar to that observed at much lower energies, showing that the $rho-omega$ phase difference does not depend significantly on photon energy. The $rho^0$ differential cross section $dsigma/dt$ exhibits a clear diffraction pattern, compatible with scattering from a gold nucleus, with 2 minima visible. The positions of the diffractive minima agree better with the predictions of a quantum Glauber calculation that does not include nuclear shadowing than with a calculation that does include shadowing.
Production of the $K^{ast}(892)^0$ and $bar{K}^{ast}(892)^0$ resonances was studied via their $K^+ pi^-$ and $K^- pi^+$ decay modes in central Pb+Pb, Si+Si, C+C and inelastic p+p collisions at 158agev ($sqrt{s_{NN}}$ = 17.3 GeV) with the NA49 detector at the CERN SPS. Transverse momentum and rapidity distributions were measured and total yields were estimated. The yield of $K^{ast}$ exceeds that of $bar{K}^{ast}$ by about a factor of two in nucleus-nucleus reactions. The total yield ratios $< K^{ast} >/< K^+ >$ and $< bar{K}^{ast} >/< K^->$ are strongly suppressed in central Pb+Pb compared to p+p, C+C and Si+Si collisions in agreement with the expected attenuation of these short-lived resonance states in the hadronic phase of the expanding fireball. The UrQMD model, although incorporating such a scenario, does not provide a quantitative description of the experimental results. The statistical hadron gas model assuming the same freeze-out parameters for stable hadrons and resonances overestimates the $< K^{ast} >/< K >$ ratios in central Pb+Pb collisions by about a factor of 2.5.
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