Spin alignment measurement of vector mesons produced in high energy collisions

Abstract

This review covers the recent experimental development on spin alignment measurements of $K^{\ast 0}$ and $\varphi$ vector mesons in heavy-ion and pp collisions at RHIC and LHC energies. Measurements in $e^{+}{e}^{-}$ collisions at LEP energies are also discussed. Spin alignment of vector mesons are studied by measuring the second diagonal element ${\rho }_{00}$ of spin density matrix. The ${\rho }_{00}$ is obtained by measuring the angular distribution of vector meson decay daughter with respect to the quantization axis in vector meson rest frame. Measured ${\rho }_{00}$ values for vector mesons are found to be larger than 1/3 at high momentum in $e^{+}{e}^{-}$ collisions at LEP energies, suggesting the preferential production of vector meson with helicity zero state from the fragmentation process. The ${\rho }_{00}$ values are found to be smaller than 1/3 (${\rho }_{00}$ = 1/3 implies no spin alignment) for $K^{\ast 0}$ and $\varphi$ vector mesons at low transverse momentum in Pb--Pb collisions at $\sqrt{s_{\mathrm{NN}}}$ = 2.76 TeV. This observations are qualitatively consistent with the expectation from models which attribute the spin alignment effect due to polarization of quarks in the presence of large initial angular momentum in non-central heavy-ion collisions and its subsequent hadronization by the process of recombination. No significant spin alignment effect is observed for $K_S^0$ (spin = 0) in mid-central Pb--Pb collisions and for vector mesons in pp collisions. However, the preliminary results of ${\rho }_{00}$ for $\varphi$ mesons are larger than 1/3 at intermediate $p_{\mathrm{T}}$ in Au--Au collisions at RHIC energies and can be attributed to the presence of $\varphi$ meson field. Although there is evidence of spin alignment effect of vector mesons in heavy-ion collisions but the measured effect is surprisingly larger in context of hyperon polarization. Therefore these results will trigger further theoretical study.

25 pages, 13 figures

References in corpus (8)

• An Introduction to PYTHIA 8.2
• Angular momentum conservation in heavy ion collisions at very high energy
• Collective longitudinal polarization in relativistic heavy-ion collisions at very high energy
• Local spin alignment of vector mesons in relativistic heavy-ion collisions
• Improved quark coalescence model for spin alignment and polarization of hadrons
• Global polarization of QGP in non-central heavy ion collisions at high energies
• The Spin Alignment of Vector Mesons in High Energy pp Collisions
• The effect of hadronic scatterings on the measurement of vector meson spin alignments in heavy-ion collisions

Cited by in corpus (11)

• Relativistic second-order dissipative spin hydrodynamics from the method of moments
• Spin alignment of vector mesons in heavy-ion collisions
• Spin alignment of vector mesons by glasma fields
• Pseudo-gauges and relativistic spin hydrodynamics for interacting Dirac and Proca fields
• Lorentz-covariant nonlocal collision term for spin-1/2 particles
• Spin alignment of vector mesons by second-order hydrodynamic gradients
• Transverse and longitudinal spin alignment from color fields in heavy ion collisions
• Impact of strong magnetic field, baryon chemical potential, and medium anisotropy on polarization and spin alignment of hadrons
• Anisotropic linear waves and breakdown of the momentum expansion in spin magnetohydrodynamics
• First-order spin magnetohydrodynamics
• Heavy quark polarization anisotropy as a novel probe of fireball geometry