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Register a new userDifferent space-time freeze-out picture -- an explanation of different $Lambda$ and $bar{Lambda}$ polarization?
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Thermal vorticity in non-central Au+Au collisions at energies $7.7 leq sqrt{s} leq 62.4$ GeV is calculated within the UrQMD transport model. Tracing the $Lambda$ and $bar{Lambda}$ hyperons back to their last interaction point we were able to obtain the temperature and the chemical potentials at the time of emission by fitting the extracted bulk characteristics of hot and dense medium to statistical model of ideal hadron gas. Then the polarization of both hyperons was calculated. The polarization of $Lambda$ and $bar{Lambda}$ increases with decreasing energy of nuclear collisions. The stronger polarization of $bar{Lambda}$ is explained by the different space-time distributions of $Lambda$ and $bar{Lambda}$ and by different freeze-out conditions of both hyperons.
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With the PICR hydrodynamic model, we study the polarization splitting between $Lambda$ and $bar{Lambda}$ at RHIC BES energy range, based on the meson field mechanism. Our results fit to the experimental data fairly well. Besides, two unexpected effect emerges: (1) the baryon density gradient has non-trivial and negative contribution to the polarization splitting; (2) for 7.7 GeV Au+Au collisions within the centrality range of 20%-50%, the polarization splitting surprisingly increases with the centrality decreases. The second effect might help to explain the significant signal of polarization splitting measured in STARs Au+Au 7.7 Gev collisions.
High energy heavy-ion collisions in laboratory produce a form of matter that can test Quantum Chromodynamics (QCD), the theory of strong interactions, at high temperatures. One of the exciting possibilities is the existence of thermodynamically distinct states of QCD, particularly a phase of de-confined quarks and gluons. An important step in establishing this new state of QCD is to demonstrate that the system has attained thermal equilibrium. We present a test of thermal equilibrium by checking that the mean hadron yields produced in the small impact parameter collisions as well as grand canonical fluctuations of conserved quantities give consistent temperature and baryon chemical potential for the last scattering surface. This consistency for moments up to third order of the net-baryon number, charge, and strangeness is a key step in the proof that the QCD matter produced in heavy-ion collision attains thermal equilibrium. It is a clear indication for the first time, using fluctuation observables, that a femto-scale system attains thermalization. The study also indicates that the relaxation time scales for the system are comparable to or smaller than the life time of the fireball.
We consider $Lambda$ and $bar{Lambda}$ production in a wide range of proton scattering experiments. The produced $Lambda$ and $bar{Lambda}$ may or may not contain a diquark remnant of the beam proton. The ratio of these two production mechanisms is found to be a simple universal function $r = [ kappa/(y_p - y) ]^i$ of the rapidity difference $y_p - y$ of the beam proton and the produced $Lambda$ or $bar{Lambda}$, valid over four orders of magnitude, from $r approx 0.01$ to $r approx 100$, with $kappa = 2.86 pm 0.03 pm 0.07$, and $i = 4.39 pm 0.06 pm 0.15$.
The Lambda and Lambda-bar polarizations in muon neutrino charged current interactions have been measured in the NOMAD experiment. The event sample (8087 reconstructed Lambdas and 649 Lambda-bars) is more than an order of magnitude larger than that of previous bubble chamber experiments, while the quality of event reconstruction is comparable. For the Lambda hyperons we observe negative polarization along the W-boson direction which is enhanced in the target fragmentation region: Px(xF < 0) = -0.21 +- 0.04 (stat) +- 0.02 (sys). In the current fragmentation region we find Px(xF > 0) = -0.09 +- 0.06 (stat) +- 0.03(sys). A significant transverse polarization (in the direction orthogonal to the Lambda production plane) has been observed for the first time in a neutrino experiment: Py = -0.22 +- 0.03 (stat) +- 0.01 (sys). The dependence of the absolute value of Py on the Lambda transverse momentum with respect to the hadronic jet direction is in qualitative agreement with the results from unpolarized hadron-hadron experiments. The polarization vector of Lambda-bar hyperons measured for the first time in neutrino interactions is found to be consistent with zero.
In this paper we study transverse polarization of $Lambda$ hyperons in single-inclusive leptonic annihilation. We show that when the transverse momentum of the $Lambda$ baryon is measured with respect to the thrust axis, a transverse momentum dependent (TMD) factorization formalism is required and the polarization is generated by the TMD polarizing fragmentation function (TMD PFF), $D_{1T}^perp$. However, when the transverse momentum of the $Lambda$ baryon is measured with respect to the momentum of the initial leptons, a collinear twist-3 formalism is required and the polarization is generated by the intrinsic collinear twist-3 fragmentation function $D_{T}$. Thus while these measurements differ from one another only by a change in the measurement axis, they probe different distribution functions. Recently, Belle measured a significant polarization in single-inclusive $Lambda$ baryon production as a function of the transverse momentum with respect to the thrust axis. However, this data can in principle be re-analyzed to measure the polarization as a function of the transverse momentum of the $Lambda$ baryon with respect to the lepton pair. This observable could be the first significant probe of the function, $D_{T}$. In this paper, we first develop a TMD formalism for $Lambda$ polarization; we then present a recent twist-3 formalism that was established to describe $Lambda$ polarization. Using the TMD formalism, we demonstrate that the $Lambda$ polarization at OPAL and Belle can be described using the twist-2 TMD factorization formalism. Finally, we make a theoretical prediction for this polarization in the collinear twist-3 formalism at Belle.
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