Λ0nonpolarization: Possible signature of quark matter

Abstract

The polarization of the lambda hyperons, produced in protonand nucleus-induced interactions over a aside range of incident energies and target masses, is examined. The persistence and constancy of the polarization up to the highest energy measured is consistent with the absence of quark-gluon plasma in these reactions. It is suggested that, in ultrarelativistic heavy ion colhsions, the formation of quark-gluon plasma in the central region F11result in the diminishing of the polarization of central rapidity lambdas.The strong polarization of singly and doubly strange particles, [roduced in p-induced interactions, is well established. ' ' In the case of the A hyperon, produced in the elementaryand dominantproduction channel NN~KNA, the polarization (and spin} is carried by the strange quark, the ud quarks forming a spin-zero system.The polarization is thought to be the result of a Thomas precession effect in the quark recombination process, taking into account SU(6} symmetry of hadron wave functions. " Another semiclassical model'2 predicts the polarization to be the result of the correlation between the "inherent" transverse momentum and the spin of the s quark, produced by a tunneling (soft) process in the color field, where pertubative quantum chromodynamics (@CD) is not applicable.In p+ p and p+ A collisions the number of participating nucleons is not more than the very few interacting hadrons in the primary collisions, while the size of the interaction zone is considerably smaller than the nuclear size, too small to speak of a confined plasma in an extended volume. Furthermore, even the most energetic of the interactions, in which the polarization was measured, does not create the necessary conditions (temperature and energy density) for the formation of quark-gluon plasma. (In p+ p interactions at Ws =63 GeV the energy density is less than 0.5 GeV/fms. ) It is, therefore, interesting to examine the state of the lambda polarization in ultrarelativistic nucleus-nucleus collisions, where a phase transition to quark matter is conjectured to occur. The speculation that a phase transition may be accompanied by symmetry breakingreflected in a change of the lambda polarization anisotropywas first mentioned by Stock. ' We intend to reconsider this idea in view of the polarization systematics available.

Globally Polarized Quark-Gluon Plasma in NoncentralA+ACollisions

“…of the valence quarks in a polarized nucleon [3,4,5], or fragmentation functions of transverse polarized quarks [6] as well as twist-3 parton correlations in nucleons [7]. It has also been suggested [8,9] that hyperon polarization could disappear due to the formation of QGP.In this Letter, we show that parton interaction in noncentral heavy-ion collisions leads to a global quark polarization along the opposite direction of the reaction plane,as determined by the nuclear impact parameter b. This global polarization is essentially a local manifestation of the global angular momentum of the colliding system through interaction of spin-orbital coupling in QCD.…”

“…of the valence quarks in a polarized nucleon [3,4,5], or fragmentation functions of transverse polarized quarks [6] as well as twist-3 parton correlations in nucleons [7]. It has also been suggested [8,9] that hyperon polarization could disappear due to the formation of QGP.…”

Global Λ polarization in high energy collisions

“…Afterwards, more experimental research was launched continuously, including nucleon collisions and heavy ion collisions [3][4][5][6][7][8][9]. Theoretical studies have also been under way synchronously with the experiments [10][11][12][13][14][15][16][17][18][19].…”

Longitudinal $\bar{\Lambda}_0$ polarization in heavy-ion collisions as a probe for QGP formation

“…It is interesting to note that, the momentum shift due to multiple-scattering ofΛ 0 's in the nuclear medium produces a small increase in the polarization as a function of p T in the low momentum region. Another possible observable which would exhibit strong changes in the presence of the QGP phase is the transverse polarization of the Λ 0 's [6,12]. However, in this case it is expected a decrease of its value with respect to what happens in p-p interactions since transverse polarization is related to the production mechanisms of the Λ 0 .…”