Evidence of Spin-Orbital Angular Momentum Interactions in Relativistic Heavy-Ion Collisions

Abstract

The first evidence of spin alignment of vector mesons (K Ã0 and ϕ) in heavy-ion collisions at the Large Hadron Collider (LHC) is reported. The spin density matrix element ρ 00 is measured at midrapidity (jyj < 0.5) in Pb-Pb collisions at a center-of-mass energy (ffiffiffiffiffiffiffi ffi s NN p) of 2.76 TeV with the ALICE detector. ρ 00 values are found to be less than 1=3 (1=3 implies no spin alignment) at low transverse momentum (p T < 2 GeV=c) for K Ã0 and ϕ at a level of 3σ and 2σ, respectively. No significant spin alignment is observed for the K 0 S meson (spin ¼ 0) in Pb-Pb collisions and for the vector mesons in pp collisions. The measured spin alignment is unexpectedly large but qualitatively consistent with the expectation from models which attribute it to a polarization of quarks in the presence of angular momentum in heavy-ion collisions and a subsequent hadronization by the process of recombination.

Zilch vortical effect, Berry phase, and kinetic theory

“…The helicity transport of guage fields is especially interesting in light of experimental measurements of hadron spin polarization in off-central heavy-ion collisions at RHIC and LHC [64][65][66]. The final state polarization follows the spin polarization of quarks and gluons in the QGP which, in turn, is in correspondence with the helicity transport due to the chiral effects.…”

Highlights from the ALICE collaboration

“…As a consequence, the spin of vector mesons such as K * 0 is expected to be aligned with that motion, which is what is observed in measurements of the spin density matrix element 00 in Pb-Pb collisions in Fig. 3 [9,10]. It is worth noting that the magnitude of the effect is significantly more pronounced than the Λ polarization measurements by ALICE and STAR [11,12], which poses an as-of-yet unresolved challenge to theorists.…”

Spin alignment of vector mesons in high energy pp collisions

“…III B, we calculate the spin alignment of vector meson in pp → VX using Eqs. (6) and (8). We present our predictions in the following.…”

“…Among them, the S LL component is directly related to the probability for the third component of spin to take zero that is called the spin alignment. The spin alignment of vector mesons has been measured in e þ e − annihilations and other high energy reactions [1][2][3][4][5][6][7][8].…”

“…2) No decay parameter is involved in the twobody strong decay of the vector meson so that there is no uncertainty caused by the decay parameter [26] and the measurement efficiency is high. Measurements have been carried out on the spin alignment and also the off-diagonal components in high energy reactions [1][2][3][4][5][6][7][8]. We have in particular data on the spin alignment with relatively high accuracy from experiments at large electron-position collider (LEP) [1][2][3][4].…”