Abstract
The first evidence of spin alignment of vector mesons ( and ) in heavy-ion collisions at the Large Hadron Collider (LHC) is reported. The spin density matrix element is measured at midrapidity () in Pb-Pb collisions at a center-of-mass energy () of 2.76 TeV with the ALICE detector. values are found to be less than ( implies no spin alignment) at low transverse momentum () for and at a level of and , respectively. No significant spin alignment is observed for the meson () in Pb-Pb collisions and for the vector mesons in 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.
- Received 21 November 2019
- Revised 25 February 2020
- Accepted 27 May 2020
DOI:https://doi.org/10.1103/PhysRevLett.125.012301
Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.
© 2020 CERN, for the ALICE Collaboration