Initial spin fluctuations in heavy-ion collisions and where to find them

¹ Theoretical Physics Department, CERN, 1211 Geneva 23, Switzerland
² Department of Physics and Astronomy, University of Florence, Via G. Sansone 1, 50019 Sesto Fiorentino, Italy

Published online: 17 April 2026

Abstract

Collective spin phenomena in the final states of heavy-ion collisions are typically understood to originate from vorticity and shear in the quark-gluon plasma (QGP). Here, we ask whether spin could already be present in the initial condition of the collisions. In particular, we argue that if a spin density exists at the beginning of the QGP expansion, it should experience event-by-event fluctuations due to the finite number of participant nucleons. In this contribution, we propose a simple model of fluctuating spin initial conditions for event-by-event spin hydrodynamics based on the Glauber Monte Carlo paradigm. We postulate that, if the net spin of the events is conserved from the initial to the final state, then initial state fluctuations of spin should manifest in specific spin correlations of Λ hyperons. Within our picture, we predict that this signal is much larger in central O+O collisions than in central Pb+Pb collisions.