Characterizing the initial state and dynamical evolution in XeXe and PbPb collisions using multiparticle cumulants

For the first time, correlations among mixed-order moments of two or three flow harmonics —(𝑣𝑘𝑛,𝑣𝑙𝑚) and (𝑣𝑘𝑛, 𝑣𝑙𝑚, 𝑣𝑞𝑝), with 𝑘, 𝑙, and 𝑞 denoting the respective orders—are measured in xenon–xenon (XeXe) collisions and compared with lead–lead (PbPb) results, providing a novel probe of collective behavior in heavy ion colli sions. These measurements compare a nearly spherical, doubly-magic 208Pb nucleus to a triaxially deformed 129Xe nucleus, emphasizing the sensitivity to initial-state geometry fluctuations arising from nuclear deformation. The dependence of these results (𝑣𝑛, 𝑛 = 2,3,4) on the shape and size of the nuclear overlap region is studied. Comparisons between 𝑣2, 𝑣3, and 𝑣4 demonstrate the importance of 𝑣3 and 𝑣4 in exploring the nonlinear hydrodynamic response of the quark-gluon plasma (QGP) to the initial spatial anisotropy. The results constrain initial-state model parameters that influence the evolution of the QGP. The CMS detector was used to collect XeXe and PbPb data at nucleon-nucleon center-of-mass energies of √𝑆𝑁𝑁 =5.44 and 5.36 TeV, respectively. Correlations are extracted using multiparticle mixed-harmonic cumulants (up to eight-particle cumulants) wi