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 —(v_n^k,v_m^l) and (v_n^k,v_m^l,v_p^q), 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 collisions. 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 (v_n , n=2,3,4) on the shape and size of the nuclear overlap region is studied. Comparisons between v ₂, v ₃, and v ₄ demonstrate the importance of v ₃ and v ₄ 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 S_NN=5.44 and 5.36 TeV, respectively. Correlations are extracted using multiparticle mixed-harmonic cumulants (up to eight-particle cumulants) with charged particles in the pseudorapidity range | η | ' 2.4 and transverse momentum range 0.5 ' p_T ' 3 GeV/c.