Title:Probing Initial State Clustering through Photon Anisotropic Flow in 7A TeV $^{16}$O+$^{16}$O Collisions at the LHC

Abstract:The presence of $\alpha$ clustered structures in light nuclei can enhance the initial spatial anisotropies in relativistic nuclear collisions relative to those arising from nuclei with uniform density distributions. Thus, observables that are strongly sensitive to the initial geometry can be a more efficient probe of the clustered structures than observables dominated by final state dynamics. We investigate the collisions of $\alpha$ clustered oxygen nuclei at $\sqrt{s_{NN}}=7$A TeV at the LHC using the GLISSANDO initial state model along with the MUSIC event-by-event hydrodynamical framework. The tetrahedral $\alpha$ clustered structure of $^{16}$O leads to significantly larger initial triangular eccentricity $\epsilon_3$ than collisions with uniform density distributions especially in the most central events. The spatial eccentricity $\epsilon_2$ is found to be relatively less sensitive to the initial state clustered structure. The production of thermal photons is estimated to be only marginally influenced by clustering for both central as well as peripheral collisions. In contrast, the photon triangular flow coefficient $v_3(p_T)$ is strongly affected by initial state clustering resulting in substantially larger values in both central and peripheral collisions. An experimental determination of photon anisotropic flow together with the ratios of flow coefficients in $^{16}$O+$^{16}$O collisions therefore expected to provide valuable insight into the possible clustered structure in light nuclei and also to constrain parameters in theoretical modeling.