Probing initial state clustering through photon anisotropic flow in 7A TeV $$^{16}$$O+$$^{16}$$O collisions at the LHC

Abstract The presence of $\alpha$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> mml:miα</mml:mi> </mml:math> 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 $\alpha$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> mml:miα</mml:mi> </mml:math> clustered oxygen nuclei at $\sqrt{s_{NN}}=7$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> mml:mrow mml:msqrt mml:msub mml:mis</mml:mi> mml:mrow mml:miNN</mml:mi> </mml:mrow> </mml:msub> </mml:msqrt> mml:mo=</mml:mo> mml:mn7</mml:mn> </mml:mrow> </mml:math> A TeV at the LHC using the GLISSANDO initial state model along with the MUSIC event-by-event hydrodynamical framework. The tetrahedral $\alpha$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> mml:miα</mml:mi> </mml:math> clustered structure of $^{16}$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> mml:mmultiscripts mml:mrow/ mml:mrow/ mml:mn16</mml:mn> </mml:mmultiscripts> </mml:math> O leads to significantly larger initial triangular eccentricity $\epsilon _3$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> mml:msub mml:miϵ</mml:mi> mml:mn3</mml:mn> </mml:msub> </mml:math> than collisions with uniform density distributions especially in the most central events. The spatial eccentricity $\epsilon _2$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> mml:msub mml:miϵ</mml:mi> mml:mn2</mml:mn> </mml:msub> </mml:math> 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 $v_3(p_T)$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> mml:mrow mml:msub mml:miv</mml:mi> mml:mn3</mml:mn> </mml:msub> mml:mrow mml:mo(</mml:mo> mml:msub mml:mip</mml:mi> mml:miT</mml:mi> </mml:msub> mml:mo)</mml:mo> </mml:mrow> </mml:mrow> </mml:math> 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 $^{16}$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> mml:mmultiscripts mml:mrow/ mml:mrow/ mml:mn16</mml:mn> </mml:mmultiscripts> </mml:math> O+ $^{16}$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> mml:mmultiscripts mml:mrow/ mml:mrow/ mml:mn16</mml:mn> </mml:mmultiscripts> </mml:math> O collisions therefore expected to provide valuable insight into the possible clustered structure in light nuclei and also to constrain parameters in theoretical modeling.