Speaker
Description
Direct photons escape the collision zone with negligible final-state interaction and retain information about the evolving QCD medium. Their low-momentum yield and elliptic flow $v_2$, however, remain challenging to describe simultaneously. Standard calculations treat the incoming partons as uncorrelated. We ask whether correlations surviving from the early non-equilibrium stage leave a measurable photon signal and contribute to the yield--flow tension.
We develop a general description of correlated parton pairs and follow their photon emission through a realistic collision evolution. Pair correlations do more than rescale the overall emission rate: their angular structure produces characteristic, sign-changing modifications of the photon spectrum. The yield and $v_2$ therefore probe complementary aspects of the same underlying correlations.
We compare the calculation with 0--20% PHENIX Au--Au data at $\sqrt{s_{NN}}=200$ GeV and ALICE Pb--Pb data at $2.76$ TeV. Within each collision system, one correlation pattern, kept unchanged between the yield and $v_2$ analyses, describes both observables more successfully than the uncorrelated baseline. This result supports partonic pair correlations as a candidate contribution to direct-photon phenomenology, without excluding other medium or emission effects. It also shows that photon measurements, although single-particle observables, can constrain two-particle structure and provide a new view of early-time hydrodynamization.
[1]Xingjian Lu, Shuzhe Shi. arXiv:2604.27926
[2]Xingjian Lu, Shuzhe Shi. arXiv:2606.02414
[3]Xingjian Lu, Shuzhe Shi. arXiv:2608.xxxxx
