The collective behavior of hadronic particles has been observed in high multiplicity proton-lead collisions at the Large Hadron Collider, as well as in deuteron-gold collisions at the Relativistic Heavy-Ion Collider. In this work we present the first calculation, in the hydrodynamic framework, of thermal photon radiation from such small collision systems. Owing to their compact size, these systems can reach temperatures comparable to those in central nucleus-nucleus collisions. The thermal photons can thus shine over the prompt background, and increase the low $p_T$ direct photon spectrum by a factor of 2–3 in 0%–1% $p+\mathrm{Pb}$ collisions at 5.02 TeV. This thermal photon enhancement can therefore serve as a signature of the existence of a hot quark-gluon plasma during the evolution of these small collision systems, as well as validate hydrodynamic behavior in small systems.

• Received 8 May 2015

DOI:https://doi.org/10.1103/PhysRevLett.116.072301