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Direct photons: promises kept, broken - and open
The formation, evolution and properties of the strongly interacting medium created in heavy ion collisions is one of the fundamental problems in modern physics. Direct photons offer a unique opportunity to study the "history" of those collisions, since there are physics mechanisms creating photons at all stages, and once created, photons tend to leave the interaction region unscathed. In fact, a few decades ago direct photons, as penetrating probes, were considered the most promising tools (along with J/Psi) to prove the formation and study the properties of the quark-gluon plasma.
As often happens, Nature, while somewhat predictable, didn't fully cooperate. Although direct photons played a role in discovering the QGP, it was not the dominant one. For the same reason they are penetrating probes, direct photons are also relatively rare, mostly buried in a large background, and very challenging to measure precisely. Moreover, photons from the various proven or conjectured sources are difficult to disentangle. As a consequence, the measured direct photon spectra couldn't discriminate well between models: many scenarios with very different initial conditions could reproduce them reasonably well.
All this changed with the first measurements of azimuthal asymmetries (flow). None of the models could simultaneously describe the measured large yields and large flow, comparable to the hadronic one. The problem, quickly dubbed "direct photon puzzle" triggered immense intellectual effort - including this very workshop - and even starts to question some of our current thinking on the origin and evolution of the QGP, putting direct photon measurements once again center-stage. This also makes revival of some "open promises" - investigation of certain signatures not fully explored yet - very timely. In this colloquium we will briefly review the direct photon milestones of the past decades, survey where we are today and try to guess what could and should be done next in order to explore the full physics potential offered by direct photons.