Why direct photons ?

• [Definition: direct photons = photons not coming from hadronic decays]

• Advantages:

• - Weakly interacting. Travel unaffected to detectors after production.

• - Direct coupling to (point-like) scatterings.

• - Good theoretical understanding: γ – quark coupling precisely known (QED)

• - “Clean” probes in many QCD environments (pp, pA, AA).

• pp collisions:

• - Access to (polarized) gluon PDF (via qg Compton).

• - Testing ground of pQCD. Baseline reference for AA collisions.

• pA collisions:

• - Access to nuclear modifications of gluon PDF

• - Initial-state multiple scattering (Cronin)

• - Cold nuclear-matter effects

• AA collisions:

• t<~0.2 fm/c: perturbative photons unaffected by QCD medium (monitoring jet quenching: yields and correlations).

• t<~0.6 fm/c: secondary (“cascading”) photons – thermalization mechanism

• t<~5 fm/c: thermal photons – access to medium T and EoS (!)

• g-g correlations: space-time size of radiating source