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From Color Glass Condensate to Quark Gluon Plasma through the event horizon

We propose a new thermalization scenario for heavy ion collisions which at sufficiently high energies implies the phase transition to the quark--gluon plasma. The key ingredient of our approach is the Hawking--Unruh effect: an observer moving with an acceleration $a$ experiences the influence of a thermal bath with an effective temperature $T = a / 2π$, similar to the one present in the vicinity of a black hole horizon. For electric charges moving in external electromagnetic fields of realistic strength, the resulting temperature appears too small to be detected. However for partons in strong color fields the effect should be observable: in the Color Glass Condensate picture, the strength of the color-electric field is $E \sim Q_s^2/g$ ($Q_s$ is the saturation scale, and $g$ is the strong coupling), the typical acceleration is $a \sim Q_s$, and the heat bath temperature is $T = Q_s / 2π\sim 200$ MeV. In nuclear collisions at sufficiently high energies the effect can induce a rapid thermalization over the time period of $τ\simeq 2π/Q_s \simeq 1\ {\rm fm}$ accompanied by phase transitions. We consider a specific example of chiral symmetry restoration induced by a rapid deceleration of the colliding nuclei. We argue that parton saturation in the initial nuclear wave functions is a necessary pre--condition for the formation of quark--gluon plasma. We discuss the implications of our "black hole thermalization" scenario for various observables in relativistic heavy ion collisions.