Radiative energy loss of high-energy quarks in finite-size nuclear matter and quark-gluon plasma

The induced gluon radiation of a high-energy quark in a finite-size QCD medium is studied. For a sufficiently energetic quark produced inside a medium we find the radiative energy loss ΔE_q∝L², where L is the distance traveled by quark in the medium. It has a weak dependence on the initial quark energy E_q. The L² dependence turns to L¹ as the quark energy decreases. Numerical calculations are performed for a cold nuclear matter and a hot quark-gluon plasma. For a quark incident on a nucleus we predict ΔE_q≈0.1E_q(L/10fm)^β, with β close to unity.