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∝L 2, where L is the distance traveled by quark in the medium. It has a weak dependence on the initial quark energy E q. The L 2 dependence turns to L 1 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.