Reacceleration of the fragmentation residues in the reactions $^{197}$Au+$^{197}$Au/$^{27}$Al at 0.5 and 1 $A$ GeV

We present the results of the first experiments dedicated to the precise measurements of the longitudinal momenta of the projectile fragments in the reactions $^{197}$Au+$^{197}$Au at 0.5 and 1 $A$ GeV, and $^{197}$Au+$^{27}$Al at 0.5 $A$ GeV performed with the high-resolution magnetic spectrometer FRS at GSI-Darmstadt. It is observed, that projectile residues with masses above A$_{res} \quad >$ 150 produced in very peripheral collisions are decelerated in the reaction and follow the well established Morrissey systematic [1], however the mean velocities of the lighter residues level off, and progressively increase with decreasing mass and even reach a positive value with respect to the original projectile velocity (for A$_{res }<$ 85). The observed reacceleration phenomenon and its behavior with respect to the different beam energy and/or different collision geometry thus significantly improves our understanding of the spectator response to the participant blast which has been just recently theoretically predicted [2] and experimentally observed [3]. References: [1] D.J.Morrissey, Phys. Rev. C39, 460 (1989). [2] L.Shi, P.Danielewicz, and R.Lacey, Phys. Rev. C64, 034601 (2001). [3] M.V.Ricciardi et al., Phys. Rev. Lett. 90, 2123021 (2003).