Electromagnetic dissociation of relativistic ${}_{}{}^{28}{}_{}{}^{}\mathrm{Si}$ into p${+}^{27}$Al

We report a direct measurement of the final-state energy spectrum in the electromagnetic dissociation of ${}_{}{}^{28}{}_{}{}^{}\mathrm{Si}$ into p+ ${}_{}{}^{27}{}_{}{}^{}\mathrm{Al}$ at an energy of 14.6 GeV/nucleon. The final-state energy is obtained through a calculation of the p${\mathrm{-}}^{27}$Al invariant mass in kinematically reconstructed events. The final-state energy spectrum for all targets is peaked near the isovector giant-dipole resonance in ${}_{}{}^{28}{}_{}{}^{}\mathrm{Si}$ and the dependence of the magnitude of the cross section on target charge confirms that the excitation is largely electromagnetic. By exploiting the expected scaling behavior on target Z and A, the background from nuclear interactions is evaluated and subtracted, leaving a pure electromagnetic dissociation final-state energy distribution. This distribution is well reproduced by simulated events, in which the photon spectrum calculated in the Weiszäcker-Williams approximation is combined with experimental data on the photonuclear reaction ${}_{}{}^{28}{}_{}{}^{}\mathrm{Si}$(γ,p) ${}_{}{}^{27}{}_{}{}^{}\mathrm{Al}$, and slight differences are observed only at low final-state energy.