Manifestation of quark effects in nuclei via bremsstrahlung analysis in the proton-nucleus scattering

As is known, the incoherent emission of photons is dominant comparing to the coherent one in proton-nucleus scattering. The incoherent bremsstrahlung is highly sensitive to the magnetic moments of nucleons in nuclei. According to the quark-meson coupling model, the nucleon magnetic moments in nuclei are enhanced relative to those in vacuum, originating from the quark structure of nucleons. On such basis and within new model, we for the first time investigate possibilities of observing quark effects in nuclei by analyzing bremsstrahlung photons in nuclear reactions. At first, we calibrate the model without the quark effects for experimental data of bremsstrahlung in proton-nucleus scattering (TAPS Collaboration data for 197 Au at proton beam energy 190 MeV). Next, we calculate the differential cross sections including the in-medium enhanced nucleon magnetic moments due to the quark effects in this nucleus and observe the slight difference between the spectra with and without the quark effects. Such result is found for the first time, confirming possibilities of observing the quark effects in the spectra of bremsstrahlung in proton-nucleus scattering. Investigating this phenomenon for nuclei from heavy to light, we find that 18 C has minimal in-coherent contribution (quark effects should be minimal) comparing with the other carbon isotopes. Difference in ratios between spectra for 18 C and those for 12 C with and without the quark effects is clearly observed. Thus, we establish new physical observable for quark effects in nuclei measurable in the bremsstrahlung in reactions, which was not studied before theoretically and experimentally.