NUCLEON RESONANCES IN NUCLEI AND QUARK LEVELS

The effect of the nuclear medium on the excitation mechanism of nucleo n resonances is investigated in the framework of the constituent quark model (CQM). Calculations of the total strength of the M1 and E1 exci tations in a two-nucleon system are performed by means of a two-centre quark potential model in which the intercluster barrier is allowed to vary with the relative distance. The results show that the resonance excitation in the deuteron is not modified with respect to the free nu cleon case. In contrast, when the internucleon distance assumes typica l nuclear values, the pattern of the quark levels leads to a similar t o 10% damping of the D-13 resonance, leaving the Delta excitation unaf fected, in agreement with the recent photoabsorption measurements.