Constituent quarks in nuclear matter and the Nolen-Schiffer anomaly

We investigate the density dependence of the neutronproton mass difference using the Nambu and Jona-Las-inio model in combination with the Isgur-Karl constituent quark model. The decrease of the constituent quark masses with increasing density reduces the n-p mass difference in the proper way to help resolving the Nolen-Schiffer anomaly. We point out however, that in the presence of vector interactions, this effect is less pronounced than previously suggested.