Photon E1 sum rules for quark matter

Photoabsorption by nucleon E1 excitation is suggested as a means to test quark mobility in nuclear matter. To this end the E1 sum rules are exhibited for 3 non‐relativistic quarks in a harmonic oscillator model. An excellent fit is obtained to photopion production on free nucleons with an r.m.s. radius 〈r〉≊0.35 F and a constituent quark mass μ=0.37 GeV. Extension of this harmonic oscillator model without additional parameters leads to a Regge trajectory slope of dm2/dJ≊1.1 GeV2, which is just the empirically extracted value. Since γ‐γ scattering has not been observed on nuclear matter in the nucleon E1 region (Eγ=600–800 GeV), some discussion is presented of corresponding M1 measurements in the △ region (Eγ=200–400 MeV). In this discussion the γ‐γ backscattering contributions of individual nucleons are treated as incoherent.