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Delta-to-N-gamma Coulomb Quadrupole Amplitude in PQCD

Ahmad Idilbi·Xiangdong Ji·Jian-Ping Ma
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Abstract

We present a leading-order pQCD calculation of the helicity-flip $Δ\to Nγ^*$ matrix element $G_0$ (Coulomb quadrupole amplitude $C2$), taking into account the transverse momenta of the quarks and the contribution from the gluons. In the large $Q^2$ limit, its scaling behavior acquires a double-logarithmic correction $\log^2{(Q^2/Λ^2)}$ compared with the standard scaling analysis, due to the contribution from the orbital motion of the small-$x$ partons. Based on this and on the latest JLab experimental results of the $C2-M1$ ratio $R_{SM}$ at $Q^2$ = 3 $\sim$ 4 GeV$^2$, we make a phenomenological prediction for the latter at higher values of $Q^2$.

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Ahmad Idilbi, Xiangdong Ji, Jian-Ping Ma. 2003-08-01. Delta-to-N-gamma Coulomb Quadrupole Amplitude in PQCD. https://doi.org/10.1103/physrevd.69.014006

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