Q^2-evolution of Delta N gamma form factors up to 4 (GeV/c)^2 from JLab data

Abstract

We present the results on the ratios $E_{1+}^{(3/2)}/M_{1+}^{(3/2)}$ and $S_{1+}^{(3/2)}/M_{1+}^{(3/2)}$ for the ${\gamma }^{\ast }N\to \Delta (1232)$ transition at $Q^2\le 4(GeV/c{)}^2$ extracted from the $p(e,e^{\prime }p){\pi }^0$ cross section using two approaches: dispersion relations and modified version of unitary isobar model. The obtained results are in good agreement with the results of other analyses obtained using truncated multipole expansion at $Q^2=0.4,0.525,0.65,0.75,0.9,1.15,1.45,1.8(GeV/c{)}^2$ and within dynamical and unitary isobar models at $Q^2=2.8,4(GeV/c{)}^2$. According to obtained results the ratio $E_{1+}^{(3/2)}/M_{1+}^{(3/2)}$ remains small in all investigated region of $Q^2$ with very unclear tendency to cross zero above $2(GeV/c{)}^2$. The absolute value of the ratio $S_{1+}^{(3/2)}/M_{1+}^{(3/2)}$ is clearly increasing with increasing $Q^2$, while it should be a constant value in the pQCD asymptotics. So, at $Q^2\le 4(GeV/c{)}^2$ there is no evidence of approaching pQCD regime for these ratios. None of the soft approaches gives satisfactory description of the obtained results.

Comments: 5 pages, 1 figure, invited talk at the Workshop on Exclusive Processes at High Momentum Transfer, May 15-18, 2002, Jefferson Lab