<‘γ*N→Δtransition form factors: A new analysis of data onp(e,e′p)π0atQ2=2.8and4.0 (GeV/c)2

Abstract

Recent JLab data of the differential cross section for the reaction ${p(e,e}^{\ensuremath{'}}p){\ensuremath{\pi}}^{0}$ in the invariant mass region of $1.1<W<1.4 \mathrm{GeV}$ at four-momentum transfer squared ${Q}^{2}=2.8$ and $4.0 (\mathrm{GeV}{/c)}^{2}$ are analyzed with two models, both of which give an excellent description of most of the existing pion electroproduction data below $W<1.5 \mathrm{GeV}.$ We find that at up to ${Q}^{2}=4.0 (\mathrm{GeV}{/c)}^{2},$ the extracted helicity amplitudes ${A}_{3/2}$ and ${A}_{/2}$ remain comparable with each other, implying that hadronic helicity is not conserved at this range of ${Q}^{2}.$ The ratios ${E}_{1+}{/M}_{1+}$ obtained show, starting from a small and negative value at the real photon point, a clear tendency to cross zero, and to become positive with increasing ${Q}^{2}.$ This is a possible indication of a very slow approach toward the pQCD region. Furthermore, we find that the bare helicity amplitude ${A}_{1/2}$ and ${S}_{1/2},$ but not ${A}_{3/2},$ starts exhibiting the scaling behavior at about ${Q}^{2}>~2.5(\mathrm{GeV}{/c)}^{2}.$