$Q^2$ Dependence of Quadrupole Strength in the ${\gamma }^{\ast }p\to {\Delta }^{+}(1232)\to p{\pi }^0$ Transition

Abstract

Models of baryon structure predict a small quadrupole deformation of the nucleon due to residual tensor forces between quarks or distortions from the pion cloud. Sensitivity to quark versus pion degrees of freedom occurs through the $Q^2$ dependence of the magnetic ($M_{1+}$), electric ($E_{1+}$), and scalar ($S_{1+}$) multipoles in the ${\gamma }^{\ast }p\to {\Delta }^{+}\to p{\pi }^0$ transition. We report new experimental values for the ratios $E_{1+}/M_{1+}$ and $S_{1+}/M_{1+}$ over the range $Q^2$= 0.4-1.8 GeV${}^2$, extracted from precision $p(e,e^{\prime }p){\pi }^{\circ }$ data using a truncated multipole expansion. Results are best described by recent unitary models in which the pion cloud plays a dominant role.

5 pages, 5 figures, 1 table. To be published in Phys. Rev. Lett. (References, figures and table updated, minor changes.)

References in corpus (5)

• Dynamical Study of the Delta Excitation in N(e,e' pi) Reactions
• Determination of the E2/M1 Ratio in the γN \to Δ(1232) Transition from a Simultaneous Measurement of p(\vecγ,p)π^0 and p(\vecγ,π^+)n
• Measurement of the recoil polarization in the p (\vec e, e' \vec p) pi^0 reaction at the Δ(1232) resonance
• The electroproduction of the $\Delta$(1232) in the chiral quark-soliton model
• $\Delta$(1232) electroproduction amplitudes in chiral soliton models of the nucleon

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