Electro-Excitation of the Δ. Spherical Asymmetry of the Nucleon: The Oops/Bates Program

Abstract

The issue of hadron deformation has been a subject of intense scientific interest during the last two decades. The detailed study of the N→Δ transition is the best method of experimental investigation of this issue. A brief overview of the experimental and theoretical landscape on the field will be presented followd by a presentation of the most recent results from the Bates N→Δ program. The Bates data yield precise quadrupole to dipole amplitude ratios: EMR=(-2.3±0.3stat+sys±0.6model)% and CMR=(-6.1±0.2stat+sys±0.5model)% for [Formula: see text]. The derived amplitudes give credence to the conjecture of deformation in hadrons favoring, at low Q2, the dominance of mesonic effects.

Nucleon toΔtransition form factors withNF=2+1domain wall fermions

“…This can be understood in chiral effective theory, which predicts different chiral behavior for the two quantities. The nonzero values calculated in QCD are in accord with the experimental determinations [8,38,[40][41][42][43] and confirm a deviation from spherical symmetry in the Nucleon-∆ system.…”

“…the corresponding R EM evaluated in the rest frame of the ∆ baryon (p = 0) is depicted for the fine DWF lattice as well as for the hybrid action [28]. The experimentally available results from [38][39][40][41][42][43] are also shown.…”

The Electromagnetic Nucleon to Delta Transition in Chiral Effective Field Theory

“…The data-points shown are from refs. [20] (cross), [21] (diamonds), [22] (circles) and [4] (square).…”

“…see ref. [4] and references given therein), which has access to a lot more hadron structure properties than just the N ∆-transition current of eq.(1). However, we compare our results to experimental data assuming the approximate relations…”

Improved empirical parametrizations of theγ*N→Δ(1232)andγ*N<

“…[22] includes data for finite Q 2 from Refs. [23][24][25], and the world data average of G E at Q 2 = 0, extracted from the particle data group (PDG) result for G E /G M at Q 2 = 0 [26].…”