Induced proton polarization forπ0electroproduction atQ2=0.126 

Abstract

We present a measurement of the induced proton polarization P n in 0 electroproduction on the proton around the ⌬ resonance. The measurement was made at a central invariant mass and a squared four-momentum transfer of Wϭ1231 MeV and Q 2 ϭ0.126 GeV 2 /c 2 , respectively. We measured a large induced polarization, P n ϭϪ0.397Ϯ0.055Ϯ0.009. The data suggest that the scalar background is larger than expected from a recent effective Hamiltonian model. ͓S0556-2813͑98͒02012-3͔ PACS number͑s͒: 13.88.ϩe, 13.60.Le, 13.60.Rj, 14.20.Gk At low Q 2 , the N→⌬ transition is dominated by the magnetic dipole amplitude. In a simple SU͑6͒ model in which all the quarks occupy S states in the N and ⌬ wave functions, the N→⌬ transition is a spin flip of a single quark. If the quarks are allowed to occupy D states as well as S states in the N or ⌬ wavefunctions, then electric and Coulomb quadrupole transitions are allowed ͓1,2͔. The ratios of these quad-*Present address:

Measurement of the LT-asymmetry in πelectroproduction at the energy of the Δ(1232)-resonance

“…The extraction of the quadrupole ratios from the measured cross sections is non-trivial. For the R SM discussed here, it is more reliable at lower Q 2 where M 1+ dominance is more pronounced than at higher Q 2 and single [22,23] and double polarization results [24,25,26] are already available in addition to unpolarized recent measurements [20,21,27,28,29,30]. The low Q 2 results are almost all compatible with each other, yielding R SM ≃ −6 %, cf.…”

Q2-evolution of nucleon-to-resonance transition form factors in a QCD-inspired vector-meson-dominance model

“…In particular, the data on the transitions N ∆(1232), N N (1440), N N (1520), N N (1535) were obtained up to squared momentum transfer Q 2 = 6 GeV 2 in exclusive experiments carried out in such facilities as JLab, MITBates, MAMI and others [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22] (see also review [23]). Although there is a total absence of exclusive data above Q 2 = 6 GeV 2 , a large body of inclusive measurements by SLAC is available for the resonance ∆(1232) (up to almost 10 GeV 2 ) and N (1535) (up to 21 GeV 2 ) (see [24], review [25] and references therein).…”

Lowest- Q2 measurement of the γp → Δ reaction: Probing the pionic contribution

“…Measurements of the E2 amplitude from photopion reaction experiments with polarized photons have been reported [4,5]. Electroproduction experiments at JLab [6,7] for Q 2 from 0.4 to 4.0 (GeV/c) 2 , at Bates [8][9][10][11] at Q 2 = 0.127 (GeV/c) 2 and Mainz [12][13][14] at Q 2 = 0.127, 0.20 (GeV/c) 2 have been published. The present result at Q 2 = 0.060 (GeV/c) 2 is the lowest-Q 2 value probed to date in modern, precision electroproduction.…”