Gamma-Transition, Yet Again

Ahrens, L; Yamin, Sumera; Ratner, L. G.

doi:10.2172/1131570

Cited by 20 publications

(39 citation statements)

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“…A least-squares fit leads to G^O) -1.35 ±0.11, which is compatible with the pdecay value (1.257±0.003 [17]), and M,* = 1.15 ±0.27 GeV/c 2 which is compatible both with the last neutrino result [18] (M A =1.09 ±0.03 ±0.02 GeV/c 2 ) and with the most recent electroproduction experiment [4,19] (MA =0.96 ±0.03 GeV/c 2 and M A =0.97 ±0.06 GeV/c 2 , respectively). A general discussion about the determination of MA will be given in a forthcoming publication [20].…”

Section: And D(t)=[2mg a (T) + Tg P (T)]/mlsupporting

confidence: 75%

Axial and pseudoscalar nucleon form factors from low energy pion electroproduction

et al. 1993

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The cross section for exclusive zr + electroproduction on the proton has been measured near threshold for the first time at two different values of the virtual photon polarization (e-0.2 and e~0.7). Using the low energy theorem for this reaction we deduce the axial and pseudoscalar weak form factors GA and G P at |f | =0.073, 0.139, and 0.179 (GeV/c) 2 . The slope of G A agrees with the value obtained in neutrino experiments. Gp satisfies the pion pole dominance hypothesis, which is thus verified for the first time in this range of transfer.PACS numbers: 13.60. Le, 11.40.Ha, 14.20.Dh The electroweak form factors provide a significant test of our understanding of the nucleon structure. Continuous efforts are devoted to their experimental determination but, as compared to the electromagnetic form factors of the nucleon (Ff'^Ff'"), the axial {GA) and pseudoscalar (Gp) weak form factors are still poorly known. In particular, very little is known about Gp which is very sensitive to the pion cloud of the nucleon. This Letter reports the first determination of both GA and Gp at / =-0.073, -0.139, and -0.179 (GeV/c) 2 , using near threshold n + electroproduction on the proton with detection of the pion and electron in coincidence.To fix our conventions we write the matrix element of the axial current between nucleon states of momenta p\ and/?2 U == (pi -pi)(1) From p decay and muon capture one can determine the weak form factors in the range |/1 -0-0.01 (GeV/c) 2 , but at larger / the only direct information comes from neutrino scattering on nuclei. From these experiments, only the mass parameter MA of the dipole parametrization of GA can be obtained, assuming that the vector and magnetic weak form factors can be taken from electron scattering using the isotriplet hypothesis and that Gp is given by pion pole dominance [1], which we write in the form Gp(t) = -2MG A (t)/U-mZ). The well established approximate chiral symmetry of strong interactions allows us to write a low energy theorem [1,2] which relates the low energy pion electroproduction amplitude to GA, Gp, F p,n , and Ff ,n , up to corrections which vanish in the chiral limit (pion mass going to zero). For our purpose, F p>n and Ff'" are known with sufficient accuracy. Therefore, a measurement at the same t for two values of the virtual photon polarization e allows a simultaneous determination of GA and Gp. Previous experiments (see Ref.[3] for a list of references) performed the measurement only at a single value of e except in Ref. [4] where the neutral pions were not separated from the charged ones. Therefore the determination of GA relied either on the pion pole dominance hypothesis for Gp or on a model for the estimation of the neutral pion contribution. Our experiment is the first exclusive experiment which uses two values of e (e~0.2 and £~~0.7) different enough to allow an independent determination of GA and Gp. Up to now, the latter is known only at the muon point [5]: G P =0.082 ±0.018 MeV _1 c 2 at t 0.0112 (GeV/c) 2 . In the following, p\, pi, q, and...

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Section: And D(t)=[2mg a (T) + Tg P (T)]/mlsupporting

confidence: 75%

Axial and pseudoscalar nucleon form factors from low energy pion electroproduction

et al. 1993

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“…Moreover, there is a strong correlation between the extracted value of g s A and the axial mass M A obtained from dipole fits to the axial-vector form factor [6,7]. Indeed, the world-average value of M A (1.032 ± 0.036 GeV) that the BNL group has used is significantly different from the one [1.09 ± 0.03 (stat)±0.02 (syst) GeV] extracted later from a charged-current experiment [8]. Thus, this large difference in M A is sufficient to change the value of g s A extracted from the BNL experiment and the conclusion of a nonzero strange-quark content from our previous neutral-current study [7].…”

Section: Introductionmentioning

confidence: 80%

Relativistic nuclear structure effects in quasielastic neutrino scattering

1995

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Charged-current cross sections are calculated for quasielastic neutrino and antineutrino scattering using a relativistic meson-nucleon model. We examine how nuclear-structure effects, such as relativistic random-phaseapproximation (RPA) corrections and momentum-dependent nucleon selfenergies, influence the extraction of the axial form factor of the nucleon. RPA corrections are important only at low-momentum transfers. In contrast, the momentum dependence of the relativistic self-energies changes appreciably the value of the axial-mass parameter, M A , extracted from dipole fits to the axial form factor. Using Brookhaven's experimental neutrino spectrum we estimate the sensitivity of M A to various relativistic nuclear-structure effects. *

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“…Using the neutrino spectrum from the charged-current experiment at BNL [3] the resulting cross sections are shown in Fig. 2 where we used an axial mass M A = 1.09 GeV.…”

Section: Resultsmentioning

confidence: 99%