Pion Cloud Contributions to the Proton’s Weak Magnetic Form Factor

We report on the contributions to the nucleon weak magnetic and electric form factors arising from isospin-breaking form factors. The relativized quark model used in the calculations includes isospin impurities in the large basis nucleon wavefunctions and uses constituent quark currents with both Pauli and Dirac components. Significant contributions are found to arise from the Pauli component when it is interpreted as arising from pion-baryon loops. Including these pion cloud contributions reduces the strange magnetic form factor extracted from the recent SAMPLE measurement of the proton’s weak neutral magnetic form factor at low Q2 by 35 to 52%. Typeset using REVTEX 1 There has been considerable interest for several years now in the possibility of measuring the effects of the strange quark-antiquark (ss) pair in the nucleon using the parity-violating asymmetry in elastic electron scattering from the nucleon [1]. This year the first measurement of this asymmetry was reported on by the SAMPLE Collaboration [2] and clearly demonstrates the feasibility of seeing asymmetries of a few parts per million. The experiment was performed at the MIT/Bates Linear Accelerator Center using 200 MeV incident electrons elastically scattered from the proton at backward angles with an average Q of 0.1 (GeV/c). With this choice of kinematics the neutral weak form factor for the proton GM is enhanced relative to the corresponding electric term and can be extracted from the asymmetry using the relation A = σR − σL σR + σL = −F 2 πα √ 2 × { ǫGγEG Z E + τG γ MG Z M − 12(1− 4sin θW )ǫ GγMG Z A ǫ(GγE) 2 + τ(GγM) 2 }