Near threshold neutral pion electroproduction at high momentum transfers.

摘要

Pion photo- (gammap → piN ) and electroproduction (gamma*p → piN) close to threshold have been the subject of experimental and theoretical study since the 1950s. The predictions for near threshold cross sections have been studied for momentum transfers Q 2 1 GeV2/c2 theoretically via the Low Energy Theorems (LETs). Additionally, these LETs give predictions for the axial form factor GA(Q 2) via the charged pion process near threshold. The LETs provide transition amplitude predictions for both neutral and charged pion reactions using chiral symmetry and current algebra arguments near threshold. Previous experimental measurements from Saclay, Mainz and others have shown consistency with the LET predictions at low momentum transfers Q2 1 GeV2/c2.;Recently, new extensions to these LETs at high Q 2 have been introduced by Pobylista et al. (2001) and then by Braun et al. (2008). In the work of Braun et al., the near threshold pion electroproduction transition amplitudes are written in terms of new form factors. These new generalized form factors GpN1 (Q2) and GpN2 (Q2) are obtained on the current quark basis and have been predicted using light cone sum rules (LCSRs) in the chiral limit (mpi → 0) for the reactions ep → eNpi in the Q 2 range of 1 - 10 GeV2/c2. Additionally, the predictions include a prescription to access the axial form factor GA near threshold at these high momentum transfers for the charged pion reaction.;An experiment at Jefferson Lab was conducted using the CLAS detector to measure near threshold neutral pion electroproduction ep → eppi0 as a function of Q 2. This is the first measurement of this process in the near threshold region of W from 1.08 to 1.16 GeV and at momentum transfers Q2 ≈ 2 - 5 GeV2/c2 . The differential angle integrated cross sections were measured and the associated structure functions and S-wave multipoles were extracted. The cross section, extracted multipole amplitudes and generalized form factors Gp0p1 (Q2) and Gp0p2 (Q2) are presented along with the predictions from Braun et al.