Undergraduate Research at Jefferson Lab

One Pion Exchange Models in Leading Neutron Production

Student: Joshua McKenney

School: North Carolina State University

Mentored By: Nobuo Sato, Chueng Ji and Wally elnitchouk

In addition to the three valence quarks which traditionally account for the makeup of the proton, "sea" quarks are constantly popping in and out of existence inside the proton. If a particular pair of sea quarks, namely dd, appears, then the proton can behave as if it were composed of a pion and neutron at very large length scales. In deep inelastic scattering (DIS) experiments, ep→enX reactions are presumed to be dominated by a one-pion-exchange (OPE) mechanism, where the pionic component of the proton interacts with the incident particle. The cross section for such reactions can in principle be calculated with π+ loop diagrams. However, these diagrams lead to logarithmically divergent integrals in the absence of a regulating form factor at the pion-nucleon vertex, and in addition, since the pion is not a point particle, pion cross sections cannot be accurately described without such a form factor. This study seeks to place constraints on the functional form of pion-nucleon form factors, which can be parametrized in terms of a cutoff parameter Λ. A survey of potential form factor candidates was carried out, and a handful were selected to undergo more careful scrutiny. By scaling theoretical predictions with the proton and pion structure functions (F2p and F2π, respectively), calculated cross section values were compared with DIS measurements from the ZEUS and H1 collaborations at HERA. Least-squares Χ² minimization was used to optimize Λ values and to determine the viability of different form factors over various Bjorken-x and photon virtuality (Q2) values, as well as to fit F2π. Preliminary findings suggest that the Pauli-Villars model most accurately represent existing data, with Λ=0.2 GeV. The so-called Bishari flux is ruled out as a universal model despite its widespread use. The extracted F2π functions at low x are consistent with existing parametrizations. These findings provide useful predictions for the upcoming tagged DIS experiment E12-15-006 at Jefferson Lab, as well as the potential to make more universal predictions in nuclear reactions involving pions in general.