Polarization and dilepton anisotropy in pion–nucleon collisions

Enrico Speranza,Bengt Friman,Miklós Zétényi

doi:10.1016/j.physletb.2016.11.015

Abstract

Hadronic polarization and the related anisotropy of the dilepton angular distribution are studied for the reaction $\pi N \rightarrow Ne^+ e^-$. We employ consistent effective interactions for baryon resonances up to spin-5/2, where non-physical degrees of freedom are eliminated, to compute the anisotropy coefficients for isolated intermediate baryon resonances. It is shown that the spin and parity of the intermediate baryon resonance is reflected in the angular dependence of the anisotropy coefficient. We then compute the anisotropy coefficient including the $N(1520)$ and $N(1440)$ resonances, which are essential at the collision energy of the recent data obtained by the HADES collaboration on this reaction. We conclude that the anisotropy coefficient provides useful constraints for unravelling the resonance contributions to this process.

Highlights

Dilepton production in hadronic reactions provides information on the electromagnetic properties of hadrons
This implies that dilepton production in nuclear collisions can furnish information on the in-medium spectral functions of vector mesons
The aim of the present paper is to explore the reaction π N → R → Ne+e−, where R is the intermediate baryon resonance, in terms of effective Lagrangian models at the center-of-momentum (CM) energy of the HADES experiment

Summary

Introduction

Dilepton production in hadronic reactions provides information on the electromagnetic properties of hadrons. In case of an unpolarized nucleon target, spin-1/2 intermediate resonances are unpolarized, and there is no preferred direction in the CM frame In this case all observables are independent of the scattering angle, i.e., the angle θγ ∗ of the virtual photon in the CM frame. Intermediate resonances of spin ≥ 3/2 have a nontrivial polarization, implying an angular anisotropy in the CM frame In this case, observables show a nontrivial dependence on the scattering angle θγ ∗ , which reflects the quantum numbers of the resonance. The determination of the quantum numbers of the baryon resonances produced in pion–nucleon collisions is important for gaining a deeper understanding of hadron–hadron interactions in general and dilepton production in hadronic collisions in particular.

Cross section and anisotropy coefficient

The model

Results

Summary and outlook