Hard Exclusive Electroproduction of $\pi^+\pi^-$ Pairs

TL;DR

This study measures angular distributions of pion pairs produced in high-energy electron scattering, revealing interference effects between different pion states and insights into quark flavor dynamics at the parton level.

Contribution

First measurement of Legendre moments in pion pair electroproduction, providing new data on interference effects and flavor structure in exclusive processes.

Findings

Legendre moments vary with invariant mass, indicating interference between pion states.

Data suggest significant higher twist effects at Q^2 ≈ 3 GeV^2.

Interference patterns depend on pion angular distributions and polarization states.

Abstract

Hard exclusive electroproduction of $\pi^+\pi^-$ pairs off hydrogen and deuterium targets has been studied by the HERMES experiment at DESY. Legendre moments $<P_1>$ and $<P_3>$ of the angular distributions of $\pi^+$ mesons in the center-of-mass frame of the pair have been measured for the first time. Their dependence on the $\pi^+\pi^-$ invariant mass can be understood as being due to the interference between relative $P$-wave (isovector) and $S,D$-wave (isoscalar) states of the two pions. % The increase in magnitude of $<P_1>$ as Bjorken $x$ increases is interpreted in the framework of Generalized Parton Distributions as an enhancement of flavour {non-singlet} $q\bar{q}$ exchange for larger values of $x$, which leads to a sizable admixture of isoscalar and isovector pion pairs. % In addition, the interference between {$P$-wave} and {$D$-wave} states separately for transverse and longitudinal pion pairs has been studied. The data indicate that at {$\la Q^2 \ra = 3 $ GeV$^2$} higher twist effects can be as large as the leading-twist longitudinal component