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

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

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