Hard exclusive electroproduction of pi(+)pi(-) pairs

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 qq 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 in the f(2)(1270) region at = 3 GeV2 higher-twist effects can be as large as the leading-twist longitudinal component. (C) 2004 Published by Elsevier B.V.