Hard exclusive electroproduction of π+π- pairs at HERMES

Hard exclusive electroproduction of pi(+)pi(-) pairs on 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 (q) over bar 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 < Q(2)> = 3GeV(2), higher-twist effects can be as large as the leading-twist longitudinal component.