Hard exclusive electroproduction of π+π− pairs

Hard exclusive electroproduction of π + π − 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 π + mesons in the center-of-mass frame of the pair have been measured for the first time. Their dependence on the π + π − 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 ¯ 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 〉 = 3 GeV 2 higher-twist effects can be as large as the leading-twist longitudinal component.