Hard Exclusive Electroproduction of π+π- Pairs at Hermes

Hard exclusive electroproduction of π⁺π^- pairs on hydrogen and deuterium targets has been studied by the HERMES experiment at DESY. Legendre moments < P ₂ > 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 > and < P ₃ > as Bjorken x increases is interpreted in the framework of Generalized Parton Distributions as an enhancement of flavour non-singlet qbar 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 f₂(1270) region at < { Q}² ; > = 3 {GeV}^{2} , higher-twist effects can be as large as the leading-twist longitudinal component.