Polarisation in Deeply Virtual Meson Production
نویسنده
چکیده
We discuss two aspects of polarisation in hard exclusive meson production: the leading-twist selection rule for the meson helicity, and the different partial waves of a ππ-pair which may or may not be due to the decay of a ρ. 1 Helicity in vector meson production The first half of this talk is about exclusive electroproduction of a ρ (or any other light vector meson), ep → ep + ρ, in the Bjorken limit where the photon virtuality Q 2 = −q 2 becomes large, while x B = Q 2 /(2p · q) and the invariant momentum transfer t = (p − p ′) 2 remain fixed. There is a factorisation theorem 1 stating that in this limit the γ * p amplitude factorises into a hard photon-parton scattering and non-perturbative quantities, namely skewed quark and gluon distributions in the proton and a q ¯ q distribution amplitude of the meson. This is shown in Fig. 1, where also four-momenta are defined. The factorisation theorem provides a solid basis for extracting information on the quark and gluon structure of hadrons from this process, in particular for measuring skewed parton distributions. The description of the amplitude in terms of these quantities is of course only accurate up to power corrections in 1/Q, and in a data analysis it is essential to investigate how close one is to the asymptotic regime at a given value of Q 2. It is at this point that helicity selection rules are of great value.
منابع مشابه
Exclusive Vector Meson Production and Deeply Virtual Compton Scattering at HERA ∗
Exclusive vector meson production and deeply virtual Compton scattering are ideally suited reactions for studying the structure of the proton and the transition from soft to hard processes. The main experimental data obtained at HERA are summarised and presented in the light of QCD approaches.
متن کاملDiffractive Vector Meson Production at Hermes
The decay distributions of 0 and mesons diiractively produced by tagged virtual photons from the 27.5 GeV HERA positron beam impinging on an internal 3 He target have been measured with the Hermes spectrometer. Both the scattered positron and the two outgoing hadrons from the vector meson decay were detected in coincidence, and exclusive elastic events selected for analysis. The decay angular d...
متن کاملExclusive vector meson production at HERA from QCD with saturation
Following recent predictions that the geometric scaling properties of deep inelastic scattering data in inclusive γp collisions are expected also in exclusive diffractive processes, we investigate the diffractive production of vector mesons. Using analytic results in the framework of the BK equation at non-zero momentum transfer, we extend the QCD-inspired saturation model to parametrise the mo...
متن کاملHARD DIFFRACTION IN VECTOR MESON PRODUCTION AT HERA On behalf of the H1 and ZEUS Collaborations
Results on elastic vector meson production at HERA are presented in the framework of perturbative QCD. The energy dependence of the cross section for J/ψ photoproduction and for ρ electroproduction is studied. A full polarisation analysis, including the measurement of the 15 elements of the spin density matrix and of R = σL/σT , is presented for ρ electroproduction. Finally heavier vector meson...
متن کاملذخیره در منابع من
با ذخیره ی این منبع در منابع من، دسترسی به آن را برای استفاده های بعدی آسان تر کنید
عنوان ژورنال:
دوره شماره
صفحات -
تاریخ انتشار 1999