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Register a new userDielectron production in pion-nucleon reactions and form factor of baryon transition within the time-like region
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Dielectron production in reactions $pi^- p rightarrow n e^+e^-$ and $pi^- p rightarrow n e^+e^- gamma$ at energies less than 1 GeV is studied assuming electron-positron pair production to occur in the virtual time-like photon splitting process. Theoretical predictions of the effective mass distribution of dielectrons and their angular dependence are presented. Extraction of the electromagnetic form factor of baryon transition in the time-like region from future experiments of the HADES Collaboration is discussed.
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Dielectron production in the $pi N$ interaction at not large energies is studied. The dominant contribution of the $Delta$-isobar creation in the intermediate state at incident pion momenta of about 0.3-0.4 GeV$/$c is shown. The experimental distributions over the angle and effective mass $M_{e^+e_-}$ of the $e^+e^-$ pair are described satisfactorily. This stimulated us to present theoretical predictions for the $M_{e^+e_-}$ distribution in the process $pi^- prightarrow ne^+e^-$ at different incident momenta, which could be verified, for example, by the HADES experiments.
New precise experimental information on $sigma_{tot}(e^+e^- to pi^+ pi^-)$ is transferred into the space-like region, by taking advantage of the analyticity. As a result a rigorous pion electromagnetic form factor behavior is obtained. The latter with some existing model predictions is compared.
Recent BaBaR data on the pion transition form factor, whose Q^2 dependence is much steeper then predicted by asymptotic Quantum Chromodynamics (QCD), have caused a renewed interest in its theoretical description. We present here a formalism based on a model independent low energy description and a high energy description based on QCD, which match at a scale Q_0. The high energy description incorporates a flat pion distribution amplitude, phi(x)=1, at the matching scale Q_0 and QCD evolution from Q_0 to Q>Q_0. The flat pion distribution is connected, through soft pion theorems and chiral symmetry, to the pion valance parton distribution at the same low scale Q_0. The procedure leads to a good description of the data, and incorporating additional twist three effects, to an excellent description of the data.
The possibility to compute nucleon electromagnetic form factors in the time-like region by analytic continuation of their space-like expressions has been explored in the framework of the Skyrme model. We have developed a procedure to solve analytically Fourier transforms of the nucleon electromagnetic current and hence to obtain form factors defined in all kinematical regions and fulfilling the first-principles requirements. The results are discussed and compared to data, both in space-like and time-like region.
It has been pointed out that the recent BaBar data on the pi gamma^* -> gamma transition form factor F_{pi gamma}(Q^2) at low (high) momentum transfer squared Q^2 indicate an asymptotic (flat) pion distribution amplitude. These seemingly contradictory observations can be reconciled in the k_T factorization theorem: the increase of the measured Q^2F_{pi gamma}(Q^2) for Q^2 > 10 GeV^2 is explained by convoluting a k_T dependent hard kernel with a flat pion distribution amplitude, k_T being a parton transverse momentum. The low Q^2 data are accommodated by including the resummation of alpha_s ln^2x, x being a parton momentum fraction, which provides a stronger suppression at the endpoints of x. The next-to-leading-order correction to the pion transition form factor is found to be less than 20% in the considered range of Q^2.
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