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We investigate the soft handbag contribution to two-photon annihilation into pion or kaon pairs at large energy and momentum transfer. The amplitude is expressed as a hard γγ → q¯q subprocess times a form factor describing the soft... more
We investigate the soft handbag contribution to two-photon annihilation into pion or kaon pairs at large energy and momentum transfer. The amplitude is expressed as a hard γγ → q¯q subprocess times a form factor describing the soft transition from q¯q to the meson pair. We find the calculated angular dependence of the cross section in good agreement with data, and extract annihilation form factors of plausible size. A key prediction of the handbag mechanism is that the differential cross section is the same for charged and neutral pion pairs, in striking contrast with what is found in the hard scattering approach. 1
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The process p¯p → Λ + c ¯ Λ − c is investigated within the handbag approach. It is shown that the dominant dynamical mechanism, characterized by the partonic subprocess uū → c¯c, factorizes in the sense that only the subprocess contains... more
The process p¯p → Λ + c ¯ Λ − c is investigated within the handbag approach. It is shown that the dominant dynamical mechanism, characterized by the partonic subprocess uū → c¯c, factorizes in the sense that only the subprocess contains highly virtual partons, a gluon to lowest order of perturbative QCD, while the hadronic matrix elements embody only soft scales and can be parameterized in terms of helicity flip and non-flip generalized parton distributions. Modelling these parton distributions by overlaps of light-cone wave functions for the involved baryons we are able to predict cross sections and spin correlation parameters for the process of interest. 1
The present status of forward dispersion relations for pp scattering is investigated. It is stressed that at low energies the situation is not yet settled, precise total cross-section and Coulomb-interference data are required for in... more
The present status of forward dispersion relations for pp scattering is investigated. It is stressed that at low energies the situation is not yet settled, precise total cross-section and Coulomb-interference data are required for in particular at very low energies (pL < 0.3 GeV/c). Furthermore, possible applications of non-forward dispersion relations are discussed.
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Analyzing power, polarizations of final-state particles, and spin transfer in the fragmentation region are discussed. Experimental results, many of which are presented in other papers from this symposium, are exhibited; and an efficient... more
Analyzing power, polarizations of final-state particles, and spin transfer in the fragmentation region are discussed. Experimental results, many of which are presented in other papers from this symposium, are exhibited; and an efficient way to parametrize these findings is addressed. Some theoretical ideas on transverse polarizations are set forth. It is concluded that no satisfying theoretical understanding of the data on transverse polarizations of hyperons in inclusive reactions has yet been achieved.
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In any application of dispersion relations to kaon-nucleon and nucleon-nucleon scattering the existence of an unphysical part of the left hand cut (LHC) is a challenge. If one is, for example, interested in predictions of real parts of... more
In any application of dispersion relations to kaon-nucleon and nucleon-nucleon scattering the existence of an unphysical part of the left hand cut (LHC) is a challenge. If one is, for example, interested in predictions of real parts of scattering amplitudes or more generally if one uses dispersion relations as constraints in phase shift or amplitude analyses in order to resolve ambiguities, one has to assume some thing about the contributions from the unphysical part of the LHC which clearly weakens the power of the dispersion relations. On the other hand, if an amplitude, that is real as well as imaginary part, for a given t value and for a given energy range is known one may turn around the procedure and use the dispersion relation as a tool to obtain information on the discontinuity of the LHC. The easiest and often used method to exploit this idea is to approximate the discontinuity by a sum of poles and to determine their coupling constants and eventually also their masses. In nucleon-nucleon scattering this method has been applied for example by Bugg (l) and by Verwest et al. (2). With the advent of total cross-section data in pure spin states, however, it turned out by a detailed study performed by us (3) that this pole approximation is not sufficient. We found clear evidence for contributions from the 3π continuum. In the following I will report on that investigation. I will start with a description of the available information on the discrepancy functions and will proceed with a qualitative discussion of the physics of the LHC. Thereafter the discontinuity of the LHC will be discussed quantitatively. I will end up with some conclusions.
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We analyse the exclusive channel $p\pb\to\gamma \pi^0$, assuming handbag dominance. The soft parts are parametrized in terms of CGLN amplitudes for the $q\qb\to\gamma \pi^0$ transition and form factors for the $p\pb\to q\qb$ ones, the... more
We analyse the exclusive channel $p\pb\to\gamma \pi^0$, assuming handbag dominance. The soft parts are parametrized in terms of CGLN amplitudes for the $q\qb\to\gamma \pi^0$ transition and form factors for the $p\pb\to q\qb$ ones, the latter represent moments of Generalized Distribution Amplitudes. We present a combined fit to Fermilab data from E760 taking simultaneously into account information from other exclusive reactions, especially from $p\pb\to \gamma\gamma$ data. Overall a nicely consistent picture emerges, such that one can hope, that our theoretical analysis will be reliable also for the kinematics of GSI/FAIR, which hopefully will provide much more precise and complete data. Consequently, data from this facility should improve our knowledge both on the proton-antiproton distribution amplitudes and the pion production mechanism.
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Theoretically it is well founded that the large momentum-transfer region of exclusive processes like pp → 2 hadrons shotdd be dominated by perturbative QCD1. Or, to be more precise, the corresponding scattering amplitude is known to be a... more
Theoretically it is well founded that the large momentum-transfer region of exclusive processes like pp → 2 hadrons shotdd be dominated by perturbative QCD1. Or, to be more precise, the corresponding scattering amplitude is known to be a convolution of a process specific, perturbatively calculable hard scattering amplitude — which is the sum of all tree diagrams — with process independent quark distribution amplitudes — which account for the nonperturbative formation of quarks into hadrons. For the reactions we are interested in, namely pp → B f B; f , where B f is a baryon containing at least one heavy quark f(= s,c,b) a full calculation along this so called “hard scattering scheme” would require an enormous effort due to the tremendous number of diagrams (O(105)) contributing. This huge number of diagrams can be reduced considerably by assuming baryons to be bound states of a pointlike quark and a spatially extended diquark. As an additional advantage one thereby includes non-perturbative (higher twist) effects which, as e.g. spin measurements have revealed2, are still present in the available large momentum-transfer exclusive scattering data. The existence of diquarks, although not deduced from QCD, is also strongly suggested by many effects they can explain such as baryon production in hard collisions, scaling violations in the structure functions of deep inelastic lepton-hadron scattering, or static properties of baryons. Recently it has been shown3 that a consistent description of the electromagnetic proton form factor and γγ → pp cross sections can be achieved within the hard scattering scheme including diquarks.