903 research outputs found
What is Measured in Hard Exclusive Electroproduction?
We examine the relation between amplitudes measured in exclusive
lepto-production and the quark content of the nucleon. We show that in the
limit of high energy and small t, the natural interpretation of amplitudes
measured in these hard exclusive processes is in terms of the quark content of
the meson cloud and not the target itself. In this regime, Regge amplitudes
will make a significant contribution to these exclusive amplitudes. This leads
to violation of QCD scaling.Comment: 6 pages, 2 figure
Identification of non-ordinary mesons from the dispersive connection between their poles and their Regge trajectories: the f0(500) resonance
We show how the Regge trajectory of a resonance can be obtained from its pole
in a scattering process and analytic constraints in the complex angular
momentum plane. The method is suited for resonances that dominate an elastic
scattering amplitude. In particular, from the rho(770) resonance pole in
pion-pion scattering, we obtain its linear Regge trajectory, characteristic of
ordinary quark-antiquark states. In contrast, the f0(500) pole -the sigma
meson- which dominates scalar isoscalar pion-pion scattering, yields a
non-linear trajectory with a much smaller slope at the f0(500) mass.
Conversely, imposing a linear Regge trajectory for the f0(500), with a slope of
typical size, yields an elastic amplitude at odds with the data. This provides
strong support for the non-ordinary nature of the sigma meson.Comment: 8 pages, 4 figure
Illuminating the 1/x moment of parton distribution functions
The Weisberger relation, an exact statement of the parton model, elegantly
relates a high-energy physics observable, the 1/x moment of parton distribution
functions, to a nonperturbative low-energy observable: the dependence of the
nucleon mass on the value of the quark mass or its corresponding quark
condensate. We show that contemporary fits to nucleon structure functions fail
to determine this 1/x moment; however, deeply virtual Compton scattering can be
described in terms of a novel F_{1/x}(t) form factor which illuminates this
physics. An analysis of exclusive photon-induced processes in terms of the
parton-nucleon scattering amplitude with Regge behavior reveals a failure of
the high Q^2 factorization of exclusive processes at low t in terms of the
Generalized Parton-Distribution Functions which has been widely believed to
hold in the past. We emphasize the need for more data for the DVCS process at
large t in future or upgraded facilities.Comment: 11 pages, 3 figures, invited contribution to the 11th International
Conference on Meson-Nucleon Physics and the Structure of the Nucleon, sept.
10th-14th 2007, Juelich, German
Regge trajectory of the f_0(500) resonance from a dispersive connection to its pole
We report here our results on how to obtain the Regge trajectory of a
resonance from its pole in a scattering process by imposing analytic
constraints in the complex angular momentum plane. The method, suited for
resonances that dominate an elastic scattering amplitude, has been applied to
the {\rho}(770) and the f_0(500) resonances. Whereas for the former we obtain a
linear Regge trajectory, characteristic of ordinary quark-antiquark states, for
the latter we find a non-linear trajectory with a much smaller slope at the
resonance mass. This provides a strong indication of the non-ordinary nature of
the sigma meson.Comment: 4 pages, 2 figures, to appear in the proceedings of the "Seventh
International Symposium on Chiral Symmetry in Hadrons and Nuclei
Separation of Soft and Hard Physics in DVCS
A model for deeply virtual Compton scattering, based on analytical light-cone
hadron wave functions is presented and studied at energies currently accessible
at Jefferson Laboratory and DESY. It is shown that poles and perpendicular
vector components play an important role at Q^2 < 10 GeV^2. A Q^2 suppressed
diagram has to be included at these low energies, but becomes negligible above
10 GeV^2. Future prospects and developments of this model are discussed.Comment: 6 pages, 2 figures, uses ws-procs9x6.cls, to appear in Proceedings of
the Exclusive Processes at High-t Workshop at Jefferson Lab, May 15-18 200
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