192 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
The Validity of Charge Symmetry for Parton Distributions
Recent measurements of the Gottfried sum rule have focused attention on the
possibility of substantial flavor symmetry breaking in the proton sea. This is
confirmed by pp and pD Drell-Yan processes measured at FNAL. Theoretical models
used to `prove' flavor symmetry breaking rely on the assumption of charge
symmetric parton distributions. Substantial charge symmetry violation [CSV]
could affect current tests of flavor symmetry. In this review we examine the
possibility of CSV for parton distributions. We first give definitions for
structure functions without making the usual assumption of charge symmetry.
Next we make estimates of CSV for both valence and sea quark distributions. We
list a set of relations which must hold if charge symmetry is valid, and review
current experimental limits on CSV. We propose a series of experimental tests
of charge symmetry. These tests could either detect CSV, or they could provide
stronger upper limits on CSV effects. We discuss CSV contributions to sum
rules, and we propose sum rules which could differentiate between flavor
symmetry, and charge symmetry violation in nuclear systems.Comment: 74 pages, including 26 figure
Evidence for Substantial Charge Symmetry Violation in Parton Distributions
In principle one can test the validity of charge symmetry for parton
distributions by comparing structure functions measured in neutrino and charged
lepton deep inelastic scattering. New experiments make such tests possible;
they provide rather tight upper limits on parton charge symmetry violation
[CSV] for intermediate Bjorken x, but appear to show evidence for CSV effects
at small x. We examine two effects which might account for this experimental
discrepancy: nuclear shadowing corrections for neutrinos, and strange quark
contributions s(x) unequal to sbar(x). We show that neither of these two
corrections removes the experimental discrepancy between the structure
functions. We are therefore forced to consider the possibility of a
surprisingly large CSV effect in the nucleon sea quark distributions.Comment: 10 pages, 3 postscript figure
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