arXiv subjects

J. T. Londergan

Publications and source records attributed to J. T. Londergan.

48 records · Page 3Linked to original sources

arXiv1998

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.

hep-ph↗

arXiv1998

Flavor and Charge Symmetry in the Parton Distributions of the Nucleon

Recent calculations of charge symmetry violation(CSV) in the valence quark distributions of the nucleon have revealed that the dominant symmetry breaking contribution comes from the mass associated with the spectator quark system.Assuming that the change in the spectator mass can be treated perturbatively, we derive a model independent expression for the shift in the parton distributions of the nucleon. This result is used to derive a relation between the charge and flavor asymmetric contributions to the valence quark distributions in the proton, and to calculate CSV contributions to the nucleon sea. The CSV contribution to the Gottfried sum rule is also estimated, and found to be small.

nucl-th↗

arXiv1998

Validity of Flavor Symmetry and Charge Symmetry for Parton Distributions

Recent experimental measurements of the Gottfried Sum Rule, and pp and pD Drell-Yan processes, suggest significant violation of flavor symmetry in the proton sea. This interpretation rests on the assumption of parton charge symmetry. Our model calculations suggest charge symmetry violation [CSV] for parton valence distributions of a few percent. Precision measurements of structure functions in muon and neutrino experiments allow us to set rather stringent experimental limits on CSV in certain kinematic regions. In another region, these experiments suggest substantial CSV effects. We suggest experiments which could test parton CSV.

hep-ph↗

arXiv1998

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.

hep-ph↗

arXiv1998

Chiral Dynamics of Low-Energy Kaon-Baryon Interactions with Explicit Resonance

The processes involving low energy $\bar{K}N$ and $Yπ$ interactions (where $Y= Σ$ or $Λ$) are studied in the framework of heavy baryon chiral perturbation theory with the $Λ$(1405) resonance appearing as an independent field. The leading and next-to-leading terms in the chiral expansion are taken into account. We show that an approach which explicitly includes the $Λ$(1405) resonance as an elementary quantum field gives reasonable descriptions of both the threshold branching ratios and the energy dependence of total cross sections.

nucl-th↗

arXiv1998

Shadowing in neutrino deep inelastic scattering and the determination of the strange quark distribution

We discuss shadowing corrections to the structure function $F_2$ in neutrino deep-inelastic scattering on heavy nuclear targets. In particular, we examine the role played by shadowing in the comparison of the structure functions $F_2$ measured in neutrino and muon deep inelastic scattering. The importance of shadowing corrections in the determination of the strange quark distributions is explained.

hep-ph↗

arXiv1997

Using Drell-Yan Processes to Probe Nucleon and Meson Structure Functions

We investigate how Drell-Yan processes can be used to measure the magnitude of flavor symmetry violation in the proton sea. We examine the utility of the following beams: protons, charged pions, and charged kaons. In each case we present an approximate expression for the Drell-Yan asymmetry. Using currently available parton distributions, we locate those kinematic regions which provide the greatest information on the quantity $\bar{d}^p(x) - \bar{u}^p(x)$. If sufficiently intense $K^+$ beams were available, they could provide an efficient measurement of this quantity. Finally we present and discuss sets of sum rules for the Drell-Yan processes.

hep-ph↗

arXiv1997

Testing Parton Charge Symmetry at HERA

There are strong theoretical indications that the minority valence quark distributions in the nucleon may break charge symmetry by as much as 3-5%. We show that a comparison of $e^{\pm} D$ deep inelastic scattering through the charged current, for example at HERA, could provide a direct test of this effect. This measurement is also be sensitive to an intrinsic component of the strange quark sea which leads to $s \neq \bar{s}$.

hep-ph↗

arXiv1996

Kaon-Nucleus Drell-Yan Processes and Kaon Structure Functions

We investigate the information which could be obtained from Drell-Yan processes with sufficiently intense beams of charged kaons on isoscalar targets. It is found that combinations of $K^+$ and $K^-$ Drell-Yan measurements on isoscalar nuclear targets would allow one to extract the kaon sea quark distributions. These cross sections are also sensitive to the strange valence quark distribution in the kaon, although one would need a significant increase over the presently available kaon fluxes in order to extract this quantity with sufficient accuracy.

hep-ph↗

arXiv1996

Probing Charge-Symmetry-Violating Quark Distributions in Semi-Inclusive Leptoproduction of Hadrons

Recent experiments by the HERMES group at HERA are measuring semi-inclusive electroproduction of pions from deuterium. We point out that by comparing the production of $π^+$ and $π^-$ from an isoscalar target, it is possible, in principle, to measure charge symmetry violation in the valence quark distributions of the nucleons. It is also possible in the same experiments to obtain an independent measurement of the quark fragmentation functions. We review the information which can be deduced from such experiments and show the ``signature'' for charge symmetry violation in such experiments. Finally, we predict the magnitude of the charge symmetry violation, from both the valence quark distributions and the pion fragmentation function, which might be expected in these experiments.

hep-ph↗

arXiv1995

Composite vertices that lead to soft form factors

The momentum-space cut-off parameter $Λ$ of hadronic vertex functions is studied in this paper. We use a composite model where we can measure the contributions of intermediate particle propagations to $Λ$. We show that in many cases a composite vertex function has a much smaller cut-off than its constituent vertices, particularly when light constituents such as pions are present in the intermediate state. This suggests that composite meson-baryon-baryon vertex functions are rather soft, i.e., they have Λconsiderably less than 1 GeV. We discuss the origin of this softening of form factors as well as the implications of our findings on the modeling of nuclear reactions.

nucl-th↗

arXiv1995

DRELL-YAN PROCESSES AS A PROBE OF CHARGE SYMMETRY VIOLATION IN THE PION AND NUCLEON

We extend earlier investigations of charge symmetry violation in the valence quark distributions of the nucleon, and make similar estimates for the pion. The sensitivity of pion-induced Drell-Yan measurements to such effects is then examined. It is shown that combinations of $π^+$ and $π^-$ data on deuterium and hydrogen are sensitive to these violations, and that the pion and nucleon charge symmetry violating terms separate as a function of $x_π$ and $x_N$ respectively. We estimate the background terms which must be evaluated to extract charge symmetry violation.

hep-ph↗