arXiv subjects

Mark W. Paris

Publications and source records attributed to Mark W. Paris.

24 records · Page 2Linked to original sources

arXiv2003

Spin asymmetries for confined Dirac particles

We study the Bjorken `x' (or equivalently Nachtmann `xi') dependence of the virtual photon spin asymmetry in polarized deep inelastic scattering of electrons from hadrons. We use an exactly solved relativistic potential model of the hadron, treating the constituents as independent massless Dirac particles confined to an infinitely massive force center. The importance of including the p-wave components of the Dirac wave function is demonstrated. Comparisons are made to the observed data on the proton by taking into account the observed flavor dependence of the valence quark distribution functions.

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arXiv2003

Electromagnetic response of confined Dirac particles

The eigenstates of a single massless Dirac particle confined in a linear potential are calculated exactly by direct solution of the Dirac equation. The electromagnetic structure functions are calculated from the Dirac wave functions of the ground and excited states of the particle by coupling to its conserved vector current. We obtain the longitudinal and transverse structure functions as a function of y=nu-q, where nu and q are the energy and momentum transferred to the target in its rest frame. At values of q>~2.5 GeV, much larger than the characteristic energy scale ~440 MeV of the confining potential, the response exhibits y scaling, a generalization of Bjorken scaling. We compare the exact structure functions with those obtained from the ground state wave functions in the plane wave impulse approximation. The deviation from the Callan-Gross relation is compared with the parton model prediction.

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arXiv2002

Spacelike and timelike response of confined relativistic particles

Basic theoretical issues relating to the response of confined relativistic particles are considered including the scaling of the response in spacelike and timelike regions of momentum transfer and the role of final state interactions. A simple single particle potential model incorporating relativity and linear confinement is solved exactly and its response is calculated. The response is studied in common approximation schemes and it is found that final state interactions effects persist in the limit that the three-momentum transferred to the target is large. The fact that the particles are bound leads to a non-zero response in the timelike region of four-momentum transfer equal to about 10% of the total strength. The strength in the timelike region must be taken into account to fulfill the particle number sum rule.

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arXiv2001

Final state interaction contribution to the response of confined relativistic particles

We report studies of the response of a massless particle confined by a potential. At large momentum transfer q it exhibits \tilde{y} or equivalently Nachtmann ξscaling, and acquires a constant width independent of q. This width has a contribution from the final state interactions of the struck particle, which persists in the q->\infty limit. The width of the response predicted using plane wave impulse approximation is smaller because of the neglect of final state interactions in that approximation. However, the exact response may be obtained by folding the approximate response with a function representing final state interaction effects. We also study the response obtained from the momentum distribution assuming that the particle is on the energy shell both before and after being struck. Quantitative results are presented for the special case of a linear confining potential. In this case the response predicted with the on-shell approximation has correct values for the total strength, mean energy and width, however its shape is wrong.

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arXiv2001

Scaling of space and timelike response of confined relativistic particles

The response of a relativistic particle bound in a linear confining well is calculated as a function of the momentum and energy transfer, q, ν. At large values of |q| the response exhibits scaling in the variable y=ν-|q|, which is proportional to the Nachtmann variable, ξ. The approach to scaling is studied at smaller values of |q|. Scaling occurs at ν~|q| at relatively small |q|, and its validity extends over the entire ξrange as |q| increases; this behavior is observed in electron-proton scattering. About 10% of the response at large |q| is in the timelike region where ν>|q|, and it is necessary to include it to fulfill the particle number sum rule. The Gross-Llewellyn Smith and Gottfried sum rules are discussed in the context of these results.

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arXiv2000

Quantum Monte Carlo calculations of six-quark states

The variational Monte Carlo method is used to find the ground state of six quarks confined to a cavity of diameter R_c, interacting via an assumed non-relativistic constituent quark model (CQM) Hamiltonian. We use a flux-tube model augmented with one-gluon and one-pion exchange interactions, which has been successful in describing single hadron spectra. The variational wave function is written as a product of three-quark nucleon states with correlations between quarks in different nucleons. We study the role of quark exchange effects by allowing flux-tube configuration mixing. An accurate six-body variational wave function is obtained. It has only ~13% rms fluctuation in the total energy and yields a standard deviation of ~<.1%; small enough to be useful in discerning nuclear interaction effects from the large rest mass of the two nucleons. Results are presented for three values of the cavity diameter, R_c=2, 4, and 6 fm. They indicate that the flux-tube model Hamiltonian with gluon and pion exchange requires revisions in order to obtain agreement with the energies estimated from realistic two-nucleon interactions. We calculate the two-quark probability distribution functions and show how they may be used to study and adjust the model Hamiltonian.

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