132 research outputs found
Structure functions in the bag model
In this paper we present calculations of nucleon structure functions in the three-dimensional MIT bag model. The nucleon wave functions are modified by the Peierls Yoccoz projection in order to give eigenstates of the total momentum operator. Pair creation by the probe is taken into account. Without this the quark distributions would not obey normalization requirements. The quark distributions have vanishing support for x>1. The effect of one-gluon exchange, yielding the NâÎ mass splitting, is incorporated. This has significant effects on the du ratio as well as the spin-dependent g1(x) of the neutron. Finally, the results are compared to data after allowing for perturbative QCD evolution.A. W. Schreiber, A. I. Signal, and A. W. Thoma
QCD evolution of the spin structure functions of the neutron and proton
Models of the long-distance behavior of QCD can be used to make predictions for the twist-two piece of the current correlation function appearing in deep-inelastic scattering. The connection between the model predictions, valid at a four-momentum transfer squared Q2 of the order of 1 (GeV/c)2, and the data, collected at much higher Q2, is provided by perturbative QCD. We present here the details of a method, previously used for spin-averaged lepton scattering, applied to the spin-dependent case. We then use the method to make predictions for the spin structure functions gprotonl and gneutronl.A. W. Schreiber, A. W. Thomas, and J. T. Londerga
Pion cloud of the nucleon and its effect on deep-inelastic structure
In this paper we present the calculation of deep-inelastic scattering structure functions of nucleons dressed by pions. The calculation is performed within the convolution model. We present analytic results for quark and antiquark distributions of a given flavor and spin and numerical results of those structure functions that are likely to be most affected by the dressing of the nucleon. We allow for the probe scattering off both nucleons and Îâs. Diagrams where it scatters off the pions themselves are not taken into account. For the numerical results we look only at combinations of structure functions where these do not contribute.A. W. Schreiber, P. J. Mulders, A. I. Signal, and A. W. Thoma
Variational Field Theoretic Approach to Relativistic Scattering
Nonperturbative polaron variational methods are applied, within the so-called
particle or worldline representation of relativistic field theory, to study
scattering in the context of the scalar Wick - Cutkosky model. Important
features of the variational calculation are that it is a controlled
approximation scheme valid for arbitrary coupling strengths, the Green
functions have all the cuts and poles expected for the exact result at any
order in perturbation theory and that the variational parameters are
simultaneously sensitive to the infrared as well as the ultraviolet behaviour
of the theory. We generalize the previously used quadratic trial action by
allowing more freedom for off-shell propagation without a change in the
on-shell variational equations and evaluate the scattering amplitude at first
order in the variational scheme. Particular attention is paid to the
-channel scattering near threshold because here non-perturbative effects can
be large. We check the unitarity of a our numerical calculation and find it
greatly improved compared to perturbation theory and to the zeroth order
variational results.Comment: 26 pages, Latex, 5 postscript figures embedded with epsf, submitted
to Nucl. Phys.
Mapping the x dependence of the axial anomaly in polarised deep inelastic scattering
We discuss the role of the U(1) axial anomaly in the spin structure functions
of the nucleon, with particular emphasis on how one might determine its x
dependence in present and future deep inelastic scattering experiments. We
focus on the C-odd spin structure function g3 and also the deuteron structure
function g1^d.Comment: 11pages Latex, 6 Figs. appended as .ps files after main text,
Cavendish preprint HEP-93/
Non-perturbative structure of the polarized nucleon sea
We investigate the flavour and quark-antiquark structure of the polarized
nucleon by calculating the parton distribution functions of the nucleon sea
using the meson cloud model. We find that the SU(2) flavor symmetry in the
light antiquark sea and quark-antiquark symmetry in the strange quark sea are
broken, {\it i.e.} \Delta\ubar < \Delta \dbar and \Delta s < \Delta \sbar.
The polarization of the strange sea is found to be positive, which is in
contradiction to previous analyses. We predict a much larger quark-antiquark
asymmetry in the polarized strange quark sea than that in the unpolarized
strange quark sea. Our results for both polarized light quark sea and polarized
strange quark sea are consistent with the recent HERMES data.Comment: RevTex, 17 pages plus 8 PS figure
First Order Variational Calculation of Form Factor in a Scalar Nucleon--Meson Theory
We investigate a relativistic quantum field theory in the particle
representation using a non-perturbative variational technique. The theory is
that of two massive scalar particles, `nucleons' and `mesons', interacting via
a Yukawa coupling. We calculate the general Euclidean Green function involving
two external nucleons and an arbitrary number of external mesons in the
quenched approximation for the nucleons. The non-perturbative renormalization
and truncation is done in a consistent manner and results in the same
variational functional independent of the number of external mesons. We check
that the calculation agrees with one-loop perturbation theory for small
couplings. As an illustration the special case of meson absorption on the
nucleon is considered in detail. We derive the radius of the dressed particle
and numerically investigate the vertex function after analytic continuation to
Minkowski space.Comment: 28 pages standard LaTeX, 13 uuencoded postscript figures embedded
with psfig.st
Effects of combined administration of FK 506 and the purine biosynthesis inhibitors mizoribine or mycophenolic acid on lymphocyte DNA synthesis and T cell activation molecule expression in human mixed lymphocyte cultures
Our objective was to obtain new information on the in vitro antilymphocytic action of the cytokine synthesis inhibitor FK 506 and the purine biosynthesis inhibitors mycophenolic acid (MPA; the active moiety of RS61443) and mizoribine (MZB) when used alone or in combination. When added at the initiation of six-day human mixed lymphocyte cultures (MLC), FK 506, MPA or MZB exhibited dose-dependent inhibition of T-lymphocyte DNA synthesis. FK 506, however, was 100-fold more potent than MPA, and 10000-fold more potent than MZB. Combination of FK 506 with either MPA or MZB, each at suboptional concentrations, produced no more than additive inhibitory effects on 3H thymidine incorporation. Two-colour flow cytometric analysis of lymphocytes revealed that none of the drugs affected cell surface activation molecule expression (CD25 = IL-2R 55 kD α-chain, HLA-DR or CD71 = transferrin receptor [TR]) on allostimulated CD4+ or CD8+ cells harvested at three days of culture. By day six, however, all three agents, at levels which markedly inhibited proliferation, suppressed the expression of activation markers on both CD4+ and CD8+ cells. Also at day six, inhibition of activation molecule expression on CD4+ cells was achieved with the combination of FK 506 and either MPA or MZB at concentrations which, on their own, were ineffective. These data provide new, additional information on the in vitro antilymphocytic action of FK 506, MPA and MZB when used alone and in combination. © 1993
Charge Symmetry Violating Contributions to Neutrino Reactions
The NuTeV group has measured charged and neutral current reactions for
neutrinos on iron targets. Ratios of these cross sections provide an
independent measurement of the Weinberg angle. The NuTeV value for sin^2
theta_W is three standard deviations larger than the value measured in other
electroweak processes. By reviewing theoretical estimates of parton charge
symmetry violation (CSV), we study CSV contributions to the NuTeV measurement.
We conclude that charge symmetry violating effects should remove roughly 30% of
the discrepancy between the NuTeV result and other determinations of sin^2
theta_W.Comment: 15 pages, 2 figures; Fig 2 replaced [CSV distribution calculated at
low Q^2, evolved to 20 GeV^2]; table II change
A portable high power diode laser-based single-stage ceramic tile grout sealing system
By means of a 60 W high power diode laser (HPDL) and a specially developed grout material the void between adjoining ceramic tiles has been successfully sealed. A single-stage process has been developed which uses a crushed ceramic tile mix to act as a tough, inexpensive bulk substrate and a glazed enamel surface to provide an impervious surface glaze. The single-stage ceramic tile grout sealing process yielded seals produced in normal atmospheric conditions that displayed no discernible cracks and porosities. The single-stage grout is simple to formulate and easy to apply. Tiles were successfully sealed with power densities as low as 200 kW/mm2 and at rates of up to 600 mm/min. Bonding of the enamel to the crushed ceramic tile mix was identified as being primarily due to van der Waals forces and, on a very small scale, some of the crushed ceramic tile mix material dissolving into the glaze. In terms of mechanical, physical and chemical characteristics, the single-stage ceramic tile grout was found to be far superior to the conventional epoxy tile grout and, in many instances, matched and occasionally surpassed that of the ceramic tiles themselves. What is more, the development of a hand-held HPDL beam delivery unit and the related procedures necessary to lead to the commercialisation of the single-stage ceramic tile grout sealing process are presented. Further, an appraisal of the potential hazards associated with the use of the HPDL in an industrial environment and the solutions implemented to ensure that the system complies with the relevant safety standards are given
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