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F. M. Steffens

Publications and source records attributed to F. M. Steffens.

32 records · Page 2Linked to original sources
arXiv1999

Meson Cloud and SU(3) Symmetry Breaking in Parton Distributions

We apply the Meson Cloud Model to the calculation of nonsinglet parton distributions in the nucleon sea, including the octet and the decuplet cloud baryon contributions. We give special attention to the differences between nonstrange and strange sea quarks, trying to identify possible sources of SU(3) flavor breaking. A analysis in terms of the $κ$ parameter is presented, and we find that the existing SU(3) flavor asymmetry in the nucleon sea can be quantitatively explained by the meson cloud. We also consider the $Σ^+$ baryon, finding similar conclusions.

hep-ph↗
arXiv1999

Scheme Independence of $g_1^p (x, Q^2)$

We work with two general factorization schemes in order to explore the consequences of imposing scheme independence on $g_1^p (x, Q^2)$. We see that although the light quark sector is indifferent to the choice of a particular scheme, the extension of the calculations to the heavy quark sector indicates that a scheme like the $\bar{MS}$ is preferable.

hep-ph↗
arXiv1999

A New Analysis of Charge Symmetry Violation in Parton Distributions

To date, the strongest indication of charge symmetry violation in parton distributions has been obtained by comparing the $F_2$ structure functions from CCFR neutrino data and NMC muon data. We show that in order to make precise tests of charge symmetry with the neutrino data, two conditions must be satisfied. First, the nuclear shadowing calculations must be made explicitly for neutrinos, not simply taken from muon data on nuclei. Second, the contribution of strange and charm quarks should be calculated explicitly using next-to-leading order [NLO] QCD, and the ``slow rescaling'' charm threshold correction should not be applied to the neutrino data. When these criteria are satisfied, the comparison is consistent with charge symmetry within the experimental errors and the present uncertainty in the strange quark distribution of the nucleon.

hep-ph↗
arXiv1999

Charm in the Nucleon

A next-to-leading order analysis of inelastic electroproduction of charm is performed using an interpolating scheme which maps smoothly onto massless QCD evolution at large Q^2 and photon-gluon fusion at small Q^2. In contrast with earlier analyses, this scheme allows the inclusion of quark and target mass effects and heavy quark thresholds, as well as possible non-perturbative, or intrinsic, charm contributions. We find no conclusive evidence in favor of an intrinsic charm component in the nucleon, although several data points which disagree with perturbative QCD expectations will need to be checked by future experiments.

hep-ph↗
arXiv1998

Spin Dependent Parton Distributions in a Bound Nucleon

The determination of the spin distribution functions for the neutron requires an understanding of the changes induced by a nuclear medium. We present the first estimates of the changes induced in the spin-dependent, valence parton distributions of the proton and neutron bound in $^3$He and $^6$Li. For $^3$He the result is quite reassuring, in that the off-shell correction to $g_{1n}$ is very small. On the other hand, in $^6$Li the corrections to the spin distributions in the bound proton are sufficiently large that they should be taken into account if one aims for a precision of better than 10%. We provide a simple parametrization of the off-shell changes in the valence spin distributions of $^3$He and $^6$Li, which can be used with any conventional estimate of nuclear binding and Fermi motion corrections.

nucl-th↗
arXiv1998

Heavy Quarks in Polarized Structure Functions

Quark mass effects are included in the calculation of polarized structure functions. In particular, the validity of fixed order perturbation theory and of massless evolution is studied in the framework of heavy quarks structure functions. The polarized version of the ACOT and MRRS interpolating schemes for the evolution of massive quarks distributions is also developed and studied. The different behaviours of the various approaches in $x$ and $Q^2$ are shown.

hep-ph↗
arXiv1997

Measuring anomalous ``spin'' in elastic e-p or ν-p and deep inelastic e-p scattering

We obtain a general rule that the O(1/log m_h) term due to the current \bar{h}γ_μγ_5h of a mass-m_h quark h in f-flavour theory is -3\bar{g}^2_f(m_h)/{2π^2(33-2f)} times the flavour singlet current of the residual (f-1)-flavour theory, where \bar{g}_f is the f-flavour running coupling constant in a mass-independent renormalization scheme. The rule is applied to the Ellis-Jaffe moment below and well above charm threshold, and to low-energy Z^0-exchange amplitudes. The singlet axial charge of the proton common to these experiments is both scale and gauge invariant, but is related to the axial anomaly and the ``gluon spin'' by a non-perturbative renormalization factor.

hep-ph↗
arXiv1996

The Flavor Asymmetry of the Nucleon Sea

We re-examine the effects of anti-symmetry on the anti-quarks in the nucleon sea arising from gluon exchange and pion exchange between confined quarks. While the effect is primarily to suppress anti-down relative to anti-up quarks, this is numerically insignificant for the pion terms.

nucl-th↗
arXiv1996

On the Interpretation of the NA51 Experiment

We study the $p-n$ Drell-Yan asymmetry, recently measured by the NA51 collaboration, and conclude that the value quoted by their experiment only sets a lower limit on the asymmetry of the proton sea. In particular, we notice that charge symmetry breaking between the proton and the neutron may produce corrections which should be taken into account.

hep-ph↗
arXiv1995

The Role of Strange and Charm Quarks in the Nucleon Spin Structure Function

We perform an analysis of the relation between the factorization scale and the masses of the quarks in the calculation of the hard gluon coefficient in polarized deep inelastic scattering. Particular attention is paid to the role of strange and charm quarks at finite momentum transfer. It is found that for the momentum transfer of the present experiments, the contribution from the charm quark is significant.

hep-ph↗
arXiv1995

A Study of Structure Functions for the Bag Beyond Leading Order

There has recently been surprising progress in understanding the spin and flavor dependence of deep inelastic structure functions in terms of the same physics needed in the simple quark models used for hadronic spectroscopy. However, the corresponding scale is usually very low, casting doubt on the use of leading order $QCD$ evolution. We show that the conclusions are not significantly altered if one goes to next-to-leading order. In particular, the excellent agreement with unpolarized and polarized valence quark distributions is retained.

hep-ph↗
arXiv1995

Mesonic Corrections to the Shape of Quark Distributions

We compute the full x - dependence of the proton and neutron spin structure functions in the MIT bag model, including the effect of gluon exchange and the meson cloud. Impressive agreement is found for x larger than 0.1, where polarised gluons are not expected to play a significant role.

hep-ph↗
arXiv1993

Vibrational Aspects of the SU(2) Skyrmion

We treat the Skyrme model with the breathing mode in a situation involving two quartic terms. It is seen that there is a new limit for large $e$ due to the breathing mode not found in the usual rotating hedgehog.

hep-ph↗