arXiv subjects

Stefano Forte

Publications and source records attributed to Stefano Forte.

170 records · Page 10Linked to original sources

arXiv1993

The Light-flavor structure of the nucleon

Recent data on the Gottfried sum and less recent ones on the pion-nucleon sigma term seem to disagree with naive parton-based expectations on the light (i.e., up, down and strange) quark content of the nucleon. We show that these discrepancies are resolved if nonperturbative contributions are included in the analysis of the data. These appear both in the computation of matrix elements of operators, and in their QCD scale dependence, and depend strongly on the quantum numbers of the given state.

hep-ph↗

arXiv1993

Anomalous Evolution of the Gottfried Sum

We discuss nonperturbative QCD evolution of nonsinglet nucleon structure functions, with particular application to the Gottfried sum. We show that the coupling of the quark partons to bound state mesons leads to nonperturbative contributions to the Altarelli--Parisi equations which, due to the axial anomaly, result in a strong scale dependence of nonsinglet structure functions for values of $Q^2$ around the nucleon mass scale. We compute specifically the evolution of the first moment of the quark distribution, and find that it is sufficient to explain recent experimental data which indicate a violation of the Gottfried sum rule.

hep-ph↗

arXiv1993

Relativistic Quantum Theory with Fractional Spin and Statistics

These lectures discuss the formulation of quantum mechanics with fractional spin and statistics in 2+1 dimensions in a relativistic setting, emphasizing the path-integral approach. The non-relativistic theory is reviewed from a path-integral viewpoint. The group-theoretical underpinnings of relativistic fractional spin are discussed, then the path-integral quantization of spin and of massive fermions without using spinors is reviewed. The path integral for a system of n relativistic particles with fractional spin is constructed, the spin-statistics relation and the Lorentz and Poincare' representation content of physical states are discussed. Some problems in formulating a field theory with fractional statistics are presented, and their resolution in the operator cocycle approach is reviewed.

hep-th↗

arXiv1993

A Dynamical Resolution of the Sigma Term Puzzle

We propose a resolution of the puzzle posed by the discrepancy between the pion-nucleon sigma term inferred from pion-nucleon scattering, and that deduced from baryon mass splittings using the Zweig rule. We show that there is a significant hypercharge-dependent dynamical contribution to baryon masses, not hitherto included in the analysis, which may be estimated using the scale Ward identity, and computed by solution of the Schwinger-Dyson equation for the quark self-energy. We find that the discrepancy is completely resolved without the need for any Zweig rule violation.

hep-ph↗

arXiv1993

Anomaly-Induced Magnetic Screening in 2+1 dimensional QED at Finite Density

We show that in 2+1 dimensional Quantum Electrodynamics an external magnetic field applied to a finite density of massless fermions is screened, due to a $2+1$-dimensional realization of the underlying $2$-dimensional axial anomaly of the space components of the electric current. This is shown to imply screening of the magnetic field, i.e., the Meissner effect. We discuss the physical implications of this result.

hep-th↗

arXiv1993

The sigma term and the quark number operator in QCD

We discuss the relationship of the forward matrix element of the operator $\barψψ$, related to the so-called sigma term, to the quark number. We show that in the naive quark model in the canonical formalism these quantities coincide in the limit of small average quark momenta. In the QCD parton model defined through light-front quantization this result is preserved at leading perturbative order but it receives radiative corrections. We analyze the theoretical and phenomenological consequences of this result, which provides a bridge between a current algebra quantity, the sigma term, and a deep-inelastic quantity, the parton number.

hep-ph↗

arXiv1992

Violation of the Bjorken Sum Rule in QCD

We show that the Bjorken sum rule for the first moment of the polarized nucleon structure function $g_1$ is not necessarily satisfied in QCD if the axial anomaly is taken into account. We argue that nonperturbative QCD effects should lead to a violation of the sum rule of the order of the percentage isospin violation in current quark masses.

hep-ph↗

arXiv1992

Small Angle Polarization in High Energy P--P Scattering Through Nonperturbative Chiral Symmetry Breaking

We show that a large anomalous contribution due to nonperturbative instanton-like gluonic field configurations to the axial charge of the proton implies high-energy spin effects in $p-p$ elastic scattering. This is the same mechanism which is responsible for anomalous baryon number violation at high energy in the standard model. We compute the proton polarization due to these effects and we show that it is proportional to the center-of-mass scattering angle with a universal (energy-independent) slope of order unity.

hep-ph↗