45792390
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) 6 ¯(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. PACS: 13.60.Hb, 13.15.+g, 12.40.Vv, 11.40.Ha
55909393
Quarks in finite nuclei
We describe the development of a theoretical description of the structure of finite nuclei based on a relativistic quark model of the structure of the bound nucleons which interact through the (self-consistent) exchange of scalar and vector mesons.
119930609
The EMC spin effect
Attempts to understand the breakdown of the Ellis-Jaffe sum-rule observed by the European Muon Collaboration have led a new insight into the QCD parton model. The authors provide an overview of this work with emphasis on the role of the U(1) axial anomaly. Particular attention is given to the role that future experimental work might play in resolving the key theoretical questions.
54008667
Anatomy of relativistic pion loop corrections to the electromagnetic nucleon coupling
We present a relativistic formulation of pion loop corrections to the coupling of photons with nucleons on the light front. Vertex and wave function renormalization constants are computed to lowest order in the pion field, including their nonanalytic behavior in the chiral limit, and studied numerically as a function of the ultraviolet cutoff. Particular care is taken to explicitly verify gauge invariance and Ward-Takahashi identity constraints to all orders in the mexpansion. The results are used to compute the chiral corrections to matrix elements of local operators, related to moments of deep-inelastic structure functions. Finally, comparison of results for pseudovector and pseudoscalar coupling allows the resolution of a longstanding puzzle in the computation of pion cloud corrections to structure function moments. I. INTRODUCTION The importance of chiral symmetry in hadron physics has been understood for more than 50 years. As an explicit Lagrangian representation of the approximate chiral sym- metry that had been recognized in low-energy pion- nucleon interactions, Gell-Mann and Levy (1) constructed the extremely successful linear sigma model. In that model the pion couples to the nucleon through pseudoscalar coupling, while an additional scalar (� ) field is also coupled linearly. At a more formal level, Gell-Mann (2) proposed SUð2 Þ� SUð2Þ as an exact algebra for the charges associated with the Hamiltonian governing the strong interaction, even though chiral symmetry was not an exact symmetry of that Hamiltonian. On the basis of current algebra one can show very gen- erally that the amplitude for pion scattering or production must vanish as the four-momentum of the pion vanishes (3,4). Within the linear sigma model, this important result for low-energy pion-nucleon scattering, for example, is only possible through a subtle cancellation of two large contri- butions, the first involving pion emission and absorption through the pseudoscalar (PS) coupling, and the second involvingexchange in the t channel. Keeping track of the necessary cancellations between such large terms in the linear sigma model is tedious and for that reason modern formulations of chiral effective field theory tend to prefer a Lagrangian formulation based on a nonlinear realization of chiral symmetry (5-8). In such a formulation the natural �NN vertex involves pseudovector (PV) coupling and the vanishing of pion-nucleon scattering amplitudes as the pion four-momentum vanishes emerges trivially. In this work, motivated by the phenomenological simplicity of enforcing soft-pion theorems, we focus on the case of PV coupling. However, since the linear realization of chiral symmetry is still used in the literature, for completeness we also compare our results with those for PS coupling. More recently, chiral symmetry and the pion cloud of the nucleon have been shown to play a central role in under- standing various flavor and spin asymmetries in quark distribution functions measured at high energies. Most prominent of these has been the SU(2) flavor asymmetry in the proton sea, with the large excess of � d quarks over �
122821193
The NJL‐jet model for quark fragmentation functions
A description of fragmentation functions which satisfy the momentum and isospin sum rules is presented. We concentrate on the pion fragmentation function, taking into account cascade‐like processes in a generalized jet‐model approach. Numerical results obtained in this NJL‐jet model are presented.
122078954
Proton pole contribution in proton decay
We show that nucleon pole terms contribute significantly to proton decay, increasing the branching ratio for e+π0 and νRπ+ modes. The total lifetime is decreased to the point where keeping the SU(5) value of the lifetime below the experimental upper bound forces us to the choice of QCD scale parameter and proton radius parameter which are near the edge of the allowed region.
18353661
Super-Sample CMB Lensing
Lensing of the cosmic microwave background by modes that are larger than the size of the survey dilates intrinsic scales in the temperature and polarization fields and coherently shifts their observed power spectra with respect to the ensemble or all-sky mean. The effect can be simply encapsulated as a contribution to the power spectrum covariance matrix in accordance with the lensing trispectrum or as an additional parameter, the mean convergence in the field, for parameter estimation. It should be included for upcoming surveys that precisely measure acoustic polarization features deep into the damping tail at multipoles of $\ensuremath{\ell}\ensuremath{\gtrsim}1500$ with less than 10% of the sky. Its omission may lead to seemingly conflicting values for the angular scale of the sound horizon, which may then provide erroneous cosmological parameters when compared to baryon acoustic oscillation measurements.
19019014
On the flavour structure of the constituent quark
We discuss the dressing of constituent quarks with a pseudoscalar meson cloud within the effective chiral quark model. symmetry breaking effects are included explicitly. Our results are compared with those of the traditional meson cloud approach in which pions are coupled to the nucleon. The pionic dressing of the constituent quarks explains the experimentally observed violation of the Gottfried sum rule and leads to an enhanced non-perturbative sea of pairs in the constituent quark and consequently in the nucleon. We find 2.5 times more pions and 10 - 15 times more kaons in the nucleon than in the traditional picture. The asymmetry obtained here is concentrated at somewhat smaller x and the ratio is somewhat different than in the traditional meson cloud model of the nucleon.