I review recent progress in understanding the structure of the nucleon sea and the role of the nucleon’s pion cloud. In particular, I discuss the consequences of the pion cloud for the d̄ − ū asymmetry in the proton, the neutron’s electric form factor, and the proton’s electric to magnetic form factor ratio.

A natural SL(2,Z)-invariant generalization of the Veneziano amplitude in type IIB superstring theory is investigated. It includes certain perturbative and non-perturbative (D-instanton) contributions, and it reduces to the correct expressions in different limits. The singularities are poles in the s-t-u channels, corresponding to the exchange of particles with a mass spectrum coinciding with th...

The presentation is concerned with higher twist corrections of nonpartonic origin to semi-inclusive observables in the kinematical region relevant for the HERMES experiment. We demonstrate a strong impact of the VDM-like interaction and the exclusive production of ρ meson on the extraction of the d̄ − ū asymmetry from charged pion DIS multiplicities. We also show that it is the exclusive-ρ chann...

Consider the Cauchy problem for a hyperbolic n × n system of conservation laws in one space dimension: ut + f(u)x = 0, u(0, x) = ū(x). (CP ) Relying on the existence of a continuous semigroup of solutions, we prove that the entropy admissible solution of (CP) is unique within the class of functions u = u(t, x) which have bounded variation along a suitable family of space-like curves.

Consider the hyperbolic system of conservation laws ut + F (u)x = 0. Let u be the unique viscosity solution with initial condition u(0, x) = ū(x), and let u be an approximate solution constructed by the Glimm scheme, corresponding to the mesh sizes ∆x, ∆t = O(∆x). With a suitable choice of the sampling sequence, we prove the estimate ∥∥uG(t, ·)− u(t, ·)∥∥ L1 = o(1) · √ ∆x ∣∣ ln(∆x)∣∣.

The parareal algorithm seeks to extract parallelism in the time-integration direction of time-dependent differential equations. While it has been applied with success to a wide range of problems, it suffers from some stability issues when applied to non-dissipative problems. We express the method through an iteration matrix and show that the problematic behavior is related to the non-normal str...

The normal mode approach to investigating the stability of a parallel shear flow involves the superposition of a small wavelike perturbation on the basic flow. Its evolution in space and/or time is then determined. In the linear inviscid theory, if ū(y) is the basic velocity profile, then a singularity occurs at critical points yc, where ū = c, the perturbation phase speed. This is plausible in...

Violation of the Gottfried sum rule was suggested by the New Muon Collaboration in measuring proton and deuteron F2 structure functions. The finding triggered many theoretical studies on physics mechanisms for explaining the antiquark flavor asymmetry ū − d̄ in the nucleon. Various experimental results and proposed theoretical ideas are summarized. Possibility of finding the flavor asymmetry in ...

In this paper we study genericity for the following parameterized class of nonlinear programs: minimize fu(x) := f(x)− 〈u, x〉 subject to x ∈ S, where f : R → R is a polynomial function and S ⊂ R is a closed semialgebraic set, which is not necessarily compact. Assume that the constraint set S is regular. It is shown that there exists an open and dense semialgebraic set U ⊂ R such that for any ū ...