arXiv subjects

Paul F. Mende

Publications and source records attributed to Paul F. Mende.

7 recordsLinked to original sources

arXiv1994

High Energy String Collisions in a Compact Space

When high energy strings scatter at fixed angle, their amplitudes characteristically fall off exponentially with energy, ${\cal A} \sim \exp(-s \times const.)$. We show that in a compact space this suppression disappears for certain kinematic configurations. Amplitudes are power-law behaved and therefore greatly enhanced. In spacetime this corresponds to fixed-angle scattering, with fixed transfer in the compact dimensions. On the worldsheet this process is described by a stationary configuration of effective charges and vortices with vanishing total energy. It is worldsheet duality---and not spacetime duality---that plays a role.

hep-th↗

arXiv1994

Form Factors in the Heavy Quark and Chiral Limit: Pole dominance in $\bar B\to πe \barν_e$

It is shown that the form factors in the semileptonic decay of ground state heavy mesons to light pseudoscalar mesons are dominated by the vector meson pole at all momentum transfers. First, the general approaches to modeling form factors in terms of quark model, pole dominance, or hybrid shapes are reviewed. It is then shown that in the combined limits of heavy quarks, chiral symmetry, and large~$\Nc$ the form factor is precisely pole-dominated. It is also shown that in a two-dimensional QCD model pole-dominance occurs for any heavy quark mass. Corrections to this limit are discussed and, to illustrate the nature of the approximations, an explicit exact calculation of the approach to the limit in a two-dimensional model is given.

hep-ph↗

arXiv1993

Exact Heavy to Light Meson Form Factors in the Combined Heavy Quark, Large $N_c$ and Chiral Limits

We demonstrate that the form factors of local operators between a heavy meson state (like the~$B$) and a light pseudoscalar state (like the pion) are given exactly by a single pole form in the combined heavy quark, large $N_c$ (number of colors) and chiral limits. We discuss the deviations from this exact result from finite heavy quark masses, non-zero light quark masses and finite $N_c$. We comment on some implications of this result.

hep-ph↗

arXiv1992

Semiclassical Tunneling in 1+1 Dimensional String Theory

We describe time-dependent tunneling of massless particles in 1+1 dimensional string field theory. Polchinski's description of the classical solutions in terms of the Fermi sea is used to identify the leading instanton contribution connecting the two half-lines. The field theory lagrangian is proportional to $1/g^2$, where $g$ is the string coupling constant, but the $S$-matrix for tunneling from one half-line to the other behaves as $\exp(-C/g)$. We note the constant~$C$ involves curious boundary contributions and observe that a prescription connecting the two half-lines unifies treatments of the Fermi level above and below the barrier. We also note the relation to recent work of Brustein and Ovrut.

hep-th↗

arXiv1992

On Constraints for Heavy-Meson Form Factors

We examine the recent work of de Rafael and Taron where model-independent bounds on the Isgur-Wise function are presented. We first argue that the bounds cannot hold in as much generality as implied. We show that the effects of resonances omitted in their discussion (such as heavy-heavy ``onium'' states below threshold) modify the bound. The resulting bound is much weaker but may be useful where the size of the additional contribution may be computed or estimated.

hep-ph↗

arXiv1992

String Theory at Short Distance and the Principle of Equivalence

Point particles fall freely along geodesics; strings do not. In string theory all probes of spacetime structure, including photons, are extended objects and therefore always subject to tidal forces. We illustrate how string theory modifies the behavior of light in weak gravitational fields and limits the applicability of the principle of equivalence. This gives in principle a window on the short-distance structure of geometry in quantum gravity where one can see in a model-independent way how some of its predictions differ from those of classical gravity. We compare this with the lessons of high-energy string scattering.

hep-th↗

arXiv1992

Heavy Mesons in Two Dimensions

The large mass limit of QCD uncovers symmetries that are not present in the QCD lagrangian. These symmetries have been applied to physical (finite mass) systems, such as B and D mesons. We explore the validity of this approximation in the 't Hooft model (two-dimensional QCD in the large-N approximation). We find that the large mass approximation is good, even at the charm mass, for form factors, but it breaks down for the pseudoscalar decay constant.

hep-ph↗