Marco Fabbrichesi

Neutrino mass and lepton mixing hierarchies and future oscillation experiments

Abstract Future experiments searching for neutrino oscillations are discussed on the assumption of the existence of a hierarchy in the neutrino masses. Different scenarios for the hierarchy of coupling among lepton families are considered. Relations and inequalities among transition probabilities are derived. The inequalities follow the pattern in the hierarchy of coupling among lepton generations. Simpsons 17 keV neutrino is discussed. It is shown that if such a heavy neutrino exist, the probability of the transition v e → v τ will not exceed approximately 2×10 −2 independently of the nature (Dirac or Marjorana) and the number of the neutrino mass eigenstates.

Supergravity and matrix theory do not disagree on multi-graviton scattering

We compare the amplitudes for the long-distance scattering of three gravitons in eleven dimensional supergravity and matrix theory at finite N. We show that the leading supergravity term arises from loop contributions to the matrix theory effective action that are not required to vanish by supersymmetry. We evaluate in detail one type of diagram - the setting sun with only massive propagators - reproducing the supergravity behavior.

Scattering of a massless particle by the “sun” in superstring theory

Abstract We consider the multiple scattering of a massless state of the superstring on a composite target, which we shall call sun, made of N = 1040 massive scalar states. Tree level amplitudes are enhanced by the large combinatory factor. By looking into the large-s and small-t asymptotic limit, we find the first (Schwarzschild-like) correction to Newtons law.

The gravitational field of string matter when the dilaton is massive

Abstract We study numerically the gravitational field of a star made of massive and neutral string states for the case in which the dilaton is massive. The solution exhibits very simple scaling properties in the dilaton mass. There is no horizon and the singularity is surrounded by a halo (the physical size of which is inversely proportional to the dilaton mass) where the scalar curvature is very large and proportional to the square of the dilaton mass.

Strong anti-gravity: Life in the shock wave

Abstract Strong anti-gravity is the vanishing of the net force between two massive particles at rest, to all orders in Newtons constant. We study this phenomenon and show that it occurs in any effective theory of gravity which is obtained from a higher-dimensional model by compactification on a manifold with flat directions. We find the exact solution of the Einstein equations in the presence of a point-like source of strong anti-gravity by dimensional reduction of a shock-wave solution in the higher-dimensional model.

The Gravitational field of string matter

Abstract We analyze the static gravitational field of a body (the string star) made of a large number of massive scalar states of the superstring. We consider two cases, one in which these states are neutral string excitations, massive already in ten dimensions, and one in which their masses (and charges) originate in the process of compactification on tori. In both cases the gravitational field is modified by the presence of other massless fields that couple to the string star matter. These fields, and the strength of their couplings, are fixed by superstring perturbation theory and may be infered by studying the scattering of a neutral massless test particle. We compute the deflection angle up to second order in Newtons constant, and by a comparison with field theory explicitly show which among the various massless fields of the superstring are excited by the string star matter. The perturbative series can now be resummed by finding the exact solution to the classical equations of motion of the corresponding low-energy action. We discuss the exact solutions in some detail.

Graviton scattering in matrix theory and supergravity

I briefly review recent work on the comparison between two and three graviton scattering in supergravity and matrix theory