Alessandro Bellini

Superstring Regge amplitudes and emission vertices

Abstract We calculate (super) string high-energy amplitudes with external leading Regge trajectories in terms of new emission vertices with continuous spin and off-shell momenta. Such vertices are constructed as quasi-local operators from the short-distance product of primary fields and used to calculate the four-graviton Regge amplitude, relevant to gravitational scattering at planckian energies.

Superstring Regge amplitudes and graviton radiation at Planckian energies

Abstract We introduce in (super) string theory new emission vertices for external (leading) Regge trajectories and we use them to calculate tree amplitudes with off-shell reggeized gluons and gravitons. Such Regge-Gribov vertices arise as quasi-local operators in the short-distance product of primary fields and generate coherent reggeon states. We present detailed calculations in the three- and four-reggeon cases for both the bosonic string and the superstring in the Neveu-Schwarz formalism. In particular, we derive the graviton emission current associated with high-energy scattering, which also incorporates internal line insertions. The corresponding squared current shows the absence of Coulomb singularities associated with gravitational radiation.

Practical experiences in interconnecting LANs via satellite

We present an experiment in interconnecting LANs via a satellite link and describe the individual components involved in the experiment. The project was developed in two phases: a) design and realisation of a satellite access scheme that supports real-time and non real-time traffic with a signal fading countermeasure, called FODA/IBEA-TDMA; b) interconnection of LANs where real-time and non real-time applications run. The experiment was presented the first time in June 1994 as a demo in which the Eutelsat satellite was used (in the 12/14 GHz band) to exchange data between two LANs in Pisa and Florence, while video and audio applications running on PCs connected the two sites. The demo was repeated a few weeks later and the Intelsat satellite was used in the 20/30 GHz band.

Superstring elastic scattering at s⩾1019 GeV: The covariant loop calculus approach

We study quantum gravity by considering the elastic scattering of two on-shell string ground states in the large-s, fixed-t asymptotic limit. The amplitude is computed by means of the covariant loop calculus which turns out to be a powerful method in the study of such a Regge regime. The leading contribution in powers of s is shown to factorize at any g-loop order in a product of g +1 tree amplitudes times the expectation value of a factorized operator. The cumulative effect of these contributions - once it has been resuummed - restores unitarity in the theory and gives a deflection angle in agreement with the linear approximation to general relativity. In the process, it is possible to determine the normalization dictated by unitarity of all terms in the topological expansion. Next, the first sub-leading contribution is calculated at the two-loop order, the so-called H-diagram. A resummation of such terms gives rise to the first non-linear correction in the deflection angle. Our results are in agreement with previous work by Amati, Ciafaloni and Veneziano in the framework of Regge-Gribov techniques. The correspondence between the two approaches is discussed.

(2+1)-Gravity with moving particles in an instantaneous gauge

Abstract By defining a regular gauge which is conformal-like and provides instantaneous field propagation, we investigate classical solutions of (2+1)-Gravity coupled to arbitrarily moving point-like particles. We show how to separate field equations from self-consistent motion and we provide a solution for the metric and the motion in the two-body case with arbitrary speed, up to second order in the mass parameters.

Superstring one-loop and gravitino contributions to planckian scattering

Abstract Corrections to the semiclassical approximation in nearly forward planckian energy collisions are reconsidered. Starting from the one-loop superstring amplitude, we are able to disentangle the first subleading high-energy contribution at large impact parameters, and we thus directly compute the one-loop correction to the superstring eikonal. By comparing this result with previous ones by Amati, Ciafaloni and Veneziano (ACV) for pure gravity, we identify one-loop gravitino contributions which agree with previous results by Lipatov. We finally argue, on the basis of analyticity and unitarity, that gravitinos do not contribute at all the large-distance two-loop ACV correction, which thus acquires a universal “classical” interpretation.