G. Veneziano

Can spacetime be probed below the string size

Abstract Strings may be explored, through a scattering process at planckian energies, down to distances of the order of the string size. We show that this is possible if the energy is not so extreme to cause a gravitational instability and when the scattering angle approaches some critical value from below. Above this angle, the distance starts increasing, thus departing from the usual position-momentum uncertainty relation, and in no instance is the resolution smaller than the string length. This suggests that below the Planck scale the very concept of space-time changes meaning.

U(1) Without Instantons

Wittens recent proposal that the U(1) problem might be solved in 1/N expanded QCD, is shown to be automatically consistent with expected θ dependences and anomalous Ward identities, if a (modified) Kogut-Susskind mechanism is used. Ward identities are algebraically saturated for large N. A sort of “partial conservation of the U(1) current” is found to hold for the “η” field.

The Pre-Big Bang Scenario in String Cosmology

I review various aspects of the pre-big-bang scenario and of its main open problems, with emphasis on the role played by the dilaton. Since the dilaton is a compelling consequence of string theory, tests of this scenario are direct tests of string theory and also, more generally, of Planck scale physics.

Superstring Collisions at Planckian Energies

Abstract By evaluating and resumming the large-s behaviour of string loops to all orders we are able to obtain an explicitly unitary operator for the light (closed) superstring S-matrix above planckian energies: it is dominated by graviton exchange at large impact parameters and by absorption at small impact parameters. In an intermediate, eikonal region a semiclassical description emerges as if, for small deflection angles at least, each string was moving in a static Schwarzschild metric.

Pre - big bang in string cosmology

The duality-type symmetries of string cosmology naturally lead us to expect a pre-big-bang phase of accelerated evolution as the dual counterpart of the decelerating expansion era of standard cosmology. Several properties of this scenario are discussed, including the possibility that it avoids the initial singularity and that it provides a large amount of inflation. We also discuss how possible tracks of the pre-big-bang era may be looked for directly in the spectral and ”squeezing” properties of relic gravitons and, indirectly, in the distorsion they induce on the cosmic microwave background.

An Effective Lagrangian for the Pure N=1 Supersymmetric Yang-Mills Theory

Abstract An effective lagrangian for the pure, N =1, supersymmetric Yang-Mills theory is proposed by suitably modifying that of QCD. The quantum breaking of scale and chiral invariance by the corresponding anomalies generates a massive Wess-Zumino supermultiplet while preserving supersymmetry. The large- N c limit is discussed for an SU ( N c ) gauge group.

SCALE FACTOR DUALITY FOR CLASSICAL AND QUANTUM STRINGS

Abstract Duality under inversion of the cosmological scale factor is discussed both for the classical motion of strings in cosmological backgrounds and for genus zero, low energy effective actions. The string-modified, Einstein-Friedman equations are then shown to possess physically-inequivalent, duality-related solutions which generically describe the “decay” of an initial, perturbative, flat D =10 superstring vacuum towards a more intersting strong-coupling state through an appealing pre-big-bang cosmological scenario.

Chiral estimate of the electric dipole moment of the neutron in quantum chromodynamics

Abstract Current algebra for CP violating strong interactions is investigated. In particular, the neutron electric dipole moment D n is shown to behave as θm π 2 1n m π 2 for small pion mass m π and CP violating parameter θ. This logarithm is explicitly calculable: it contributes 5.2×10 −16 θ cm to D n . This result is somewhat larger than a previous O ( m π 2 ) estimate based on the bag model.

Chiral Dynamics in the Large n Limit

We construct an effective lagrangian describing the low-energy spectrum and dynamics of the pseudoscalar nonet in the large Ncolour limit of QCD. Effects of the axial anomaly and of a possible non-zero vacuum angle are incorporated together with corrections of order mqNc/GL.

Classical and quantum gravity effects from Planckian energy superstring collisions

We argue that superstring collisions at Planckian energies and small deflection angles are calculable through an evaluation and resummation of string loops in flat space-time. This leads to a systematic expansion of the collision in classical general relativity and quantum string effects. Our results explicitly show how unitarity constraints are recovered through loop corrections and how flat metric calculations reveal the generation of nontrivial geometries by energetic particles. Finite-string-size effects can be evaluated in some regimes and are found to modify classical relativity expectations even at impact parameters much larger than the string scale. Conversely, we find that, through the resummation of large genus contributions, intermediate distance physics provides important classical relativity effects even in the hard, fixed angle regime.