M. Ciafaloni

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.

High energy factorization and small-x heavy flavour production

Abstract We have recently proposed a form of high-energy factorization in order to find the small- x behaviour of heavy mass production in QCD. This factorization is k ⊥ -dependent and provides all leading ln x corrections to the coefficient function in various kinds of single- k ⊥ and double- k ⊥ processes. Here we investigate in detail its application to heavy flavour production in lepton and hadron initiated processes. We find that the resummation procedure yields large correction factors with respect to the lowest-order result, and we compute their explicit form in various cases.

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.

Jet structure and infrared sensitive quantities in perturbative QCD

Abstract Infrared sensitive measurements in hard scattering include x - and p ⊥ -distributions at the phase space boundary (where soft emision is depressed) and jet multiplicity distributions (where it is emphasized). We review QCD results for these quantities and the corresponding infrared resummation techniques. Particular emphasis is put on the structure of the jet final state and its phenomenological consequences.

Coherence effects in initial jets at small Q2/s

Abstract The QCD evolution of initial jets (corresponding to space-like partons) is analyzed for small Bjorken x. Coherent effects are described by a gluon emission current which also embodies internal line insertions to double log accuracy. It is found that gluons are emitted off thr space-like leg in the order of their angles with respect to the jet axis. The modified branching vertex probability is given and differences with previous prescriptions are pointed out. The relation to transverse momentum ordering in structure function is also discussed.

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.

Gluon contributions to small χ heavy flavour production

Abstract We show that the large QCD corrections to heavy flavour production due to small χ logarithms can be evaluated and resummed in closed form, including the coefficient factor. The argument is based on a k ⊥ -dependent factorization of the Born and structure function terms which is ascribed to Regge behaviour. The photoproduction case is investigated in detail. At high energies the total cross section turns out to be much larger than that obtained by a naive renormalization group approach.

A Treatment of Hard Processes Sensitive to the Infrared Structure of QCD

Abstract We propose a modified jet evolution equation which resums large corrections to the usual leading logarithmic approximation when phase-space constraints expose the singular infrared structure of QCD. The modification, which consists simply of a rescaling of the argument of the running coupling constant, is based on perturbative arguments verified at the fourth-order level. Processes analyzed by this method include the quark (Sudakov) form factor, the large moments of structure and fragmentation functions, the asymptotic behaviour of multiplicities and the clustering of final quanta in colourless systems which occupy finite regions of (momentum and position) phase space.

Renormalization group improved small x equation

We propose and analyze an improved small-

Jet Multiplicity and Soft Gluon Factorization

x