Aspects of Two-Photon Physics at Linear e+e- Colliders

We discuss various reactions at future e+e- and gamma-gamma colliders involving real (beamstrahlung or backscattered laser) or quasi--real (bremsstrahlung) photons in the initial state and hadrons in the final state. The production of two central jets with large pT is described in some detail; we give distributions for the rapidity and pT of the jets as well as the di--jet invariant mass, and discuss the relative importance of various initial state configurations and the uncertainties in our predictions. We also present results for `mono--jet' production where one jet goes down a beam pipe, for the production of charm, bottom and top quarks, and for single production of W and Z bosons. Where appropriate, the two--photon processes are compared with annihilation reactions leading to similar final states. We also argue that the behaviour of the total inelastic gamma-gamma cross section at high energies will probably have little impact on the severity of background problems caused by soft and semi--hard (`minijet') two--photon reactions. We find very large differences in cross sections for all two--photon processes between existing designs for future e+e- colliders, due to the different beamstrahlung spectra; in particular, both designs with >1 events per bunch crossing exist.Comment: 51 pages, 13 figures(not included

How To Find Charm in Nuclear Collisions at RHIC and LHC

Measurements of dilepton production from charm decay and Drell-Yan processes respectively probe the gluon and sea quark distributions in hadronic collisions. In nucleus-nucleus collisions, these hard scattering processes constitute a `background' to thermal contributions from the hot matter produced by the collision. To determine the magnitude and behavior of this background, we calculate the hard scattering contribution to dilepton production in nuclear collisions at RHIC and LHC at next to leading order in perturbative QCD. Invariant mass, rapidity and transverse momentum distributions are presented. We compare these results to optimistic hydrodynamic estimates of the thermal dilepton production. We find that charm production from hard scattering is by far the dominant contribution. Experiments therefore can measure the gluon distribution in the nuclear target and projectile and, consequently, can provide new information on gluon shadowing. We then illustrate how experimental cuts on the rapidity gap between the leptons can aid in reducing the charm background, thereby enhancing thermal information.Comment: 32 pages, latex, 19 figure

Empirical Determination of the Very High Energy Heavy Quark Cross Section from Non-Accelerator Data

To cosmic rays incident near the horizon the Earth's atmosphere represents a beam dump with a slant depth reaching 36000~g~cm$^{-2}$ at $90^\circ$. The prompt decay of a heavy quark produced by very high energy cosmic ray showers will leave an unmistakable signature in this dump. We translate the failure of experiments to detect such a signal into an upper limit on the heavy quark hadroproduction cross section in the energy region beyond existing accelerators. Our results disfavor any rapid growth of the cross section or the gluon structure function beyond very conservative estimates based on perturbative QCD.Comment: (8 pages, latex file, 3 figures available upon request) MAD/PH/74