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Abstract. Physics and programming aspects are discussed for a Fortran 77 Monte Carlo program to simulate complete events in deep inelastic leptonnucleon scattering. The parton level interaction is based on the standard model electroweak cross sections, which are fully implemented in leading order for any lepton of arbitrary polarization, and different parametrizations of parton density functions can be used. First order QCD matrix elements for gluon radiation and boson-gluon fusion are implemented and higher order QCD radiation is treated using parton showers. Hadronization is performed using the Lund string model, implemented in Jetset/Pythia. Rapidity gap events are generated through a model based on soft colour interactions.

We have developed the Monte Carlo simulation program Jewel 1.0 (Jet Evolution With Energy Loss), which interfaces a perturbative final state parton shower with medium effects occurring in ultrarelativistic heavy ion collisions. This is done by comparing for each jet fragment the probability of further perturbative splitting with the density-dependent probability of scattering with the medium. A simple hadronisation mechanism is included. In the absence of medium effects, we validate Jewel against a set of benchmark jet measurements. For elastic interactions with the medium, we characterise not only the medium-induced modification of the jet, but also the jet-induced modification of the medium. Our main physics result is the observation that collisional and radiative medium modifications lead to characteristic differences in the jet fragmentation pattern, which persist above a soft background cut. We argue that this should allow to disentangle collisional and radiative parton energy loss mechanisms by measuring the n-jet fraction or a class of jet shape observables.

Production of muons and neutrinos in cosmic ray interactions with the atmosphere has been investigated with Monte Carlo models for hadronic interactions. The resulting conventional muon and neutrino fluxes (from π and K decays) agree well with earlier calculations, whereas our prompt fluxes from charm decays are significantly lower than earlier estimates. Charm production is mainly considered as a well defined perturbative QCD process, but we also investigate a hypothetical nonperturbative intrinsic charm component in the proton. The lower charm rate implies better prospects for detecting very high energy neutrinos from cosmic sources. IntroductionThe flux of muons and neutrinos at the earth has an important contribution from decays of particles produced through the interaction of cosmic rays in the atmosphere (for a recent introduction see [1]). This has an interest in its own right, since it reflects primary interactions at energies that can by far exceed the highest available accelerator energies. It is also a background in studies of neutrinos from cosmic sources as attempted in large neutrino telescopes, such as Amanda [2], Baikal [3], Dumand [4] and Nestor [5].Here we present a detailed study of muon and neutrino production in cosmic ray interactions with nuclei in the atmosphere using Monte Carlo simulations [6].At GeV energies the atmospheric muon and neutrino fluxes are dominated by 'conventional' sources, i.e. decays of relatively long-lived particles such as π and K mesons. This is well understood from earlier studies [7,8,9], with which our investigations agree. With increasing energy, the probability increases that such particles interact in the atmosphere before decaying. This implies that even a small fraction of short-lived particles can give the dominant contribution to high energy muon and neutrino fluxes. These 'prompt' muons and neutrinos arise through semi-leptonic decays of hadrons containing heavy quarks, most notably charm.Available data in the multi-TeV energy range, obtained with surface and underground detectors (see e.g. refs. [10][11][12][13]), are still too discrepant to draw definitive conclusions on the flux of 1 thunman@tsl.uu.se 2 ingelman@tsl.uu.se, also at DESY, Hamburg.