18.15.1} High energy model improvements

 In all the developments described in this paragraph and also of some in
 other paragraphs, J. Ranft always acted as the main mentor and source
 of theoretical and often practical support. Even when he did not
 contributed to the code directly, his ideas, help and suggestions were
 essential part of its development.

 The two models developed by the Leipzig group were initially improved
 by removing a number of known bugs and approximations (mainly, but not
 only, in the kinematics). In the years 1990-1991 all hyperons and
 anti-hyperons were added as possible projectiles, and most important,
 nuclear effects, previously restricted to Fermi momentum, were
 expanded and treated more accurately, with an explicit treatment
 of the nuclear well potential, the inclusion of detailed tables
 of nuclear masses to account for nuclear binding energy, a consistent exact
 determination of nuclear excitation energy and an overall "exact"
 conservation of energy and momentum on an event-by-event basis. These
 changes were the minimal modifications required for introducing a sensible
 evaporation module and related low energy particle production: they made
 up the first stage of upgrade of the intermediate and high energy event
 generator and were performed by Ferrari and Sala.

 In the following years, negative Binomial multiplicity distribution,
 correlations between primary interactions and cascade particles and
 better energy-angle distributions were implemented.
 Sea quark distributions were updated, new distributions
 were used for the number of primary collisions using an improved Glauber
 cascade approach, and Reggeon mediated interactions (single chains) were
 introduced at the lower energy end of the application range of the Dual
 Parton Model. An initial improvement of the diffraction treatment as well
 of the hadronisation algorithm were performed. These developments ended
 up in the 1993 version, which represented the second stage of the high energy
 generator development (and which was made available to GEANT3 users, see
 later).

 Several major changes were performed on both the intermediate and high
 energy hadron generator in the years 1994-1996 by Ferrari and Sala.
 The latter was extensively improved, bringing its results into much better
 agreement with available experimental data from as low as 4 GeV up to
 several hundreds of GeV. A fully new treatment of transverse momentum
 and of all DPM in general was developed, including a substantially
 improved version of the hadronisation code and a new driver
 model for managing two-chain events. The existing treatment of
 high-energy photonuclear reactions, previously already based on the
 VMD model [Bau78] but in an approximate way, was improved by implementing the
 contribution of all different vector mesons, as well as the quasielastic
 contribution. The simulation of diffractive events was completely reworked
 distinguishing between resonant, single-chain and two-chain events, and
 a smeared mass distributions for resonance was introduced.
 This version of the model was completed in 1996 and performed very well
 together with the new "sophisticated" PEANUT when applied to a variety
 of problems, ranging from radiation protection, to cosmic ray showers
 in the atmosphere and to the test beam of the ATLAS calorimeters.

 The latest round of improvements originated by the new interest of
 Ferrari and Sala for neutrino physics, triggered by
 their participation in the ICARUS experiment and resulted in several
 improvements in the high-energy interaction model.
 In 1998, a new chain fragmentation/hadronisation scheme was
 put to use, and a new diffraction model was worked out once
 more according to rigorous scaling, including low mass diffraction
 and antibaryon diffraction. In 1999, charm production was set up by
 Ranft and Ferrari (reasonable at least for integrated rates),
 and charmed particle transport and decay were introduced.
 The chain building algorithm was thoroughly revised to ensure a
 continuous transition to low energies, and a significant reworking was
 done on the chain hadronisation process, providing a smooth and physically
 sound passage to chains made up by only two particles, resulting in an
 overall better description of particles emitted in the fragmentation
 region. This model was thoroughly benchmarked against data taken at WANF
 by NOMAD and the particle production data measured by SPY. It constituted
 the basis for all calculations performed for CNGS, both in the early
 physics design stage and later in the optimisation and engineering studies.

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