Experiment Proposal - opera - Infn

5 Analysis of the electronic detectors data and brick finding
5.1 Event generators and cross sections
Neutrino-nucleon interactions are generated through the JETTA and RESQUE packages [69] developed
in the framework of the CHORUS Collaboration. JETTA is based on the LEPTO model for deepinelastic
neutrino interactions. The Fermi motion of the nucleons is treated by using a simple Fermi gas
model in which nucleons are considered as free fermions in a finite volume. The reinteractions of hadrons
while crossing the nucleus are not taken into account. Hadronic fragmentation at the primary vertex is
accomplished according to the LUND string model, with some tuning in order to reproduce experimental
neutrino data. For DIS ν τ interactions the mass of the τ in the final state and its polarization are taken
into account.
The RESQUE package simulates quasi-elastic interactions and resonance production in neutrino
scattering. Nuclear effects such as Fermi motion and Pauli suppression are taken into account, but not
nuclear reinteractions. The simulation of quasi-elastic events is based on the calculation performed by
Llewellyn-Smith [70]. The resonance production cross section is computed by using a simplified version
of the Rein and Seghal model [71], which simulates various states up to a W value of 2 GeV without
taking into account the interference terms. The effects of the mass of the τ on the kinematic of the final
state and its polarization are also taken into account. A more complete simulation of the resonances
based on [71] is being developed also including the effects of the τ mass at the level of dynamics.
5.2 Detector simulation
A GEANT 3.21 simulation has been performed (AIDA) which takes into account the detector geometry
and the materials. All the primary tracks from the neutrino interaction in the target are generated
according to the event generators and traced through the entire apparatus taking into account all relevant
physics processes. Secondaries down to 1 MeV kinetic energy for electrons and γ’s, and down to 10 MeV
for all other particles are also traced. Information about each particle crossing the active detectors is
recorded, to provide data input for the event reconstruction programs. This information, classified as
hits, includes detector identifier, crossing position, energy deposition, particle kinematics, etc.
Signals produced in the scintillator strips are simulated assuming readout at both ends through 1 m
of extra fibre and taking into account the light attenuation along the fibres. In order to validate the
simulation of the electronic detectors, AIDA has been used to simulate the setup used in the test beam
measurements. In one of these tests, 15 GeV/c pions were sent into a brick wall and the shower centre
of gravity was measured by means of scintillator strip planes. The comparison of the differences between
the true position of the beam in one transverse coordinate and the reconstructed one is shown in Fig. 73.
The agreement gives us confidence in the simulation algorithms.
In addition, a detailed simulation of the bricks has been performed to study the event topology in the
vertex region, to estimate the physics background and to study different brick configurations. A standard
GEANT 3.21 software requiring high performance in terms of tracking step length, position resolution
and machine precision was used for this purpose.
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