Romanian contributions to the ATLAS experiment

The ATLAS experiment of the LHC programme at CERN entered the phase of commissioning and integration, in view of the first data taking in 2007. A group of physicists and engineers from IFIN-HH and ITIM-Cluj is actively involved since 1993 in the design, construction, tests and simulation of the ATLAS detector, as well as in the investigation of several physical processes at LHC. We first make a brief overview of the Romanian contributions and prospects of future work for the hadronic calorimeter with scintillating tiles (TILECAL), the Online Software component of the ATLAS Trigger/DAQ/DCS system and the development of a testbed GRID facility allowing to take part in the ATLAS distributed computing environment based on the LCG software. We present then in more details recent results on the analysis of the TILECAL response to pions and protons and the investigation of the ATLAS potential for the discovery of heavy charged leptons. We show that the data yielded by the 2002 beam tests with final modules confirm the different response and performances of the calorimeter to pions and protons. The simulation code GEANT4 is consistent with the experimental results concerning the ratio between the pion and proton response and the linearity and resolution of pions and protons only if the physics library LHEP3.6 is used. This is a very important result in the present phase, when the validation of the physics implemented in GEANT4 is a priority of ATLAS. We finally discuss the production at LHC of exotic charged heavy leptons predicted by various models with an extended gauge sector. Both Drell-Yan and gluon-gluon production processes are considered. We present the ATLAS discovery potential, for the decay channel with lepton and dijets in the final state, as a function of the heavy lepton and Z' masses. We also show that the inclusion of QCD radiative corrections to lepton production by gluon fusion reduces the unphysical dependence of the production rate on the renormalization and factorization scales. (authors)