ATLAS-IFJ-TAUHAD · Towards observation of hadronic tau decays in the ATLAS experiment with the first LHC collisions: key for observability of the New Physics and Higgs boson(s)

Final Activity Report Summary - ATLAS-IFJ-TAUHAD (Towards observation of hadronic tau decays in the ATLAS experiment with the first LHC collisions: New Physics and Higgs boson(s))

For the purpose of studying the most fundamental structure of matter a particle accelerator probing deeper into matter than ever before, the Large Hadron Collider (LHC), is being built at CERN near Geneva. It is foreseen to begin operation in the year 2008. The most important challenge of the high energy particle physics in the next decade will be to confirm with this facility existing theory of the origin of masses of elementary particles and the mechanism responsible for the electroweak symmetry breaking, namely to discover the Higgs boson. Finding Higgs particle and measurement of its properties is therefore a primary goal in the physics programme of ATLAS and CMS, two largest collaborations at the LHC. The ATLAS experiment is a world collaboration of about 1800 physicists and engineers, from 34 countries. The detector is the largest and the most complex ever made. The main goal of the project was continuation of an active participation of the fellow in the world-wide program of finalising strategies for analysis of the data to be collected at the LHC collider. The observation of tau leptons play important role in the physics to be observed: Higgs physics, supersymmetry, etc. The tau observation with the ATLAS experiment will be possible already at the start of the data taking in year 2008 with large statistics of channels: W -> tau nu and Z->tau tau. Those processes used as control channels will be important for detector calibration, background normalisation for Higgs searches, tuning reconstruction algorithms. Preparation for performing of such an analysis on the first LHC data was one of the objectives of the project. Sensitivity to all those channels depends strongly on the efficiency of the tau identification thus the aim of this project was development of the algorithms for reconstruction and identification of taus. There are two tau reconstruction and identification algorithms developed in the ATLAS experiment. The first, base-line one uses calorimetric clusters as a seed. The work of the fellow concerned the second one, based on different concept. Its innovation is the use of reconstructed track as a seed and the energy flow technique to define energy scale of reconstructed candidates. This new approach is more difficult and sophisticated than the universal base-line one, but better suited for the physics goals of interest within the presented project. The algorithm was implemented into the ATLAS experiment official object-oriented framework, ATHENA, allowing other scientists from collaboration to use it freely. The integrated algorithm was also used in the official, worldwide production of the ATLAS Monte Carlo data, which will be used by physicists to create publications showing the level of understanding performance of the detector just before starting to collect data. After implementation, the algorithm has been re-optimised to obtain better reconstruction and identification efficiency. The main improvements were: reconstruction of candidates with two tracks to recover large fraction of lost three-prong candidates, including more discriminating variables like secondary vertex information, tools for rejection of fake candidates built from electrons or muons. Also development in terms of better tau identification was made, the new multivariant tau identification was introduced (neural network, PDE-RS) giving better signal / background separation. During implementation and development of the algorithm it's validation on Monte Carlo physics samples was continued. Also, the ATLAS reconstruction and simulation software go toward the final version to be used for the first data taking it is under constant development. This requires the maintenance of the implemented algorithm and evaluation of its performance. During the project the fellow supervised the diploma student in his work on hadronic calorimeter calibration using E/p of single pions from hadronic decays of taus. The main part of that method is to distinguish between one-prong decays of tau leptons with and without accompanying neutral pions. The method can be used as well for studies of the tau polarisation.

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