• ATLAS
• LHC
• Optimal Filtering
• TileCal
• validation

The Large Hadron Collider (LHC) that is operating at the CERN laboratory is a
proton-proton collider that will provide one bunch collision each 25 ns at a nominal
center-of-mass energy of 14 TeV and at a peak luminosity of 10^34 cm−2 s−1 . These
conditions allow to investigate the Standard Model predictions and to test many
critical areas like the Higgs Boson mechanism in the framework of a wide range of
scenarios.

Currently the LHC is being tuned and produces collisions at s = √7 TeV with a
luminosity of about 10^31 cm−2 s−1 .
The experimental conditions however impose severe constraints on the detector
structures, electronics and performances in order to cope with the huge amount of
data and to be able to select the interesting events with reasonable precision at the
Bunch Crossing frequency of 40 MHz.
ATLAS is a detector situated on the LHC ring. This thesis deals with its central
hadronic calorimeter, TileCal, which is a sampling calorimeter with steel as absorber
material and plastic scintillator tiles as active medium; groups of tiles and steel plates
form the TileCal cells. The cells are coupled to wave-lenght-shifter bers, which
transport the scintillation light to two photomultipliers. The output signals are then
properly shaped and digitized by the front-end electronics, which is composed of
about 10000 channels.
The Tile Calorimeter is complemented by a triple calibration system that allows
equalization and monitoring of the signal at various stages. A radioactive source,
pushed by an hydraulic system across the calorimeter, generates a signal in the
scintillator tiles, allowing to equalize the response of each cell; a laser pulse, injected
at the input of the photomultiplier, is used to test the stability of the complete
readout chain; finally a dedicated Charge Injection System allows to test the response
of the front-end electronics for each channel.
A dedicated algorithm, the Non-Iterative Optimal Filtering method (OF-NI), is
executed in Digital Signal Processors and is responsible for the online reconstruction
of the signal time and amplitude. The reconstruction rate for the whole calorimeter
cannot exceed the Level 1 Trigger output rate of 100 kHz, otherwise the event signal
is completely lost. This is far from a simple effort due the large amount of background
events.
In this thesis the focus is on validation aspects of the OF-NI reconstruction for
the TileCal signal, mainly using the tools provided by the Charge Injection System; a
preliminary analysis of the signal reconstruction in proton collision is also presented.
It will be shown that a good understanding of both the hardware and the al-
gorithm implementation is required in order to validate the reconstruction and to
evaluate the systematics induced on the signal amplitude and time.
In the last part of the thesis a timing monitor is presented which allows to control
the stability and the performances of the signal reconstruction.
This work is part of the contribution to the Tile Signal Reconstruction
And Validation Task Force, a group dedicated to the validation of the online
signal reconstruction in TileCal.