Our tasks, responsibility and preliminary status quo
in ATLAS experiment

Introduction

The main goal of the ATLAS experiment is looking for the experimental evidence of Higgs particle and the investigation of the properties of matter in the region 1 TeV (physics beyond Standard Model, MSSM, SUSY). This last point after HERA results is more actual and attractive.
Calorimeters will play a crucial role at LHC. In contrast to other detectors their intrinsic resolution improves with energy, which makes them very suitable detectors at high energy machines. An important aspect of the Hadronic End-Cap (HEC) is its ability to detect muons, and to measure any radiative energy loss.
The Hadronic End-Cap is designed to provide coverage for hadronic showers in the range 1.55 < eta < 3.2. The HEC detector elements are located in the end-cap cryostat at either end of the ATLAS tracking volume. It is a liquid argon (LAr) sampling calorimeter with copper-plate absorber (with the thickness 25/50 mm). This technology was selected as it allows a simple mechanical design to be produced that is radiation resistant and covers the required area in a cost-effective way.
The gaps between the copper plates (8.5 mm) are instrumented with a read-out structure forming an electrostatic transformer (EST), see fig 1.

Figure 1: Hadronic end-cap EST (cell)
This structure optimizes the signal-to-noise ratio while reducing the high-voltage requirements and ionization pile-up, and limiting the effect of failure modes such as high-voltage sparks and shorts.
The signals are amplified and summed employing the concept of "active pads": the signals from two consecutive pads are fed into separate amplifiers (based on GaAs electronics) and summed to read-out channels. The use of cryogenic GaAs preamplifiers provides the optimum signal-to-noise ratio for the HEC. The boards with the GaAs chips are mounted on the outer radius of the HEC. The use of cryogenic GaAs preamplifiers provides the optimum signal-to-noise ratio for the HEC.

Our primary goal in ATLAS experiment is the proposal and the development of the calibration system for HEC calorimeter.

Our responsibilities can be described by the following way:

Juridical background

In 1996 was signed Interim Memorandum of Understanding for ATLAS experiment (period two years 1996-1997) and in July 1998 was signed Memorandum of Understanding (MoU) for ATLAS experiment up to 2005. These two acts represent the basic juridical background of our present activities in ATLAS experiment.
According to MoU of the ATLAS experiment, which from our side was signed by the secretary of the Ministry of education of Slovak republic, our financial obligations during whole time period of the detector construction are following:

- core expenses: 100 000,- CHF (today we reimbursed 6 x 12 500,- CHF)

- core expenses expressed as in-kind invoices: 100 000,- CHF (today we reimbursed by cash 5 000,- CHF)

hardware contribution into liquid argon subdetectors, especially into the construction of the hadronic end-cap calorimeter: 304 000,- CHF

The proposal and the development of the calibration system for HEC calorimeter is serious and difficult task. HEC is make up from two symmetric calorimeters in the wheel structure HEC1 and HEC2, each wheel has 32 modules, see fig. 2, where the one module of HEC is shown.

Figure 2: Hadronic end-cap module
One wheel has roughly 4 x 7 m size. On the fig. 3 you can see the first from four HEC wheels

Figure3: One wheel of HEC
Pad structure defines lateral segmentation in (eta, phi) = 0.1 x 0.1. LAr gap is instrumented with 1 PAD board and 2 EST boards. From the electronic point of view HEC has 92 160 full independent read-out channels, which is approximately two-time more than in the up today largest liquid argon calorimeter which is used in H1 in DESY, Hamburg.
Our project from the view on the development and realization of the calibration procedures for HEC is possible to divide into three parts, which in the present time status quo is following:

Hardware

We continued in the development and testing of the calibration system for the modules assembled in five different laboratories (München, LPI Moscow, ITEP Moscow, Protvino and Vancouver).
The calibration system was developed and tested in 1999-2000 test runs. In fig. 4 you can see the calibration signal (left) and the prediction for the ionization signal (right) together with the residuals with the respects to the fit (lower figures).

Figure 4: The calibration signal (left) and the prediction for the ionization signal (right) together with the residuals with the respects to the fit (lower figures)
We participate also on the design and laboratory tests of the Front End Board (FEB) for Atlas experiment. The FEB, see fig. 5,

Figure 5: The view on the FEB
is going to be used for the EM and HEC part of the Atlas calorimeter. As described in the TDR of the liquid argon (LAr) calorimeter, the FEBs contain the electronics for amplifying, shaping, sampling, pipeling, and digitizing the LAr calorimeter signals. The FEB electronics e..g. must handle the signal dynamic range of about 16 bits without contributing more than 0.2%.

We have produced also a set of print boards with the different patterns for the wiring of the final modules of HEC. Parameters of the board divided a calibration signal on 3 pads are crucial for the accuracy calibration signal distribution. A dividing ratio of the front calibration distribution board for all channels is shown in figure fig. 6.

Figure 6: Dividing ratio of the Front Module Calibration Distribution Boards of HEC

We finished HV and signal boards production. All these elements are the parts of HEC modules and this year also the first of four large wheels of the HEC calorimeter has been successfully assembled and rotated into its final position on schedule - see fig. 3. To assemble a wheel have been taken modules that have already been cold tested, do the final electrical testing and locate them onto the HEC wheel assembly table. All mechanical and electrical tests have been successfully passed, it means also Košice elements are satisfied.
In period 2001-2002 we began start the filter box production, see fig. 7,

Figure 7: Filter box
all needed quality criteria.
The filter boxes will be used in all ATLAS calorimeters, e.g. on the fig. 8

Figure 8: ATLAS barrel cryostat
the ATLAS barrel cryostat with our filter boxes (see fig. 9 in more detail) is shown.

Figure 9: Detail view: the filter box in the ATLAS barrel cryostat

Software

In 1998 the Cleland method for the noise suppression in the calorimeter readout was successfully applied. The improvement of the signal/noise ratio can be seen in fig. 10.

Figure 10: The noise reduction for 100 GeV electron deposits in HEC module 0 a) before and b) after filter applying

In fig. 11 is shown the linearity in the four different impact points in the test modules with the points corresponding to uncalibration and calibration data for the various electrons energies. The improvement is markant.

Figure 11: Response linearity of module 0 in four different impact points. The full points - before and, triangle - after calibration

The new function describing the response of the whole testing electronics for various signal's types (e..g. calibration one or signal from particle) had been calculated and implemented, which also using less number of free parameters the measured signals describes very well.

Monte-Carlo simulation

In Košice now we have full present in stand-alone mode the ATLAS software, which is based on OO-object oriented software. We take participation in Monte-Carlo simulation of HEC beam set-up, in the cluster reconstruction in LAr using standard Athena ATLAS tool and in this time we try to use last GEANT4 version for the missing ET/jets physics in HEC, e.g. in fig. 12 the comparision between GEANT3.21 and GEANT4 simulation has been shown.

Figure 12: The energy resolution of electrons in EMBarrel

Into generator PYTHIA6.2 has been putted new process with the production two top quarks and we began the analyze with two top production including background calculation.

Our tasks, responsibility and preliminary status quo in ATLAS experiment

Our tasks, responsibility and preliminary status quo
in ATLAS experiment