Steffen Kappler

CONSTRUCTION, TEST AND COMMISSIONING OF THE TRIPLE-GEM TRACKING DETECTOR FOR COMPASS

The Small Area Tracking system of the COMPASS experiment at CERN includes a set of 20 large area, fast position-sensitive Gas Electron Multiplier detectors, designed to reliably operate in the harsh radiation environment of the experiment. We describe in detail the design, choice of materials, assembly procedures and quality controls used to manufacture the devices. The test procedure in the laboratory, the performance in test beams and in the initial commissioning phase in the experiment are presented and discussed.

High rate X-ray imaging using multi-GEM detectors with a novel readout design

Abstract Modern micropattern gas counters are able to detect soft X-rays with high spatial resolution. Based on the principle of gas amplification, this detector type has a wide dynamic range and provides energy selection during image reconstruction. One of the most recent developments is the Multi-GEM, an efficient, high-rate capable micropattern gas detector. Owing to the separation of gas amplification and readout stage, this detector allows high flexibility in the geometry of the readout structure. With a two-dimensional projective readout, GEM detectors have shown good capability in providing X-ray absorption radiographies of small mammals. We present a detector with a novel design where the readout board consists of micropads connected to three layers of strips in such a way that two-dimensional spatial reconstruction and an unambiguous resolution of events with multiple hits is possible.

Electron collection and ion feedback in GEM-based detectors

We report the results of systematic experimental investigations on electron transmission and ion feedback in a single gas electron multiplier (GEM) detector for the operating region close to unity gain. Critical factors for obtaining good collection are the transverse diffusion coefficient and the hole diameter. A unit gain GEM can be used for instance as first element in a cascade to improve collection efficiency and ion feedback suppression in time projection chambers.

Triple GEM tracking detectors for COMPASS

The small-area tracker of COMPASS, a high-luminosity fixed target experiment at CERNs SPS, includes a set of 20 large-size (31 /spl times/ 31 cm/sup 2/) gas electron multiplier (GEM) detectors. Based on gas amplification in three cascaded GEM foils, these devices permit to obtain high gain and good spatial resolution even at very high particle fluxes. A two-coordinate projective readout yields, for each track, highly correlated signal amplitudes on both projections, allowing to resolve multiple hits in high occupancy regions close to the central deactivated area of 5 cm diameter. At the same time, the material exposed to the beam is minimized. Splitting the amplification in three cascaded stages permits to achieve a gain of /spl sim/ 8000, necessary for efficient (> 98%) detection of minimum ionizing particles on both coordinates, already at relatively moderate voltages across individual GEM foils. As a consequence, the probability of a gas discharge to occur when a heavily ionizing particle enters the detector volume, is reduced by more than one order of magnitude at a given gain compared to the initially foreseen double GEM structure. In conjunction with other strategies resulting from extensive R&D on discharge phenomena, we were able to further reduce both the energy and the probability of such breakdowns. In order to completely exclude permanent damage to the front-end chip by the rare event of a discharge fully propagating to the readout strips, an external electronic protection circuit is used. The operational characteristics of these detectors were examined both in the laboratory and in the beam, where a spatial resolution for minimum ionizing particles of (46 /spl plusmn/ 3) /spl mu/m and a time resolution of /spl sim/ 15 ns were achieved. For the 2001 run of COMPASS, a total of 14 triple GEM detectors have been installed. First results from the commissioning phase in the high-intensity /spl mu/ beam are presented.

Study of various anode pad readout geometries in a GEM-TPC

The use of a time projection chamber (TPC) based on gas electron multipliers (GEMs) as a central tracker in particle-physics experiments is being studied by several groups. Compared to the conventional multi-wire readout this combination offers a number of advantages such as intrinsically suppressed ion feedback, high granularity and decoupling of gas amplification stage and readout geometry. The fast signal of GEMs and the reduction of the transverse diffusion in parallel magnetic fields give a good spatial resolution. However, for short drift distances and high magnetic fields, this may lead to signal sizes much narrower than the pad size and thus to a degradation of the spatial resolution. We have studied the use of different pad geometries to improve the performance of the detector in the small diffusion limit

The PAX toolkit and its applications at Tevatron and LHC

At the CHEP03 conference, we launched the Physics Analysis eXpert (PAX), a C++ toolkit released for the use in advanced high energy physics (HEP) analyses. This toolkit allows to define a level of abstraction beyond detector reconstruction by providing a general, persistent container model for HEP events. Physics objects such as particles, vertices and collisions can easily be stored, accessed and manipulated. Bookkeeping of relations between these objects (like decay trees, vertex and collision separation, etc.) including deep copies is fully provided by the relation management. Event container and associated objects represent a uniform interface for algorithms and facilitate the parallel development and evaluation of different physics interpretations of individual events. So-called analysis factories, which actively identify and distinguish different physics processes and study systematic uncertainties, can easily be realized with the PAX toolkit. PAX is officially released to experiments at Tevatron and LHC. Being explored by a growing user community, it is applied in a number of complex physics analyses, two of which are presented here. We report the successful application in studies of tt~ production at the Tevatron and Higgs searches in the channel tt~H at the LHC and give a short outlook on further developments.

A fast Tracker for COMPASS based on the GEM

Abstract The small area tracking system of COMPASS, a new high-luminosity fixed target experiment at CERNs SPS accelerator, comprises 20 large-size triple GEM (Gas Electron Multiplier) detectors. It was completed and fully operational for the first time for the 2002 muon run. We report on the performance of these chambers in the high-intensity beam and give first results on efficiencies as well as spatial and time resolutions measured for the full set of detectors under various beam conditions.

Studies With a GEM-TPC Prototype for the ILC: Dependencies of Spatial Resolution for Short Drift Distances in a 4 T Magnetic Field

Time Projection Chambers (TPCs) with a micro pattern gas amplification stage are best suited for high precision tracking at a future linear collider. Among the major advantages of this technology are the vanishingly small distortions due to EtimesB effects. In order to prove the functionality of this concept, and to study the behavior of such TPCs in high magnetic fields, a small cylindrical TPC prototype has been equipped with two Gas Electron Multipliers (GEMs) and a micro pad readout plane. We discuss results of a test of this setup in high magnetic fields at DESY, where the prototype was placed into a solenoidal magnet with a field of up to 5.5 T. The studies include measurements of transverse and longitudinal spatial resolution using cosmic rays. Special interest is given to dependencies on drift distance, pad geometry, sampling rate, and on various cuts on angles and chi2. We also briefly describe a new method of data analysis and show that the spatial resolution can be improved by taking only a smaller fraction of all data points. With the used gas mixture of Ar:CH4:CO2-93:5:2, transverse spatial resolutions down to 100mum can be achieved

New Applications of PAX in Physics Analyses at Hadron Colliders

The PAX (Physics Analysis eXpert) toolkit assists physicists in the analysis and interpretation step of a particle physics research project. Its aim is to provide a new level of abstraction beyond detector reconstruction which facilitates code reuse and unification. PAX makes use of fourvector arithmetics and offers sophisticated relation management and memory management functionality. This paper gives an overview of the toolkit and reports about its application in the hadron collider experiments CDF and CMS.