4. MEMORIA CONSTRUCTIVA DEL ESTADO REFORMADO
5.6. DB HE AHORRO DE ENERGÍA
The honeycomb tracker has achieved its original aims; it makes stand-alone reconstruc- tion of 3-d tracks possible with good resolution. These tracks have been used to align the diamond tracker paddles with respect to each-other and the other sub-detectors. Before applying the alignment determined with the honeycomb tracker, the momentum resolution of the hadron-spectrometer was found to be:
∆p
p =
q
0.352+ (0.038p/1 GeV)2 .
After applying the new alignment, the momentum resolution was found to be [232]: ∆p
p =
q
0.352+ (0.025p/1 GeV)2 .
In this, the first term is due to multiple scattering and the second incorporates the spatial resolution and alignment effects. The introduction of the honeycomb tracker has lead to a significant improvement in the momentum determination of hadrons. This improve- ment could even be extended to the period before the installation of the honeycomb tracker. The alignment obtained with the honeycomb data in the 1997 run was in good agreement with that of the 1996 run. As the alignment between these different periods was consistent, the same alignment of the diamond tracker paddles was applied to the data of the 1995 run.
Chapter 4
Track finding in emulsion
In Chapter 2 the setup of theCHORUSexperiment has been introduced, including a hardware-based method to scan emulsion. This chapter describes the development of a new approach undertaken atCERN. The principles in this approach were to use off-the-shelf hardware and to use software as much as possible. This required the development of track-finding code for the particular case of emulsion images. Elec- tronic detectors usually yield information in one or two projections; emulsion yields 3-D position information. Electronic detectors typically measure only a few track points; emulsion tracks typically contain many hits. The difficulty with emulsion is that these hits are burried in a large number of background hits.
An algorithm that could efficiently find tracks in this high noise environment was developed. Although originally written for track finding in emulsion, the algo- rithm and its tools could be used in more general applications and have, therefore, been implemented as an object-oriented C++ toolkit. Part of this chapter is a copy of a published paper describing this toolkit [233]. In this chapter, the algo- rithms and implementations of the track-finding classes and the container classes developed for fast searching in multi-dimensional spaces are presented. The track- finding efficiency, estimated using a Monte-Carlo simulating, is also presented. The expected performance of the algorithm has been investigated. The tracking code was originally designed to reconstruct all tracks. However, in the scan-back stage of event location, a track-selector like approach (section 2.9.4) is sufficient and faster. This was also implemented in software and is described in section 4.5. Finally, the application to real emulsion data is presented.
114 track finding in emulsion
4.1
Introduction
Automatic emulsion scanning with computer-controlled microscope stages and digital read-out and processing of emulsion images was pioneered by the thefkenlaboratory in Nagoya (Japan). As is described in section 2.9, the Nagoya approach to emulsion scanning is based on a brute force, hardware based, track-finding system which examines a fixed set of 16 images. Originally, only a track with known slope could be located automatically. With the development of ever faster hardware, this restriction disappeared because the hardware could simply check for many slopes.
When one examines the emulsion-scanning strategies used in chorus in detail (sec- tion 2.10), three different stages can be distinguished: scan-back, net-scan and eye-scan. These stages differ in the area and thickness scanned and whether all tracks or only a single track is being searched for. During scan-back (section 2.10.2), a single track is looked for and the area scanned is large on the interface sheets but very small on the target sheets. During net-scan (section 2.10.3), all tracks are reconstructed and the area is large. In both stages, it is sufficient to examine only a thin slice of one emulsion surface to find all interesting tracks. The exception is scan-back on the interface sheets where both surfaces need to be scanned.
The net-scan procedure has a short-coming which becomes apparent for events with a secondary vertex or kink. Net-scan is comparable to electronic tracking detectors in the sense that tracks and vertices are reconstructed from a few measurements along the paths of the tracks. The complete particle track or the actual vertex is not seen. From the net-scan data alone, it is impossible to tell if a secondary vertex was caused by a decay or an interaction. The net-scan procedure can also not distinguish between the decay of a charged or neutral short-lived particle if the decaying particle does not cross the upstream surface of at least one emulsion plate. These limitations re-introduced human- eye scanning in the emulsion analysis. The advantage of net-scan is that now only a small sample of events needs to be scanned at a well known location in the emulsion and with a partially known topology. During such eye-scanning, one to several plates are examined through their full thickness and the tracks and vertices of interest (some of which are already known) are inspected, measured and registered in a computer readable format. Eye-scan corresponds thus to a scanning stage with small to medium areas but full thickness.
During the development of the scanning and track-finding hardware in Nagoya, the optics and the limitation to 16 images have never changed. So even though the scanning speed has increased several orders of magnitude (including theccd camera speed), the optics still limit the field of view to about 150µm×150µm and the hardwired 16 image limit restricts the scanning to emulsion slices of around 100µm thick. Historically of course, the track-selector was designed for doing scan-back only; it was the increased scanning speed which led to the development of the net-scan procedure. Net-scan is probably close to the best that can be done for full automatic event reconstruction given the limitations of the hardware and a given time frame determining the time that can be spent on each event.
Within thecernscanning laboratory, the idea took root to redevelop automatic scan- ning techniques, keeping the ideas that had already been developed while avoiding known limitations and human eye-scanning as much as possible. The guiding principles in these developments were to use up-to-date instrumentation and off-the-shelf electronic com-
ponents wherever possible and implement as much as possible all pattern recognition in software. Using off-the-shelf components and implementing pattern recognition in soft- ware, allows one to profit directly from Moore’s law,1while avoiding the long development
time and relative inflexibility of home-built designated hardware.
The main subject of this chapter is the software developed for track-finding in a set of emulsion images. Another pattern-recognition problem improved in thecern devel- opments was the location of reference points on the emulsion plates. This has already been addressed in section 2.10.1. In the next sections, some of the new instrumentation developed for the cern scanning laboratory will be briefly described, before returning to the main subject with a discussion of the characteristics of emulsion data and the constraints these place on a tracking algorithm.