Tracking Detectors in the COMPASS Experiment

Before turning to the individual detectors in the COMPASS setup, we shall discuss the information obtained from the detectors. Each particle detector measures coordinates and time of particle traversal. The detectors are planar and oriented orthogonally to the beam axis along which they are installed. Most detector planes measure a single coordinate, except for the pixelized GEM and Micromegas detectors which measure two coordinates. The one-dimensional detectors are arranged in stations combined from sev- eral planes oriented along thexandyaxis, and – in order to resolve left-right ambiguities and ambiguities resulting from different tracks hitting the same detectors strip – at 20 or 45 degree angles with respect to these axes. Neglecting the extension along the beam axis, each station can then measure a point in space.

Very Small Area Trackers These are the detectors placed in the vicinity trajectory of

the (undeflected) beam. Requirements on these are high-rate capability and very good spatial resolution for efficient vertexing.

Scintillating Fiber (SciFi) stations are detectors built from several layers of wavelength shifting fibers which are used as scintillator medium. These are arranged along the various spatial directions. These detector can stand high rates on the order of3·106Hz per fiber, are highly efficient (>96%) and deliver good time resolution (on the order of

400ps). Besides tracking, they are also used as parts of the beam trigger. A total of eight stations of varying size were installed in the spectrometer.

Silicon detectors are used as vertex detector in the immediate vicinity of the target. Their role as vertex detectors is due to their spatial resolution: up to 8µm (rms) can be achieved and a time resolution of the order of1.3ns (rms). The silicon detectors are semiconductor detectors where traversing particles excite electrons and holes into the valence band. A current due to these electrons is then detected. The silicon detectors were cooled to cryogenic temperatures (liquid nitrogen) to minimize radiation induced efficiency losses.

Additionally, the newly developed pixel readout for the central area of the GEM detectors, allowed their use in very small angle tracking. This is discussed below.

Small Area Trackers These cover the area close to the beam at distances from around

2.5cm to40cm. Here, again high spatial resolution is required as well as radiation hard- ness, combined with a small material budget along the beam direction. Two types of the detectors were used for this purpose: gas-electron mulatiplier (GEM) detectors and micromesh gaseous (Micromegas) detectors. COMPASS was the first large-scale exper- iment to use either type of detector. Both detectors are gaseous detectors separating the drift from the amplification stage. In the Micromegas detectors, ionization electrons drift towards a fine metallic mesh, the eponymous micro mesh. Beyond the mesh, the electrons are accelerated leading to the typical ionization cascade of gaseous detectors. The readout allows for a spatial resolution of 90µm (rms) while a temporal resolution of 9ns (rms) could be obtained over the active area of 40×40cm2 with a circular dead zone of radius5cm in the central region.

The GEM detectors employed by COMPASS [Ketzer et al., 2004] use three layers of GEM foils to ensure safe operation at high rates by a reduced conversion time. The GEM detectors provide a spatial resolution of70µm (rms) and a temporal resolution of

12ns over the active area of31×31cm2. Again, the detector center was not powered. In 2008 a new pixelized GEM detector was installed in the COMPASS experiment, allowing use of the detector even in the central region. These were installed in five stations, one upstream of the first spectrometer magnet SM1, adding redundancy in the very central region of the spectrometer, and two upstream and downstream of SM2, each. In 2009 a pixelized Micromegas station was also installed for testing purposes upstream of SM1.

Large Area Trackers Finally, the area away from the beam was covered with several

types of wire drift detectors. Next to SM1 three drift chambers with an active area of

180×127cm2and 176 pairs of wires running at opposite-sign high voltages were installed, providing a spatial resolution of 190µm. Wire detectors rely on an external time source to determine the distance of particle impacts from the wires. Therefore they do not provide time measurements on their own.

A number of multiwire proportional chambers is installed over the whole spectrometer. The active area is depending on the station either178×120cm2 or178×80cm2. Wires are spaced apart by2mm, and the total resolution is1.6mm.

Another type of tracking detectors in COMPASS are the straw tube drift cham- bers [Bychkov et al., 2006], straw detectors for short. In these, each individual wire is placed in a surrounding tube, ensuring the mechanical stability over the large area. The active area of323×280cm2_{is covered with tubes of diameter6.14mm in the central}

region and9.65mm in the region away from the beam. The inner part achieves a spatial resolution of190µm. The straw detectors are installed as the last part of tracking before particle identification in each spectrometer stage, i.e. right upstream of the RICH in the large-angle spectrometer and upstream of ECAL2 in the small-angle spectrometer.

active area is500×250cm2_{, giving a resolution of0.5mm. Its large wire spacing of4cm}

leads to huge dead time, requiring a dead zone in the central 50 – 100 cm.

Hodoscopes Mostly for the purpose of muon identification and triggering, COMPASS

is also equipped with several large trigger hodoscopes consisting of scintillators. The resulting light is read out by photomultipliers.

These trackers combined lead to a reconstruction performance on the order of 90 %

measured as the ratio of reconstructed tracks over generated tracks in Monte Carlo calculations for tracks with momentum above a lower limit of approx. 1GeV. A weak point of the setup is the lack of redundancy in the very small area tracking. In the 2008 setup, only three tracking planes are available in central region of the spectrometer upstream of SM1: two Silicon stations and one PixelGEM detector. Especially for displaced decays such as those of the long-lived neutralK_S0, efficiency drops dramatically for decays downstream of the silicon detectors. We shall now discuss the reconstruction of individual tracks, from which we shall turn to vertexing and the associated questions concerning the reconstructions ofK_S0 and other intermediate particles.