2.2 Detector Overview

The detector and its front-end electronics are situated 100 m underground in the experimental cavern (see figure 2.3).

Figure 2.3: The DELPHI experimental hall. This view shows the counting houses, which are in front of the detector itself.

The layout of DELPHI is illustrated in figure 2.4.

Figure 2.4: Schematic layout of the 1994-5 DELPHI detector, showing a cutaway view of the barrel and the $-z$ endcap, and a `standard woman' for scale. Note that the Forward Chambers A are actually fixed to the Time Projection Chamber, but shown here on the front of the endcap for clarity.

The detector has a cylindrical geometry consisting of successive layers of charged tracking detectors (Vertex Detector, Inner Detector, Time Projection Chamber (TPC), and outside the Cherenkov detector (Barrel RICH), the Outer Detector at a radius of 2 m), followed by electromagnetic and hadron calorimetry (provided respectively by the High-density Projection Chamber and instrumented magnet yoke), and finally muon chambers (at 5 m radius). There is a similar arrangement in the endcaps (forward tracking chambers, RICH, electromagnetic and hadron calorimetry, and muon chambers at 5 m on either side of the interaction point). Additional scintillators and muon chambers between barrel and endcaps, and low-angle calorimeters (mainly for measuring Bhabhas) aim to provide near- $4\mathrm{\pi}$ solid-angle coverage. The tracking chambers use a solenoidal magnetic field of $1.2$ tesla, provided by a superconducting electromagnet just inside the hadron calorimeter.

The primary coordinate scheme [21] used by the DELPHI collaboration has the $z$-axis along the electron beam direction (parallel to the detector's central magnetic field), horizontal $x$-axis pointing towards the centre of LEP, and vertical $y$-axis pointing upwards, so that $(x,y,z)$ make a right-handed Cartesian system. Given the cylindrical symmetry of the detector (and the processes it measures) it is often more convenient to use a cylindrical or spherical coordinate system with radial and polar coordinates, $R$ and $\theta$, giving respectively the perpendicular distance and the angle from the $z$-axis. The azimuthal coordinate, $\phi$, gives the angle from the $x$-axis in the $x$$y$ projection. Due to DELPHI's symmetry about the $x$$y$ plane, quoted polar angles $\theta < 90\ensuremathbox{^\circ}$ will imply also the reflection in the $x$$y$ plane ( $180\ensuremathbox{^\circ}- \theta$), unless otherwise stated.

Tables 2.2 and 2.3 summarize the characteristics of the main detector components described in the following sections. Particular attention is paid to the Vertex Detector and Muon Chambers which play an important rôle in the analysis presented in chapter 4. More details of the detector design may be found in [22,14] and references given below. A review of the performance of DELPHI in its first six years of operation is given in [23]. The Slow Controls of each detector component are described in section 3.2, with table 3.1 listing the gas mixtures used.