Particle Detection
4.4 Detector Systems and Experiments at Accelerators
4.4.1 Examples of Detectors for Fixed-Target Experiments
In a fixed-target experiment, relativistic effects make the interaction products highly collimated. In such experiments then, in order to enhance the possibility of detection in the small-xT (xT =pT/√
s, wherepTis the momentum component perpendicular to the beam direction), different stages are separated by magnets opening up the charged particles in the final state (lever arms).
The first detectors along the beam line should be nondestructive; at the end of the beam line, one can have calorimeters. Two examples are given in the following;
the first is a fixed-target experiment from the past, while the second is an almost fixed-target detector presently operating.
152 4 Particle Detection
Fig. 4.26 A configuration of the European Hybrid Spectrometer (a fixed-target detector at the CERN Super Proton Synchrotron). From M. Aguilar-Benitez et al., “The European hybrid spec- trometer,” Nucl. Instr. Methods 258 (1987) 26
4.4.1.1 The European Hybrid Spectrometer at the SPS
The European Hybrid Spectrometer EHS was operational during the 1970s and in the beginning of the 1980s at the North Area of CERN, where beams of protons were extracted from the SPS (Super Proton Synchrotron)11 accelerator at energies ranging from 300 to 400 GeV. Such particles might possibly generate secondary beams of charged pions of slightly smaller energies by a beam-dump and a velocity selector based on magnetic field. EHS was a multi-stage detector serving different experiments (NA16, NA22, NA23, NA27). Here we describe a typical configuration;
Fig.4.26shows a schematic drawing of the EHS setup.
In the figure, the beam particles come in from the left. Their direction is determined by the two small wire chambers U1 and U3. From the collision point inside a rapid cycling bubble chamber (RCBC; the previously described LEBC is an example, with a space resolution of 10µm) most of the particles produced enter the downstream part of the spectrometer.
The RCBC acts both as a target and as a vertex detector. If an event is triggered, stereoscopic pictures are taken with 3 cameras and recorded on film.
The momentum resolution of the secondary particles depends on the number of detector element hits available for the track fits. For low momentum particles, typicallyp<3 GeV/c,length and direction of the momentum vector at the collision point can be well determined from RCBC. On the other hand, tracks withp>3 GeV/c have a very good chance to enter the so-called first lever arm. This is defined by the group of four wire chambers W2, D1, D2, and D3 placed between the two magnets M1 and M2. Very fast particles (typically with momentump>30 GeV/c) will go through the aperture of the magnet M2 to the so-called second lever arm, consisting of the three drift chambers D4, D5, and D6.
To detect gamma rays, two electromagnetic calorimeters are used in EHS, the intermediate gamma detector (IGD) and the forward gamma detector (FGD). IGD is placed before the magnet M2. It has a central hole to allow fast particles to proceed to the second lever arm. FGD covers this hole at the end of the spectrometer. The
11A synchrotron is a particle accelerator ring, in which the guiding magnetic field (bending the particles into a closed path) is time dependent and synchronized to a particle beam of increasing kinetic energy. The concept was developed by the Soviet physicist Vladimir Veksler in 1944.
IGD has been designed to measure both the position and the energy of a shower in a two-dimensional matrix of lead-glass counters 5 cm ×5 cm in size, each of them connected to a PMT. The FGD consists of three separate sections. The first section is the converter (a lead-glass wall), to initiate the electromagnetic shower.
The second section (the position detector) is a three-plane scintillator hodoscope. The third section is the absorber, a lead-glass matrix deep enough (60 radiation length) to totally absorb showers up to the highest available energies. For both calorimeters, the relative accuracy on energy reconstruction isΔE/E0.1/√
E⊕0.02.
The spectrometer included also three detectors devoted to particle identification:
the silica-aerogel Cherenkov detector (SAD), the ISIS chamber measuring specific ionization, and the transition radiation detector TRD.
4.4.1.2 LHCb at LHC
LHCb (“Large Hadron Collider beauty”) is a detector at the Large Hadron Collider accelerator at CERN. LHCb is specialized in the detection of b-hadrons (hadrons containing a bottom quark). A sketch of the detector is shown in Fig.4.27.
Fig. 4.27 Sketch of the LHCb detector. Credit: CERN
154 4 Particle Detection Although, in strict terms, LHCb is a colliding beam experiment, it is done as a fixed-target one: the strongly boostedb-hadrons fly along the beam direction, and one side is instrumented.
At the heart of the detector is the vertex detector, recording the decays of the b particles, which have typical lifetimes of about 1 ps and will travel only about 10 mm before decaying. It has 17 planes of silicon (radius 6 cm) spaced over a meter and consisting of two disks (in order to measure radial and polar coordinates) and provides a hit resolution of about 10 and 40µm for the impact parameter of high momentum tracks.
Downstream of the vertex detector, the tracking system (made of 11 tracking chambers) reconstructs the trajectories of emerging particles. LHCb’s 1.1 T super- conducting dipole spectrometer magnet (inherited from the DELPHI detector at LEP, see later) opens up the tracks.
Particle identification is performed by two ring-imaging Cherenkov (RICH) detec- tor stations. The first RICH is located just behind the vertex detector and equipped with a 5 cm silica aerogel and 1 m C4F10gas radiators, while the second one consists of 2 m of CF4gas radiator behind the tracker. Cherenkov photons are picked up by a hybrid photodiode array.
The electromagnetic calorimeter, installed following the second RICH, is a
“shashlik” structure of scintillator and lead read out by wavelength-shifting fibers. It has three annular regions with different granularities in order to optimize readout. A lead-scintillator preshower detector improves electromagnetic particle identification.
The hadron calorimeter is made of scintillator tiles embedded in iron. Like the electromagnetic calorimeter upstream, it has three zones of granularity. Downstream, shielded by the calorimetry, are four layers of muon detectors. These are multigap resistive plate chambers and cathode pad chambers embedded in iron, with an addi- tional layer of cathode pad chambers mounted before the calorimeters. Besides muon identification, this provides important input for triggering.
There are four levels of triggering. The initial (level 0) decisions are based on a high transverse momentum particle and use the calorimeters and muon detectors.
This reduces by a factor of 40 the 40 MHz input rate. The next trigger level (level 1) is based on vertex detector (to look for secondary vertices) and tracking informa- tion, and reduces the data by a factor of 25 to an output rate of 40 kHz. Level 2, suppressing fake secondary decay vertices, achieves further eightfold compression.
Level 3 reconstructsBdecays to select specific decay channels, achieving another compression factor of 25. Data are written to tape at 200 Hz.