Figure 2.1: The HERA-B detector.
2.1 The HERA-B detector
The B0mesons are produced by a wire target in the proton beam halo. To collect a suffi-cient number ofGolden Decays, four proton–nucleon interactions per bunch crossings are needed on average. In every bunch crossing there are about 200 tracks of charged parti-cles in the detector. This leads to a large track density and to irradiation conditions only comparable to the ones in the forthcoming LHC experiments. Because HERA-B is a fixed target experiment, the whole detector is built as a forward spectrometer (see Figure2.1) with aperture of 220 mrad which is almost 4π in the rest frame of the proton–nucleon system. To ensure that the decay products originate from a B0, its decay vertex (= sec-ondary vertex) i.e. its decay length, must be known precisely. Thus one can distinguish directly produced particles from decay products. This is done by the silicon vertex de-tector. The momentum of the charged particles is determined using the magnet and the tracking chambers. The decay particles – electrons, muons, kaons, and pions – are
iden-tified with the help of the electro-magnetic calorimeter, the muon system, and the ring imaging ˇCerenkov counter.
2.1.1 The target
The target (see Figure 2.2) consists of two sets of four wires placed in the halo of the HERA proton beam. Each of them can be separately inserted and retracted in steps of 0.05 mm [10]. Several different materials are used as target wire. During the run 2000 the
proton beam
target wires steering
colimator
target wire
indensity
beam profile
no wire
with wire X
Figure 2.2: On the left side is a schematic drawing halo wire target, consisting of two sets of four wires (only one set is drawn). The right side shows how the reduces the beam tails.
following materials samples were investigated: Ti, Al, W and C. All wires have a cross section of 50×500µm2except for the carbon wire, which has a size of 50×1000µm2. The wire target has the effect of a collimator in the sense that it scrapes away beam tails, the protons on the border of the phase space, which would probably leave the beam anyhow.
Thus it effectively reduces the size of the beam. It does not deteriorate the beam quality for the electron–proton experiments H1 and Zeus and their luminosity stays (almost) the same.
2.1.2 The vertex detector
The vertex detector is composed of silicon micro strip detectors. Its main purpose is to separate the different “events” (proton target interaction) in every bunch crossing and to measure the decay length of the B0-meson. To minimize the dead material in front of it, it is operated in the evacuated vertex vessel where the vacuum is almost as good as in the beam tube. Only a seamless 200µm thin aluminium cap separates it from the HERA ring vacuum. Seven of the eight vertex detector layers are in the vessel and the last one is located right behind it. To minimize radiation damage, they are retracted during the injection of the proton beam. Therefore for every proton fill a new online alignment has to be performed.
2.1 The HERA-B detector 21
2.1.3 The magnet
The particle momenta are measured with a normal conducting magnet in conjunction with the tracking chambers. The integrated field amounts to 2.2 Tm. The distance be-tween the magnet center and the target is 4.5 m. 85% of the K0ss have decayed within this flight path and can therefore be identified and analysed.
2.1.4 The outer tracker
As the particle density decreases with 1/r2 (r = distance from the beam axis) (see [4]
p. 188, formula 25.6), HERA-B is composed of two tracking systems. The inner tracker discussed in greater details in Section 2.2, covers the region 45×45 cm2 to 50×50 cm2 along the beam pipe. The outer tracker covers the remaining part of the acceptance re-gion. It consists of honeycomb drift chambers with a cell size of 5 mm in the inner and 10 mm in the outer region. For an accurate momentum measurement the track resolu-tion should be better in the bending plane than perpendicular to it. Hence there are no detectors with wires along the in horizontal direction. Only some chambers are tilted by a stereo angle of±5◦to enable a relatively inaccurate measurement of the y-position.
Additionaly, detectors tilted by 90◦would lead to a lot of ambiguities in the pattern recog-nition.
2.1.5 The RICH
Behind the magnet and the main part of the tracking system, there is a large ring imaging Cerenkov counter (RICH). The task of the RICH is to separate charged kaons from pions,ˇ electrons, and protons in a momentum region from a few GeV to about 50 GeV. The main part of the RICH consists of a big tank filled with C4F10 gas. When a charged particle passes the tank, it emits light, which is reflected against two mirrors and detected by a photomultiplier. From the radius of the ring of photon impact points the opening angle of the light cone and further the velocity of the particle is reconstructed. Together with the momentum from the tracking system this information leads to a mass determination.
2.1.6 The electromagnetic calorimeter
The main purpose of the ECAL is to produce the pretrigger (see Section2.1.9) for electrons from the Golden Decay. It is divided into an inner, a middle, and an outer section, with segmentations to ensure an occupancy of at most 10%. The calorimeter modules are sampling plastic scintillators absorber sandwiches. They are read out via optical fibers by photomultipliers. The absorber material in the inner region is tungsten to keep the Moli`ere radius low; in middle and outer region, where the track density is lower, lead is used.
2.1.7 The TRD
To separate electrons from hadrons, a transition radiation detector is placed in front of the inner part of the ECAL. A charged particle passing the boundary between two media of different dielectric constants emits transition radiation. The rate strongly depends on the Lorentz factor γ, i.e. the lighter the particle the more light is emitted. In this way hadrons faking an electron from a Golden Decay can be strongly suppressed.
2.1.8 The muon system
The penetrating muons are separated from the hadrons by the use of a massive absorber.
Three iron/concrete absorber blocks alternate with the muon chambers. No absorber is between the two last chamber layers. Hits from these two layers are used asµ-pretrigger (see Section2.1.9).
2.1.9 The trigger
The event input rate of 10 MHz and the limited data logging rate of 50 Hz at HERA-B put some strong requirements on the background rejection factor, which the trigger has to achieve. The main trigger method is to look for J/ψ from the Golden Decay. This is done in a five stage process:
Pretrigger The pretrigger delivers the starting points (Regions of Interest = RoI) for the track search in the following trigger level. RoIs are large clusters in the ECAL or multiple hits in the last two muon chamber layers.
FLT The first level trigger starts looking for tracks at the RoIs. It moves upstream from tracking plane to tracking plane towards the magnet. From the position and the angle of these track the corresponding momentum is estimated assuming it comes from the nominal vertex. If the invariant mass fo two tracks combined lies in the region of the J/ψmass, the event is accepted. The FLT’s implementation consists of the track finding units (TFU), the track parameter units (TPU), and the track decision unit (TDU).
SLT In the second level trigger the tracks found in the first level are refitted using more exact data from the tracking chambers. The cut on the J/ψ is narrowed. If a sec-ondary vertex is found, the event is directly sent to the fourth level trigger. Digital signal processors (Sharcs) are used as second level buffer and to take the trigger decision.
TLT A refined reconstruction with correct treatment of multiple scattering is done at the third trigger level. Secondary vertices are searched. Additionally physics channels that do not necessary contain a secondary vertex are selected at this level.
2.1 The HERA-B detector 23
4LT The fourth level trigger performs a full event reconstruction. The final event selec-tion and various monitoring tasks are carried out before the events are written to tape. The TLT and 4LT is implemented as a computing farm of 200 Linux PCs.
proton
TFU TFU TFU TFU TFU TFU TFU
MUON
(ECAL) (ITR & OTR)inner & outer tracker magnet
Figure 2.3: The principle of first level trigger. Tracks of decayingJ/ψare searched starting from the back towards the target. Hits in the muon system or the electromagnetic calorimeter are used as starting points (= pretrigger).