COMPASS detector for the initial hadron programme

dissociation, a Monte Carlo based simulation was done using COMGEANT. COMGEANT is the interface to the GEANT3.21 [50] software, including the geometrical properties of the COMPASS detector. The GEANT program simulates interactions and decays of particles and resonances in the detector.

The detector for the initial hadron programme in COMPASS should be similar to the detector that will be used for the muon programme in order to reduce the ”dead time” when switching between physics programmes. Nevertheless, the detector should be suited for the characteristics of the physics programmes of interest.

0 500 1000 1500 2000 -0.004 0 0.004 0.008 0 400 800 1200 0 0.002 0.004 0.006 0.008 0.01 b) a) θi θo [rad] θ [rad] θ (θo - i) Entries Entries

Figure 4.7: Primakoff reaction: a) polar angles θi of the incoming π− and θ0 of the outgoing

π−. b) scattering angleθ0−θi of the outgoing π−.

different components. The setup described is the official setup for operations in 2001. The official setup is the result of an agreement of the different detector groups in the COMPASS technical board meeting on the 25th of January 2001.

Some explanation of the different positions for the detectors is given here. The scin- tillating fibres hodoscopes 1 and 2, placed upstream of the target, should monitor the beam. The purpose of the veto, upstream of the target, is to ensure that the beam spot is not larger than the target itself. Further upstream of the target, two more detectors are present, consisting of two silicon microstrip stations (stations 1 and 2). Each station will have 4 projections: 0, 90, +10 and -10 deg. These two stations have a lever arm of 50 cm, which is necessary to measure the angle of the incoming pion. Two more stations of silicon detectors (stations 3 and 4) will be placed downstream of the target also with 50 cm lever arm. They will measure the angle of the outgoing pion. The angle difference between the incoming and outgoing pion is a very characteristic variable for the Primakoff reaction. The silicon detectors, with a typical spatial resolution of 14µm, will determine the char- acteristic angle for the Primakoff reaction with high accuracy, as will be shown later. The silicon stations 3 and 4 will also provide some tracking between the target and the first spectrometer magnet (SM1). The veto box will be situated around the target. It has a hole for the incoming beam and a hole for the outgoing very forward particles. For Pri- makoff reactions, the recoil momentum of the target is very small, less than 0.02 GeV/c2 (see 4.6.a). In this case, no track at all should be recorded in the veto box. The events with this signature could come from Primakoff reaction thus they will be analyzed. For diffractive processes, the recoil momentum of the target is also small, less than 1 GeV/c2 (see 4.2). In this case, the recoil proton should hit the veto at a large angle. Events with one hit in the veto at large angle could be a signature for diffractive events.

For tracking, several track segments are required. The first lies between the target and the first magnet, another track segment is located between both magnets, and a last track segment lies between the second magnet and the electromagnetic calorimeter. For this purpose, different tracking detectors are distributed along the beam line. To cover all of the acceptance, tracking detectors of different sizes are used. The main tracking detectors

4.3. ACCEPTANCE STUDIES 77

Detector Location along Lateral displacement name beam axis [cm] [cm]

Scintillating fibre station 1 -800 – Scintillating fibre station 2 -290 –

Veto 1 -100 –

Silicon microstrip detector 1 -80 – Silicon microstrip detector 2 -30 –

Veto box -10 to 10 –

Target 0 –

Silicon microstrip detector 3 30 –

Silicon microstrip detector 4 80 –

Scintillating fibre station 3 164 –

Scintillating fibre station 4 215.5 –

Saclayµω 234.5 –

Saclay DC 250.5 –

SM1 262 to 438 –

Silicon microstrip detector 5 450 –

1st straw submodule 519 –

2nd straw submodule 531 –

Scintillating fibre station 5 555 –

RICH1 560-900 – GEM 1 913 1 MWPCA* 925 1 ECAL 1 937-1115 – HCAL 1 1115-1280 – GEM 2 1301 1.5

Scintillating fibre station 6 1308 1.5

MWPCA 1 1320 1.5

GEM 3 1486 2

MWPCA 2 1500 2

SM2 1530 to 1930 –

MWPCA 3 1980 4.5

Scintillating fibre station 7 2000 4.7

MWPCA 4 2020 5

MWPCA 5 2040 5

GEM 4 2064 5

Scintillating fibre station 8 3150 16

GEM 5 3166 16

MWPCA 6 3180 16.5

ECAL 2 3226 to 3476 16.5

Table 4.1: Location of the components of the COMPASS detector for the initial hadron pro- gramme.

for the initial hadron programme are the GEMs, the silicon and the MWPCs. The Saclay

the hadron programme discussed. In the region where these detectors are situated, the angles of the particles considered are very small. The majority of these particles will pass through the holes or the insensitive regions of these detectors. Due to the present design of the tracking algorithm, at least three orientations are required to consider a detector as tracking detector. The scintillating fibres, only having two orientations (0 and 90 degrees) are not considered as trackers for the moment. The GEM detectors have an insensitive area 50 mm in diameter at the centre. The only detectors that could cover this hole for tracking would have been the scintillating fibres. Since these detectors are not used for tracking, the COMPASS detector has holes for certain particle momenta at certain angles. The gamma detector in COMPASS, the ECAL2, has also a hole for the deflected beam. This hole will cause a loss in acceptance for Primakoff and diffractive events, as it will be shown later in section 4.3.2.

The actual hadron setup finishes after ECAL2 even though other detectors are existing behind ECAL2. These detectors will be used in the muon programme. A detector still missing in the described setup is the scintillator hodoscope that will be used for trigger purposes. This will be situated at the end of the hadron setup, in front of the electromag- netic calorimeter (ECAL2). The acceptance for Primakoff and diffractive processes using these detectors will be shown in the next sections.

Material budget

Crucial for the physics programmes is the amount of material introduced by the different detectors. Survival probabilities of particles can be studied directly from the COMGEANT Monte Carlo simulations.

A general event generator was developed to generate gammas with random angle ϕ

and different transversal momenta p_⊥ following a flat distribution. Studies of the survival probability of these particles in the COMPASS detector were carried out using this gen- eral event generator. A gamma detector placed at 30 m from the target, the ECAL2, is responsible for the detection of the gammas from diffractive and Primakoff processes.

Figure 4.8 shows the survival probability for a gamma (integrated over the entire range of longitudinal and transverse momenta). The number of gammas along the detector will be reduced due to the gamma conversions in the different detectors.

The major gamma loss comes from the conversions in the RICH1 detector. The RICH1 detector contains 3.4 m of C4F10 gas along the beam line, which corresponds to 10.5 % of radiation length. To avoid gamma conversions in this region, a beam pipe has to be installed in the RICH1 detector. This pipe should contain a light gas such as He to reduce the amount of material.