The future CBM experiment at GSI

The Superconducting Dipole Magnet

The Silicon Tracking System

Transition radiation detector (TRD)

The ring Imaging Cherenkov Detector

Resistive Plate Counters (RPC)

Electromagnetic Calorimeter (ECAL) 

Recently a new collaboration has been formed at GSI to design and build the Compressed Baryonic Matter (CBM) Experiment at the future accelerator facility of GSI. The main task of the CBM experiment is to identify both leptons and hadrons and to detect rare probes in a heavy ion environment. The apparatus should be able to investigate the heaviest collision systems at a broad range of incident beam energies. The experiment has to fulfill the following requirement:

i.              Large acceptance

ii.        Fast and radiation hard detectors 

iii.           Lepton and hadron identification

iv.       High resolution secondary vertex determination

v.        A highly selective trigger scheme 

Figure 1: Schematic detector setup at the future facility of GSI. The first detector is an extended version of the currently used electron spectrometer HADES. The second detector comprises a big dipole magnet, a RICH detector, a TRD detector and a RPC TOF wall. Both detectors will be used for the lower and higher beam energy range available [1].

The CBM experiment will be performed with two detector systems [1, 2] (figure 1). The first device is the HADES detector, which will be used to measure dilepton pairs and hadrons in the beam range up to about 7xA GeV.  The second detector (Compressed Baryonic Matter, CBM) will measure dilepton pairs and hadrons up to beam energies of about 40xA GeV. It consists of a superconducting dipole magnet with a Silicon tracker inside, a RICH detector, a TRD detector and RPC TOF wall. An electromagnetic calorimeter (ECAL) is added to the layout, shown in figure 2, in order to improve the pion suppression capability of the setup and to be able to measure real photons and neutral mesons decaying into photons. The CBM target is placed at the entrance of the magnet while the HADES target is placed at the entrance of the spectrometer. Figure 2 depicts the present layout of the CBM experimental setup.


Figure 2: Geometry of the CBM experiment

The experimental setup consists of the following detector components:

         Dipole magnet for bending the particles (according to their momentum and charge) and  ray deflection

         A set of Silicon pixel and strip detectors in the field of a super-conducting magnet and close to the target. These detectors allow to determine particle trajectory with high accuracy and vertex reconstruction, which is crucial for the measurement of charmed D mesons. These detectors provide a vertex resolution of about 50 m.

         A Ring imaging Cherenkov detector (RICH) is spilt into two. The first RICH serves for the identification of electrons from the decays of low-mass vector mesons ( ). The second RICH is optional. It might be used for the separation of high-energy pions and kaons in order to improve the identification of high-energy D mesons.

         Transition radiation detector (TRD) for identifying high-energy electrons.

         Resistive plate counters (RPC) for time-of-flight measurement.

         Diamond pixel detector for time-of-flight signal 

         The electromagnetic calorimeter (ECAL) measures electrons, photons and muons.


Figure 3: The experimental setup consisting of the dipole magnet, the RICH, the TRD detector and the RPC TOF wall [2].

Figure 3 displays a sketch of the proposed setup. The CBM setup is optimized for heavy-ion collisions in the beam energy range from about 8 to 45xA GeV. Experiments on Dilepton production at beam energies from 2 to about 8xA GeV will be carried out with the HADES spectrometer, which will be installed in front of the CBM target (see figure 1)

The Superconducting Dipole Magnet

The goal is to build a dipole magnet with a large aperture (1x1 m2) and a small stray field. The momentum of charged particles is derived from their track curvature in the magnetic dipole field. The target and the Silicon Tracking Station (STS) are placed in the magnetic field, which deflect most of the abundantly produced -electrons. In order to restrict the size of the tracking detectors inside the field, the field region should be confined within 1 m along the beam axis. A simple estimate of the coil requirements assuming a perfect yoke to close the field lines results in 2 coils with 800000 A*turns each to provide a field of 2T. This requires a superconducting coil.

The Silicon Tracking System

The silicon tracking system will be used to perform a measurement of the charge particle trajectories in the magnetic field of the dipole up to emission angles of 25o. The track information should allow to determine the momentum with an accuracy of better than 1% and secondary decay vertices with a resolution of about 50 m. The simulation reaction rates will be up to 107/s with a multiplicity of 1000 charged particles per event. The tracking system has to fulfill the following requirements:

         High granularity: to determined the granularity required in such a high rate experiment, both the average local track density and the intrinsic dead time of the detector has to be taken into account. The high granularity required for small laboratory polar angles can be achieved by silicon pixel detectors. 

         Fast read out to avoid pile up

         Radiation hardness

         Operation in high -ray background 

Transition radiation detector (TRD)

Transition radiation detector (TRD) is used to improve the identification of electrons with respect to pion for momentum above 1 GeV/c. The TRD will allow to study various aspects of dielectron physics, among them the production of quarkonium state (J/ and ), as well as the production of open charm.

Figure 4: The geometry of one TRD module. The geometric proportions and the field lines in the Drift Chamber (DC) are accurate. Schematic signals produced by a pion and an electron are shown.

The TRD is composed of a radiator and a photon detector; the photon detector is a Drift Chamber with 3 cm drift zone and an amplification region of about 6 mm. A cross-section of a segment of one module of the TRD is shown in figure 4 along with a schematic illustration of the signals detected by the DC from a pion and an electron. The field lines in the DC, depicted in figure 4 are calculated with GARFIELD [3] for the dimensions mentioned above. 

The ring Imaging Cherenkov Detector

The ring imaging Cherenkov detector based on currently available technologies can cover the full acceptance range. It provides either lepton identification in the full momentum range or can be used to separate high momentum pions from kaons with limited lepton identification depending on the radiator gas.

A ring imaging Cherenkov detector will serve two purposes:

1.    Lepton identification up to the highest momentum (approx. 10 GeV/c).

2.    Separation of fast pions and kaons starting from momentum of 5 GeV/c.

The basic design features are:

n      Radiator-gas = counter gas

n      No window between UV detector and radiator

n      Mirror: float glass 1.8 mm thick, 36 segments

n      UV-photon detector: solid CsI photocathode

Resistive Plate Counters (RPC)

The large area Resistive Plate Counters (RPC) provide the stop signal for the time-of-flight measurement, which allows the identification of low momentum particles. A TOF distance of 10 m results in a time difference of 400 ps for pions and kaons of 3 GeV/c momentums. The development of RPC has made significant progress during the last few years [4, 5]. The RPC array has an area of 120 m2 with a granularity of about 35000 individual detectors. Their area ranges from 4 cm2 at small deflection angles to 100 cm2 at larger angles.  

Electromagnetic Calorimeter (ECAL) 

The electromagnetic calorimeter will be used to measure real photons, neutral mesons decaying into photons, electrons and muons. Particular emphasis is put on a good energy resolution and spatial resolution for reconstruction of short-lived heavy particles (J/, D, , mesons) decay products.

References

[1] P. Senger, Compressed Baryonic Matter Experiment, 24-29 Sept. 2001 J. Phys. G, Nucl. Part. Phys. 28 (2002) 186

[2] P. Senger, Int. Workshop XXX on Ultrarelativistic Heavy-Ion Collisions, Hirschegg/Austria, 13-19 Jan. 2002

[3] R. Veenhof, Nucl. Instr. Meth. in Phys. Res. A419, pp.726-730, 1998

[4] P. Fonte A. Smirnitski and M.C.S. Williams, Nucl. Instr. Meth A 443 (2000) 201

[5] P. Fonte, R. Ferreira-Marques, J. Pinhao, N. Carolino and A. Policarpo, Nucl. Instr. Meth A 449 (2000) 295