BaBar  Experiment

In the Stanford Linear Accelerator Center (SLAC), the PEPII machine, a e⁺e^- collider with e⁺  beam energy 3.1 GeV and e^- beam energy 9.0 GeV, was built up in the base of original e⁺e^- collider, PEP. The design luminosity is 3x10³³ cm^-2 s^-1. It can produce huge amount of B mesons, therefore, is called as B Factory. The whole cost of the B factory, including the BaBar detector, is 1.77 billion US dollars.

e⁺  and e^- beams are circulated in two different rings and collide in interaction region. Different energies of e⁺ and e^- beams mean that the center of mass of the collision products will rapidly move along the direction of higher energy beams. The most possible products of the collision in PEPII is neutral B pair. The neutral B meson has relatively long life time, it will decay after flighting a certain distance, which will be further extended due to the Lorents boost effect caused by the movement of the center of mass. In this case, the CP asymmetry in  system become measurable. A detailed study will be carried out in PEPII/BaBar based on its higher luminosity, asymmetric collision and powerful detection capability.

BaBar detector is assemblied in the only interaction region of PEPII. The BaBar Collaboration consists of more than 400 physicists and engineers from 9 countries (Canada, China, France, Germany, Italy, Norway, Russia, UK and USA).

After the completion of the PEPII commissioning at Feb.1999, the BaBar detector weighing 1200 tons was moved into the PEPII interaction region, the final examinations for the detector was carried out, and the physics run was started. In the end of Feb. 1999, the peak luminosity reached 5.2x10³² cm² s^-1.

The BaBar detector is optimized in its design to meet the requirements of its physics goals:  

1. Silicon Vertex Tracker: The distance of decay points between two B mesons is measured with Silicon Vertex Tracker, in which the silicon microstrips are perpendicular to or parallel with the beam direction. The detector has 5 sensitive layers with the radii of 3.2 to 14.4 cm. The Silicon Vertex Tracker measures the Z coordinates of decay points with the accuracy of 90 μ, and also measure the hit points of charged particle’s tracks within 92% solid angle.

2. Drift Chamber: The measurements of the charged particle’s tracks are carried out by Silicon Tracker together with Drift Chamber surrounded with 1.5 T solenoidal magnetic field. The inner and outer radius is 23.6 and 80.9 cm, respectively. The momentum range of charged particles inside Drift Chamber which can be reconstructed is 0.1-4.5 GeV/c. The Drift chamber, filled with Helium-isobutane (80% - 20%) to suppress the multiple scattering, reaches rather high spacial resolution (120-140 microns). The high precision is reached for the charged particle momentum measurements due to the high spacial resolution of the Drift Chamber and a 1.5T field generated by a superconducting solenoidal magnet.

3. Particle ID: One of the key points of the BaBar detector is to have powerful PID capability. Electrons are identified with the CsI Electromagnetic Calorimeter and the Drift Chamber. The large iron structure is segmented and instrumented with Resistive Plate Counters (RPC), called the Instrumented Flux Retune (IFR), to provide the identifications for neutral K mesons and other neutral hadrons, and for muons with energy above 0.5 GeV. The DIRC ( Detection of Internally Refleted Cherenkov ) is designed to provide excellent kaon identification. The DIRC is a ring-image cherenkov detector, covering a 87% solid angle. The cherencov lights are inner-reflected in 4.8 meters long quartz bar and detected by 13000 photomultiplier’s array outside the magnetic iron yoke.

4. CsI Electro-Magnetic Calorimeter: The average photon energy in B decay is about 200 MeV. Therefore, it is important to have low detection threshold photon energy (~20 MeV). At the same time, good energy resolution is necessary to suppress backgrounds. This leads to a design of EMC consisting of 6800 pieces of CsI crystals. Such kind of calorimeter is currently operated successfully in CLEO-II detector at Cornell e⁺e^- collider CESR.