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 areperpendicular 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.

The Purpose of BaBar Experiment

　　The BaBar experiment is one of the most attractive projects within a decade in high energy physics, whose main purpose is to generate a plenty amount of　 events at Υ (4S) resonance, to study the decay of B0 meson into CP intrinsic states systematically and measure the asymmetries. The standard model predicted there exists large asymmetries in the decays of B0 to CP intrinsic states. Therefore, this measurement can test the prediction of the standard model to the CP violation, or can find new source of CP violation. The research of the CP violation is one of the frontiers with significant theoretical interest, which is very important to the understanding of the structure and interactions of elementary particles, to the explanation of universe consisting of mainly matter rather than anti-matter.

　　In addition to this main physics goal, due to the high luminosity and the excellent performances of PEPII/BaBar, many subjects, such as the decays of beauty and charm mesons, τ leptons, the two-photon processes, the spectroscopy, etc., can be studied and new phenomena can be searched, with much higher statistics and much better precisions. Many measurements which are of significant importance in two frontiers of current particle physics researches -- the precision test of the standard model and the search for new physics beyond the standard model --, can be carried out, for instance, the determination of CKM matrix elements, the measurement of　 mixing (at Υ(5S) resonance) and　 mixing, the measurement of　 mixing and its CP violation, the accurate measurement of the form factor in the semi-leptonic decays of charmed mesons, the precise determination of the pseudo-scalar coupling constants fD, fDs, the studies of excited charmed meson spectroscopy, the measurement of τ neutrino mass (or, upper limit), etc.

　　BaBar experiment is one of the cooperative items in the PRC/U.S. Cooperative Program in High Energy Physics between the Chinese Academy of Sciences (CAS) and the Department of Energy (DOE) of U.S.. The PEP-II has begun its physics run since April 1999 and currently is running at the desired energy region with the designed luminosity, and BaBar detector has collected huge amount of experimental data. Some important physics results can be expected in the near future, which will have big impact on the current understanding of the particle world. IHEP scientists have joined the BaBar collaboration to study the τ-charm physics. Such type of cooperation has the advantage of directly joining the research work in the physics frontier with most advanced machine and detector, and quickly obtaining the significant physics results with the highest level.

Intended research subjects of IHEP group

　　The research subjects IHEP (BaBar) group intended to implement is in the τ-charm physics and its related physics tool packages. This strategy is based on the following consideration: We can quickly start our researches using the experiences obtained in the BEPC/BES experiment; with the fresh experiences obtained in the BaBar's work, the researches in BEPC/BES can be improved to a new level. Therefore, the researches on τ-charm physics at BEPC/BES and PEPII/BaBar can take advantages of each other.

　　Copious subjects on τ-charm physics can be studied in BaBar experiment, some of them are listed as follows:

　　1. Researches on D and DS leptonic decays, determination of fD and fDs

　　2. Researches on D and DS semi-leptonic decays, determination of Vcs,Vcd

　　3. Researches on charmed mesons hadronic decays, studies of mutual effect of perturbative and non-perturbative interactions

　　4. Measurement of τ-nutrino mass or its upper limit with better accuracy

　　5. Researches on CP violation in τ decays

　　6. Measurement of Michel parameters in τ decays

　　7. Measurement of τ electric dipole moment

 　　  ................................................................

　　In order to study these physics subjects, some physics tool packages must be pre-studied, such as

　　1. D,DS tagging

　　2. identification of τ decay events, study of τ decay's character

　　3. study of missing energy in τ decays due to its neutrino's escape

 　  .........................................................

　　We will choose some of these subjects based on available manpower, study them in detail, and get meaningful physics results in possibly a short period. For this purpose, IHEP has formed a team (IHEP BaBar group) including 6 physicists, two of them have been sent to SLAC, to join the BaBar experiment data taking, to get familiar with BaBar detector and its software system, to participate in the physics tool and data analysis programming, to prepare setting a BaBar data analysis environment at IHEP. At the IHEP site, we will gradually establish a PC system with Linux version BaBar data analysis environment, in order to carry out BaBar data analysis at IHEP.