BaBar experiment
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The BaBar experiment is an international collaboration of more than 550 physicists and engineers investigating CP-violation effects using the BaBar particle detector at the Stanford Linear Accelerator Center, which is operated by Stanford University in California.
BaBar was set up to study the difference between matter and antimatter — CP-violation. CP is the product of the Charge and Parity symmetries. In classical physics both symmetries are conserved, but this is not the case at sub-atomic scales. BaBar focuses on the study of CP-violation in the B meson system. The name of the experiment is derived from the nomenclature for B meson (B) its anti-particle (B-bar), and is also a reference to the experiment's mascot — Babar the Elephant.
If the CP symmetry holds, the decay rate of B meson particles and their anti-particles should be equal. Analysis of the BaBar results showed this was not the case — in the summer of 2002, definitive results were published based on the analysis of 87 million B/B-bar meson-pair events, clearly showing the decay rates were not equal. Consistent results were found by the Belle experiment at the KEK laboratory in Japan.
CP-violation was already predicted by the Standard Model of particle physics, and well established in the neutral kaon system. The BaBar experiment has increased the accuracy to which this effect has been experimentally measured. Currently results are in agreement with the standard model, but further investigation of a greater variety of decay modes may reveal discrepancies in the future.
The BaBar detector is a multi-layer particle detector. Its large solid angle coverage (near hermetic), vertex location with precision on the order of tens of micrometres (provided by a silicon vertex detector), good pion-kaon separation at multi-GeV momenta (provided by a novel Cherenkov detector), and few-percent precision electromagnetic calorimetry (CsI(Tl) scintillating crystals) allow a list of other scientific searches apart from CP violation in the B system.[1] Studies of rare decays and searches for exotic particles and precision measurements of bottom and charm mesons and tau leptons are possible.
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[edit] Detector Description
BaBar is a cylindrically shaped detector with the interaction region at the center. In the interaction region 9 GeV electrons collide with 3.1 GeV positrons to produce a center-of-mass energy of 10.58 GeV, corresponding to the Upsilon(4S) resonance. The Upsilon(4S) decays immediately into a pair of B mesons — half the time B+B- and half the time B0 anti-B0. To detect the particles there are a series of subsystems arranged cylindrically around the interaction region. These subsystems are as follows, in order from inside to outside:
- Silicon Vertex Tracker (SVT) Made from 5 layers of double-sided silicon strips, the SVT records charged particle tracks very close to the interaction region inside BaBar.
- Drift Chamber (DCH) Less expensive than silicon, the 40 layers of wires in this gas chamber detect charged particle tracks out to a much larger radius, providing a measurement of their momenta. In addition, the DCH also measures the energy loss of the particles as they pass through matter, dE/dx.
- Detector of Internally Reflected Cherenkov Light (DIRC) The DIRC is composed of 144 quartz bars which radiate and focus Cherenkov light to differentiate between kaons and pions.
- Electromagnetic Calorimeter (EMC) Made from 6580 CsI crystals, the EMC identifies electrons and positrons, and allows for the reconstruction of the particle tracks of photons (and thus π0s) and KLs, which are electrically neutral.
- Magnet The Magnet produces a 1.5 Tesla field inside the detector, which bends the tracks of charged particles allowing deduction of their mass.
- Instrumented Flux Return (IFR) The IFR is designed to return the flux of the 1.5 T magnet, so it is mostly iron but there is also instrumentation to detect muons and KLs. The IFR is broken into 6 sextants and two endcaps. Each of the sextants has empty spaces which held the 19 layers of Resistive Plate Chambers (RPC), which were replaced in 2004 and 2006 with Limited Streamer Tubes (LST) interleaved with brass. The brass is there to add mass for the interaction length since the LST modules are so much less massive than the RPCs. The LST system is designed to measure all three cylindrical coordinates of a track: which individual tube was hit gives the φ coordinate, which layer the hit was in gives the ρ coordinate, and finally the z-planes atop the LSTs measure the z coordinate.
[edit] Notable Events
On 9 October 2005, BaBar recorded a record luminosity just over 1x1034 cm-2s-1 delivered by the PEP-II positron-electron collider.[2] This represents 330% of the luminosity that PEP-II was designed to deliver, and was produced along with a world record for stored current in an electron storage ring at 1732 mA, paired with a record 2940 mA of positrons. "For the BaBar experiment, higher luminosity means generating more collisions per second, which translates into more accurate results and the ability to find physics effects they otherwise couldn’t see."[3]
[edit] Datataking Record
| Run | Dates | Total Integrated Luminosity (fb-1) |
|---|---|---|
| 1 | 22 Oct 1999 - 28 Oct 2000 | 23.61 |
| 2 | 2 Feb 2001 - 30 Jun 2002 | 70.30 |
| 3 | 8 Dec 2002 - 27 Jun 2003 | 35.79 |
| 4 | 17 Sep 2003 - 31 Jul 2004 | 111.97 |
| 5 | 16 Apr 2005 - 17 Aug 2006 | 148.58 |
| Total (so far) | 22 Oct 1999 - 17 Aug 2006 | 390.25 |
[edit] Institutions Involved
Laboratoire de Physique des Particules
Universita di Bari
Institute of High Energy Physics, Beijing
Lawrence Berkeley National Laboratory
Ruhr Universitat Bochum
University of British Columbia
Budker Institute of Nuclear Physics
University of California, Berkeley
University of California at Irvine
University of California at Los Angeles
University of California at Riverside
University of California at San Diego
University of California at Santa Barbara
University of California at Santa Cruz
California Institute of Technology
Universitat Dortmund
Technische Universitat Dresden
Universita di Ferrara
Laboratori Nazionali di Frascati dell'INFN
Universita di Genova
Universitat Karlsruhe
Laboratoire de l'Accelerateur Lineaire
Lawrence Livermore National Laboratory
Queen Mary, University of London
University of London, Royal Holloway and Bedford New College
Massachusetts Institute of Technology
Universita di Milano
Universita di Napoli Federico II
National Institute for Nuclear Physics and High Energy Physics, Netherlands
Universita di Padova
Laboratoire de Physique Nucleaire et de Hautes Energies
Universita di Perugia
Universita di Roma La Sapienza
Rutherford Appleton Laboratory
DSM/Dapnia, CEA/Saclay
Stanford Linear Accelerator Center
State University of New York, Albany
Universita di Trieste
Universitat de Valencia-CSIC
University of Wisconsin-Madison
[edit] External links
- Official BaBar Website
- BaBar Public Home Page
- Report of 2001 announcement about detection of CP violation
- WIRED3 Web Display shows BaBar collision results live
[edit] Sources
- ^ BaBar Collaboration, B. Aubert. et al., Nucl. Instrum. Methods A 479, 1 (2002).
- ^ Daily PEP-II-delivered and BaBar-recorded luminosities. (bar chart) Accessed 11 October 2005.
- ^ Dynamic Performance from SLAC B-Factory. Accessed 11 October 2005.
