BaBar:
The BaBar experiment is an international collaboration of more than 550 physicists and engineers studying the subatomic world at energy of approximately ten times the rest mass of a proton. Its design was motivated by the investigation of CP-violation effects. BaBar is located at the Stanford Linear Accelerator Center, which is operated by Stanford University for the Department of Energy in California.
BaBar was set up to quantify the disparity between matter and antimatter, using a measure known as CP-violation. CP is symmetry obtained by combining (by multiplication) Charge and Parity symmetries, each of which are conserved separately except in weak interactions. 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 CP symmetry holds, the decay rate of B meson particles and their anti-particles should be equal. Analysis of secondary particles produced in the BaBar detector 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 consistent 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.
The BaBar detector ceased operation on April 7th 2008, but data analysis will continue for some time to come.
[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.
- 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 momentum.
- 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] Data record
| Run |
Dates |
Total Integrated Luminosity (fb-1) |
| 1 |
22 October 1999 - 28 October 2000 |
23.61 |
| 2 |
2 February 2001 - 30 June 2002 |
70.30 |
| 3 |
8 December 2002 - 27 June 2003 |
35.79 |
| 4 |
17 September 2003 - 31 July 2004 |
111.97 |
| 5 |
16 April 2005 - 17 August 2006 |
148.58 |
| Total (so far) |
22 October 1999 - 17 August 2006 |
390.25 |
[edit] Institutions involved
[edit] See also
- ^ 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.
- ^ University of Warwick Elementary Particle Physics group. Accessed 28 January 2008.
[edit] External links
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