BaBar experiment

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. [BaBar Collaboration, B. Aubert. et al., Nucl. Instrum. Methods A 479, 1 (2002).] 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 sometime to come.

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 K_Ls, 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 K_Ls. 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.

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. [ [http://bbr-onlwww.slac.stanford.edu:8080/babarrc/RecLumPerDay-Run_5.gifDaily PEP-II-delivered and BaBar-recorded luminosities] . (bar chart) Accessed 11 October 2005.] 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." [ [http://www.slac.stanford.edu/front_page_continue.html Dynamic Performance from SLAC B-Factory] . Accessed 11 October 2005.]

Data record

Institutions involved

* Laboratoire d'Annecy-le-vieux de Physique des Particules
* Universitat de Barcelona
* University of Bari
* Institute of High Energy Physics, Beijing
* University of Bergen
* Lawrence Berkeley National Laboratory
* University of Birmingham
* Ruhr-Universität Bochum
* University of Bristol
* University of British Columbia
* Brunel University
* 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
* University of Cincinnati
* University of Colorado
* Colorado State University
* Dortmund University of Technology
* Technische Universität Dresden
* Ecole Polytechnique
* University of Edinburgh
* University of Ferrara
* Laboratori Nazionali di Frascati dell'INFN
* University of Genoa
* Harvard University
* Universität Heidelberg
* Imperial College London
* University of Iowa
* Iowa State University
* Johns Hopkins University
* Universität Karlsruhe (TH)
* Laboratoire de l'Accelerateur Lineaire
* Lawrence Livermore National Laboratory
* University of Liverpool
* Queen Mary, University of London
* University of London, Royal Holloway and Bedford New College
* University of Louisville
* University of Manchester
* University of Maryland
* University of Massachusetts
* Massachusetts Institute of Technology
* McGill University
* University of Milan
* University of Mississippi
* Universite de Montreal
* Mount Holyoke College
* University of Naples Federico II
* National Institute for Nuclear Physics and High Energy Physics, Netherlands
* University of Notre Dame
* Ohio State University
* University of Oregon
* University of Padua
* Laboratoire de Physique Nucleaire et de Hautes Energies
* University of Pennsylvania
* University of Perugia
* Universita di Pisa
* Prairie View A&M University
* Princeton University
* University of Rome La Sapienza
* Universität Rostock
* Rutherford Appleton Laboratory
* DSM/Dapnia, CEA/Saclay
* University of South Carolina
* Stanford Linear Accelerator Center
* Stanford University
* State University of New York, Albany
* University of Tennessee
* University of Texas at Austin
* University of Texas at Dallas
* Universita di Torino
* University of Trieste
* Universitat de Valencia-CSIC
* University of Victoria
* University of Warwick [ [http://www2.warwick.ac.uk/fac/sci/physics/research/epp/exp/babar University of Warwick Elementary Particle Physics group] . Accessed 28 January 2008.]
* University of Wisconsin-Madison
* Yale University

ee also

*B-Factory
*B-Bbar oscillation

Notes

External links

* [http://www.slac.stanford.edu/BFROOT/ Official BaBar Website]
* [http://www-public.slac.stanford.edu/babar/ BaBar Public Home Page]
* [http://physicsweb.org/articles/world/14/8/9 Report of 2001 announcement about detection of CP violation]
* [http://www.youtube.com/watch?v=dj7gCZTEoq0 YouTube Video of the BaBar Control Room]