BaBar experiment

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The BaBar experiment, or simply BaBar, is an international collaboration of more than 500 physicists and engineers studying the subatomic world at energies of approximately ten times the rest mass of a proton (~10 

SLAC National Accelerator Laboratory, which is operated by Stanford University for the Department of Energy in California
.

Physics

BaBar was set up to understand the

Parity symmetry (P symmetry), 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 the B meson (symbol
B
) and its antiparticle (symbol
B
, pronounced B bar). The experiment's mascot was accordingly chosen to be Babar the Elephant
 :).

If CP symmetry holds, 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 (
K
/
K
 meson pairs). 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 multilayer

tau leptons
are possible.

The BaBar detector ceased operation on 7 April 2008, but data analysis is ongoing.

Detector description

At the bottom of the image, two straight lines originate from a single point (the event origin), separate by an angle of 30 or so degrees. The two line cross two grids of squares (detector grids) placed on top of each other, separated by some distance. The grid squares crossed by the lines are highlighted in different color, corresponding to the detection of the particles which crossed them.
Principle of silicon vertex detectors: the particles' origin, where the event that created them occurred, can be found by extrapolating backwards from the charged regions (red) left on the sensors.

The BaBar detector is cylindrical with the interaction region at the center. In the interaction region, 9 

GeV electrons collide with 3.1 GeV antielectrons (sometimes called positrons) to produce a center-of-mass collision energy of 10.58 GeV, corresponding to the
ϒ
(4S)
resonance. The
ϒ
(4S) decays immediately into a pair of B mesons – half the time
B+

B−
 and half the time
B0

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:

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. See
Bethe-Bloch formula
.
  • Detector of Internally Reflected Cherenkov Light (DIRC)
The DIRC is composed of 144 fused silica bars which radiate and focus Cherenkov radiation to differentiate between kaons and pions.
Made from 6580 CsI crystals, the EMC identifies electrons and antielectrons, which allows for the reconstruction of the particle tracks of photons (and thus of neutral pions (
π0
)) and of "long Kaons" (
K
L), which are also electrically neutral.
The Magnet produces a 1.5 T 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 long kaons. 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 1 × 1034 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 1.73 A, paired with a record 2.94 A 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]

In 2008, BaBar physicists detected the lowest energy particle in the bottomonium quark family, ηb. Spokesperson Hassan Jawahery said: "These results were highly sought after for over 30 years and will have an important impact on our understanding of the strong interactions."[4]

In May 2012 BaBar reported[5] that their recently analyzed data may suggest deviations from predictions of the Standard Model of particle physics. The experiments see two particle decays, and , happen more often than the Standard Model predicts. In this type of decay, a B meson decays into a D or D* meson, a tau-lepton and an antineutrino.[6] While the significance of the excess (3.4 sigma) is not enough to claim a break from the Standard Model, the results are a potential sign of something amiss and are likely to impact existing theories. In 2015 results from

LHCb and the Belle experiment strengthen the evidence (to 3.9 sigma) of possible physics beyond the Standard Model in these decay processes, but still not at the gold standard 5 sigma level of significance.[7]

Data record

Run Period Integrated luminosity[8]
(fb−1)
1 22 October 1999 – 28 October 2000 22.93
2 2 February 2001 – 30 June 2002 68.19
3 8 December 2002 – 27 June 2003 34.72
4 17 September 2003 – 31 July 2004 109.60
5 16 April 2005 – 17 August 2006 146.61
6 25 January 2007 – 4 September 2007 86.06
7 13 December 2007 – 7 April 2008 45.60
Total 22 October 1999 – 7 April 2008 513.70

See also

Notes

  1. S2CID 117579419
    .
  2. ^ Daily PEP-II-delivered and BaBar-recorded luminosities (bar chart).[dead link] Accessed 11 October 2005.
  3. ^ Dynamic Performance from SLAC B-Factory. Accessed 11 October 2005. Archived October 16, 2005, at the Wayback Machine
  4. ^ Physicists Discover New Particle: the Bottom-most 'Bottomonium' 2008-07-10, Accessed 2009-08-02
  5. S2CID 20896961
    .
  6. ^ BaBar data hint at cracks in the Standard Model (EScienceNews.com).
  7. ^ 2 Accelerators Find Particles That May Break Known Laws of Physics. Sept 2015
  8. S2CID 33933422
    .

External links
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