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The BaBar Experiment
A team of around 600 physicists and engineers from ten countries built a
huge particle detector at SLAC (the Stanford Linear Accelerator Centre in
California) to measure the decay of B mesons and their anti-particles, B-bar
mesons.
The detector, which was named BaBar after these particles, weighed 1,200 tons, is 6 metres long and 6 metres in diameter. Some 75 institutions from Canada, France, Germany, Italy, Norway, Russia, Spain, Taiwan, the United Kingdom and the United States all collaborated on the project. At the peak of its activity around 90 British particle physicists and engineers from eleven institutions took part in the experiment. The UK institutions involved
were:
- University of Birmingham
- University of Bristol
- Brunel University
- University of Edinburgh
- University of Liverpool
- University of Manchester
- Imperial College of Science, Technology and Medicine
- Queen Mary and Westfield College
- Royal Holloway, University of London
- University of Warwick
- Rutherford Appleton Laboratory of the STFC
Aims of the BaBar experiment
A view of the BaBar detector
Credit: DOE
In the very early moments after the Big Bang, the universe should have
contained equal amounts of matter and antimatter. When matter and antimatter
particles meet, they annihilate each other. Yet, the universe we see around us
is made up almost entirely of matter. The results of the BaBar experiment cast
light on the puzzle of how the matter we see in our universe survived this
primordial mutual annihilation. The BaBar experiment has made the first
observation of charge-parity (CP) violation in the B meson system and
determined the associated parameters. "CP violation" is the term given to the
subtle effect where particles and their anti-particles differ minutely in their
properties. CP violation reveals itself through small differences in the rates
at which the B and B-bar mesons decay when produced from a particle called the
Upsilon(4S). There are two vital ingredients in the measurement of the CP
violation parameters:
- Identifying the B or B-bar as having decayed to particles with a definite
CP value
- Distinguishing between the other B or B-bar by identifying the particles
into which they decay
An impression of the BaBar detector
Credit: Daryl Oshatz/Ernest Orlando Lawrence Berkeley
National Laboratory
To make these identifications, the detector needed to measure the energy of
each particle in the decays and locate the B and B-bar decay points by
measuring their position to a precision of about 100 microns using a silicon
vertex detector. B mesons decay in about one trillionth of a second and before
BaBar physicists had not been able to produce enough of them to observe CP
violation. The new particle accelerator, PEP-II, at SLAC changed all of that.
PEP-II provided a "B-Factory" which is capable of generating enormous numbers
of B mesons. It did this by colliding intense beams of electrons and positrons
250 million times every second. This produced about 10 B and B-bar mesons per
second. The velocities of the electrons and positrons were been chosen to
ensure that the B and B-bar mesons which were generated on impact have enough
momentum to travel about 1/4 millimetre before decaying. This allowed the
lifetime of each one to be measured. The BaBar detector surrounded the point of
collision and was capable of measuring precisely the paths of the particles
from the decays.
In 2008, BaBar stopped taking data to look for CP-Violation and started a
unique project to search for Dark Matter and Light Higgs particles from the
decays of the Upsilon(3S) and Upsilon(2S).
The UK contribution to BaBar
The UK groups constructed the forward end cap calorimeter and the
signal-processing and triggering electronics for the whole calorimeter. The
forward end cap is a crucial element of the detector, as the asymmetry in the
energy of the electron and positron beams results in a high density of
particles in the forward region.
Major contributions were also made to all aspects of the associated
software, including data acquisition, event reconstruction and physics analysis
programmes.
A view of the B Factory particle accelerator inside its tunnel
Credit: DOE
Investment
BaBar was a collaborative effort, with the major financial contribution from
the USA. The US Department of energy provided around $180M for the construction
of the electron positron collider, PEP-II, at no cost to the BaBar experimental
collaboration. The UK contribution to the detector construction, which came
from STFC's predecessor PPARC, was £8.26M. This provided for staff at the
ten UK groups, equipment, travel, and consumables. UK participation during the
operational phase of the experiment is funded from the PPARC and now STFC
particle physics programme.
Timescale and successes
The first B and B-bar pair was recorded on 19th May 1999. The
experiment stopped taking data on the Upsilon(4S) in December 2007. In January
2008, the experiment switched to running on the Upsilon(3S) for 2 months,
followed by 1 month running on the Upsilon(2S). The last data was taken on
7th April 2008 but it will take some years to fully analyse.
Approximately 500 million B and B-bar particles have been recorded, more than
30 times the previous generation of experiments. The Upsilon(3S) and
Upsilon(2S) samples are at least 10 times larger than the previous record.
Images
-
Babar Detector - a view of the BaBar detector (Credit: DOE)
-
Babar Detector - an impression of the BaBar detector. (Credit: Daryl Oshatz/Ernest Orlando Lawrence Berkeley National Laboratory)
-
PEP II - a view of the B Factory particle accelerator inside its tunnel (Credit: DOE)
-
PEP II - an aerial view of the PEP II facility
Further information
More information on the BaBar project can be found on the BaBar public
information website (link opens in a new window).
Page last updated: 11 November 2009
by Charlotte Jamieson
