The Babar Experiment at SLAC the Babar Detector Is Located at the PEP-II Electron-Positron Collider at the SLAC Laboratory in California

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The BaBar Experiment at SLAC The BaBar detector is located at the PEP-II electron-positron collider at the SLAC laboratory in California. The BaBar collaboration is studying CP violation using B mesons. CP violation allows a difference between particles and antiparticles and hence is thought to explain the matter-antimatter asymmetry of the Universe. In 2001 BaBar made the world’s first observation of this phenomenon in B decays through the measurement of the parameter sin2β of the CKM matrix. The measurement of CP violation in sin2β is done by looking for a difference between B and anti-B meson decays. These can only be different if matter and antimatter are not exactly “equal- but-opposite” and this is one of the consequences of CP violation. The figure below shows the rate of B decays to a particular final state (here J/Ψ KS) compared to anti-B decays as a function of their lifetime. This shows a very clear difference between the two and was the first measurement to demonstrate “indirect” CP violation in B meson decays. The BaBar detector was completed and started data-taking in 1999. Since that time, it has recorded around 600 million B meson decays. The detector (shown above) consists of tracking, particle identification and calorimeter subdetectors. In particular, the novel particle identification device is based on observation of Cherenkov radiation from charged particles passing through quartz bars. The Cherenkov light is detected in the large prominent ring (above) at the end of the detector. BaBar has a wide physics programme other than CP violation, for example searches for new particles. The plot below shows the peak from a new particle, called the Y(4260), discovered by BaBar in 2005. The nature of this particle is not yet understood. The Imperial group is heavily involved in measurements of charmless decays. An example of a computer reconstruction of a charmless B decay observed in the BaBar detector is shown above. These decays are sensitive to different CKM parameters from “indirect” CP violation, and allow estimates of the CKM angle parameters sin2α and γ. So far, only one charmless mode has shown significant “direct” CP violation so these parameters are not yet accurately determined. However, BaBar is scheduled to continue data-taking until 2008 and is expected to quadruple its data sample over that time. This will allow many more such modes to be accurately measured..

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BABAR Studies Matter-Antimatter Asymmetry in Τ Lepton Decays
BABAR studies matter-antimatter asymmetry in τττ lepton decays Humans have wondered about the origin of matter since the dawn of history. Physicists address this age-old question by using particle accelerators to recreate the conditions that existed shortly after the Big Bang. At an accelerator, energy is converted into matter according to Einstein’s famous energy-mass relation, E = mc 2 , which offers an explanation for the origin of matter. However, matter is always created in conjunction with the same amount of antimatter. Therefore, the existence of matter – and no antimatter – in the universe indicates that there must be some difference, or asymmetry, between the properties of matter and antimatter. Since 1999, physicists from the BABAR experiment at SLAC National Accelerator Laboratory have been studying such asymmetries. Their results have solidified our understanding of the underlying micro-world theory known as the Standard Model. Despite its great success in correctly predicting the results of laboratory experiments, the Standard Model is not the ultimate theory. One indication for this is that the matter-antimatter asymmetry allowed by the Standard Model is about a billion times too small to account for the amount of matter seen in the universe. Therefore, a primary quest in particle physics is to search for hard evidence for “new physics”, evidence that will point the way to the more complete theory beyond the Standard Model. As part of this quest, BABAR physicists also search for cracks in the Standard Model. In particular, they study matter-antimatter asymmetries in processes where the Standard Model predicts that asymmetries should be very small or nonexistent.
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