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Research with the Birmingham BaBar group

Staff 1998-2009: Paul Bright-Thomas, Kelly Ford, Theresa Harrison, Chris Hawkes, Andrew Kirk, Steve O'Neale, David Smith, Nitesh Soni, Alan Watson, Nigel Watson
Students 1998-2008: Matthew Barrett, Kelly Ford, Alistair Hart, David Knowles, Sian Morgan, Richard Penny, Pablo del Amo Sanchez

Where has all the antimatter gone? In the early universe, matter and antimatter were created in equal amounts: pairs of electrons and positrons, pairs of quarks and antiquarks, and so on. Today, all we find in the universe is the matter which makes up the atoms of stars, planets and postgraduate students. All the antimatter has disappeared.

The excess of matter over antimatter is thought to have built up during the evolution of the universe. It can arise from a subtle, but fundamental difference between the properties of matter and antimatter, known as CP violation. This matter-antimatter asymmetry had been observed in particle physics experiments since the 1960's, but only as a tiny effect in the decay of a particle called the neutral K-meson.

In order to understand better the differences between matter and antimatter we need to study CP violation thoroughly by making precise measurements of its effects. Standard electroweak theory predicts that these effects should be much larger in the decays of B mesons, which are similar to, but heavier than K mesons. However, the CP violation effects can be observed only in certain rare decay modes of B mesons, and so many hundreds of millions of B decays must be recorded and measured to study the phenomenon.

Aerial view of SLAC

Aerial view of the SLAC accelerator complex

The BaBar experiment, at the SLAC National Accelerator Laboratory in California, was one of the first experiments designed to make these measurements. The BaBar detector is situated on the PEP-II electron-positron collider facility, or "B-factory", so called because it was able to produce the enormous numbers of B mesons which are needed for precise CP violation studies. The experiment started running in 1999. The B-factory exceeded its design luminosity (the rate of electron-positron collisions) already in the year 2000, and has now recorded about 500 million B anti-B pairs. BaBar has measured sin(2beta), a basic parameter of CP violation, with 5% precision (arXiv:0902.1708 [hep-ex]). BaBar's measurements in many different B decay channels provide a comprehensive test of the theory of CP violation. General studies of the physics of B mesons, as well as that of charm mesons, tau leptons and quantum chromodynamics (QCD), are also possible with unprecedented accuracy.

The BaBar experiment was performed by a world-wide collaboration of several hundred physicists. Birmingham was one of ten UK institutes involved. The names of the academic and research staff members and the seven PhD students who worked with the Birmingham BaBar Group between 1998 and 2008 are listed above. Our primary physics goal was the study of the decays of charged and neutral B mesons to three particles that do not contain charm quarks, so-called charmless three-body B decays to final states consisting of pions and/or kaons. These can be studied by means of the Dalitz Plot technique, giving sensitivity to quantum interference between different decay channels. Birmingham students and postdocs were among the principal authors of the following recent papers: Phys. Rev. D72, 072003 (2005) (hep-ex/0507004), Phys. Rev. D73, 031101(R) (2006) (hep-ex/0508013), Phys. Rev. D74, 032003 (2006) (hep-ex/0605003), Phys. Rev. D79, 051101(R) (2009) (arXiv:0811.1979 [hep-ex]) and Phys. Rev. D80, 112001 (2009) (arXiv:0905.3615 [hep-ex]).

The precise measurements of B decays made by the BaBar experiment at SLAC, together with the Belle experiment at KEK in Japan, have provided experimental confirmation of the theory of CP violation, which forms a part of the Standard Model of Particle Physics. The work of the two Japanese physicists, Makoto Kobayashi and Toshihide Maskawa, who devised this theory in 1972, was recently recognised by the award of half of the 2008 Nobel Prize in Physics. The BaBar Collaboration has prepared a statement explaining the meaning and importance of this work.