While proton accelerators provide the most powerful means for exploring the quantum universe, electron accelerators are the most precise. When protons smash together, the most interesting interaction occurs between one constituent from each projectile. All the other components in the collision, however, create a lot of debris that partially obscures the primary collision. When an electron and its opposite, a positron, smash together, there is no debris. This creates a very clean collision. The powerful proton collisions provide the big picture; an electron accelerator zooms in on the details.
From 1999 to 2008, the BaBar experiment at SLAC's PEP-II electron-positron collider had investigated CP violation in the B sector. In 2002 the BaBar experiment provided definitive results that the decay rates of B and anti-B mesons differed, a result which contributes to our knowledge of how matter and antimatter behave differently and why there are so many kinds of particles in the Universe. The BaBar experiment's results on CP violation confirmed the predictions of the so-called Standard Model of high energy physics and showed that the inclusion of three generations of quarks in the model is essential to describe Nature. The three generation formulation of the Standard Model by Kobayashi and Maskawa won a portion of the 2008 Nobel Prize in physics and the BaBar experiment, as well as its rival in Japan, the KEK-B facility at Tsukuba, were cited for their accurate and crucial data. In addition to studies of CP violation, the BaBar experiment at SLAC has pursued a broad program of research on particles containing bottom or charm quarks. Today the BaBar collaboration is actively engaged in data analysis.
The next step in highly accurate electron-positron collider physics is the construction of facilities with 50-100 times the intensities of the PEP-II and the KEK-B accelerators. Projects in Italy and Japan are planned for this new era which will produce such copious data that very rare processes can be studied in detail and new processes that are go beyond the Standard Model can be discovered. These experiments will complement the experiments underway at CERN's Large Hadron Collider and the various facilities together should lead the way to our deeper understanding of symmetries and particles that underlie the physical world.
Looking ahead, the International Linear Collider, a proposed electron-positron collider, or a similar machine--perhaps a muon collider--could serve as the next step in discovering new physics at high energies comparable to the Large Hadron Collider. A lepton collider will allow scientists to home in on the new landscape that the LHC will initially chart. Together, these discovery machines will bring the quantum universe into unprecedented focus.
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