B Factory

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B FACTORY

A B factory is a particle collider dedicated to producing B mesons. A meson is a bound state of a quark and an antiquark. A B meson contains a heavy bottom antiquark together with a light up or down quark while a B̄ meson contains a bottom quark and light antiquark. Whereas up and down quarks are stable, bottom quarks decay with a lifetime of just 1.5 pico-seconds (1.5 trillionths of a second). B mesons can decay in many different ways. Measurements of the pattern of decay rates have led physicists to a much deeper understanding of the fundamental properties of quarks and of the electroweak force.

B mesons also possess the remarkable ability to change, or oscillate, from matter to antimatter. This happens when a B⁰ meson, the bound state of a b antiquark and a d quark, turns into a B̄⁰ meson, consisting of a b quark and a d antiquark. B⁰-B̄⁰oscillations were first observed in 1984, and this property together with the relatively long lifetime of the B meson led to the realization that it might be possible to observe an asymmetry between matter and antimatter in B meson decays. This asymmetry, known as CP violation, is required to explain how the universe evolved from a matter-antimatter symmetric configuration just after the Big Bang into the present matter-dominated state. CP violation was first observed in 1974 by James Cronin and Val Fitch in decays of K with K⁰ mesons. However, further experiments K⁰ mesons did not reveal the source of CP violation, and thus the possibility of studying it in another, heavier, quark system was very attractive.

The first B factory was the Cornell Electron-Positron Storage Ring (CESR). CESR began operation in 1979, following the discovery of the b quark by Leon Lederman and colleagues at Fermilab in Batavia, Illinois, in 1977. A storage ring in Germany called DORIS was upgraded in the early 1980s so it could also produce B mesons. At CESR and DORIS, equal-energy positron and electron beams were collided at a center-of-mass energy of 10.58 GeV/c². At this energy, B meson production is resonantly enhanced, occurring in a quarter of all interactions. Another advantage is that B mesons are pair-produced with no additional particles to complicate the event. A disadvantage of the symmetric B Factory is that the B mesons are almost at rest and travel only 0.03 mm on average before decaying. This distance is too short to be measured with present detector technology, making it impossible to study time-dependent effects, such as lifetimes, at a symmetric B factory.

CP violation appears as a time-dependent difference in the B⁰ and B̄⁰ decay rates to special final states known as CP eigenstates. This time dependence occurs because of a special quantum coherence between a pair of B mesons that are resonantly produced. The CP asymmetry may only be observed if one can measure the time between the two B meson decays and determine whether the CP eigenstate decay occurred first. The other B , called the tagging B , is identified as a B⁰ or B̄⁰ . The time difference between the two B decays is plotted separately for B⁰ and B̄⁰ tagged events, and a shift between the two distributions is evidence of CP violation (Figure 1). Because the overall CP asymmetry will average to zero if the time between the decays of the two B mesons is not measured, this interesting phenomenon could not be studied at symmetric B factories.

In 1987 Piermaria Oddone of the Lawrence Berkeley National Laboratory proposed the asymmetric B factory (ABF) as a means to combine the copious and clean B meson production of an e +e - B factory with the lifetime information required for CP violation studies. In an ABF, the electron and positron beams have unequal energy, but the total energy available in the center of mass is tuned to

FIGURE 1

The construction of two ABFs started in 1994 and both were completed in 1999. The PEP-II ABF was built at the Stanford Linear Accelerator Center (SLAC) in California. In Japan the KEK-B ABF was built at the Japanese High-Energy Research Organization (KEK) in Tskuba, Japan. PEP-II and KEK-B are very similar in design. To achieve high interaction rates, both require very-high-intensity electron and positron beams, typically 1 ampere or more of stored current for each beam. Both reused existing electron-positron storage rings that had formerly operated as symmetric rings with much higher energies and lower currents, and both required the construction of an additional ring in order to store the beam of lower energy. The designs differed somewhat in how the beams are brought into collision. KEK-B employs a small crossing angle between the two beams to separate them after colliding, while in PEP-II the beams are collided head-on and separated by means of strong permanent dipole magnets located close to the interaction point.

PEP-II and KEK-B began operations in 1999 and soon achieved very high interaction rates. In the first year of operation, PEP-II produced approximately 20 million pairs of B mesons, whereas KEK-B produced about 10 million. In the future both machines expect to reach a rate of 100 million B meson pairs per year. The B meson decay products are recorded with large detectors that surround the electron-positron collision point. The PEP-II detector is called BaBar and the KEK-B detector is called Belle; both detectors were built by large international teams of physicists and are designed to record the charged tracks as well as the photons produced in B meson decays. The data are analyzed to reconstruct the B mesons from their decay products. The decay rates of B⁰mesons are compared to the corresponding B̄⁰ decay rates as a function of the time between decays to look for evidence of a time-dependent CP-violating asymmetry. In the summer of 2001 the BaBar and Belle collaborations both reported significant evidence of CP violation in B mesons.

The 2002 results from BaBar and Belle are in agreement with the predictions of the Standard Model. However, current theories of baryogenesis cannot explain the amount of matter in the universe within the context of Standard Model CP violation. The future goal of the BaBar and Belle experiments is to measure CP violation in B decays more accurately and in a number of different decays in an effort to find an inconsistency that will point to new physics beyond the Standard Model.