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Measuring B(KOPIO)

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Symmetry Violation

In 1967 Sakharov showed that a matter-dominated universe such as the one in which we live (rather than one with equal parts of matter and anti-matter) could occur if a set of simple principles that involve symmetry violation were obeyed. The symmetries here are related to charge, space and time. Since 1964, in experiments involving the decays of neutral K mesons, charge-parity (CP) and time reversal (T) symmetry violations have been observed lending support to the consistency of Sakharov's hypothesis. However, in the context of the SM, these effects fail to explain the matter dominance effect by many orders of magnitude, leading physicists to search for entirely new effects that could have profound implications for our understanding of the universe.

Since CP symmetry violation is one of the most important outstanding issues in the study of elementary particle physics with profound implications on the relationship between the quarks, and possibly also on the origin of matter in the universe, it is one of the main areas of worldwide activity in particle physics. While CP symmetry violation had only been observed in K meson decays, during the past year, great international efforts involving several thousand researchers and new accelerators and detectors have uncovered similar effects using the heavier B meson.

Nevertheless, it has become clear that the single most incisive measurement in the study of CP symmetry violation would be the branching ratio (or rate of decay) of a neutral K meson () decaying to a pi meson () and a neutrino anti-neutrino pair, represented by the symbols


and the acronym KOPIO. Discovery and study of this reaction is the main focus of the proposed KOPIO experiment at Brookhaven National Laboratory. The decay mode is unique because it is completely dominated by direct CP violation. Since theoretical uncertainties are extremely small, measurement of the branching ratio B(KOPIO) (or the fraction of decays which proceed by reaction (1)) will provide the standard against which all other measures of CP violation will be compared, and even small deviations from the expectation derived from SM predictions or from other measurements, e.g. in the B meson sector, will unambiguously signal the presence of new physics. Using current estimates of SM parameters, B(KOPIO) is expected to lie within the range , i.e. only three out of every 100 billion reactions.

The importance of this measurement is related to the extreme precision with which the SM can predict the branching ratio in the context of current knowledge. Although the KOPIO reaction is extremely rare, the new experiment has been designed with unprecedented sensitivity and has been accepted at the Alternating Gradient Synchrotron (AGS) accelerator of Brookhaven National Laboratory (New York). Because new technologies proposed for the KOPIO experiment represent an improvement in experimental sensitivity of a factor of more than 100,000 over previous instruments, it will open a vast window of discovery with the potential to transform our picture of nature. Employing new technology and achieving remarkable sensitivity, KOPIO has the capability of discovering entirely unanticipated phenomena, or of inferring discrepancies in existing theory by comparison with B and K-meson experiments. In the history of science, transformations in our understanding have often followed the introduction of new instruments, such as when Galileo’s telescope revealed details of the planets and the inner workings of the solar system.

> CONTINUE: Measuring B(KOPIO)



Last updated January 24, 2006 by Gary Schroeder.