The Fermilab Proposal P996: Precision Measurement of [formula omitted

Abstract

The high precision measurement of the ultra-rare K + → π + ν ν ¯ decay at Fermilab would be one of the most incisive probes of quark flavor physics this decade. The dramatic physics reach of a precision measurement of K + → π + ν ν ¯ is due to three factors. 1) The Standard Model prediction for the K + → π + ν ν ¯ and K L 0 → π 0 ν ν ¯ branching fractions are broadly recognized as theoretically robust to the 2-4% level. No other loop-dominated quark process can be predicted with this level of certainty. 2) The K + → π + ν ν ¯ branching fraction is highly suppressed in the Standard Model to the level of less than 1 part in 10 billion. This suppression allows physics beyond the Standard Model to contribute noticeably to the branching fraction with enhancements of up to factors of 5 above the Standard Model level. 3) The certainty with which the Standard Model contribution to K + → π + ν ν ¯ is known permits a 5 σ discovery potential for new physics even for enhancements of the branching fraction as small as 20%. This sensitivity is unique in quark flavor physics and probes essentially all models of new physics that couple to quarks within the reach of the LHC. Further, precision measurement of K + → π + ν ν ¯ is sensitive to many models of new physics far beyond the direct mass reach of the LHC. The experimental challenge of measuring K + → π + ν ν ¯ at the 1 in 10-billion Standard Model rate has been met successfully. Several events of the K + → π + ν ν ¯ process have been clearly observed at BNL. Operating the Tevatron after Run-II as a 120 GeV high-duty factor synchrotron “Stretcher” offers the opportunity to reach more than two orders of magnitude greater sensitivity yielding a 1000-event experiment based on incremental improvements to the techniques refined and firmly demonstrated at BNL. The Fermilab Stretcher would be a unique facility that would provide ideal properties for such rare-decay experiments, allowing the demonstrated performance of the AGS experiment to be extrapolated with confidence to an experiment driven by the Fermilab Stretcher.