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A high precision measurement of the ultra-rare K{sup +} {yields} {pi}{sup +} {nu}{bar {nu}} decay at Fermilab would be one of the most incisive probes of quark flavor physics this decade. Its dramatic reach for uncovering new physics is due to several important factors: (1) The branching ratio is sensitive to most new physics models which extend the Standard Model to solve its considerable problems. (2) The Standard Model predictions for the K{sup +} {yields} {pi}{sup +} {nu}{bar {nu}} and K{sub L}{sup 0} {yields} {pi}{sup 0} {nu}{bar {nu}} branching fractions are broadly recognized to be theoretically robust at the 5-10% level. Only a precious few accessible loop-dominated quark processes can be predicted with this level of certainty. (3) The K{sup +} {yields} {pi}{sup +} {nu}{bar {nu}} branching fraction is highly suppressed in the Standard Model to the level &lt; 10{sup -10} (&lt;1 part in 10 billion). This suppression allows physics beyond the Standard Model to contribute dramatically to the branching fraction with enhancements of up to factors of 5 above the Standard Model level. (4) The certainty with which the Standard Model contribution to K{sup +} {yields} {pi}{sup +} {nu}{bar {nu}} can be predicted will permit a 5{sigma} discovery potential for new physics even for enhancements of the branching fraction as small as 35%. This sensitivity is unique in quark flavor physics and allows probing of essentially all models of new physics that couple to quarks within the reach of the LHC. Furthermore, a high precision measurement of K{sup +} {yields} {pi}{sup +} {nu}{bar {nu}} is sensitive to many models of new physics with mass scales well beyond the direct reach of the LHC. The experimental challenge of suppressing backgrounds to enable measurement of K{sup +} {yields} {pi}{sup +} {nu}{bar {nu}} at the 1 in 10-billion Standard Model rate has been met successfully. Several events of K{sup +} {yields} {pi}{sup +} {nu}{bar {nu}} decay have been clearly observed at BNL by using a carefully refined technique involving stopped low-energy kaons. Recently, it has become evident that the Fermilab Main Injector (MI) accelerator, running at about 95 GeV with a moderate duty factor to produce kaons, presents an opportunity to extend this approach by two orders of magnitude in sensitivity. The first order of magnitude improvement comes from the substantially brighter source of low energy kaons, and the second arises from incremental improvements to the experimental techniques firmly established at BNL. The proposed experiment at Fermilab, ORKA, will yield a precision of 5% for the K{sup +} {yields} {pi}{sup +} {nu}{bar {nu}} branching ratio measurement, which is comparable to the uncertainty of the Standard Model prediction. Opportunities for further advances to attain even higher precision would be made possible by the advent of Project X

ORKA: Measurement of the $K^ \to \pi^+ \nu \bar{\nu}$ decay at Fermilab