Appendix F: Precision physics experiments with kaon beams
The charged kaon experiment enabled by the Tevatron Stretcher and Project X could deliver a precision measurement of the K+→π+νv branching fraction that matches the small theoretical uncertainty. In parallel, the charged kaon experiment can probe many other decay channels including a precision measurement of K+→e+ν and a search for K+→π+μe which are both uniquely incisive probes of Terascale physics and beyond. The neutral kaon experiment driven by the enormous 8 GeV beam power provided by Project X could discover and measure the ultrarare KL→π0νv process. As the sensitivity reaches down to the Standard Model branching ratio of 3×10-11, several extensions of the Standard Model would be excluded or, even better, new Terascale phenomena would be detected. Upon attaining the Standard Model sensitivity the neutral kaon experiment then becomes sensitive to very high mass scale (>1000 TeV/c2) through precision measurement of the KL→π0νv branching fraction.
Driving the charged kaon experiment with the high-duty-factor Tevatron Stretcher simultaneously reduces detector rates by a factor of three and reduces the proton tax on the Main Injector neutrino program from 30 percent to fi ve percent. The lower detector rates reduce the technical risk of the charged kaon experiment and support scaling of the experiment to much higher sensitivities in the Project-X era. The high-energy separated charged-kaon beam based on ILC crab-cavity technology drives this next step in ultrarare K+→π+νv sensitivity with samples of 100-200 decays per year within reach prior to Project X coming online. Project X could then further increase the rare-decay sensitivities by a factor of three while maintaining a small fi ve percent tax on the Main Injector neutrino program. Meanwhile at CERN the NA48 collaboration is working to advance a K+→π+νv experiment that could collect about 50 events per year early in the next decade. The key technical elements of the charged-kaon experiment have been reviewed worldwide in detail, and an experiment could be developed into a reviewable project with a year of effort.
Estimates of kaon flux driven from 8 GeV Fermilab proton sources suggest that a compelling staged neutral-kaon program could be developed that first reaches Standard Model sensitivity of the KL→π0νv process with an experiment driven by the Booster, followed by a precision measurement enabled by the 8 GeV proton fl ux provided by Project X. Across the globe a similar staged KL→π0νv program is being pursued in Japan, where Stage I at KEK is now complete. There are plans for Stage II at J-PARC which could reach Standard Model sensitivity sometime in the next decade. Key technical issues of the neutral kaon experiment are actively being pursued worldwide, and an experiment based at Fermilab could be developed into a reviewable project with a year of effort.