Testing the Unitarity of the CKM Matrix with a Space-Based Neutron Decay Experiment

Abstract

If the Standard Model is correct, and fundamental fermions exist only in the three generations, then the CKM matrix should be unitary. However, there remains a question over a deviation from unitarity from the value of the neutron lifetime. We discuss a simple space-based experiment that, at an orbit height of 500 km above Earth, would measure the kinetic-energy, solid-angle, flux spectrum of gravitationally bound neutrons (kinetic energy K<0.606 eV at this altitude). The difference between the energy spectrum of neutrons that come up from the Earth&#39;s atmosphere and that of the undecayed neutrons that return back down to the Earth would yield a measurement of the neutron lifetime. This measurement would be free of the systematics of laboratory experiments. A package of mass $<25$ kg could provide a 10^{-3} precision in two years.