The search for K+ → π+νν-bar. New results from Brookhaven E-787

The decay K+ → π+νν-bar is a flavor-changing neutral current interaction, forbidden to first order and highly suppressed to all orders by the GIM mechanism in the Standard Model. Hadronic uncertainties in the predicted branching ratio can be minimized by normalizing to the related decay K+ → π0e+ν. The large top quark mass makes diagrams with virtual top quarks the largest contributor to the branching ratio. The result is that K+ → π+νν-bar is an extremely clean way to measure the product of Cabibbo-Kobayashi-Maskawa (CKM) matrix elements /V*tsVtd/. The current constraints on this product come from measurements of other CKM elements such as Vub and unitarity, as well as from CP violation and B-B-bar mixing (with considerable theoretical uncertainty). Such constraints give a Standard Model prediction of the branching ratio of Bcalc (K+ → π+νν-bar) = (0.6-3) x 10-10 for three neutrino flavors. Experimental results on this branching ratio are presented. (author)