New Bounds on Leptoquark Couplings

We derive new bounds on scalar leptoquark couplings from K^0 K^0 bar, D^0 D^0 bar and B^0 B^0 bar mixing. Although leptoquarks contribute to these processes only at one loop, their contribution is large, due to the lack of GIM cancelation. Our bounds have two important features: (i) They bound g^4/M^2, in contrast to the hitherto known bounds on g^2/M^2, and are consequently stronger at high masses. (ii) The bound from D^0 D^0 bar mixing is the first FCNC bound in the up sector for chirally-coupled leptoquarks, and is similar in strength to the K^0 K^0 bar and B^0 B^0 bar bounds. Together, these bounds strongly constrain any leptoquark that couples to left handed quarks.