NEW BOUNDS ON LEPTOQUARKS

We derive new bounds on scalar leptoquark couplings from K0-K0BAR, D0- D0BAR, and B0-B0BAR mixing. Although leptoquarks contribute to these p rocesses only at one loop, their contribution is large, due to the lac k of Glashow-Iliopoulos-Maiani cancellation. Our bounds have two impor tant features: (i) They bound g4/M2, in contrast to the hitherto known bounds on g2/M2, and are consequently stronger at high masses. (ii) T he bound from D0-D0BAR mixing is the first flavor changing neutral cur rent bound in the up sector for chirally coupled leptoquarks, and is s imilar in strength to the K0-K0BAR and B0-B0BAR bounds. Together, thes e bounds strongly constrain any leptoquark that couples to left-handed quarks.