We derive new bounds on scalar leptoquark couplings from K⁰-K⁰, D⁰-D⁰, and B⁰-B⁰ mixing. Although leptoquarks contribute to these processes only at one loop, their contribution is large, due to the lack of Glashow-Iliopoulos-Maiani cancelation. Our bounds have two important features: (i) They bound g⁴/M², in contrast to the hitherto known bounds on g²/M², and are consequently stronger at high masses. (ii) The bound from D^0-D⁰ mixing is the first flavor changing neutral current bound in the up sector for chirally coupled leptoquarks, and is similar in strength to the K^0-K⁰ and B^0-B⁰ bounds. Together, these bounds strongly constrain any leptoquark that couples to left-handed quarks.
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Miriam Leurer (1993) studied this question.