New bounds on leptoquarks

We derive new bounds on scalar leptoquark couplings from ${\mathit{K}}^0$-K${\mathrm{¯}}^0$, ${\mathit{D}}^0$-D${\mathrm{¯}}^0$, and ${\mathit{B}}^0$-B${\mathrm{¯}}^0$ 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 ${\mathit{g}}^4$/${\mathit{M}}^2$, in contrast to the hitherto known bounds on ${\mathit{g}}^2$/${\mathit{M}}^2$, and are consequently stronger at high masses. (ii) The bound from ${\mathit{D}}^{0\mathrm{-}}$D${\mathrm{¯}}^0$ mixing is the first flavor changing neutral current bound in the up sector for chirally coupled leptoquarks, and is similar in strength to the ${\mathit{K}}^{0\mathrm{-}}$K${\mathrm{¯}}^0$ and ${\mathit{B}}^{0\mathrm{-}}$B${\mathrm{¯}}^0$ bounds. Together, these bounds strongly constrain any leptoquark that couples to left-handed quarks.