Constraints on New Physics from $K \to πν\barν$

Abstract

We study constraints on new physics from the recent measurement of Br ($K^+ \to π^+ ν\bar ν)$ by the E787 and E949 Collaborations. In our analysis we consider two models of new physics: $(a)$ extra down type singlet quark model (EDSQ) and $(b)$ R-parity violating Minimal Supersymmetric Standard Model(MSSM). We find that $K^+\to π^+ ν\bar ν$ along with other processes like $K_L \to μ^+ μ^-, ε^\prime/ε$ provide useful bounds on the parameter $U_{sd}$, characterizing the off-diagonal $Z-d-\bar s $ coupling of model $(a)$. The bounds on the ${\rm Re}(U_{sd})$ from $(K_L \to μ^+μ^-)_{\rm SD}$ and ${\rm Im}(U_{sd})$ from $ε^\prime/ε$ are so tight that the branching ratio of $K^+\to π^+ ν\bar ν$ can exceed the standard model value by at most a factor of two For model b), we also obtain stringent bounds on certain combinations of product of two $\lp_{ijk}$ couplings originating from $L$ number violating operator $ L_i Q_j D^c_k $ using $K^+ \to π^+ ν\bar ν$ and $K_L \to μ^+ μ^-$ processes. Even with the stringent constraints on $U_{sd}$ (in model $(a)$) and on products of two $\rp$couplings (model $(b)$), we find that the branching ratios for $K_L \to π^0 ν\bar ν$ and $K_L \to π^0 e^+ e^-$ can be substantially different in both the above models from those predicted in the standard model.