Neutral kaon mixing from new physics: matrix elements in $N_f=2+1$ QCD

TL;DR

This paper computes matrix elements relevant for constraining new physics models through kaon mixing, using lattice QCD with improved staggered fermions and continuum extrapolation, achieving about 5-6% accuracy.

Contribution

First lattice QCD calculation of $ abla S=2$ four-fermion operator matrix elements with 2+1 flavors, providing crucial inputs for new physics constraints.

Findings

Matrix elements determined with 5-6% errors.

Discrepancies found with other fermion discretizations for some matrix elements.

Results enable tighter constraints on models of new physics.

Abstract

We present results for matrix elements of $\Delta S=2$ four-fermion operators arising generically in models of new physics. These are needed to constrain such models using the measured values of $\varepsilon_K$ and $\Delta M_K$. We use lattice QCD with 2+1 flavors of improved staggered fermions on lattices generated by the MILC collaboration. We extrapolate to the continuum from three lattice spacings ranging down to $a\approx 0.045\;$fm. Total errors are $\sim 5-6%$, arising primarily from our use of one-loop matching between lattice and continuum operators. For two of the matrix elements, our results disagree significantly from those obtained using different fermion discretizations.