Continuum Limit Latice Calculation of Direct CP-violation in Kaon Decays

PI Christopher Kelly , Brookhaven National Laboratory
Co-PI Norman Christ, Columbia University
Masaaki Tomii, University of Connecticut
Amarjit Soni, Brookhaven National Laboratory
Tianle Wang, Brookhaven National Laboratory
Kelly ALCC Image

The constraints on the apex of the CKM unitarity triangle in the ρ −η plane. The pink band (ε 0 ) is the new constraint obtained through our calculation. The proposed research is a vital step towards reducing the error band by a factor of ten over the next decade.

Project Description

CP-violation (CPV) is necessary to explain the asymmetry between the amount of mater and antimater in the observable Universe. CPV is highly suppressed in the Standard Model of particle physics and the predicted amount appears insufficient to account for the measured amount of mater, hence a precise comparison between experiment and Standard Model theory is a promising path to uncovering new physics. In the late 1990's, impressive experiments at FNAL (DOE-funded) and CERN succeeded in measuring with 15% errors the quantity ϵ′, which characterizes the elusive direct CPV in 𝐾𝐾→ππ decay, a tiny, part-per-million effect that is highly sensitive to new physics. Unfortunately, a correspondingly precise theoretical calculation was not possible until recently due to large, low-energy non-perturbative effects intractable to traditional theory approaches. Latice QCD is the only known method to compute these with controllable errors, by directly simulating the theory in a finite, discretized box using Monte Carlo techniques on supercomputers.

This research team, along with other members of the RBC & UKQCD collaborations, performed the first complete latice calculation of the decay in 2015, and in 2020 published an improved result with significantly beter control over the systematic errors. Both results agree with experiment within errors, but at present these are sizeable, O(39%), and completely dominated by systematic effects. Separate research is underway to develop strategies for addressing two of the largest systematic errors, those associated with isospin-breaking/electromagnetic effects, and the Wilson coefficients that encapsulate the high-energy weak-interaction physics. This research project aims to address the last of the three largest errors in the earlier work: the finite latice spacing effects resulting from using a single, somewhat coarse latice spacing. While these are estimated to be of O(15%), smaller than the other dominant errors, this estimate has substantial uncertainty and so it is vital to repeat the calculation with a second (finer) latice spacing and perform a continuum extrapolation to reduce/remove this source of error. This requires significant computational resources only available through DOE leadership-class facilities.