Isospin 0 and 2 two-pion scattering at physical pion mass using all-to-all propagators with periodic boundary conditions in lattice QCD

Blum, Thomas; Boyle, Peter A.; Bruno, Mattia; Hoying, Daniel; Izubuchi, Taku; Jin, Luchang; Jung, Chulwoo; Kelly, Christopher; Lehner, Christoph; Meyer, Aaron S.; Soni, Amarjit; Tomii, Masaaki (2023). Isospin 0 and 2 two-pion scattering at physical pion mass using all-to-all propagators with periodic boundary conditions in lattice QCD. Physical review. D - particles, fields, gravitation, and cosmology, 107(9) American Physical Society 10.1103/PhysRevD.107.094512

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We present a lattice calculation of the K→ππ matrix elements and amplitudes with both the ΔI=3/2 and 1/2 channels and ϵ′, the measure of direct CP violation. We use periodic boundary conditions (PBC), where the correct kinematics of K→ππ can be achieved via an excited two-pion final state. To overcome the difficulty associated with the extraction of excited states, our previous work [Z. Bai et al., Standard Model prediction for direct CP violation in K→ππ decay, Phys. Rev. Lett. 115, 212001 (2015); R. Abbott et al., Direct CP violation and the ΔI=1/2 rule in K→ππ decay from the Standard Model, Phys. Rev. D 102, 054509 (2020).] successfully employed G-parity boundary conditions, where pions are forced to have nonzero momentum enabling the I=0 two-pion ground state to express the on shell kinematics of the K→ππ decay. Here instead we overcome the problem using the variational method which allows us to resolve the two-pion spectrum and matrix elements up to the relevant energy where the decay amplitude is on shell. In this paper we report an exploratory calculation of K→ππ decay amplitudes and ϵ′ using PBC on a coarser lattice size of 243×64 with inverse lattice spacing a−1=1.023  GeV and the physical pion and kaon masses. The results are promising enough to motivate us to continue our measurements on finer lattice ensembles in order to improve the precision in the near future.

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