The role of Lattice QCD in searches for violations of fundamental symmetries and signals for new physics

Abstract

Executive summary 2 BSM scenarios may involve new particles and interactions that originate at very high-energy scales, or alternatively may involve new physics at relatively low scales with extremely feeble couplings [10]. This whitepaper focuses on the former class of BSM scenarios, for which an effective field theory (EFT) framework can be adopted to encompass all SM extensions that respect the SM gauge group and have the same low-energy particle content as the SM [11][12][13]. One challenge that often needs to be dealt with in carrying out the program in Fundamental Symmetries is that the SM processes that are sensitive to new physics often involve hadrons and nuclei as initial, final, and/or intermediate states at low energies, demanding that SM and ab initio quantum many-body calculations be performed in a highly nonperturbative regime.The Department of Energy (DOE)-funded USQCD collaboration, as the unified collaboration of the majority of the LQCD collaborations in the U.S., has long identified the role of LQCD studies in enabling and advancing research at the Intensity Frontier. The significant results produced by the collaboration in the past two decades in constraining the Cabibbo-Kobayashi-Maskawa (CKM) matrix quark-mixing matrix of the SM, and in constraining the quantities relevant to tests of CP violation in the Kaon sector, mark major accomplishments for the LQCD community, and signify the essential role of LQCD in complimenting both experiment and theory effort in HEP research. Such a role has been identified in other sectors in which there is, or is expected to be, hints of deviation from the SM and of new physics. In particular, compared to the year 2013 when the Collaboration's previous whitepapers were released, in 2018 the Collaboration has dedicated three separate whitepapers to the role of LQCD in research at the Intensity Frontier: 1 i) Opportunities for LQCD in quark and lepton flavor physics: Given the persisting tensions between theoretical predictions based on the SM and and experimental measurements in B meson decays at the LHC-B and the anomalous magnetic moment of the muon at the Brookhaven National Laboratory, and the continuing improvement in determinations of the CP violating parameters in Kaon and recently the heavy-meson sectors, there is a pressing need for reliable SM predictions of corresponding quantities from LQCD, to a precision level that is comparable to experimental determinations. Progress made by the USQCD collaboration in all these areas, the challenges ahead, and the plans forward are detailed in a companion whitepaper, see [6].ii) LQCD and neutrino-nucleus scattering: The large experimental enterprise in neutrino physics, in particular the upcoming Deep Underground Neutrino Experiment (DUNE) at Fermilab aims to shed light on CP violation in the lepton sector. However, reliable determinations of the Pontecorvo-Maki-Nakagawa-Sakata (PMNS) lepton-mixing matrix require accurate knowledge of neutrino-nucleus interactions. The neutrino-nucleon and neutrinonucleus scatt...

Renormalized approach to neutrinoless double- β decay

“…Nevertheless, this approach explicitly demonstrates the necessity of including g NN ν at LO. An alternative is to match this counterterm to results from a direct nonperturbative QCD calculation, something which is imaginable with lattice QCD (LQCD) methods, both for light Majorana exchange [54][55][56][57] and TeV-scale LNV mechanisms [58,59].…”

“…It can in principle be extracted by matching the scattering amplitude for nn → pp e − e − in χEFT to LQCD. Such LQCD calculations are extremely challenging [57], but are beginning to be investigated. For instance, Ref.…”

Expectation values of coherent states for SU(2) Lattice Gauge Theories

“…To understand their quantisation, a promising avenue comes in the study of Lattice Gauge Theories (LGT) [1][2][3][4][5][6]. On the one hand, high performance computations in lattice quantum chromodynamics are the main tool to aid the experiments in particle and nuclear physics [7,8]. On the other hand, LGT provides a theoretical framework that is ideally suited to make an impact on models trying to explore new physics beyond and within the standard model [9,10].…”

Supersymmetric proton decay revisited

“…A simulation at the physical point is on-going [76], which is expected to reduce the systematic uncertainty associated with the chiral extrapolation. All in all, a precision with < 10% uncertainty is expected to be achieved in 5 years [77].…”