Session W04: Mini-Symposium: Lattice QCD Inputs for BSM Searches

Lattice QCD Inputs for BSM Searches

I will give an overview of recent progress in lattice QCD studies relevant to searches for physics beyond the Standard Model. In the precision era of lattice QCD for single-hadron systems, calculations with complete systematic control are playing an important role in precision tests of the Standard Model; I will discuss examples including tensions in heavy flavour physics, the recent lattice QCD calculations of key contributions to muon g-2, and determinations of the quark masses and strong coupling which complement experimental studies at the LHC. In contrast to the precision of lattice QCD for single hadron systems, for light nuclear systems we are in the very early era of first reliable lattice QCD calculations. As a result, there is also an important and emerging role for lattice QCD in the interpretation of low-energy precision experiments. In this arena, I will outline how the interplay of lattice QCD with effective field theories is providing improved predictions with impact spanning from neutrino physics experiments to direct searches for dark matter.   

Nuclear Effective Theory of μ→e Conversion

The coming decade promises exceptional experimental progress in tests of charged lepton flavor violation (CLFV), with branching ratio sensitivities for μ→e conversion searches expected to improve by more than four orders of magnitude due to efforts at Fermilab (Mu2E) and J-PARC (COMET). To support this progress, significant theoretical advances are required in order to connect low-energy tests of μ→e conversion to candidate UV theories of CLFV. Here we describe an effective theory of μ→e conversion formulated at the non-relativistic nucleon level which represents the most general constraint that this process can place on the underlying CLFV operators and provides the foundation to match onto effective theories at higher scales. This formulation provides a clear factorization of the CLFV physics from the nuclear physics (in analogy with standard-model processes like β decay and μ capture), delineating what can and cannot be learned about CLFV operator coefficients from elastic μ→e conversion. Using state-of-the-art shell model wave functions, we derive bounds on operator coefficients from existing μ→e conversion and μ→eγ results, and estimate the improvement in these bounds if Mu2e, COMET, and MEG II reach their design goals.   

Finite-Volume Pionless Effective Field Theory for Few-Nucleon Systems with Differentiable Programming

Finite-volume pionless effective field theory is an efficient framework with which to perform the extrapolation of finite-volume lattice QCD calculations of multi-nucleon spectra and matrix elements to infinite volume and to nuclei with larger atomic number. In this contribution, a new implementation of this framework based on correlated Gaussian wavefunctions optimized using differentiable programming and using a solution of a generalised eigenvalue problem is discussed. This approach is found to be more efficient than previous stochastic implementations of the variational method, as it yields comparable representations of the wavefunctions of nuclei with atomic number A <= 6 with an order of magnitude fewer terms.Future applications to infinite-volume extrapolations of nuclear matrix elemetns will also be discussed.   

Uncertainty quantification in pionless effective field theory

Effective field theories (EFTs) offer a rigorous connection of quantum chromodynamics to the low energy regime of nuclear structure and interactions. While high-quality effective interactions have been produced that can successfully replicate experimental data, they suffer from the same shortcoming that plagues much of theoretical nuclear physics: a lack of error estimation. Currently, EFT models have been used in calculations with rudimentary uncertainty quantification, but rigorous and systematic treatments are still missing. To remedy this, we have introduced an implementation of Markov Chain Monte Carlo (MCMC) for EFT parameter estimation. The parameter estimation is then propagated to calculations of observables. Further, treatment of uncertainty from the truncation of the EFT is also included for a more complete examination of the error.   

Quantum Monte Carlo calculations of electroweak observables

Experiments searching for neutrinoless double beta (0νββ) decay and measuring beta decay spectra will address important questions within fundamental symmetries. To interpret new physics signals, these experiments require an accurate treatment of the underlying nuclear dynamics. Quantum Monte Carlo (QMC) methods allow one to solve the many-body Schrödinger equation while retaining the full complexity of the strongly-correlated nuclear system. Recently, QMC calculations using the Norfolk two- and three-nucleon chiral interaction and its set of consistent one- and two-body electroweak current operators have been validated with experimental data for beta decay and muon capture. In this talk, I will discuss using QMC methods with the Norfolk model to make predictions of the ⁶He beta decay spectrum and A=6 0νββ decay matrix elements. Using the various Norfolk model classes, we estimate the error on these observables arising from different choices in constructing the chiral interaction.   

Ab initio theory for heavy nuclei and the physics of neutrinos

Breakthroughs in our treatment of the many-body problem and nuclear forces are rapidly transforming modern nuclear theory into a true first-principles discipline. This allows us to address some of the most exciting questions at the frontiers of nuclear structure and physics beyond the standard model, such as the nature of dark matter and neutrino masses, as well as searches for violations of fundamental symmetries in nature.

In this talk I will briefly outline our many-body approach, the valence-space in-medium similarity renormalization group, and how recent advances now allows us to calculate converged properties of open-shell nuclei to the ²⁰⁸Pb region and beyond. In particular I will focus on connections to key topics in neutrino physics, such as converged ab initio neutrinoless double beta (0νββ) decay nuclear matrix elements (including the recently discovered leading order contact term) for all major players in global searches: ⁷⁶Ge, ¹³⁰Te, and ¹³⁶Xe. In addition I will discuss correlations of 0νββ decay matrix elements with those of double Gamow-Teller and double electron capture will be explored. Finally, ab initio calculations of neutrino scattering will be presented for all relevant target nuclei.   

Lattice QCD results for Kaon electromagnetic form factor up to Q2 ∼ 10 GeV2

Electromagnetic form factor, especially its asymptotic behavior for large momentum transfer (Q²), of kaon provides crucial insight into the partonic structure of a Nambu-Goldstone boson in strong interaction. Studies of the electromagnetic form factor of kaon up to Q² ∼ 6 GeV² are underway at the ongoing JLab12 experiment, and its measurements in an extended range of Q² ∼ 9 – 30 GeV² are planned at the future Electron Ion Collider (EIC). For the first time, we will present results for the kaon electromagnetic form factor in the range of Q² ∼ 2 – 10 GeV² from state-of-the-art lattice QCD calculations carried out using physical values of up, down and strange quark masses. These results will provide benchmark QCD predictions for model-based studies and the experimental measurements, in particular at the boundaries between the JLab12 and the EIC.