Justus Tobias Tsang

Neutrinoless double-beta ( 0 ν β β ) decays provide an excellent probe for determining whether neutrinos are Dirac or Majorana fermions. The short-range matrix elements associated with the π − → π + e e process contribute at leading order in the 0 ν β β decay channel n n → p p e e through pion exchange between nucleons. However, current lattice calculations show notable discrepancies in predicting these short-range contributions. To address this issue, we perform a lattice QCD calculation of the π − → π + e e matrix elements using domain wall fermion ensembles at the physical pion mass generated by the RBC/UKQCD Collaboration. To mitigate contamination from around-the-world effects, we develop a new method to reconstruct and subtract them directly from lattice data. We then perform nonperturbative renormalization in the regularization-independent symmetric momentum-subtraction scheme (RI/SMOM), using the ( γ μ , γ μ ) and ( q , q ) projectors. Compared with previous studies, this work reduces the uncertainties in the matrix elements and provides an independent cross-check that helps to reconcile the discrepancies among previous lattice calculations.

The analysis of lattice simulation correlation function data is notoriously hindered by the ill conditioning of the Euclidean time covariance matrix. Additionally, the isolation of a single physical state in such functions is generally affected by systematic contamination from unwanted states. In this paper, we present a new methodology based on regulated Laplace filters and demonstrate that it can be used to address both issues using state-of-the-art simulation data. Regulated Laplace filters are invertible high-pass filters that suppress local correlations in the data, and we show that they can reduce the condition number of covariance matrices by several orders of magnitude. Furthermore, Laplace filters can annihilate functions that decay exponentially with time, which can be used to alter the spectrum of a lattice correlation function. We show that this property can be exploited to significantly reduce excited-state contamination in the determination of matrix elements. The same property can also be used to constrain the spectral content of a correlation function and has the potential to form the basis of new methods to extract physical information from lattice data.

We investigate properties of the topological charge for several S U ( N C ) gauge field ensembles for N C = 4 , 5, 6 with a single fermion in the two-index antisymmetric representation, covering multiple lattice spacings at otherwise approximately constant physical parameters. Comparing the topological charge defined by the Wilson flow and the overimproved DBW2 flow we find that already at small flow times the latter stabilizes on discrete values. We provide evidence that as the lattice spacing is lowered the Wilson flow also separates into discrete sectors at earlier flow times. Adopting the DBW2 definition in the remainder of the analysis, we do not see any evidence of fractional topological charges, which could in principle appear at finite lattice spacing.

We provide the first calculation of the Euclidean long-distance window of the isospin-symmetric light-quark connected contribution to the hadronic vacuum polarization for the muon g − 2 and find a μ LD , iso , conn , ud = 411.4 ( 4.3 ) ( 2.4 ) × 10 − 10 . We also provide the currently most precise calculation of the total isospin-symmetric light-quark connected contribution, a μ iso , conn , ud = 666.2 ( 4.3 ) ( 2.5 ) × 10 − 10 in the BMW20 world, which is more than 4 σ larger compared to the data-driven estimates of Boito [] and 1.7 σ larger compared to the lattice QCD result of BMW20.

In recent years there has been impressive progress in quark flavour physics, with current efforts tackling complicated quantities such as for example inclusive decays, decays to QCD-unstable final states and radiative decays. At the same time current lattice flavour physics results are receiving a lot of attention from outside the lattice community. This requires careful scrutiny, even for well explored quantities. In this plenary contribution I review the status of flavour physics observables and, in the context of heavy flavours, suggest possible benchmark quantities that could allow to better compare intermediate results. These can in turn be used to investigate the origin of existing (and possible future) tensions.

In recent years there has been impressive progress in quark flavour physics, with current efforts tackling complicated quantities such as for example inclusive decays, decays to QCD-unstable final states and radiative decays. At the same time current lattice flavour physics results are receiving a lot of attention from outside the lattice community. This requires careful scrutiny, even for well explored quantities. In this plenary contribution I review the status of flavour physics observables and, in the context of heavy flavours, suggest possible benchmark quantities that could allow to better compare intermediate results. These can in turn be used to investigate the origin of existing (and possible future) tensions.