arXiv subjects

J. Tobias Tsang

Publications and source records attributed to J. Tobias Tsang.

17 recordsLinked to original sources

arXiv2026

Lattice Calculation of Short-Range Contributions to Neutrinoless Double-Beta Decay $π^-\toπ^+ ee$ at Physical Pion Mass

Neutrinoless double-beta ($0νββ$) decays provide an excellent probe for determining whether neutrinos are Dirac or Majorana fermions. The short-range matrix elements associated with the $π^- \to π^+ ee$ process contribute at leading order in the $0νββ$ decay channel $nn \to ppee$ 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 $π^- \to π^+ ee$ matrix elements using domain wall fermion ensembles at the physical pion mass generated by the RBC and UKQCD Collaborations. 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 a nonperturbative renormalization using the RI/SMOM scheme. 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.

hep-lat↗

arXiv2026

Discretisation effects of gradient flows in QCD-like theories on the lattice

Recent software advances now allow large-scale lattice studies of the Corrigan--Ramond large-$N_C$ limit of Yang-Mills theory coupled with a two-index antisymmetric fermion, providing a path to SUSY Yang-Mills. We are currently generating ensembles for $N_C=4,5,6$ for lattice spacings in the range $0.11 - 0.08$ fm. We report on two aspects of our work: the study of topological properties as well as estimates of discretisation effects. The first aspect is relevant since naively, fractional topological charges might be expected in our simulations. Using a gluonic definition of the topological charge combined with gradient flow, we perform an analysis of the effect of different discretisations of the kernel action, from which we identify and interpret quantitative differences between Wilson and over-improved flows such as DBW2. The second aspect is addressed by considering ratios of different reference flow times. We conclude that our current simulations might be affected by discretisation effects of order 10\%.

hep-lat↗

arXiv2026

$K π$ scattering as a step towards $B \to K^* \ell^+ \ell^-$ from Lattice QCD

Rare $b\to s\ell^+\ell^-$ decays provide some of the most sensitive tests of the Standard Model and require precise and systematically improvable hadronic input from lattice QCD. For the phenomenologically important channel $B\to K^*\ell^+\ell^-$ this entails a first-principles treatment of a resonant $Kπ$ final state together with controlled heavy-quark dynamics. We present the status of a new exploratory lattice calculation that combines a variational determination of finite-volume $Kπ$ states with the $1+J\to2$ finite-volume formalism to access the relevant matrix elements. The computation is carried out on an RBC/UKQCD domain-wall fermion ensemble with $a^{-1} \approx 2.7\,\mathrm{GeV}$ and employs a dual heavy-quark strategy, using both a relativistic heavy-quark action tuned to the physical $b$ mass and domain-wall heavy masses extrapolating from charm. All correlation functions are computed using (stochastic) distillation, providing a versatile setup that supports a broad range of heavy-to-light transitions into resonant final states. We show first two-point results for the $K^*\leftrightarrow Kπ$ system and discuss the accessible kinematic region, which allows for a controlled study at high $q^2$. The outlook for extending the calculation to lower $q^2$ and for incorporating effects from charmonium resonances is outlined.

hep-lat↗

arXiv2026

Isospin breaking corrections to the hadronic vacuum polarization with stochastic coordinate sampling

In the recent Muon g-2 Theory Initiative white paper update, the hadronic vacuum polarization (HVP) contribution -- which dominates the theoretical uncertainty -- is evaluated as an average of different lattice QCD calculations. Since lattice simulations are mostly carried out in isospin symmetric QCD, corrections due to the mass difference of the up and down quarks and the coupling to photons have to be accounted for. These isospin breaking effects are of order 1\% and can be treated as corrections to the result for the HVP contribution in isospin symmetric QCD. In the current estimate of the HVP contribution, these effects are a large source of uncertainty due to the extensive computational cost to compute all occurring Wick contractions and degrading signal-to-noise behaviour especially for quark disconnected diagrams. We present the current status of the calculation of isospin breaking corrections in the HVP contribution for the RBC/UKQCD collaborations. We use a dataset of propagators computed using stochastic coordinate sampling (SCS) to construct all necessary Wick contractions for the electromagnetic and strong isospin breaking effects. We employ different versions of QED on the lattice, such as QED$_L$, QED$_r$ and QED$_\infty$ to improve our estimate of finite-volume uncertainties.

hep-lat↗

arXiv2025

Comparison of smoothening flows for the topological charge in QCD-like theories

We investigate properties of the topological charge for several SU(NC) gauge field ensembles for NC = 4, 5, 6 with a single fermion in the two-index anti-symmetric representation, covering multiple lattice spacings at otherwise approximately constant physical parameters. Comparing the topological charge defined by the Wilson flow and the over-improved DBW2 flow we find that already at small flow times the latter stabilises 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.

hep-lat↗

arXiv2025

Smoothing properties of the Wilson flow and the topological charge

We study SU$(N_C)$ gauge theories with a single fermion in the two-index antisymmetric representation to predict the mesonic spectrum of supersymmetric $\mathcal{N}=1$ SYM theories. Using gradient flow methods, we investigate fractional topological charges in $N_C = 4$ ensembles with varying lattice spacings. We show that the use of overimproved gauge actions (specifically the DBW2 action) in the smearing kernel stabilises the values of the topological charge already at moderate values of the flow time, while this is not the case for the standard Wilson flow.

hep-lat↗

arXiv2024

Absorbing discretisation effects with a massive renormalization scheme: the charm-quark mass

We present the first numerical implementation of the massive SMOM (mSMOM) renormalization scheme and use it to calculate the charm quark mass. Based on ensembles with three flavours of dynamical domain wall fermions with lattice spacings in the range 0.11 -- 0.08 fm, we demonstrate that the mass scale which defines the mSMOM scheme can be chosen such that the extrapolation has significantly smaller discretisation effects than the SMOM scheme. Converting our results to the $\overline{\mathrm{MS}}$ scheme we obtain $\overline{m}_c(3\,\mathrm{GeV}) = 1.008(13)\,\mathrm{GeV}$ and $\overline{m}_c(\overline{m}_c) = 1.292(12)\,\mathrm{GeV}$.

hep-lat↗

arXiv2024

Kaon mixing beyond the standard model with physical masses

We present non-perturbative results for beyond the standard model kaon mixing matrix elements in the isospin symmetric limit ($m_u=m_d$) of QCD, including a complete estimate of all dominant sources of systematic error. Our results are obtained from numerical simulations of lattice QCD with $N_f = 2+1$ flavours of dynamical domain wall fermions. For the first time, these quantities are simulated directly at the physical pion mass $m_π$~$\sim$~$139\,\mathrm{MeV}$ for two different lattice spacings. We include data at three lattice spacings in the range $a = 0.11 $ - $ 0.07\,\mathrm{fm}$ and with pion masses ranging from the physical value up to 450$\,\mathrm{MeV}$. Compared to our earlier work, we have added both direct calculations at physical quark masses and a third lattice spacing making the removal of discretisation effects significantly more precise and eliminating the need for any significant mass extrapolation beyond the range of simulated data. We renormalise the lattice operators non-perturbatively using RI-SMOM off-shell schemes. These schemes eliminate the need to model and subtract non-perturbative pion poles that arises in the RI-MOM scheme and, since the calculations are performed with domain wall fermions, the unphysical mixing between chirality sectors is suppressed. Our results for the bag parameters in the $\overline{\mathrm{MS}}$ scheme at $3\,\mathrm{GeV}$ are $B_K~\equiv~\mathcal{B}_1 = 0.5240(17)(54)$, $\mathcal{B}_2 = 0.4794(25)(35)$, $\mathcal{B}_3 = 0.746(13)(17)$, $\mathcal{B}_4 = 0.897(02)(10)$ and $\mathcal{B}_5 = 0.6882(78)(94)$, where the first error is from lattice uncertainties and the second is the uncertainty due to the perturbative matching to $\overline{\mathrm{MS}}$.

hep-lat↗

arXiv2023

Charm quark mass using a massive nonperturbative renormalisation scheme

We present a first numerical implementation of a massive nonperturbative renormalisation scheme, RI/mSMOM, in the study of heavy quarks using the domain-wall fermion action. In particular, we calculate renormalisation constants for fermion bilinears at non-vanishing heavy-quark masses and compare the approach to the continuum of the renormalised charm-quark mass with that from a mass-independent scheme.

hep-lat↗

arXiv2023

B-physics from Lattice Gauge Theory

We discuss the main issues in dealing with heavy quarks on the lattice and shortly present the different approaches used. We discuss a selection of computations covering first the b-quark mass and the B(s) meson decay constants as the consolidated results (neglecting isospin breaking corrections). In the second part we consider recent calculations of form factors for tree-level semileptonic decays with emphasis on the tensions between the results produced by different collaborations. We propose benchmark quantities and tests suited to investigate the origin of such tensions. Finally, we review computations of the bag parameters parameterising neutral meson mixing and provide an overview on a few recent developments in the field.

hep-lat↗

arXiv2023

Exclusive semileptonic $B_s\to K \ell ν$ decays on the lattice

Semileptonic $B_s \to K \ell ν$ decays provide an alternative $b$-decay channel to determine the CKM matrix element $|V_{ub}|$, and to obtain a $R$-ratio to investigate lepton-flavor-universality violations. Results for the CKM matrix element may also shed light on the discrepancies seen between analyses of inclusive or exclusive decays. We calculate the decay form factors using lattice QCD with domain-wall light quarks and a relativistic $b$-quark. We analyze data at three lattice spacings with unitary pion masses down to $268\,\mathrm{MeV}$. Our numerical results are interpolated/extrapolated to physical quark masses and to the continuum to obtain the vector and scalar form factors $f_+(q^2)$ and $f_0(q^2)$ with full error budgets at $q^2$ values spanning the range accessible in our simulations. We provide a possible explanation of tensions found between results for the form factor from different lattice collaborations. Model- and truncation-independent $z$-parameterization fits following a recently proposed Bayesian-inference approach extend our results to the entire allowed kinematic range. Our results can be combined with experimental measurements of $B_s \to D_s$ and $B_s\to K$ semileptonic decays to determine $|V_{ub}|=3.8(6)\times 10^{-3}$. The error is currently dominated by experiment. We compute differential branching fractions and two types of $R$ ratios, the one commonly used as well as a variant better suited to test lepton-flavor universality.

hep-lat↗

arXiv2022

Exploring the large-$N_c$ limit with one quark flavour

We use one-flavour QCD ($N_c=3$) as a proxy to understand $\mathcal{N}=1$ SYM. For our simulations, we use tree-level improved Wilson fermions and Symanzik improved gauge action. The hadron spectrum is obtained by using LapH smearing for different masses and simulation volumes. We also report on our efforts to increase the number of colours in our simulations, where we find that the simulations show increasing topological freezing for larger $N_c$.

hep-lat↗

arXiv2022

A lattice QCD perspective on weak decays of b and c quarks Snowmass 2022 White Paper

Lattice quantum chromodynamics has proven to be an indispensable method to determine nonperturbative strong contributions to weak decay processes. In this white paper for the Snowmass community planning process we highlight achievements and future avenues of research for lattice calculations of weak $b$ and $c$ quark decays, and point out how these calculations will help to address the anomalies currently in the spotlight of the particle physics community. With future increases in computational resources and algorithmic improvements, percent level (and below) lattice determinations will play a central role in constraining the standard model or identifying new physics.

hep-lat↗

arXiv2022

Semileptonic $D \rightarrow π\ell ν$, $D \rightarrow K \ell ν$ and $D_s \rightarrow K \ell ν$ decays with 2+1f domain wall fermions

We present the status of our project to calculate $D \to π\ell ν$, $D \to K \ell ν$ and $D_s \to K \ell ν$ semileptonic form factors using domain wall fermions for both heavy and light quarks. Our computations are performed using RBC/UKQCD's set of 2+1 flavour domain wall fermion and Iwasaki gauge field ensembles. We plan to calculate three-point functions covering the full, physically allowed kinematic range. Given that the signal decays faster than the noise, unambiguously and reliably extracting the ground state is critical for success. We include an analysis of operator diagonalisation within several possible $2 \times 2$ operator bases and find an admixture of gauged fixed wall and $\mathbb{Z} \left( 2 \right)$ wall sources to be acceptable at both zero and non-zero momentum. Initial results for semileptonic form factors are presented for first ensembles.

hep-lat↗

arXiv2021

Form factors for semileptonic $B\toπ$, $B_s\to K$ and $B_s\to D_s$ decays

We report on our determinations of $B\to π\ellν$, $B_s\to K \ell ν$ and $B_s\to D_s \ell ν$ semileptonic form factors. In addition we discuss the determination of $R$-ratios testing lepton-flavor universality and suggest an improved ratio. Our calculations are based on the set of 2+1 flavor domain-wall Iwasaki gauge field configurations generated by the RBC/UKQCD collaboration with three lattice spacings of $1/a = 1.78$, $2.38$, and $2.79\,\text{GeV}$. We use the relativistic heavy quark action for $b$ quarks and charm quarks are simulated with the Möbius domain-wall fermion action.

hep-lat↗

arXiv2021

One Flavour QCD as an analogue computer for SUSY

We numerically study QCD with a single quark flavour on the lattice probing predictions from effective field theories that are equivalent to minimal super-symmetric Yang-Mills theory in the large $N_c$ limit. The hadronic spectrum including excited states is analysed using one gauge coupling and several physical volumes and fermion masses. We use the LapH method and also compute disconnected diagrams. Lattice simulations with an odd number of Wilson fermions give rise to regions of configuration space with a negative fermionic weight entailing a sign problem. We perform a detailed analysis on the spectrum of the Wilson-Dirac operator and report on observed cases of a negative fermion determinant in our ensembles.

hep-lat↗

arXiv2020

Nonperturbative calculations of form factors for exclusive semileptonic $B_{(s)}$ decays

Precise theoretical predictions derived from the Standard Model are a key ingredient in searches for new physics in the flavor sector. The large mass and long lifetime of the $b$ quark make processes involving $b$ quarks of particular interest. We use lattice simulations to perform nonperturbative QCD calculations for semileptonic $B_{(s)}$ decays. We present results from our determinations of $B_s\to D_s \ell ν$ and $B_s\to K \ell ν$ semileptonic form factors and provide an outlook for our $B\to π\ellν$ calculation. In addition we discuss the determination of $R$-ratios testing lepton-flavor universality and suggest use of an improved ratio. Our calculations are based on the set of 2+1 flavor domain wall Iwasaki gauge field configurations generated by the RBC-UKQCD collaboration featuring three lattice spacings of $1/a = 1.78$, $2.38$, and $2.79\,\text{GeV}$. Heavy $b$-quarks are simulated using the relativistic heavy quark action.

hep-ph↗