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Justus Tobias Tsang

Publications and source records attributed to Justus Tobias Tsang.

At least 19 recordsLinked to original sources
arXiv2026

Real radiative decays of heavy pseudoscalar mesons

We report our ongoing lattice QCD study of radiative leptonic decays of the charged pseudoscalar mesons $D$, $D_s$, $B$, and $B_c \to \ell ν_\ell γ$. We carry out our analysis on a single JLQCD ensemble with lattice spacing $a=0.044~\text{fm}$. This work is a step towards a complete QCD+QED lattice calculation of these modes, aimed at reducing theoretical uncertainties in the extraction of $|V_{cd}|$ and $|V_{cs}|$ and providing first-principles estimates of the corresponding form factors in the $B$ sector.

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arXiv2025

Application of Laplace filters to the analysis of lattice time correlators

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.

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arXiv2025

Status and outlook of quark flavour physics

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.

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arXiv2023

Towards the super Yang-Mills spectrum at large $N_c$

We examine one-flavour $SU(N_c)$ gauge theories, where $N_c$ denotes the number of colors, with one fermion in the antisymmetric representation as a candidate to approximate $\mathcal{N}=1$ super Yang Mills due to their equivalence in the large-$N_c$ limit. Summarising results on spectral evaluations of $N_c=3$, we will report on the progress of dynamical calculations for $N_c>3$. We discuss cut-off effects and challenges in configuration generation.

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arXiv2023

Spectrum of QCD with one flavor: A window for supersymmetric dynamics

We compute the spectrum of the low-lying mesonic states with vector, scalar and pseudoscalar quantum numbers in QCD with one flavour. With three colours the fundamental and the two-index anti-symmetric representations of the gauge group coincide. The latter is an orientifold theory that maps into the bosonic sector of $\mathcal{N} = 1$ super Yang-Mills theory in the large number of colours limit. We employ Wilson fermions along with tree-level improvement in the gluonic and fermionic parts of the action. In this setup the Dirac operator can develop real negative eigenvalues. We therefore perform a detailed study in order to identify configurations where the fermion determinant is negative and eventually reweight them. We finally compare results with effective field theory predictions valid in the large $N_C$ limit and find reasonably consistent values despite $N_C$ being only three. Additionally,the spin-one sector provides a novel window for supersymmetric dynamics.

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arXiv2022

LapH interpolating fields with open boundary conditions

The stochastic Laplacian Heaviside (LapH) method has proven to be successful in hadronic calculations. In this work, with charm-light spectroscopy in mind, we set up and optimise the LapH procedure limiting ourselves to the evaluation of two-point mesonic correlation functions. The calculations are performed on CLS ensembles with $N_f=2+1$ Wilson-Clover fermions on a $32^3\times64$ lattice with open boundary conditions. We analyse the interplay between the LapH parameters and the boundary effects, and implement a fitting procedure to isolate excitations coming from the border.

hep-lat↗
arXiv2020

SU(3)-breaking ratios for $D_{(s)}$ and $B_{(s)}$ mesons

We present results for the $SU(3)$ breaking ratios of decay constants $f_{D_s}/f_D$ and $f_{B_s}/f_B$ and - for the first time with physical pion masses - the ratio of bag parameters $B_{B_s}/B_{B_d}$, as well as the ratio $ξ$, forming the ratio of the nonpeturbative contributions to neutral $B_{(s)}$ meson mixing. Our results are based on Lattice QCD simulations with chirally symmetric 2+1 dynamical flavors of domain wall fermions. Eight ensembles at three different lattice spacing in the range $a = 0.11 - 0.07\,\mathrm{fm}$ enter the analysis two of which feature physical light quark masses. Multiple heavy quark masses are simulated ranging from below the charm quark mass to half the bottom quark mass. The $SU(3)$ breaking ratios display a very benign heavy mass behaviour allowing for extrapolation to the physical bottom quark mass. The results in the continuum limit including all sources of systematic errors are $f_{D_s}/f_D = 1.1740(51)_\mathrm{stat}(^{+68}_{-68})_\mathrm{sys}$, $f_{B_s}/f_B = 1.1949(60)_\mathrm{stat}(^{+\hphantom{0}95}_{-175})_\mathrm{sys}$, $B_{B_s}/B_{B_d} = 0.9984(45)_\mathrm{stat}(^{+80}_{-63})_\mathrm{sys}$ and $ξ= 1.1939(67)_\mathrm{stat}(^{+\hphantom{0}95}_{-177})_\mathrm{sys}$. Combining these with experimentally measured values we extract the ratios of CKM matrix elements $|V_{cd}/V_{cs}| = 0.2164(57)_\mathrm{exp}(^{+12}_{-12})_\mathrm{lat}$ and $|V_{td}/V_{ts}| = 0.20329(41)_\mathrm{exp}(^{+162}_{-301})_\mathrm{lat}$.

hep-lat↗
arXiv2019

Semileptonic $B\toπ\ellν$, $B\to D\ellν$, $B_s\to K\ellν$, and $B_s\to D_s\ellν$ decays

We present updates for our nonperturbative lattice QCD calculations to determine semileptonic form factors for exclusive $B\to π\ellν$, $B\to D \ellν$, $B_s\to K\ell ν$, and $B_s\to D_s\ellν$ decays. Our calculation is based on RBC-UKQCD's set of $2+1$-dynamical-flavor gauge field ensembles. In the valence sector we use domain wall fermions for up/down, strange and charm quarks, whereas bottom quarks are simulated with the relativistic heavy quark action. The continuum limit is based on three lattice spacings. Using kinematical $z$ expansions we aim to obtain form factors over the full $q^2$ range. These form factors are the basis for predicting ratios addressing lepton flavor universality or, when combined with experimental results, to obtain CKM matrix elements $|V_{ub}|$ and $|V_{cb}|$.

hep-lat↗
arXiv2019

An exploratory study of heavy-light semileptonic form factors using distillation

We present our exploratory study with the aim of simulating heavy-light semileptonic form factors as part of the RBC-UKQCD charm (to bottom) physics programme. We are using a distillation-based setup as a strategy to get optimised plateaus in semileptonic $D_{(s)}$ and $B_{(s)}$ decays, and compare our results to form factors obtained from sequential $Z_2$-Wall propagators. The study is done in a centre-of-mass frame as well as in several moving frames. We use an $N_f=2+1$ domain wall fermion ensemble with a pion mass of $340$ MeV, with the aim of extending the study to a variety of other domain-wall ensembles, including physical-pion mass ensembles.

hep-lat↗
arXiv2019

Semi-leptonic form factors for $B_s \to K \ell ν$ and $B_s \to D_s \ell ν$

Semi-leptonic $B_s \to K \ell ν$ and $B_s \to D_s \ell ν$ decays provide an alternative $b$-decay channel to determine the CKM matrix elements $|V_{ub}|$ and $|V_{cb}|$ or to obtain $R$-ratios to investigate lepton flavor universality violations. In addition, these decays may shed further light on the discrepancies seen in the analysis of inclusive vs. exclusive decays. Using the nonperturbative methods of lattice QCD, theoretical results are obtained with good precision and full control over systematic uncertainties. This talk will highlight ongoing efforts of the $B$-physics program by the RBC-UKQCD collaboration.

hep-lat↗
arXiv2018

Beyond the Standard Model Kaon Mixing with Physical Masses

We present results from a calculation of beyond the standard model (BSM) kaon mixing including data physical with light quark masses. We simulate $N_f=2+1$ QCD with Iwasaki gauge and domain wall fermion action on 8 ensembles, spanning 3 lattice spacings and pion masses from the physical value up to 430MeV. The ratio of the BSM to standard model (SM) matrix elements are extracted from the correlation functions and renormalised using the RI-SMOM Rome-Southampton method with non-exceptional kinematics. The results at the physical point continuum limit are found by performing a simultaneous continuum chiral extrapolation. In this work we gain consistency with our previous results and achieve a reduction in both the statistical and systematic error.

hep-lat↗
arXiv2017

BSM Kaon Mixing at the Physical Point

We present a progress update on the calculation of beyond the standard model (BSM) kaon mixing matrix elements at the physical point. Simulations are performed using 2+1 flavour domain wall lattice QCD with the Iwasaki gauge action at 3 lattice spacings and with pion masses ranging from 430 MeV to the physical pion mass.

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arXiv2017

BSM Kaon Mixing at the Physical Point

We present new preliminary results for bag parameters and ratios of the BSM kaon mixing operators measured at the physical point. The results are obtained from simulations of domain wall fermion QCD with 2+1 flavours with an Iwasaki gauge, and now include pion and kaon masses very close to the physical point. We compare these results to our collaboration's previous results obtained from heavier simulated quarks.

hep-lat↗
arXiv2017

The decay constants ${\mathbf{f_D}}$ and ${\mathbf{f_{D_{s}}}}$ in the continuum limit of ${\mathbf{N_f=2+1}}$ domain wall lattice QCD

We present results for the decay constants of the $D$ and $D_s$ mesons computed in lattice QCD with $N_f=2+1$ dynamical flavours. The simulations are based on RBC/UKQCD's domain wall ensembles with both physical and unphysical light-quark masses and lattice spacings in the range 0.11--0.07$\,$fm. We employ the domain wall discretisation for all valence quarks. The results in the continuum limit are $f_D=208.7(2.8)_\mathrm{stat}\left(^{+2.1}_{-1.8}\right)_\mathrm{sys}\,\mathrm{MeV}$ and $f_{D_{s}}=246.4(1.3)_\mathrm{stat}\left(^{+1.3}_{-1.9}\right)_\mathrm{sys}\,\mathrm{MeV}$ and $f_{D_s}/f_D=1.1667(77)_\mathrm{stat}\left(^{+57}_{-43}\right)_\mathrm{sys}$. Using these results in a Standard Model analysis we compute the predictions $|V_{cd}|=0.2185(50)_\mathrm{exp}\left(^{+35}_{-37}\right)_\mathrm{lat}$ and $|V_{cs}|=1.011(16)_\mathrm{exp}\left(^{+4}_{-9}\right)_\mathrm{lat}$ for the CKM matrix elements.

hep-lat↗
arXiv2016

Form factors for semi-leptonic $B$ decays

Semi-leptonic $B$ decays provide promising channels to test the Standard Model, search for signs of new physics, or determine fundamental parameters like CKM matrix elements. We present an update on our calculation of short distance contributions to GIM suppressed rare $B$ decays focusing in particular on $B_s\to ϕ\ell^+ \ell^-$ decays. Furthermore we show first results for our calculation of $B_{(s)}\to D_{(s)}^{(*)}\ellν$ semi-leptonic decays involving $b\to c$ transitions. Our calculations are based on RBC-UKQCD's 2+1 flavor domain-wall fermion and Iwasaki gauge field configurations featuring three lattice spacings in the range $1.73$ GeV $\le a^{-1} \le 2.77$ GeV and pion masses down to the physical value. We calculate the form factors by simulating $b$-quarks using the relativistic heavy quark action, create light $u/d$ and $s$ quarks with standard domain-wall kernel, and use optimised Möbius domain-wall fermions for charm quarks.

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