arXiv subjects

Carlos Pena

Publications and source records attributed to Carlos Pena.

At least 19 recordsLinked to original sources

arXiv2026

Light and strange quark masses with $N_f = 2 + 1$ Wilson fermions

We report on the status of an update of our collaboration's previous computation of light and strange quark masses in QCD with $N_{f}=2+1$ dynamical flavours. Bare quark masses are extracted from CLS ensembles, using $O(a)$-improved Wilson fermions, and the mass renormalization is performed non-perturbatively in the Schr\"odinger functional scheme over a wide range of scales to make safe contact with perturbation theory. Results for five lattice spacings, down to $a\sim 0.038 \textrm{ fm}$, and pion masses reaching the physical value are included in the analysis. This allows for the exploration of different models for cutoff and chiral effects, and a controlled extrapolation to the physical point.

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arXiv2026

Heavy quark masses from step-scaling

We present a determination of the charm- and bottom-quark masses using the heavy-quark step-scaling strategy. Renormalization is performed in small volumes where relativistic bottom quarks can be simulated directly. A sequence of finite-volume simulations connects this calculation to large-volume CLS ensembles, where simulations at physical light and strange quark masses provide reliable control over low-energy hadronic physics. In all but the smallest volume, the B-scale is reached by interpolating between relativistic heavy-quark data and the static limit. The resulting quark masses are obtained with good precision, with subdominant systematic uncertainties that differ from, and thus complement, those of standard large-volume determinations.

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arXiv2026

Ground-State Extraction of Heavy-Light Meson Semileptonic Decay Form Factors

We discuss the extraction of heavy-light pseudo-scalar to light pseudo-scalar decay form factors from finite time correlation functions. We place particular emphasis on the contamination from excited states employing summed ratios and input from chiral perturbation theory. The analysis is performed on four CLS ensembles with $N_f = 2+1$ flavours of $\mbox{O}(a)$-improved Wilson fermions (presently) at the $\mathrm{SU}(3)$-symmetric point with relativistic heavy-quark masses in the charm region and above. The study presented here is part of the analysis aimed at the computation of the $B \to \pi \ell \nu$ and $B_s \to K \ell \nu$ semileptonic form factors, combining the continuum-limit relativistic results with static-limit calculations.

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arXiv2025

Hadronic physics from a Wilson fermion mixed-action approach: Setup and scale setting

We introduce a lattice QCD mixed action approach that employs Wilson-type quarks in the sea and valence sectors. The sea sector is based on gauge ensembles with $N_{\rm f}=2+1$ flavours of non-perturbatively O($a$)-improved Wilson fermions generated by the Coordinated Lattice Simulations (CLS) initiative. The parameter space of the considered ensembles encompasses five values of the lattice spacing, a range of pion masses extending down to the physical point, and large physical volumes. In the valence sector, we employ Wilson twisted-mass fermions at maximal twist, using the same massless Wilson-Dirac operator in both the sea and valence sectors. We describe the strategy applied for the required matching of the sea and valence quark masses along the target renormalised chiral trajectory. A precise universality test is then conducted by comparing the continuum-limit results of the mixed-action approach and of the unitary setup, in which the same Wilson fermion regularisation is employed in the sea and in the valence. As a key application, we conduct a scale setting procedure based on lattice determinations of the masses and decay constants of the pion and kaon, as well as the gradient flow scale $t_0$. The scale setting can consequently be performed in three distinct ways, utilising the unitary setup, the mixed action approach, and their combination. We observe that the latter combination results in enhanced control of the systematic uncertainties, thereby yielding a precise determination of the physical value of $t_0$.

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arXiv2025

A Julia Code for Lattice QCD on GPUs

We present a new GPU-based open source package to perform Lattice simulations developed in Julia. The code currently supports generation of SU(2) and SU(3) (pure gauge) configurations with different actions and boundary conditions, and is able to perform measurements of flow observables (both gluonic and fermionic) as well as different fermionic two point functions. We will show the capabilities of the package, and provide information about some measurement codes built on top of this framework.

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arXiv2024

Determination of the Gradient Flow Scale $t_0$ from a Mixed Action with Wilson Twisted Mass Valence Quarks

We perform the scale setting procedure of a mixed action setup consisting of valence Wilson twisted mass fermions at maximal twist on CLS ensembles with $N_f=2+1$ flavours of $O(a)$-improved Wilson sea quarks. We determine the gradient flow scale $t_0$ using pion and kaon masses and decay constants in the isospin symmetric limit of QCD as external physical input. We employ model variation techniques to explore the systematic uncertainties in the extraction of the ground state signal of lattice observables, as well as for the continuum-chiral extrapolations. We observe that the combined analysis of the mixed action data with that based on $O(a)$-improved Wilson valence quarks, provides an improved control of the extrapolation of $t_0$ to the physical point.

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arXiv2023

A strategy for B-physics observables in the continuum limit

In a somewhat forgotten paper [1] it was shown how to perform interpolations between relativistic and static computations in order to obtain results for heavy-light observables for masses from, say, $m_{\rm charm}$ to $m_{\rm bottom}$. All quantities are first continuum extrapolated and then interpolated in $1/m_h=1/m_{\rm heavy}$. Large volume computations are combined with finite volume ones where a relativistic bottom quark is accessible with small $am_{\rm bottom}$. We discuss how this strategy is extended to semi-leptonic form factors and other quantities of phenomenological interest. The essential point is to form quantities where the limit $m_h\to\infty$ is approached with power corrections O$(1/m_h)$ only. Perturbative corrections $\simα_s(m_h)^{γ+n}$ are cancelled in the construction of the observables. We also point out how such an approach can help to control systematics in semi-leptonic decays with just large volume data. First numerical results with $N_f = 2 + 1$ and lattice spacings down to 0.039 fm are presented in [2].

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arXiv2023

$m_B$ and $f_{B^{(\star)}}$ in $2+1$ flavour QCD from a combination of continuum limit static and relativistic results

We present preliminary results for B-physics from a combination of non-perturbative results in the static limit with relativistic computations satisfying $am_{\mathrm{heavy}}\ll 1$. Relativistic measurements are carried out at the physical b-quark mass using the Schrödinger Functional in a $0.5 \ \mathrm{fm}$ box. They are connected to large volume observables through step scaling functions that trace the mass dependence between the physical charm region and the static limit, such that B-physics results can be obtained by interpolation; the procedure is designed to exactly cancel the troublesome $α_s(m_{\mathrm{heavy}})^{n+γ}$ corrections to large mass scaling. Large volume computations for both static and relativistic quantities use CLS $N_f=2+1$ ensembles at $m_u=m_d=m_s$, and with five values of the lattice spacing down to $0.039$ fm. Our preliminary results for the b-quark mass and leptonic decay constants have competitive uncertainties, which are furthermore dominated by statistics, allowing for substantial future improvement. Here we focus on numerical results, while the underlying strategy is discussed in a companion contribution.

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arXiv2023

Hadronic physics from a Wilson fermion mixed-action approach: Charm quark mass and $D_{(s)}$ meson decay constants

We present our first set of results for charm physics, using the mixed-action setup introduced in a companion paper. Maximally twisted Wilson valence fermions are used on a sea of non-perturbatively $O(a)$-improved Wilson fermions, made up by CLS $N_{\mathrm{\scriptstyle f}}=2+1$ ensembles. Our charm-sector observables are free from $O(am_c)$ discretisation effects, without need of tuning any improvement coefficient, and show continuum-limit scaling properties consistent with leading cutoff effects of $O(a^2)$. We consider a subset of CLS ensembles -- including four values of the lattice spacing and pion masses down to 200 MeV -- allowing to take the continuum limit and extrapolate to the physical pion mass. A number of techniques are incorporated in the analysis in order to estimate the systematic uncertainties of our results for the charm quark mass and the $D_{(s)}$-meson decay constants. This first study of observables in the charm sector, where the emphasis has been on the control of the methodology, demonstrates the potential of our setup to achieve high-precision results.

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arXiv2022

Towards precision charm physics with a mixed action

We report on our first set of results for charm physics, using a mixed-action setup with maximally twisted valence fermions on CLS $N_f=2+1$ ensembles. This setup avoids the need of improvement coefficients to subtract $O(am_c)$ effects. The charm quark mass, $D$ and $D_s$ decay constants are computed on a subset of CLS ensembles, which allows to take the continuum limit and extrapolate to the physical pion mass, and assess the scaling properties. Special attention is paid to the implementation of techniques to deal with systematic uncertainties. Our results show excellent prospects for high-precision computations on the full set of ensembles.

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arXiv2021

Charmed semileptonics with twisted-mass valence quarks

Our charm program uses a mixed action with twisted-mass valence quarks over non-perturbatively improved Wilson sea quarks, in order to study various quantities in a relativistic and manifestly local framework of full QCD. The sea sector consists of $N_\mathrm{f}=2+1$ ensembles generated by the CLS initiative. Taking advantage of open boundary conditions, this allows access to fine ensembles without topological freezing. Here we focus in particular on our current progress on $D\to Kνl$ and $D\to πνl$ semileptonics. Those are first and foremost useful for the computation of the CKM matrix elements $|V_{cs}|$ and $|V_{cd}|$. We show that all discretisation effects seem to be reasonably under control with this choice of action, in particular those related to hypercubic lattice artefacts. Eventually, we obtain preliminary results of the form factors as a very smooth curve on the whole range of momentum transfer, and in particular the signal at zero $q^2$ appears to have the potential to be competitive with earlier published results.

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arXiv2020

Dissecting the $ΔI= 1/2$ rule at large $N_c$

We study the scaling of kaon decay amplitudes with the number of colours, $N_c$, in a theory with four degenerate flavours, $N_f=4$. In this scenario, two current-current operators, $Q^\pm$, mediate $ΔS=1$ transitions, such as the two isospin amplitudes of non-leptonic kaon decays for $K\to (ππ)_{I=0,2}$, $A_0$ and $A_2$. In particular, we concentrate on the simpler $K\toπ$ amplitudes, $A^\pm$, mediated by these two operators. A diagrammatic analysis of the large-$N_c$ scaling of these observables is presented, which demonstrates the anticorrelation of the leading ${\mathcal O}(1/N_c)$ and ${\mathcal O}(N_f/N_c^2)$ corrections in both amplitudes. Using our new $N_f=4$ and previous quenched data, we confirm this expectation and show that these corrections are $naturally$ large and may be at the origin of the $ΔI=1/2$ rule. The evidence for the latter is indirect, based on the matching of the amplitudes to their prediction in Chiral Perturbation Theory, from which the LO low-energy couplings of the chiral weak Hamiltonian, $g^\pm$, can be determined. A NLO estimate of the $K \to (ππ)_{I=0,2}$ isospin amplitudes can then be derived, which is in good agreement with the experimental value.

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arXiv2020

Light quark masses in N_f = 2+1 lattice QCD with Wilson fermions

We present a lattice QCD determination of light quark masses with three sea-quark flavours ($N_f = 2+1$). Bare quark masses are known from PCAC relations in the framework of CLS lattice computations with a non-perturbatively improved Wilson-Clover action and a tree-level Symanzik improved gauge action. They are fully non-perturbatively improved, including the recently computed Symanzik counter-term $b_{\rm A} - b_{\rm P}$. The mass renormalisation at hadronic scales and the renormalisation group running over a wide range of scales are known non-perturbatively in the Schrödinger functional scheme. In the present paper we perform detailed extrapolations to the physical point, obtaining (for the four-flavour theory) $m_{u/d}(2{\rm GeV}) = 3.54(12)(9)$ MeV and $m_s(2{\rm GeV}) = 95.7(2.5)(2.4)$ MeV in the $\bar{MS}$ scheme. For the mass ratio we have $m_s/m_{u/d} = 27.0(1.0)(0.4)$. The RGI values in the three-flavour theory are $M_{u/d} = 4.70(15)(12)$ MeV and $M_s = 127.0(3.1)(3.2)$ MeV.

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arXiv2019

Heavy semileptonics with a fully relativistic mixed action

The first phase of a heavy quark program based on twisted mass valence quarks has been presented at last years's lattice conference. The CLS $N_f=2+1$ ensembles were used for their fine lattice spacing, while twisting the masses is expected to reduce discretisation errors even further and allow for a fully relativistic calculation. We present our strategy and preliminary results on three point functions, corresponding to $D\to K$ and $D\toπ$ semileptonic decays. The form factors for $m_u=m_d=m_s$ quark masses obtained as a first step are shown here to be at the percent level in statistical precision at $q^2=0$.

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arXiv2019

Meson interactions at large $N_c$ from Lattice QCD

We report on the computation of the scaling of QCD observables with the number of colours, $N_c$. For this, we use dynamical configurations with four active flavours, $N_f=4$, and values of $N_c=3-6$. We study the meson masses and decay constants, and compute the leading and subleading contributions to the Low Energy Constants (LECs) of the chiral Lagrangian. We also explore $ππ$ scattering in the $I=2$ channel, and compute the $K \to π$ weak decay matrix elements. We comment on the relation of the latter to $K \to ππ$ processes and the $ΔI=1/2$ rule.

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arXiv2019

Large $N_c$ scaling of meson masses and decay constants

We perform an $\textit{ab initio}$ calculation of the $N_c$ scaling of the low-energy couplings of the chiral Lagrangian of low-energy strong interactions, extracted from the mass dependence of meson masses and decay constants. We compute these observables on the lattice with four degenerate fermions, $N_f=4$, and varying number of colours, $N_c=3-6$, at a lattice spacing of $a\simeq 0.075$ fm. We find good agreement with the expected $N_c$ scaling and measure the coefficients of the leading and subleading terms in the large $N_c$ expansion. From the subleading $N_c$ corrections, we can also infer the $N_f$ dependence, that we use to extract the value of the low-energy couplings for different values of $N_f$. We find agreement with previous determinations at $N_c=3$ and $N_f=2, 3$ and also, our results support a strong paramagnetic suppression of the chiral condensate in moving from $N_f=2$ to $N_f=3$.

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arXiv2019

Non-perturbative renormalization of O(a) improved tensor currents

We present our progress in the non-perturbative O(a) improvement and renormalization of tensor currents in three-flavor lattice QCD with Wilson-clover fermions and tree-level Symanzik improved gauge action. The mass-independent O(a) improvement factor of tensor currents is determined via a Ward identity approach, and their renormalization group running is calculated via recursive finite-size scaling techniques, both implemented within the Schrödinger functional framework. We also address the matching factor between bare and renormalization group invariant currents for a range of lattice spacings < 0.1 fm, relevant for phenomenological large-volume lattice QCD applications.

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arXiv2019

Matching of $N_f=2+1$ CLS ensembles to a tmQCD valence sector

A mixed action composed of valence quark flavours regularized with a fully-twisted tmQCD action and of $N_f=2+1$ flavours of non-perturbatively ${\rm O}(a)$-improved Wilson sea quarks is described. Two procedures for the matching of sea and valence quark masses are discussed. We report about a comparison of the continuum-limit scaling of pseudoscalar meson observables and of quark masses using the sea and valence actions.

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