arXiv subjects

Fernando Romero-López

Publications and source records attributed to Fernando Romero-López.

At least 19 recordsLinked to original sources

arXiv2026

Improving hadron creation operators for charmonium, glueballs and baryons

Constructing hadron creation operators which properly sample the finite-volume energy spectrum is a fundamental step in lattice QCD spectroscopy and scattering calculations. Operators enabling access to spectrum information from correlation functions at early temporal separations are extremely advantageous, particularly when the signal-to-noise problem is severe. We present improved operator constructions for charmonium, glueball and baryon spectroscopy which exhibit these advantages. Novelties of our work include the extension of the distillation profiles framework to baryon and multi-meson calculations, as well as an efficient implementation of glueball-like operators which retain angular momentum information from the continuum.

hep-lat↗

arXiv2026

Accessing the Wess-Zumino-Witten term using Lattice QCD

The Wess-Zumino-Witten interaction in chiral perturbation theory is a manifestation of chiral anomalies. One of its consequences is the presence of a nonvanishing $K \overline K \to π^+ π^0 π^-$ amplitude. We derive the finite-volume formalism that allows one to access this, and related, amplitudes, using the spectra of two and three particles in finite volumes, spectra that can be calculated using lattice QCD. Specifically, we present the formalism for the systems $K \overline K + 3 π$ with $I=0$ and $πK + ππK$ with $I=1/2$ and $3/2$. In addition, we determine the threshold expansions for the $2\leftrightarrow 3$ and $3\leftrightarrow 3$ K matrices that enter the formalism, and calculate the leading order predictions from chiral perturbation theory for the coefficients that enter these expansions.

hep-lat↗

arXiv2026

Variance reduction in lattice QCD observables via normalizing flows

Normalizing flows can be used to construct unbiased, reduced-variance estimators for lattice field theory observables that are defined by a derivative with respect to action parameters. This work implements the approach for observables involving gluonic operator insertions in the SU(3) Yang-Mills theory and two-flavor Quantum Chromodynamics (QCD) in four space-time dimensions. Variance reduction by factors of $10$-$60$ is achieved in glueball correlation functions and in gluonic matrix elements related to hadron structure, with demonstrated computational advantages. The observed variance reduction is found to be approximately independent of the lattice volume, so that volume transfer can be utilized to minimize training costs.

hep-lat↗

arXiv2026

Investigating the role of tetraquark operators in lattice QCD studies of the $a_0(980)$ and $κ$ resonances

The role of tetraquark operators in studying the isodoublet strange $κ$ and isovector nonstrange $a_0(980)$ scalar mesons in lattice QCD is examined using an ensemble with $m_π\approx230$ MeV and spatial extent $L$ such that $m_πL\approx4.4$. Hermitian correlation matrices using both single-meson, meson-meson, and tetraquark interpolating operators are used to extract the spectrum of finite-volume stationary states in the appropriate symmetry channels. Hundreds of local and extended tetraquark operators are explored. Determinations of the spectrum in each channel are found to be unreliable without the inclusion of at least one tetraquark operator. For example, the inclusion of tetraquark operators with isospin 1/2 and strangeness 1 quantum numbers reveals the existence of an additional energy level in the $Kη$ sub-system below the $Kη$ threshold. The implications of this on parametrizing the scattering $K$-matrix through a well-known quantization condition to extract properties of the $κ$ and $a_0(980)$ scalar meson resonances are discussed.

hep-lat↗

arXiv2026

Learning the generating functional for variance reduction in lattice QCD

The generating functional in quantum field theory provides the natural framework for constructing correlation functions as derivatives with respect to source operators. We present a methodology that leverages machine-learned normalizing flows to reduce the variance of arbitrary $N$-point correlation functions of bosonic operators in lattice gauge field theory calculations by encoding a representation of the generating functional. We show that it is possible to systematically approach noiseless estimators of correlation functions in this framework. We demonstrate this methodology with applications to calculations of glueball correlation functions and Wilson loops in Quantum Chromodynamics and Yang-Mills theory. The results show up to three orders of magnitude variance reduction.

hep-lat↗

arXiv2026

$F_K/F_π$ as a precision test of a new four flavor Domain Wall Fermion action

We present a new set of lattice QCD ensembles with four flavors of smeared Möbius Domain Wall Fermions with good chiral symmetry and small fifth-dimensional extent. A modest amount of computing resources was sufficient to generate 30 publicly-available ensembles spanning five lattice spacings and a broad range of pion masses down to physical. To scrutinize our action we determine $F_{K^{\pm}}/F_{π^{\pm}} = 1.1962(34)$, a key quantity for precision CKM unitarity tests, heralding a future of inexpensive high-precision calculations of hadronic observables with chiral fermions.

hep-lat↗

arXiv2026

Preliminary study of the $H$ dibaryon in $N_{\rm f}=2+1$ lattice QCD

We present preliminary results on the $I=0$, $S=-2$ $H$ dibaryon in $N_{\rm f}=2+1$ QCD. The calculation is performed with heavier-than-physical quarks ($m_π\approx 280$ MeV) on a single CLS ensemble. Correlation matrices are constructed using the distillation technique and the three relevant channels, $ΛΛ$, $NΞ$, $ΣΣ$, are investigated to determine the interacting spectrum relevant for $S$-wave across multiple momentum frames. The scattering amplitude is determined by solving the corresponding two-body quantization condition. These preliminary results are part of the ongoing efforts to determine the properties of di-hyperons and to establish whether the $H$ dibaryon exists down to physical quark masses.

hep-lat↗

arXiv2026

Three-body study of the $T_{cc}(3875)^+$ from lattice QCD

We discuss an ongoing first lattice study of the doubly-charmed tetraquark $T_{cc}^+$(3875) via a three-body approach. We investigate the $DDπ$ system in the $I=0$, $C=2$ sector, where the $T_{cc}^+$ appears as a pole in the $J^P = 1^+$ $DDπ$ elastic scattering amplitude. The approach automatically incorporates two-body $DD^*$ and three-body $DDπ$ effects and treats left-hand cuts due to single $π$ exchanges. Two CLS ensembles, X252 and X253, with pion mass $M_π\approx 280$ MeV, are used, and an operator set comprised of two- and three-hadron and tetraquark operators is employed to extract finite-volume energies. Additional inputs are required for the three-body finite-volume analysis, in the form of amplitudes for the $I=1$ $DD$ and $I=1/2$ $Dπ$ two-body subsystems. We present preliminary results for these subchannels and perform exploratory three-body spectra determinations for simple choices of the three-particle K-matrix $\mathcal{K}_{\text{df}, 3}$, allowing a first comparison to the lattice spectrum.

hep-lat↗

arXiv2026

Di-nucleons do not form bound states at heavy pion mass

We perform a high-statistics lattice QCD calculation of the low-energy two-nucleon scattering amplitudes. In order to address discrepancies in the literature, the calculation is performed at a heavy pion mass in the limit that the light quark masses are equal to the physical strange quark mass, $m_π= m_K \simeq 714 $ MeV. Using a state-of-the-art momentum space method, we rule out the presence of a bound di-nucleon in both the isospin 0 (deuteron) and 1 (di-neutron) channels, in contrast with many previous results that made use of compact hexaquark creation operators. In order to diagnose the discrepancy, we add such hexaquark interpolating operators to our basis and find that they do not affect the determination of the two-nucleon finite volume spectrum, and thus they do not couple to deeply bound di-nucleons that are missed by the momentum-space operators. Further, we perform a high-statistics calculation of the HAL QCD potential on the same gauge ensembles and find qualitative agreement with our main results. We conclude that di-nucleons do not form bound states at heavy pion masses and that previous identification of deeply bound di-nucleons must have arisen from a misidentification of the spectrum from off-diagonal elements of a correlation function.

hep-lat↗

arXiv2025

Bayesian Analysis and Analytic Continuation of Scattering Amplitudes from Lattice QCD

We present a novel procedure for analyzing the lattice-QCD spectrum via the finite-volume formalism to obtain constraints on multi-hadron scattering amplitudes at both real and complex energies. This approach combines a Bayesian reconstruction of the scattering amplitude on the real axis with Nevanlinna interpolation for analytic continuation to complex-valued energies. The method is non-parametric, inherently accounting for parametrization dependence within the uncertainty. We demonstrate the applicability of this approach using both toy data and real lattice-QCD data in resonant systems from the HadSpec and BaSc collaborations.

hep-lat↗

arXiv2025

Comparison of integral equations used to study $T_{cc}^+$ for a stable $D^*$

We perform a detailed comparison between three formalisms used in recent studies of $DD^*$ scattering, which aim to understand the properties of the doubly-charmed tetraquark, $T_{cc}^+(3875)$. These methods are the three-particle relativistic field theory (RFT) formalism, the two-body Lippmann-Schwinger (LS) equation with chiral effective field theory potentials, and the two-particle relativistic framework proposed by Baiao Raposo and Hansen (BRH approach). In a simplified single-channel setting, we derive the conditions under which the infinite-volume integral equations from the RFT and BRH approaches reduce to the LS form. We present numerical examples showing that differences between these methods can be largely removed by adjusting short-range couplings. We also address a number of technical issues in the RFT approach.

nucl-th↗

arXiv2025

Finite-volume formalism for $Nππ$ at maximal isospin

We extend the relativistic field theoretic finite-volume formalism to $N ππ$ scattering states at maximal isospin, $I=5/2$. As in previous work using the relativistic field theory approach, we work to all orders in a generic low-energy effective theory, and determine the quantization condition that relates finite-volume energies to intermediate K matrices, and the integral equations connecting the latter to the physical scattering amplitudes. We discuss the parametrization of the K matrices, and explain in detail the new features that arise in implementing the quantization condition due to the spin of the nucleon in combination with the use of non-degenerate particles. As a concrete example, we provide a sample numerical application including the $Δ$ resonance in the $Nπ$ subchannel. The extension to the $I=3/2$ and $1/2$ channels is more involved, due to mixing with $Nπ$ states, and we do not provide a complete formalism for these cases. We explain why $Nπ$ states cannot be included by treating the nucleon as a pole in $p$-wave $Nπ$ scattering, an approach that has been successful in studying $D D^*$ scattering using the three-particle $DDπ$ formalism. We additionally provide results for all isospins under the assumption of no two-to-three mixing, thereby laying the groundwork for a follow-up paper in which all $Nππ\leftrightarrow Nπ$ systems are fully treated. Finally, we study the singularities in $Nππ$ amplitudes arising from $Nπππ$ intermediate states, and find that our subthreshold cutoff functions must be modified to avoid such singularities.

hep-lat↗

arXiv2025

The $ππ$ scattering amplitude at large $N_\text{c}$

We study the scaling of meson-meson scattering amplitudes with the number of colors, $N_\text{c}$. We use lattice calculations in a theory with $N_\text{f}=4$ degenerate flavors, with $N_\text{c}=3-6$ and pion mass $M_π\approx 560$ MeV. We focus on three different scattering channels, two of which have the same quantum numbers as some tetraquark candidates recently found at LHCb: the $T_{cs0}^0(2900)$, $T_{c\bar{s}0}^{++}(2900)$, $T_{c\bar{s}0}^0(2900)$ and $T_{c\bar{s}1}^0(2900)$ states. Finite-volume energies are extracted using a large set of operators, containing two-particle operators with the form of two pions or two vector mesons, and local tetraquark operators. The resulting energy spectra is used to constrain the infinite-volume scattering amplitude by means of Lüscher's quantization condition. We consider polynomial parametrizations of the phase shift, as well as one-loop chiral perturbation theory (ChPT) predictions. We find that our lattice results follow the expected $N_\text{c}$ scaling and are sensitive to subleading $N_\text{c}$ corrections. In addition, we constrain the scaling of different combinations of low-energy constants from matching to large $N_\text{c}$ ChPT. The results for the channel corresponding to a $(π^+ D^+_s - K^+ D^+)$ state show evidence of a virtual bound state with energy $E_\text{virtual}=1.63(10)M_π$ for $N_\text{c}=3$, while this pole disappears at $N_\text{c}>3$. This may be connected to the exotic states found in experiment.

hep-lat↗

arXiv2025

Lattice evidence that scalar glueballs are small

This work reports the first calculation of the gravitational form factors (GFFs) of the scalar glueball, performed via lattice field theory in Yang-Mills theory at a single lattice spacing. The glueball GFFs are compared with those of other hadrons as determined in previous lattice calculations, providing strong indications that glueballs have a different gluonic structure than typical hadronic states. A mass radius of 0.263(31) fm is predicted, supporting previous suggestions that the scalar glueball is significantly smaller than other hadrons. These results point towards a potential smoking-gun characteristic to target by experimental glueball searches.

hep-lat↗

arXiv2025

Two- and three-meson scattering amplitudes with physical quark masses from lattice QCD

We study systems of two and three mesons composed of pions and kaons at maximal isospin using four CLS ensembles with $a\approx 0.063\;$fm, including one with approximately physical quark masses. Using the stochastic Laplacian-Heaviside method, we determine the energy spectrum of these systems including many levels in different momentum frames and irreducible representations. Using the relativistic two- and three-body finite-volume formalism, we constrain the two and three-meson K matrices, including not only the leading $s$ wave, but also $p$ and $d$ waves. By solving the three-body integral equations, we determine, for the first time, the physical-point scattering amplitudes for $3π^+$, $3K^+$, $π^+π^+ K^+$ and $K^+ K^+ π^+$ systems. These are determined for total angular momentum $J^P=0^-$, $1^+$, and $2^-$. We also obtain accurate results for $2π^+$, $π^+ K^+$, and $2K^+$ phase shifts. We compare our results to Chiral Perturbation Theory, and to phenomenological fits.

hep-lat↗

arXiv2025

QCD predictions for physical multimeson scattering amplitudes

We use lattice QCD calculations of the finite-volume spectra of systems of two and three mesons to determine, for the first time, three-particle scattering amplitudes with physical quark masses. Our results are for combinations of $π^+$ and $K^+$, at a lattice spacing $a=0.063\;$fm, and in the isospin-symmetric limit. We also obtain accurate results for maximal-isospin two-meson amplitudes, with those for $π^+ K^+$ and $2K^+$ being the first determinations at the physical point. Dense lattice spectra are obtained using the stochastic Laplacian-Heaviside method, and the analysis leading to scattering amplitudes is done using the relativistic finite-volume formalism. Results are compared to chiral perturbation theory and to phenomenological fits to experimental data, finding good agreement.

hep-lat↗

arXiv2025

Three-hadron dynamics from lattice QCD

Three-hadron spectroscopy is a key frontier in our understanding of the hadron spectrum. In recent years, significant formal and numerical advances have paved the way for studying three-hadron processes directly from lattice QCD, with outstanding applications including the Roper resonance and the doubly charmed tetraquark. This requires theoretical frameworks that relate finite-volume energies to infinite-volume three-particle scattering amplitudes. In this contribution, I discuss recent progress in formulating such frameworks for generic three-hadron systems, and present numerical results for three-meson systems at maximal isospin with physical quark masses, as well as our recent investigation of the three-body dynamics of the doubly charmed tetraquark, $T_{\rm cc}$.

hep-lat↗

arXiv2025

The singlet scalar state in a chiral ensemble in $SU(2)$ with two fundamental flavours

Composite Higgs models are a class of Beyond the Standard Model (BSM) models proposed to address the hierarchy and naturalness problems associated with the Standard Model (SM) Higgs. A new QCD-like strongly interacting sector based on $SU(2)$ with two fundamental flavours can be used to build a composite Higgs model which is not yet ruled out by experiment. The role of the singlet scalar resonance will affect Higgs phenomenology at the LHC. In this project our goal is to understand the properties of the singlet scalar state in the new strongly interacting sector in isolation as a first step to understanding the role of this state in composite Higgs models. We present here the first lattice results for the mass of the $σ$ in $SU(2)$ with two fundamental flavours using exponential clover Wilson fermions.

hep-lat↗