Recent anomalies observed in $ e^+e^- $ nuclear transitions of $ ^8 {{\rm{Be}}} $ , $ ^4 {\rm{He}} $ , and $ ^{12} {\rm{C}} $ by the ATOMKI collaboration may hint at the existence of a vector boson with a mass around 17 MeV, which is referred to as X17. If it exists, this boson would affect similar processes in particle physics, including the Dalitz decays of vector mesons. Recently, the BESIII collaboration measured the Dalitz decay $ D^{*0}\to D^0e^+e^- $ for the first time and reported a $ 3.5\sigma $ excess over the theoretical prediction based on the vector meson dominance (VMD) model. This excess may be another signal of X17. In this study, we investigate the possible effects of X17 on the Dalitz decays $ D^*_{(s)}\to D_{(s)}e^+e^- $ , $ B^*_{(s)}\to B_{(s)}e^+e^- $ , and $ J/\psi\to \eta_ce^+e^- $ . The required hadronic form factors are calculated within the framework of our covariant confined quark model without relying on heavy quark effective theory or the VMD model. We present predictions for the Dalitz decay widths and the ratios $ R_{ee}(V)\equiv \Gamma(V\to Pe^+e^-)/\Gamma(V\to P\gamma) $ within the standard model and in several new physics scenarios involving modifications attributed to X17. Our results are compared with other theoretical calculations.

Vietnam

C.T. Tran

Russia

M.A. Ivanov

A.T.T. Nguyen

Probing the ATOMKI X17 vector boson using Dalitz decays $\textit{V}$ → $\textit{Pe}$ $^{+}$ $\textit{e}$ $^{−}$ * Supported by Ho Chi Minh City University of Technology and Education (T2024-65)

We develop and test a spectral-density analysis method, based on the introduction of smeared energy kernels, to extract physical information from two-point correlation functions computed numerically in lattice field theory. We apply it to a <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>S</mi><mi>p</mi><mo stretchy="false">(</mo><mn>4</mn><mo stretchy="false">)</mo></math> gauge theory with fermion matter fields transforming in distinct representations, with <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msub><mi>N</mi><mi mathvariant="normal">f</mi></msub><mo>=</mo><mn>2</mn></math> Dirac fermions in the fundamental and <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msub><mi>N</mi><mtext>as</mtext></msub><mo>=</mo><mn>3</mn></math> in the 2-index antisymmetric representation. The corresponding continuum theory provides the minimal candidate model for a composite Higgs boson with partial top compositeness. We consider a broad class of composite operators, that source flavored mesons and (chimera) baryons, for several finite choices of lattice bare parameters. For the chimera baryons, which include candidate top-quark partners, we provide the first measurements, obtained with dynamical fermions, of the ground state and the lowest excited state masses, in all channels of spin, isospin, and parity. We also measure matrix elements and overlap factors, that are important to realize viable models of partial top compositeness, by implementing an innovative way of extracting this information from the spectral densities. For the mesons, among which the pseudoscalars can be reinterpreted to provide an extension of the Higgs sector of the Standard Model of particle physics, our measurements of the renormalized matrix elements and decay constants are new results. We complement them with an update of existing measurements of the meson masses, obtained with higher statistics and improved analysis. The analysis software is made publicly available, and can be used in other lattice studies, including application to quantum chromodynamics.

United Kingdom

Ed Bennett

Luigi Del Debbio

Niccolò Forzano

Ryan C. Hill

South Korea

Deog Ki Hong

Japan

Taiwan

Ho Hsiao

Jong-Wan Lee

C.J. David Lin

Biagio Lucini

France

Alessandro Lupo

Maurizio Piai

Davide Vadacchino

Fabian Zierler

Chimera baryons and mesons on the lattice: A spectral density analysis

We present a detailed investigation of the performance of transition-edge sensor (TES) microcalorimeters with $$^{163}$$ Ho atoms embedded by ion implantation, as part of the HOLMES experiment aimed at neutrino mass determination. The inclusion of $$^{163}$$ Ho atoms introduces an excess heat capacity due to a pronounced Schottky anomaly, which can affect the detector’s energy resolution, signal height, and response time. We fabricated TES arrays with varying levels of $$^{163}$$ Ho activity and characterized their performance in terms of energy resolution, decay time constants, and heat capacity. The intrinsic energy resolution was found to degrade with increasing $$^{163}$$ Ho activity, consistent with the expected scaling of heat capacity. From the analysis, we determined the specific heat capacity of $$^{163}$$ Ho to be $$(2.9 \pm 0.4 \mathrm {(stat)} \pm 0.7 \mathrm {(sys)})$$ J/K/mol at $$(94 \pm 1)$$ mK, close to the literature values for metallic holmium. No additional long decay time constants correlated with $$^{163}$$ Ho activity were observed, indicating that the excess heat capacity does not introduce weakly coupled thermodynamic systems. These results suggest that our present TES microcalorimeters can tolerate $$^{163}$$ Ho activities up to approximately 5 Bq, with only about a factor of three degradation in performance compared to detectors without $$^{163}$$ Ho. For higher activities, reducing the TES transition temperature is necessary to maintain or improve the energy resolution. These findings provide critical insights for optimizing TES microcalorimeters for future neutrino mass experiments and other applications requiring embedded radioactive sources. The study also highlights the robustness of TES technology in handling limited amounts of implanted radionuclides while maintaining high-resolution performance.

United States

Douglas Bennett

Italy

Matteo Borghesi

Pietro Campana

Rodolfo Carobene

Giancarlo Ceruti

Matteo Gerone

Marco Faverzani

Lorenzo Ferrari Barusso

Elena Ferri

Joseph Fowler

Sara Gamba

Flavio Gatti

Andrea Giachero

Marco Gobbo

Danilo Labranca

Roberto Moretti

Angelo Nucciotti

Luca Origo

Stefano Ragazzi

Dan Schmidt

Daniel Swetz

Joel Ullom

Impact of embedded $$^{163}$$ Ho on the performance of the transition-edge sensor microcalorimeters of the HOLMES experiment

We report the results of an extensive numerical study of the <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>S</mi><mi>p</mi><mo stretchy="false">(</mo><mn>4</mn><mo stretchy="false">)</mo></math> lattice gauge theory coupled to fermion matter content consisting of three (Dirac) flavors, transforming in the two-index antisymmetric representation of the gauge group. In the presence of (degenerate) fermion masses, the theory has an enhanced global <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>S</mi><mi>U</mi><mo stretchy="false">(</mo><mn>6</mn><mo stretchy="false">)</mo></math> symmetry, broken explicitly and spontaneously to its <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>S</mi><mi>O</mi><mo stretchy="false">(</mo><mn>6</mn><mo stretchy="false">)</mo></math> subgroup. This symmetry breaking pattern makes the theory interesting for applications in the context of composite Higgs models, as well as for the implementation of top partial compositeness. Alternatively, it can also provide a dynamical realization of the strongly interacting massive particle paradigm for the origin of dark matter. We adopt the standard plaquette gauge action, along with the Wilson-Dirac formulation for the fermions, and apply the (rational) hybrid Monte Carlo algorithm in our ensemble generation process. We monitor the autocorrelation and topology of the ensembles. We explore the bare parameter space, and identify the weak and strong coupling regimes, which are separated by a line of first-order bulk phase transitions. We measure two-point correlation functions between meson operators that transform as nontrivial representations of <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>S</mi><mi>O</mi><mo stretchy="false">(</mo><mn>6</mn><mo stretchy="false">)</mo></math>, and extract the ground-state masses, in all accessible spin and parity channels. We assess the size of finite volume effects, and restrict attention to measurements in which these systematic effects are negligibly small compared to the statistical uncertainties. The accuracy of our data enables us to extract the decay constants of the composite particles in the pseudoscalar, vector and axial-vector channels. In addition, we measure the mass of the first excited state for one of the channels, the vector meson, by performing a generalized eigenvalue problem analysis involving two different meson operators. Spectral quantities show a mass dependence that is compatible with the expectation that, at long distances, the theory undergoes confinement, accompanied by the spontaneous breaking of the approximate global symmetries acting on the matter fields. Finally, we discuss the continuum and massless extrapolations within the framework of Wilson chiral perturbation theory, after setting the physical scale using the gradient flow method, and compare the results to those of existing studies in the quenched approximation, as well as to the literature on closely related theories.

Meson spectroscopy in the <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>S</mi><mi>p</mi><mo stretchy="false">(</mo><mn>4</mn><mo stretchy="false">)</mo></math> gauge theory with three antisymmetric fermions

Spectral densities encode nonperturbative information that enters the calculation of a plethora of physical observables in strongly coupled field theories. Phenomenological applications encompass aspects of standard-model hadronic physics, observable at current colliders, as well as correlation functions characterizing new physics proposals, testable in future experiments. By making use of numerical data produced in a <math><mi>S</mi><mi>p</mi><mo>(</mo><mn>4</mn><mo>)</mo></math> lattice gauge theory with matter transforming in an admixture of fundamental and 2-index antisymmetric representations of the gauge group, we perform a systematic study to demonstrate the effectiveness of recent technological progress in the reconstruction of spectral densities.To this purpose, we write and test new software packages that use energy-smeared spectral densities to analyze the mass spectrum of mesons. We assess the effectiveness of different smearing kernels and optimize the smearing parameters to the characteristics of available lattice ensembles. For concreteness, we analyze the <math><mi>S</mi><mi>p</mi><mo>(</mo><mn>4</mn><mo>)</mo></math> lattice gauge theory with matter transforming in an admixture of fundamental and 2-index antisymmetric representations of the gauge group. We generate new ensembles for the theory in consideration, with lattices that have a longer extent in the time direction with respect to the spatial ones. We run our tests on these ensembles, obtaining new results about the spectrum of light mesons and their excitations. We make available our algorithm and software for the extraction of spectral densities, that can be applied to theories with other gauge groups, including the theory of strong interactions (QCD) governing hadronic physics in the standard model.

Meson spectroscopy from spectral densities in lattice gauge theories

We provide the first extensive, numerical study of the nontrivial problem of mixing between flavor-singlet composite states emerging in strongly coupled lattice field theories with matter field content consisting of fermions transforming in different representations of the gauge group. The theory of interest is the minimal candidate for a composite Higgs model that also accommodates a mechanism for top partial compositeness: the <math><mi>S</mi><mi>p</mi><mo>(</mo><mn>4</mn><mo>)</mo></math> gauge theory coupled to two (Dirac) fermions transforming as the fundamental and three as the two-index antisymmetric representation of the gauge group, respectively. We apply an admixture of APE smearing and Wuppertal smearings, as well as the generalized eigenvalue problem approach, to two-point functions involving flavor-singlet mesons, for ensembles having time extent longer than the space extent. We demonstrate that, in the region of lattice parameter space accessible to this study, both masses and mixing angles can be measured effectively, despite the presence of (numerically noisy) contributions from disconnected diagrams.

Mixing between flavor singlets in lattice gauge theories coupled to matter fields in multiple representations

We report the results of lattice numerical studies of the <math><mrow><mi>S</mi><mi>p</mi><mo>(</mo><mn>4</mn><mo>)</mo></mrow></math> gauge theory coupled to fermions (hyperquarks) transforming in the fundamental and two-index antisymmetric representations of the gauge group. This strongly coupled theory is the minimal candidate for the ultraviolet completion of composite Higgs models that facilitate the mechanism of partial compositeness for generating the top-quark mass. We measure the spectrum of the low-lying, half-integer spin, bound states composed of two fundamental and one antisymmetric hyperquarks, dubbed chimera baryons, in the quenched approximation. In this first systematic, nonperturbative study, we focus on the three lightest parity-even chimera-baryon states, in analogy with QCD, denoted as <math><msub><mi>Λ</mi><mrow><mi>CB</mi></mrow></msub></math>, <math><msub><mi>Σ</mi><mrow><mi>CB</mi></mrow></msub></math> (both with spin <math><mn>1</mn><mo>/</mo><mn>2</mn></math>), and <math><mrow><msubsup><mrow><mi>Σ</mi></mrow><mrow><mi>CB</mi></mrow><mrow><mo>*</mo></mrow></msubsup></mrow></math> (with spin <math><mn>3</mn><mo>/</mo><mn>2</mn></math>). The spin-<math><mn>1</mn><mo>/</mo><mn>2</mn></math> such states are candidates of the top partners. The extrapolation of our results to the continuum and massless-hyperquark limit is performed using formulas inspired by QCD heavy-baryon Wilson chiral perturbation theory. Within the range of hyperquark masses in our simulations, we find that <math><msub><mi>Σ</mi><mi>CB</mi></msub></math> is not heavier than <math><msub><mi>Λ</mi><mi>CB</mi></msub></math>.

Lattice investigations of the chimera baryon spectrum in the <math><mrow><mi>S</mi><mi>p</mi><mo>(</mo><mn>4</mn><mo>)</mo></mrow></math> gauge theory

We provide the first determination of the mass of the lightest flavor-singlet pseudoscalar and scalar bound states (mesons) in the Sp(4) Yang-Mills theory coupled to two flavors of fundamental fermions, using lattice methods. This theory has applications both to composite Higgs and strongly interacting dark matter scenarios. We find the singlets to have masses comparable to those of the light flavored states, which might have important implications for phenomenological models. We focus on regions of parameter space corresponding to a moderately heavy mass regime for the fermions. We compare the spectra we computed to existing and new results for SU(2) and SU(3) theories, uncovering an intriguing degree of commonality. As a by-product, in order to perform the aforementioned measurements, we implemented and tested, in the context of symplectic lattice gauge theories, several strategies for the treatment of disconnected-diagram contributions to two-point correlation functions. These technical advances set the stage for future studies of the singlet sector in broader portions of parameter space of this and other lattice theories with a symplectic gauge group.

Austria

Axel Maas

Singlets in gauge theories with fundamental matter

The present paper investigates the impact of pairing correlation on the nuclear level density (NLD) and radiative strength function (RSF) of an odd-odd and rare-earth $^{166}$Ho nucleus. By employing the exact thermal pairing (EP) solution in conjunction with both the temperature-dependent independent-particle model (EP+IPM) for the NLD and the phonon damping model (EP+PDM) for the RSF, we achieve good agreement with the experimental NLD and RSF data, which have been recently extracted by using the Oslo method. Notably, without adding any extra parameters, our calculations successfully reproduce the pygmy dipole resonances (PDR) and a portion of the strength around the region of scissor resonance (SR) in the RSF. This suggests that the SR in this nucleus may be less pronounced than that obtained in the previous analysis within the phenomenological model. These features are the consequences of the EP and the couplings of the PDM phonon to all the particle-hole (ph), particle-particle (pp), and hole-hole (hh) configurations within the EP+PDM, thus insisting on the importance of the underlying physics involved in this model. Utilizing the obtained NLD and RSF, we predict the $^{165}$Ho<math><msup><mrow><mo>(</mo><mi>n</mi><mo>,</mo><mi>γ</mi><mo>)</mo></mrow><mrow><mn>166</mn></mrow></msup></math>Ho and Maxwellian-averaged cross sections, which exhibit remarkable agreement with all the available experimental data from the Experimental Nuclear Reaction Data and Karlsruhe Astrophysical Database of Nucleosynthesis in Stars. These results reflect the imprint of the pairing correlation on those quantities.

Le Tan Phuc

Tran Vu Dong

Nguyen Dinh Dang

Bui Minh Hue

Le Thi Quynh Huong

India

Balaram Dey

Nguyen Ngoc Anh

Nguyen Quang Hung

Imprint of pairing correlation in (n, ) and Maxwellian-averaged cross sections of an odd-odd Ho nucleus

We consider the <math><mi>S</mi><mi>p</mi><mo>(</mo><mn>4</mn><mo>)</mo></math> gauge theory coupled to <math><msub><mi>N</mi><mi>f</mi></msub><mo>=</mo><mn>2</mn></math> fundamental and <math><msub><mi>n</mi><mi>f</mi></msub><mo>=</mo><mn>3</mn></math> antisymmetric flavors of Dirac fermions in four dimensions. This theory serves as the microscopic origin for composite Higgs models with <math><mi>S</mi><mi>U</mi><mo>(</mo><mn>4</mn><mo>)</mo><mo>/</mo><mi>S</mi><mi>p</mi><mo>(</mo><mn>4</mn><mo>)</mo></math> coset, supplemented by partial top compositeness. We study numerically its lattice realization, and couple the fundamental plaquette action to Wilson-Dirac fermions in mixed representations, by adopting a (rational) hybrid Monte Carlo method, to perform nontrivial tests of the properties of the resulting lattice theory. We find evidence of a surface (with boundaries) of first-order bulk phase transitions in the three-dimensional space of bare parameters (one coupling and two masses). Explicit evaluation of the Dirac eigenvalues confirms the expected patterns of global symmetry breaking. After investigating finite-volume effects in the weak-coupling phase of the theory, for the largest available lattice we study the mass spectra of the lightest spin-0 and spin-1 flavored mesons composed of fermions in each representation, and of the lightest half-integer spin composite particle made of fermions in different representations—the chimera baryon. This work sets the stage for future systematical studies of the nonperturbative dynamics in phenomenologically relevant regions of parameter space.

Michele Mesiti

Ireland

Lattice studies of the <math><mi>S</mi><mi>p</mi><mo>(</mo><mn>4</mn><mo>)</mo></math> gauge theory with two fundamental and three antisymmetric Dirac fermions