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Àngels Ramos

Publications and source records attributed to Àngels Ramos.

15 recordsLinked to original sources

Nuclear-physics-guided Gaussian Processes

Gaussian Process Regression is a powerful nonparametric Bayesian method that provides both predictions and principled uncertainty estimates in closed form. The majority of past applications have relied on agnostic priors, but physics knowledge can be systematically encoded into Gaussian Processes through physically-motivated mean functions and kernels. We exploit this capability in the context of nuclear physics, applying physics-guided Gaussian Process Regression to three problems: nucleon-nucleon scattering phase shifts, mass excesses, and the finite-temperature equation of state of dense matter. In each case, we demonstrate that encoding known theoretical structures yields substantial and systematic improvements in interpolation accuracy, uncertainty calibration, and extrapolation reliability over agnostic baselines. Our results highlight that the design of the prior, and in particular the mean function, is key for obtaining a reliable and well-calibrated Gaussian Process Regression.

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Properties of the $D_{s0}^*(2317)^\pm$ in hot and dense nuclear matter

We investigate the properties of the $D_{s0}^\ast(2317)^\pm$ in hot and dense nuclear matter using a coupled-channel molecular model built on next-to-leading-order heavy meson chiral perturbation theory. In-medium modifications to the $D_{s0}^\ast(2317)^+$ stem from changes to the $DK$ channel within the coupled $DK$-$D_s η$ system. As nuclear density increases, the $D_{s0}^\ast(2317)^+$ quasiparticle peak shifts toward lower energies and broadens, tracking the behavior of the $D$-meson spectral function. As for temperature effects, those are milder, with the thermal smearing of the Fermi surface and the melting of $Σ_c N^{-1}$ excitations in the $D$-meson spectral function shifting the $D_{s0}^\ast(2317)^+$ peak back toward its free-space mass while narrowing it. Conversely, the behavior of the $D_{s0}^\ast(2317)^-$ is governed by the $\bar D \bar K$ channel and its medium behavior is driven by the $\bar K$ spectral function. With increasing temperature, the $D_{s0}^\ast(2317)^-$ also approaches its free-space mass, but its width broadens before saturating at high temperatures. Incorporating explicit medium dependencies into the interaction kernel, driven by density and/or temperature variations in the pion decay constant, further shifts the $D_{s0}^\ast(2317)^+$ mass lower and narrows its width with temperature. As for $D_{s0}^\ast(2317)^-$, its mass also drops with temperature but its width increases. These contrasting medium behaviors offer a promising pathway to constrain the internal structure of these exotic states.

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Kaon-deuteron correlation function from an effective field theory approach

We present a study of femtoscopic correlation functions for $K^{-}d$ and $K^{+}d$ pairs, and compare our results with recent measurements by the ALICE Collaboration in both Pb-Pb and high-multiplicity $pp$ collisions. The kaon-deuteron wave functions are derived from scattering amplitudes using a unitarized chiral effective theory model describing the elementary interactions of $K^{\pm}$ mesons with nucleons. We then evaluate the $K^{\pm}d$ strong scattering amplitudes by solving the Faddeev equations within two distinct frameworks: the Impulse Approximation and the Fixed Center Approximation, which accounts for multiple scatterings. We also incorporate the long-range Coulomb effects between the kaon and the deuteron. We show that the $K^{-}d$ correlation function exhibits large sensitivity to both the size of the emitting source and the relative momentum of the pair, being heavily influenced by rescattering processes. In contrast, the $K^{+}d$ correlation function is dominated by the weakly repulsive $K^{+}N$ interaction, showing deviations from purely Coulombic behavior only at small emission source sizes. Our predictions are in agreement with the ALICE experimental data, and also with the energy-shift and width of the $1s$ level of the kaonic deuterium preliminary results from the SIDDHARTA 2 Collaboration.

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Kaon-deuteron femtoscopy from unitarized chiral interactions

We have performed a theoretical study of the correlation functions of $K^- d$ and $K^+ d$ pairs and compared them with those provided by the ALICE Collaboration from Pb-Pb collisions, and also from high-multiplicity p-p collisions in the case of $K^+ d$. In addition to implementing the effect of the Coulomb force, the $K^- d$ and $K^+ d$ wave functions are derived from the corresponding strong scattering amplitudes that are built employing a unitarized chiral model for the elementary $K^- N$ and $K^+ N$ interactions. We present results for the impulse approximation, which accounts for single-scattering processes of the kaon with the nucleons of the deuteron, as well as for the solution of the Faddeev equations in the so-called fixed center approximation, which includes multiple rescattering effects. The $K^- d$ correlation function is shown to be very sensitive to both the size of the source and the relative momentum of the interacting pair, with large deviations from the Coulomb baseline and sizable multi-step scattering contributions, effects that are tied to a ${\bar K}N$ strong interaction that is dominated by the influence of the subthreshold resonance $Λ(1405)$. In contrast, the $K^+ d$ correlation function only differs appreciably from the Coulomb one for relatively small sources, reflecting the mildly repulsive and elastic behavior of the $KN$ strong force. The calculated correlation functions are found to nicely reproduce the experimental data of the ALICE collaboration. Our study serves to reinforce the validity of the theoretical models employed and demonstrates the value of femtoscopy as a powerful tool for probing hadronic interactions involving strangeness.

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Low-mass enhancement of kaon pairs in $B^+\to\bar{D}^{(*)0}K^+\bar{K}^0$ and $B^0\to D^{(*)-}K^+\bar{K}^0$ decays

Very recently, the Belle~II Collaboration presented a measurement for the decays $B^+\to\bar{D}^{(*)0} K^+\bar{K}^0$ and $B^0\to D^{(*)-}K^+\bar{K}^0$, the bulk of observed $m(K^+ K_S^0)$ distributions showing low-mass structures in all four channels. In this work, we study the contributions of $ρ(770,1450)^+$, $a_2(1320)^+$ and $a_0(980,1450)^+$ resonances to these decay processes. The intermediate states $ρ(770,1450)^+$ are found to dominate the low-mass distribution of kaon pairs roughly contributing to half of the total branching fraction in each of the four decay channels. The contribution of the tensor $a_2(1320)^+$ meson is found to be negligible. Near the threshold of the kaon pair, the state $a_0(980)^+$ turns out to be much less important than expected, not being able to account for the enhancement of events in that energy region observed in the $B^+\to\bar{D}^{(*)0} K^+\bar{K}^0$ decays. Further studies both from the theoretical and experimental sides are needed to elucidate the role of the non-resonant contributions governing the formation of $K^+\bar{K}^0$ pairs near their threshold in these decay processes.

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Femtoscopy of $D$ mesons and light mesons upon unitarized effective field theories

Hadron femtoscopy has turned into a powerful tool for accessing space-time information of heavy-ion collisions as well as for studying final-state interactions of hadrons. Recently, heavy-flavor femtoscopy has become feasible using the ALICE detector at the LHC. We compute the correlation function of $D$ mesons and light mesons using an off-shell $T$-matrix approach to access the two-meson wave function, and predict the correlation functions involving charged $D^+, D^{*+},D_s^+$ and $D_s^{*+}$ with $π^\pm$ and $K^\pm$. From the obtained results -- all of them accessible in $p+p$ collision experiments -- we point up the case of $D^+ π^-$, which is sensitive to the lower state of the two-pole $D_0^* (2300)$ system. The presence of such poles imprints a depletion on the correlation function, which could potentially be detected in experiments. While preliminary ALICE data do not show evidence of this effect, we suggest to look into the $D_s^+ K^-$ system to explore the higher pole of the $D_0^* (2300)$, as the depletion in the correlation function is more pronounced. Using heavy-quark spin symmetry we also propose exploring the effect of the two poles of the $D_1(2430)$ and predict similar structures in the correlation functions of the $D^{*+} π^-$ and $D_s^{*+} K^-$ pairs.

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First Simultaneous K$^-$p $\rightarrow (Σ^0/Λ) \, π^0$ Cross Sections Measurements at 98 MeV/c

We report the first simultaneous and independent measurements of the K$^{-}$p $\rightarrow Σ^0 \, π^{0}$ and K$^{-}$p $\rightarrow Λ\, π^{0}$ cross sections around 100 MeV/c kaon momentum. The kaon beam delivered by the DA$Φ$NE collider was exploited to detect K$^-$ absorptions on Hydrogen atoms, populating the gas mixture of the KLOE drift chamber. The precision of the measurements ($σ_{K^- p \rightarrow Σ^0 π^0} = 42.8 \pm 1.5 (stat.) ^{+2.4}_{-2.0}(syst.) \ \mathrm{mb}$ and $σ_{K^- p \rightarrow Λπ^0} = 31.0 \pm 0.5 (stat.) ^{+1.2}_{-1.2}(syst.) \ \mathrm{mb}\,$) is the highest yet obtained in the low kaon momentum regime.

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In-medium kinetic theory of $D$ mesons and heavy-flavor transport coefficients

We extend the kinetic theory of $D$ mesons to accommodate thermal and off-shell effects due to the medium modification of the heavy-meson spectral functions. From the Kadanoff-Baym approach we derive the off-shell Fokker-Planck equation which encodes the heavy-flavor transport coefficients. We analyze the thermal width (damping rate) of $D$ mesons due to their scattering off light mesons, focusing on new in-medium effects: off-shell corrections, inelastic channels, and the contribution of the Landau cut. We obtain that the latter effect (absent for vacuum scattering amplitudes) brings sizable corrections at moderate temperatures. We discuss how the heavy-flavor transport coefficients, like the drag and diffusion coefficients, are modified in matter. We find that the $D$-meson spatial diffusion coefficient matches smoothly to the latest results of lattice-QCD calculations and Bayesian analyses at higher temperatures.

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Finite-temperature effects on D-meson properties

We study the spectroscopy and transport properties of charmed mesons in a thermal medium by applying an effective field theory based on chiral and heavy-quark symmetries in the imaginary time formalism. Relying on unitarity constraints and self-consistency we extract the in-medium properties (masses and widths) of $D$ and $D_s$ mesons and their interactions with light hadrons. We report our findings on 1) dynamically generated states, 2) thermal evolution of chiral partners, 3) in-medium scattering amplitudes, and 4) transport coefficients below the chiral restoration temperature.

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Pseudoscalar and vector open-charm mesons at finite temperature

Vacuum and thermal properties of pseudoscalar and vector charm mesons are analyzed within a self-consistent many-body approach, employing a chiral effective field theory that incorporates heavy-quark spin symmetry. Upon unitarization of the vacuum interaction amplitudes for the scattering of charm mesons off light mesons in a fully coupled-channel basis, new dynamically generated states are searched. The imaginary-time formalism is employed to extend the calculation to finite temperatures up to $T=150$ MeV. Medium-modified spectral shapes of the $D$, $D^*$, $D_s$ and $D_s^*$ mesons are provided. The temperature dependence of the masses and decay widths of the nonstrange $D_0^*$ (2300) and $D_1^*$(2430) mesons, both showing a double-pole structure in the complex-energy plane, is also reported, as well as that of the $D_{s0}^*$(2317) and $D_{s1}^*$(2460) resonances and other states not yet identified experimentally. Being the first calculation incorporating open-charm vector mesons at finite temperature in a self-consistent fashion, it brings up the opportunity to discuss the medium effects on the open charm sector under the perspective of chiral and heavy-quark spin symmetries.

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Exotic Hadrons in the $Λ_b \rightarrow J/ψ\ ϕ\ Λ$ Decay

We study the weak decay of the $Λ_b$ baryon into $J/ψ ϕ Λ$, a process that is particularly well suited to analyze the physics of some of the recently observed or theoretically predicted exotic hadrons, as one expects to see their signature in all three final two-body channels. In the $J/ψ\, ϕ$ invariant mass spectrum we study the interplay between the $X(4140)$ and the $X(4160)$ resonances. The $J/ψ\, Λ$ mass spectrum may help to identify the strange partner of the hidden-charm pentaquark recently observed by the LHCb collaboration, the existence of which has been predicted by a chiral unitary approach. We conclude that this strange pentaquark has a good chance of experimental detection if it is present in the range between $4450-4500$ MeV. Finally, in the $ϕ\, Λ$ spectrum we expect a contribution from a dynamically generated resonance at around $2160$ MeV, but with the present model parameters there is little chance for its experimental detection.

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Impact of a thermal medium on $D$ mesons and their chiral partners

We study $D$ and $D_s$ mesons at finite temperature using an effective field theory based on chiral and heavy-quark spin-flavor symmetries within the imaginary-time formalism. Interactions with the light degrees of freedom are unitarized via a Bethe-Salpeter approach, and the $D$ and $D_s$ self-energies are calculated self-consistently. We generate dynamically the $D^*_0(2300)$ and $D_s(2317)$ states, and study their possible identification as the chiral partners of the $D$ and $D_s$ ground states, respectively. We show the evolution of their masses and decay widths as functions of temperature, and provide an analysis of the chiral-symmetry restoration in the heavy-flavor sector below the transition temperature. In particular, we analyse the very special case of the $D$-meson, for which the chiral partner is associated to the double-pole structure of the $D^*_0(2300)$.

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From $\bar{K}N$ Interactions to $\bar{K}$-Nuclear Quasi-Bound States

We review the current status of our study of $K^-$-nuclear interactions and $K^-$-nuclear quasi-bound states. The adopted $K^-$-nuclear optical potential consists of two parts -- the single-nucleon one constructed microscopically from chirally motivated $\bar{K}N$ amplitudes, and a phenomenological multi-nucleon one constrained in fits to kaonic atoms data. The inclusion of multi-nucleon absorption in our calculations of $K^{-}$ quasi-bound states in many-body systems leads to huge widths, considerably exceeding the binding energies. If this feature is confirmed the observation of such states is unlikely. Finally, a development of a new microscopical model for in-medium $K^-NN$ absorption is discussed as well.

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Effects of $ΛΛ- ΞN$ mixing in the decay of $S = -2$ hypernuclei

We study the non-mesonic weak decay of the doubly-strange hypernucleus $^{ΛΛ}_6$He within a model which considers the exchange of pseudoscalar and vector mesons. Special attention is paid into quantifying the strong interaction effects, focussing on the interaction among the two Λ hyperons which induces novel weak transitions, whereby ΛΛ, ΞN and ΣΣ states decay into a hyperon-nucleon pair. The initial strangeness -2 wave function is obtained from the solution of a G-matrix equation with the input of realistic strong baryon-baryon potentials, while the final hyperon-nucleon wave functions are derived analogously from a microscopic T-matrix calculation. The new ΛΛ $\to$ Y N decay rate studied in this work, $Γ_{Λn} + Γ_{Σ^0 n} + Γ_{Σ^- p}$ , represents 3-4% of the total one-baryon induced non-mesonic decay and is remarkably affected by strong interaction effects. In particular, the relative importance of the partial decay rates, encoded in the ratio $Γ_{Λn} / (Γ_{Σ^0 n} + Γ_{Σ^- p})$, gets inverted when the mixing to ΞN states is incorporated in the initial correlated ΛΛ wave function. This sensitivity can be used experimentally to learn about the strong interaction in the strangeness -2 sector.

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A meson-baryon molecular interpretation for some $Ω_c$ excited baryons

We explore the possibility that some of the five narrow $Ω_c$ resonances recently observed at LHCb could correspond to pentaquark states, structured as meson-baryon bound states or molecules. The interaction of the low-lying pseudoscalar mesons with the ground-state baryons in the charm $+1$, strangeness $-2$ and isospin 0 sector is built from t-channel vector meson exchange, using effective Lagrangians. The resulting s-wave coupled-channel unitarized amplitudes show the presence of two structures with similar masses and widths to those of the observed $Ω_c(3050)^0$ and $Ω_c(3090)^0$. The identification of these resonances with the meson-baryon bound states found in this work would also imply assigning the values $1/2^-$ for their spin-parity. An experimental determination of the spin-parity of the $Ω_c(3090)^0$ would help in disentangling its structure, as the quark-based models predict its spin-parity to be either $3/2^-$ or $5/2^-$.

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