arXiv subjects

F. Fernández

Publications and source records attributed to F. Fernández.

At least 19 recordsLinked to original sources

arXiv2025

The Constituent Quark Model

In this chapter we give a pedagogical introduction to the constituent quark model. The explanation of magnetic moments of the nucleons was crucial to introduce an effective quark mass for light quarks that nowadays are understood as an effect of Spontaneous Chiral Symmetry Breaking in QCD. We give an overview of the first applications of the model and an introduction to the most modern developments studying states beyond the naive quark model as tetraquarks and pentaquarks.

hep-ph↗

arXiv2024

An assessment of $\mathbfΥ$-states above $\mathbf{B\bar B}$-threshold using a constituent-quark-model based meson-meson coupled-channels framework

The $Υ(10753)$ state has been recently observed by the Belle and Belle~II collaborations with enough global significance to motivate an assessment of the high-energy spectrum usually predicted by any reasonable \emph{naïve} quark model. In the framework of a constituent quark model which satisfactorily describes a wide range of properties of conventional hadrons containing heavy quarks, the quark-antiquark and meson-meson degrees of freedom have been incorporated with the goal of elucidating the influence of open-bottom meson-meson thresholds into the $Υ$ states whose masses are within the energy range of the $Υ(10753)$'s mass. It is well known that such effects could be relevant enough as to generate dynamically new states and thus provide a plausible explanation of the nature of the $Υ(10753)$ state. In particular, we have performed a coupled-channels calculation in which the bare states $Υ(4S)$, $Υ(3D)$, $Υ(5S)$ and $Υ(4D)$ are considered together with the threshold channels $B\bar{B}$, $B\bar{B}^\ast$, $B^\ast \bar{B}^\ast$, $B_s\bar{B}_s$, $B_s\bar{B}_s^\ast$ and $B_s^\ast \bar{B}_s^\ast$. Among the results we have described, the following conclusions are of particular interest: (i) a richer complex spectrum is gained when thresholds are present and bare bound states are sufficiently non-relativistic; (ii) those poles obtained in the complex energy plane do not have to appear as simple peaks in the relevant cross sections; and (iii) the $Υ(10750)$ candidate is interpreted as a dressed hadronic resonance whose structure is an equally mixture of a conventional $b\bar b$ state and $B^\ast \bar B^\ast$ molecule.

hep-ph↗

arXiv2022

Kaon spectrum revisited

The European Organization for Nuclear Research (CERN) has recently approved a world-unique QCD facility in which an updated version of the external M2 beam line of the CERN SPS in conjunction with a universal spectrometer of the COMPASS experiment is used. One of its main goals is to use highly intense and energetic kaon beams to map out the complete spectrum of excited kaons with an unprecedented precision; having a broad impact not only on low-energy QCD phenomenology, but also on many high-energy particle processes where excited kaons appear, such as the study of CP violation in heavy-meson decays studied at LHCb and Belle~II. In support of the experimental effort, the kaon spectrum is computed herein using a constituent quark model which has been successfully applied to a wide range of hadronic observables, from light to heavy quark sectors, and thus the model parameters are completely constrained. The model's prediction can be used as a template against which to compare the already collected data and future experimental findings, in order to distinguish between conventional and exotic kaon states. We also compare our results with those available in the literature in order to provide some general statements, common to all calculations.

hep-ph↗

arXiv2022

Towards the discovery of novel $B_c$ states: radiative and hadronic transitions

The properties of the $B_c$-meson family ($c\bar b$) are still not well determined experimentally because the specific mechanisms of formation and decay remain poorly understood. Unlike heavy quarkonia, i.e. the hidden heavy quark-antiquark sectors of charmonium ($c\bar c$) and bottomonium ($b\bar b$), the $B_c$-mesons cannot annihilate into gluons and they are, consequently, more stable. The excited $B_c$ states, lying below the lowest strong-decay $BD$-threshold, can only undergo through radiative decays and hadronic transitions to the $B_c$ ground state, which then decays weakly. As a result of this, a rich spectrum of narrow excited states below the $BD$-threshold appear, whose total widths are two orders of magnitude smaller than those of the excited levels of charmonium and bottomonium. In a different article, we determined bottom-charmed meson masses using a non-relativistic constituent quark model which has been applied to a wide range of hadron physical observables, and thus the model parameters are completely constrained. Herein, continuing to our study of the $B_c$ sector, we calculate the relevant radiative decay widths and hadronic transition rates between $c\bar b$ states which are below $BD$-threshold. This shall provide the most promising signals for discovering excited $B_c$ states that are below the lowest strong-decay $BD$-threshold. Finally, our results are compared with other models to measure the reliability of the predictions and point out differences.

hep-ph↗

arXiv2020

Weak B Decays into Orbitally Excited Charmed Mesons

The BaBar Collaboration has recently reported branching fractions for semileptonic decays of the $B$ meson into final states with charged and neutral $D_{1}(2420)$ and $D_{2}^{\ast}(2460)$, two narrow orbitally excited charmed mesons. We evaluate these branching fractions within the framework of a constituent quark model in two steps, one which involves a semileptonic decay and the other one mediated by a strong process. Our results are in agreement with the experimental data.

hep-ph↗

arXiv2020

Microscopic Model of Charmonium Strong Decays

Although the spectra of heavy quarkonium systems has been successfully explained by certain QCD motivated potential models, their strong decays are difficult to deal with. We perform a microscopic calculation of charmonium strong decays using the same constituent quark model which successfully describes the $c\bar{c}$ meson spectrum. We compare the numerical results with the $^{3}P_{0}$ and the experimental data. Comparison with other predictions from similar models are included.

hep-ph↗

arXiv2018

Ultrafast insulator-to-metal transition in VO$_2$ nanostructures assisted by picosecond strain pulses

Strain engineering is a powerful technology which exploits stationary external or internal stress of specific spatial distribution for controlling the fundamental properties of condensed materials and nanostructures. This advanced technique modulates in space the carrier density and mobility, the optical absorption and, in strongly correlated systems, the phase, e.g. insulator/metal or ferromagnetic/paramagnetic. However, while successfully accessing nanometer length scale, strain engineering is yet to be brought down to ultrafast time scales allowing strain-assisted control of state of matter at THz frequencies. In our work we demonstrate a control of an optically-driven insulator-to-metal phase transition by a picosecond strain pulse, which paves a way to ultrafast strain engineering in nanostructures with phase transitions. This is realized by simultaneous excitation of VO$_2$ nanohillocks by a 170-fs laser and picosecond strain pulses finely timed with each other. By monitoring the transient optical reflectivity of the VO$_2$, we show that strain pulses, depending on the sign of the strain at the moment of optical excitation, increase or decrease the fraction of VO$_2$ which undergoes an ultrafast phase transition. Transient strain of moderate amplitude $\sim0.1$% applied during ultrafast photo-induced non-thermal transition changes the fraction of VO$_2$ in the laser-induced phase by $\sim1$%. By contrast, if applied after the photo-excitation when the phase transformations of the material are governed by thermal processes, transient strain of the same amplitude produces no measurable effect on the phase state.

cond-mat.str-el↗

arXiv2018

Heavy mesons in the Quark Model

Since the discovery of the $J/ψ$, the quark model was very successful in describing the spectrum and properties of heavy mesons including only $q\bar q$ components. However since 2003, with the discovery of the $X(3872)$, many states that can not be accommodated on the naive quark model have been discovered, and they made unavoidable to include higher Fock components on the heavy meson states. We will give an overview of the success of the quark model for heavy mesons and point some of the states that are likely to be more complicated structures such as meson-meson molecules.

hep-ph↗

arXiv2016

From $J/ψ$ to LHCb pentaquark

The original charmonium two-body problem, like the $c\bar c$ structure of the $J/ψ$ meson, has become more involved in the last few years with the discovery of new resonances such as the tentative molecular state $X(3872)$ or the possible pentaquark one $P_{c}(4380)^{+}$. We discuss herein how these exotic states (and others) can be described in a unified way adding higher Fock state components to the naive quark model picture. In particular, we present our theoretical results on the pentaquark states $P_{c}(4380)^{+}$ and $P_{c}(4450)^{+}$, and on the new charmonium-like resonances $X(4140)$, $X(4274)$, $X(4500)$ and $X(4700)$ that have been reported in $2016$ by the LHCb Collaboration.

hep-ph↗

arXiv2016

From $J/ψ$ to LHCb pentaquarks

The two exotic $P_c^+(4380)$ and $P_c^+(4450)$ discovered in $2015$ by the LHCb Collaboration, together with the four resonances $X(4140)$, $X(4274)$, $X(4500)$ and $X(4700)$, reported in $2016$ by the same collaboration, are described in a constituent quark model which has been able to explain the properties of charmonium states from the $J/ψ$ to the $X(3872)$. Using this model we found a $\bar DΣ_c^*$ bound state with $J^P=\frac{3}{2}^-$ that may be identified with the $P_c^+(4380)$. In the $\bar D^*Σ_c$ channel we found three possible candidates for the $P_c^+(4450)$ with $J^P=\frac{1}{2}^-$, $\frac{3}{2}^-$ and $\frac{3}{2}^+$ with almost degenerated energies. The $X(4140)$ resonance appears as a cusp in the $J/ψϕ$ channel due to the near coincidence of the $D_{s}^{\pm}D_{s}^{\ast\pm}$ and $J/ψϕ$ mass thresholds. The remaining three $X(4274)$, $X(4500)$ and $X(4700)$ resonances appear as conventional charmonium states with quantum numbers $3^{3}P_{1}$, $4^{3}P_{0}$ and $5^{3}P_{0}$, respectively; and whose masses and widths are slightly modified due to their coupling with the corresponding closest meson-meson thresholds.

hep-ph↗

arXiv2016

LHCb pentaquarks in constituent quark models

The recently discovered $P_c(4380)^+$ and $P_c(4450)^+$ states at LHCb have masses close to the $\bar DΣ_c^*$ and $\bar D^*Σ_c$ thresholds, respectively, which suggest that they may have significant meson-baryon molecular components. We analyze these states in the framework of a constituent quark model which has been applied to a wide range of hadronic observables, being the model parameters, therefore, completely constrained. The $P_c(4380)^+$ and $P_c(4450)^+$ are studied as molecular states composed by charmed baryons and open charm mesons. Several bound states with the proper binding energy are found in the $\bar DΣ_c^*$ and $\bar D^*Σ_c$ channels. We discuss the possible assignments of these states from their decay widths. Moreover, two more states are predicted, associated with the $\bar DΣ_c$ and $\bar D^* Σ_c^*$ thresholds.

hep-ph↗

arXiv2013

B decays into radially excited charmed mesons

It has been recently argued that some longstanding problems in semileptonic B decays can be solved provided the branching ratio for the $B\to D^{\prime(\ast)}$ semileptonic decays are large enough. We have studied these decays in a constituent quark model which has been successful in describing semileptonic and non-leptonic B decays into orbitally excited charmed mesons. Our results do not confirm the hypothesis of large branching ratios for the $B\to D^{\prime(\ast)}$ semileptonic decays. In addition, we calculate the non-leptonic $B\to D'π$ decays which can provide an independent test of the form factors involved in the $B\to D^{\prime(\ast)}$ reactions.

hep-ph↗

arXiv2012

Quark model description of the Λ_c(2940)^+ as a molecular D^*N state and the possible existence of the Λ_b(6248)

The Λ_c(2940)^+ baryon is studied in a constituent quark model as a molecular state composed by nucleons and D^* mesons. A bound state with the right binding energy is found for the J^π=3/2^- channel. The partial widths Λ_c(2940)^+\to ND and Λ_c(2940)^+\to Σ_c πare calculated and the results are consistent with the experimental data. Additionally a bottom partner \bar B^*N is predicted with a mass of 6248 MeV/c^2.

hep-ph↗

arXiv2012

Renormalized Quarkonium

We extend our previous study of the vector charmonium states within a renormalization approach with boundary conditions to the full spectrum of charmonium and bottomonium. On the light of the predicted spectrum we comply to assignments suggested in the literature. A comparison with the regularized quark model is also included.

hep-ph↗

arXiv2011

Semileptonic $B$ and $B_{s}$ decays into orbitally excited charmed mesons

The BaBar Collaboration has recently reported products of branching fractions that include $B$ meson semileptonic decays into final states with charged and neutral $D_{1}(2420)$ and $D_{2}^{\ast}(2460)$, two narrow orbitally excited charmed mesons. We evaluate these branching fractions, together with those concerning $D_{0}^{\ast}(2400)$ and $D_{1}'(2430)$ mesons, within the framework of a constituent quark model. The calculation is performed in two steps, one of which involves a semileptonic decay and the other is mediated by a strong process. Our results are in agreement with the experimental data. We also extend the study to semileptonic decays of $B_{s}$ into orbitally excited charmed-strange mesons, providing predictions to the possible measurements to be carried out at LHC.

hep-ph↗

arXiv2010

Emission patterns of neutral pions in 40 A MeV Ta+Au reactions

Differential cross sections of neutral pions emitted in 181Ta + 197Au collisions at a beam energy of 39.5A MeV have been measured with the photon spectrometer TAPS. The kinetic energy and transverse momentum spectra of neutral pions cannot be properly described in the framework of the thermal model, nor when the reabsorption of pions is accounted for in a phenomenological model. However, high energy and high momentum tails of the pion spectra can be well fitted through thermal distributions with unexpectedly soft temperature parameters below 10 MeV.

nucl-ex↗

arXiv2010

The X(3872) and other possible $XYZ$ molecular states

We perform a coupled channel calculation of the $DD^*$ and $c\bar c$ sectors in the framework of a constituent quark model. The interaction for the $DD^*$ states is obtained using the Resonant Group Method (RGM) and the underlying quark interaction model. The coupling with the two quark system is performed using the $^3 P_0$ model. The X(3872) is found as a molecular state with a sizable $c\bar c$ component. A comparison with Belle and BaBar data has been done, finding a good agreement. Other possible molecular molecular states are discussed.

hep-ph↗

arXiv2008

Charm spectroscopy beyond the constituent quark model

We study the nature of the new charm and charmonium like states observed in the last few years, using a constituent quark model with an screened confinement potential. In the open charm sector we discuss different options for the $D_{sJ}(2860)$ including tetraquark configurations. In the hidden charm sector the new Y(4360) and Y(4660) are interpreted as $1^{--}$ states. The influence in the puzzling 3940 MeV region of multiquark configuration is discussed.

hep-ph↗