arXiv subjects

Juan Nieves

Publications and source records attributed to Juan Nieves.

At least 37 records · Page 2Linked to original sources

arXiv2023

Femtoscopic signatures of the lightest S-wave scalar open-charm mesons

We predict femtoscopy correlation functions for $S$--wave $D_{(s)}ϕ$ pairs of lightest pseudoscalar open charm mesons and Goldstone bosons from next-to-leading order unitarized heavy-meson chiral perturbation theory amplitudes. The effect of the two-state structure around $2300\,\text{MeV}$ can be clearly seen in the $(S,I)=(0,1/2)$ $Dπ$, $Dη$, $D_s \overline{K}$ correlation functions, while in the scalar-strange $(1,0)$ sector, the $D^\ast_{s0}(2317)^{\pm}$ state lying below the $DK$ threshold produces a depletion of the correlation function near threshold. These exotic states owe their existence to the nonperturbative dynamics of Goldstone-boson scattering off $D_{(s)}$. The predicted correlation functions could be experimentally measured and will shed light into the hadron spectrum confirming that it should be viewed as more than a collection of quark model states.

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arXiv2023

Understanding the $0^{++}$ and $2^{++}$ charmonium(-like) states near 3.9 GeV

We propose that the $X(3915)$ observed in the $J/ψ\,ω$ channel is the same state as the $χ_{c2}(3930)$, and the $X(3960)$, observed in the $D_s^+D_s^-$ channel, is an $S$-wave $D_s^+ D_s^-$ hadronic molecule. In addition, the $J^{PC}=0^{++}$ {component in the $B^+\to D^+D^-K^+$} assigned to the $X(3915)$ in the current {\it Review of Particle Physics} has the same origin as the $X(3960)$, which has a mass around 3.94~GeV. To check the proposal, the available data in the $D\bar D$ and $D_s^+ D_s^-$ channels from both $ B$ decays and $γγ$ fusion reaction are analyzed considering both the $D\bar D$-$D_s\bar D_s$-$D^*\bar D^*$-$D_s^*\bar D_s^*$ coupled channels with $0^{++}$ and a $2^{++}$ state introduced additionally. It is found that all the data in different processes can be simultaneously well reproduced, and the coupled-channel dynamics produce four hidden-charm scalar molecular states with masses around 3.73, 3.94, 3.99 and 4.23~GeV, respectively. The results may deepen our understanding of the spectrum of charmonia as well as of the interactions between charmed hadrons.

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arXiv2023

A new look at the $P_{cs}$ states from a molecular perspective

We have a look at the $P_{cs}$ states generated from the interaction of $\bar D^{(*)} Ξ^{(\prime*)}_c$ coupled channels. We consider the blocks of pseudoscalar-baryon $({\frac12}^+, {\frac32}^+)$ and vector-baryon $({\frac12}^+, {\frac32}^+)$, and find $10$ resonant states coupling mostly to $\bar D Ξ_c, \bar D^* Ξ_c,\bar D Ξ'_c, \bar D^* Ξ'_c,\bar D Ξ^*_c$ and $\bar D^* Ξ^*_c$. A novel aspect of the work is the realization that the $\bar D Ξ_c,\bar D_sΛ_c$ or $\bar D^* Ξ_c,\bar D^*_sΛ_c$ channels, with a strong transition potential, collaborate to produce a larger attraction than the corresponding states $\bar D Σ_c,\bar DΛ_c$ or $\bar D^* Σ_c,\bar D^*Λ_c$ appearing in the generation of the strangenessless $P_{c}$ states, since in the latter case the transition potential between those channels is zero. The extra attraction obtained in the $\bar D Ξ_c,\bar D^* Ξ_c$ pairs preclude the association of these channels to the $P_{cs}(4338)$ and $P_{cs}(4459)$ states respectively. Then we find a natural association of the $P_{cs}(4338)$ state coupling mostly to $\bar D^* Ξ_c$ while the $P_{cs}(4459)$ is associated to the state found that couples mostly to $\bar D Ξ'_c$. Four more states appear, like in other molecular pictures, and some of the states are degenerate in spin. Counting different spin states we find $10$ states, which we hope can be observed in the near future.

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arXiv2022

Establishing the heavy quark spin and light flavor molecular multiplets of the $X(3872)$, $Z_c(3900)$ and $X(3960)$

Recently, the LHCb Collaboration reported a near-threshold enhancement, $X(3960)$, in the $D_s^+D_s^-$ invariant mass distribution. We show that the data can be well described by either a bound or a virtual state below the $D_s^+D_s^-$ threshold. The mass given by the pole position is $(3928\pm3)$ MeV. Using this mass and the existing information on the $X(3872)$ and $Z_c(3900)$ resonances, a complete spectrum of the $S$-wave hadronic molecules formed by a pair of ground state charmed and anticharmed mesons is established. Thus, pole positions of the partners of the $X(3872)$, $Z_c(3900)$ and the newly observed $D_s^+D_s^-$ state are predicted. Calculations have been carried out at the leading order of nonrelativistic effective field theory and considering both heavy quark spin and light flavor SU(3) symmetries, though conservative errors from the breaking of these symmetries are provided.

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arXiv2022

New physics effects on $Λ_b\to Λ^*_cτ\barν_τ$ decays

We benefit from a recent lattice determination of the full set of vector, axial and tensor form factors for the $Λ_b\to Λ^*_c(2595)τ\barν_τ$ and $Λ^*_c(2625)τ\barν_τ$ semileptonic decays to study the possible role of these two reactions in lepton flavor universality violation studies. Using an effective theory approach, we analyze different observables that can be accessed through the visible kinematics of the charged particles produced in the tau decay, for which we consider the $π^-ν_τ,ρ^-ν_τ$ and $μ^-\barν_μν_τ$ channels. We compare the results obtained in the Standard Model and other schemes containing new physics (NP) interactions, with either left-handed or right-handed neutrino operators. We find a discriminating power between models similar to the one of the $Λ_b\to Λ_c$ decay, although somewhat hindered in this case by the larger errors of the $Λ_b\toΛ^*_c$ lattice form factors. Notwithstanding this, the analysis of these reactions is already able to discriminate between some of the NP scenarios and its potentiality will certainly improve when more precise form factors are available.

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arXiv2022

Is the $Λ_c(2625)^+$ the heavy quark spin symmetry partner of the $Λ_c(2595)^+$?

We use a $\mathcal{O}(Λ_\text{QCD}/m_c)$ heavy quark effective theory scheme, where only $\mathcal{O}(Λ_\text{QCD}/m_b)$ and perturbative QCD short distance corrections are neglected, to study the matrix elements of the scalar, pseudoscalar, vector, axial-vector and tensor currents between the $Λ_b$ ground state and the odd parity charm $Λ_c(2595)^+$ and $Λ_c(2625)^+$ resonances. We show that in the near-zero recoil regime, the scheme describes reasonably well, taking into account uncertainties, the results for the 24 form-factors obtained in lattice QCD (LQCD) just in terms of only 4 Isgur-Wise (IW) functions. We also find some support for the possibility that the $Λ_c(2595)^+$ and $Λ_c(2625)^+$ resonances might form a heavy-quark spin symmetry (HQSS) doublet. However, we argue that the available LQCD description of these two resonances is not accurate enough to disentangle the possible effects of the $Σ_c π$ and $Σ_c^*π$ thresholds, located only a few MeV above their position, and that it cannot be ruled out that these states are not HQSS partners. Finally, we study the ratio $\frac{dΓ[Λ_b\to Λ_{c,1/2^-}^* \ell \barν_\ell]/dq^2}{dΓ[Λ_b\to Λ_{c,3/2^-}^*\ell \barν_\ell]/dq^2}$ of the Standard Model differential semileptonic decay widths, with $q^μ$ the momentum transferred between the initial and final hadrons. We provide a natural explanation for the existence of large deviations, near the zero recoil, of this ratio from 1/2 (value predicted in the infinite heavy quark mass limit, assuming that the $Λ_{c,1/2^-}^*$ and $Λ_{c,3/2^-}^*$ are the two members of a HQSS doublet) based on S-wave contributions to the $Λ_b\to Λ_{c,1/2^-}^*$ decay amplitude driven by a sub-leading IW function.

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arXiv2022

A combined analysis of the $Z_c(3900)$ and the $Z_{cs}(3985)$ exotic states

We have performed a combined analysis of the BESIII data for both the $Z_c(3900)$ and $Z_{cs}(3985)$ structures, assuming that the latter is an SU(3) flavor partner of the former one. We have improved on the previous analysis of Albaladejo $et$ $al.$ [Phys. Lett. B 755, 337 (2016)] by computing the amplitude for the $D_1\bar{D}D^*$ triangle diagram considering both $D$ and $S$-wave $D_1D^*π$ couplings. We have also investigated effects from SU(3) light-flavor violations, which are found to be moderate and of the order of 20%. The successful reproduction of the BESIII spectra, in both the hidden-charm and hidden-charm strange sectors, strongly supports that the $Z_{cs}(3985)$ and $Z_c(3900)$ are SU(3) flavor partners placed in the same octet multiplet. The best results are obtained when an energy-dependent term in the diagonal $D^{(*)}\bar D_{(s)}^{(*)}$ interaction is included, leading to resonances (poles above the thresholds) to describe these exotic states. We have also made predictions for the isovector $Z_{c}^*$ and isodoublet $Z_{cs}^*$, $D^*\bar{D}^*$ and $D^*\bar{D}_{s}^*$ molecules, with $J^{PC}=1^{+-}$ and $J^{P}=1^{+}$, respectively. These states would be heavy-quark spin symmetry partners of the $Z_{c}$ and $Z_{cs}$. Besides the determination of the masses and widths of the $Z_c(3900)$ and $Z_{cs}(3985)$, we also predict those of the $Z_c^*$ and $Z_{cs}^*$ resonances.

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arXiv2022

Effective range expansion for narrow near-threshold resonances

We discuss some general features of the effective range expansion, the content of its parameters with respect to the nature of the pertinent near-threshold states and the necessary modifications in the presence of coupled channels, isospin violations and unstable constituents. As illustrative examples, we analyse the properties of the $χ_{c1}(3872)$ and $T_{cc}^+$ states supporting the claim that these exotic states have a predominantly molecular nature.

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arXiv2022

Coupled-channel approach to $T_{cc}^+$ including three-body effects

A coupled-channel approach is applied to the charged tetraquark state $T_{cc}^+$ recently discovered by the LHCb Collaboration. The parameters of the interaction are fixed by a fit to the observed line shape in the three-body $D^0D^0π^+$ channel. Special attention is paid to the three-body dynamics in the $T_{cc}^+$ due to the finite life time of the $D^*$. An approach to the $T_{cc}^+$ is argued to be self-consistent only if both manifestations of the three-body dynamics, the pion exchange between the $D$ and $D^*$ mesons and the finite $D^*$ width, are taken into account simultaneously to ensure that three-body unitarity is preserved. This is especially important to precisely extract the pole position in the complex energy plane whose imaginary part is very sensitive to the details of the coupled-channel scheme employed. The $D^0D^0$ and $D^0D^+$ invariant mass distributions, predicted based on this analysis, are in good agreement with the LHCb data. The low-energy expansion of the $D^*D$ scattering amplitude is performed and the low-energy constants (the scattering length and effective range) are extracted. The compositeness parameter of the $T_{cc}^+$ is found to be close to unity, which implies that the $T_{cc}^+$ is a hadronic molecule generated by the interactions in the $D^{*+}D^0$ and $D^{*0}D^+$ channels. Employing heavy-quark spin symmetry, an isoscalar $D^*D^*$ molecular partner of the $T_{cc}^+$ with $J^P=1^+$ is predicted under the assumption that the $ DD^*$-$D^*D^*$ coupled-channel effects can be neglected.

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arXiv2021

The negative-parity spin-1/2 $Λ$ baryon spectrum from lattice QCD and effective theory

The spectrum of the negative-parity spin-1/2 $Λ$ baryons is studied using lattice QCD and hadronic effective theory in a unitarized coupled-channel framework. A direct comparison between the two approaches is possible by considering the hadronic effective theory in a finite volume and with hadron masses and mesonic decay constants that correspond to the situation studied on the lattice. Comparing the energy level spectrum and $SU(3)$ flavor decompositions of the individual states, it is found that the lowest two states extracted from lattice QCD can be identified with one of the two $Λ(1405)$-poles and the $Λ(1670)$ resonance. The quark mass dependences of these two lattice QCD levels are in good agreement with their effective theory counterparts. However, as current lattice QCD studies still rely on three-quark operators to generate the physical states, clear signals corresponding to the meson-baryon scattering states, that appear in the finite volume effective theory calculation, are not yet seen.

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arXiv2021

Strange molecular partners of the $Z_c(3900)$ and $Z_c(4020)$

Quantum Chromodynamics presents a series of exact and approximate symmetries which can be exploited to predict new hadrons from previously known ones. The $Z_c(3900)$ and $Z_c(4020)$ resonances, which have been theorized to be isovector $D^* \bar{D}$ and $D^* \bar{D}^*$ molecules [$I^G(J^{PC}) = 1^-(1^{+-})$], are no exception. Here we argue that from SU(3)-flavor symmetry, we should expect the existence of strange partners of the $Z_c$'s with hadronic molecular configurations $D^* \bar{D}_s$-$D \bar{D}_s^*$ and $D^* \bar{D}^{*}_s$ (or, equivalently, quark content $c\bar{c} s \bar{q}$, with $q = u, d$). The quantum numbers of these $Z_{cs}$ and $Z_{cs}^*$ resonances would be $I(J^P)$ = $\frac{1}{2}(1^+)$. The predicted masses of these partners depend on the details of the theoretical scheme used, but they should be around the $D^* \bar{D}_s$-$D \bar{D}_s^*$ and $D^* \bar{D}^{*}_s$ thresholds, respectively. Moreover, any of these states could be either a virtual pole or a resonance. We show that, together with a possible triangle singularity contribution, such a picture nicely agrees with the very recent BESIII data of the $e^+e^-\to K^+(D_s^-D^{*0}+D_s^{*-}D^0)$.

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arXiv2021

Inclusive and exclusive neutrino-nucleus cross sections and the reconstruction of the interaction kinematics

We present a full kinematic analysis of neutrino-nucleus charged current quasielastic interactions based on the Local Fermi Gas model and the Random PhaseApproximation. The model was implemented in the NEUT Monte Carlo framework, which allows us to investigate potentially measurable observables, including hadron distributions. We compare the predictions simultaneously to the most recent T2K and MINERvA charged current (CC) inclusive, CC0$π$ and transverse kinematic-imbalance variable results. We pursuit a microscopic interpretation of the relevant reaction mechanisms, with the aim of achieving in neutrino oscillation experiments a correct reconstruction of the incoming neutrino kinematics, free of conceptual biasses. Such study is of the utmost importance for the ambitious experimental program which is underway to precisely determine neutrino properties, test the three-generation paradigm, establish the order of mass eigenstates and investigate leptonic CP violation.

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arXiv2020

New parametrization of the form factors in $\bar{B}\to D\ell\barν_\ell$ decays

A new model-independent parametrization is proposed for the hadronic form factors in the semileptonic $\bar{B}\to D\ell\barν_\ell$ decay. By a combined consideration of the recent experimental and lattice QCD data, we determine precisely the Cabibbo-Kobayashi-Maskawa matrix element $|V_{cb}|=41.01(75)\times 10^{-3}$ and the ratio $\mathcal{R}_D=\frac{\mathcal{BR}(\bar{B}\to D τ\barν_τ)}{\mathcal{BR}(\bar{B}\to D \ell \barν_\ell)}=0.301(5)$. The coefficients in this parametrization, related to phase shifts by sumrulelike dispersion relations and hence called phase moments, encode important scattering information of the $\bar{B}\bar{D}$ interactions which are poorly known so far. Thus, we give strong hints about the existence of at least one bound and one virtual $\bar B \bar D$ $S$-wave $0^+$ states, subject to uncertainties produced by potentially sizable inelastic effects. This formalism is also applicable for any other semileptonic processes induced by the weak $b\to c$ transition.

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arXiv2019

Further tests of lepton flavour universality from the charged lepton energy distribution in $b\to c$ semileptonic decays: The case of $Λ_b\to Λ_c \ell \barν_\ell$

In a general framework, valid for any $H\to H'\ell^-\barν_\ell$ semileptonic decay, we analyze the $d^2Γ/(dωd\cosθ_\ell)$ and $d^2Γ/ (dωdE_\ell)$ distributions, with $ω$ being the product of the hadron four-velocities, $θ_\ell$ the angle made by the three-momenta of the charged lepton and the final hadron in the $W^-$ center of mass frame and $E_\ell$ the charged lepton energy in the decaying hadron rest frame. Within the Standard Model (SM), $ d^2Γ/(dωdE_\ell)\propto \left(c_0(ω)+c_1(ω)E_\ell/M+c_2(ω)E^2_\ell/M^2\right)$, with $M$ the initial hadron mass. We find that $c_2(ω)$ is independent of the lepton flavor and thus it is an ideal candidate to look for lepton flavor universality (LFU) violations. We also find a correlation between the $a_2(ω)$ structure function, that governs the $(\cosθ_\ell)^2$ dependence of $d^2Γ/(dωd\cosθ_\ell)$, and $c_2(ω)$. Apart from trivial kinematical and mass factors, the ratio of $a_2(ω)/c_2(ω)$ is a universal function that can be measured in any semileptonic decay, involving not only $b\to c$ transitions. These two SM predictions can be used as new tests in the present search for signatures of LFU violations. We also generalize the formalism to account for some new physics (NP) terms. Finally, in order to illustrate our findings, we apply our general framework to the $Λ_b\to Λ_c \ell \barν_\ell$ decay. We show that a measurement of $c_2$ (or $a_2$) for $τ$ decay would not only be a direct measurement of the possible existence of NP, but it would also allow to distinguish from NP fits to $b\to cτ\barν_τ$ anomalies in the meson sector, that otherwise give the same total and differential $dΓ/dω$ widths.

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arXiv2019

On the nature of the lowest-lying odd parity charmed baryon $Λ_c(2595)$ and $Λ_c(2625)$ resonances

We study the structure of the $Λ_c(2595)$ and $Λ_c(2625)$ resonances in the framework of an effective field theory consistent with heavy quark spin and chiral symmetries, that incorporates the interplay between $Σ_c^{(*)}π-ND^{(*)}$ baryon-meson degrees of freedom and bare P-wave $c\bar ud$ quark-model states. We show that these two resonances are not HQSS partners. The $J^P= 3/2^-$ $Λ_c(2625)$ should be viewed mostly as a dressed three quark state, whose origin is determined by a bare state, predicted to lie very close to the mass of the resonance. The $J^P= 1/2^-$ $Λ_c(2595)$ seems to have, however, a predominant molecular structure. This is because, it is either the result of the chiral $Σ_cπ$ interaction, which threshold is located much more closer than the mass of the bare three-quark state, or because the light degrees of freedom in its inner structure are coupled to the unnatural $0^-$ quantum-numbers. We show that both situations can occur depending on the renormalization procedure used. We find some additional states, but the classification of the spectrum in terms of HQSS is difficult, despite having used interactions that respect this symmetry. This is because the bare quark-model state and the $Σ_cπ$ threshold are located extraordinarily close to the $Λ_c(2625)$ and $Λ_c(2595)$, respectively, and hence they play totally different roles in each sector.

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arXiv2018

$Ω_c$ excited states: a molecular approach with heavy-quark spin symmetry

The LHCb Collaboration has recently discovered five excited $Ω_c$ states with masses between 3 and 3.1 GeV, four of them corroborated by the Belle Collaboration. We analyse the dynamical generation of these states within a molecular baryon-meson model that is consistent with both chiral and heavy-quark spin symmetries. Earlier predictions within this model found five $Ω_c$ states with masses below 3 GeV. Thus, in order to study the possible identification of any of these states with the experimental ones in the correct energy region, we explore two different regularization schemes, that is, a modified regularization subtraction method and a cutoff regularization scheme. We find that at least three of the dynamically generated states can be identified with the experimental ones and have spin-parity $J=1/2^-$ or $J=3/2^-$

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arXiv2018

$Ω_c$ excited states with heavy-quark spin symmetry

We study the $C=1$, $S=-2$, $I=0$ sector, where five excited $Ω_c$ states have been recently observed by the LHCb Collaboration. We start from a recently developed unitarized baryon-meson model that takes, as bare baryon-meson interaction, an extended Weinberg-Tomozawa kernel consistent with both chiral and heavy-quark spin symmetries. This ${\rm SU(6)} \times {\rm HQSS}$ scheme leads to a successful description of the observed lowest-lying odd parity charmed $Λ_c$(2595) and $Λ_c$(2625) states, and bottomed $Λ_b$(5912) and $Λ_b$(5920) resonances. Within this model, five odd-parity $Ω_c$ states are dynamically generated, but with masses below 3 GeV, not allowing for an identification with the observed LHCb resonances. We revise this model and explore two different scenarios for the renormalization scheme, that is, using a modified common energy scale to perform the subtractions or utilizing a common ultraviolet cutoff to render finite the ultraviolet divergent loop functions in all channels. In both cases, we show that some (at least three) of the dynamically generated states can be identified with the experimental $Ω_c$, while having odd parity and $J=1/2$ or $J=3/2$. Two of these states turn out to be part of the same ${\rm SU(6)} \times {\rm HQSS}$ multiplets as the charmed and bottomed $Λ$ baryons.

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arXiv2018

Towards a new paradigm for heavy-light meson spectroscopy

Since 2003 many new hadrons, including the lowest-lying positive-parity charm-strange mesons ${D_{s0}^*(2317)}$ and ${D_{s1}(2460)}$, were observed that do not conform with quark model expectations. It was recently demonstrated that various puzzles in the charm meson spectrum find a natural resolution, if the SU(3) multiplets for the lightest scalar and axial-vector states, amongst them the ${D_{s0}^*(2317)}$ and the ${D_{s1}(2460)}$, owe their existence to the nonperturbative dynamics of Goldstone-Boson scattering off $D_{(s)}$ and $D^*_{(s)}$ mesons. Most importantly the ordering of the lightest strange and nonstrange scalars becomes natural. In this work we demonstrate for the first time that this mechanism is strongly supported by the recent high quality data on the ${B^-\to D^+π^-π^- }$ provided by the LHCb experiment. This implies that the lowest quark-model positive-parity charm mesons, together with their bottom counterparts, if realized in nature, do not form the ground-state multiplet. This is similar to the pattern that has been established for the scalar mesons made from light up, down and strange quarks, where the lowest multiplet is considered to be made of states not described by the quark model. In a broader view, the hadron spectrum must be viewed as more than a collection of quark model states.

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