Searcharxiv⌕ Search

arXiv subjects

Susana Coito

Publications and source records attributed to Susana Coito.

At least 19 recordsLinked to original sources
arXiv2020

On the origin of the $Y(4260)$

We study the relation between the $ψ(4160)$ and the $Y(4260)$ within an unitarized effective Lagrangian approach. The $Y(4260)$ arises as a manifestation of the $ψ(4160)$, when a loop-driven decay of the type $ψ(4160)\to D_s^*\bar{D}_s^*\to J/ψf_0(980)$ is enhanced by the proximity of the pole, corresponding to the $ψ(4160)$, to the \emph{almost} closed $D_s^*\bar{D}_s^*$ decay channel. Other $f_0$ resonances that may add a non-negligible contribution, by the same mechanism, are not included for simplicity, but they are not expected to change the main conclusion. Within this picture, the $Y(4260)$ is not, therefore, an independent resonance, but rather a variation of the $ψ(4160)$, which also explains why it is not seen in OZI-allowed decay channels in the experiment.

hep-ph↗
arXiv2019

$X(3872)$ as virtual companion pole of the charm-anticharm state $χ_{c1}(2P)$

We study the spectral function of the axial-vector charmonium state $χ_{c1}(2P)$ coupled to $DD^{\ast}$ mesons, by employing a quantum field theoretical approach: a pronounced enhancement close to the $D^{0}% D^{\ast0}$ threshold, to be identified with the $X(3872)$, emerges. In the complex plane we find two poles: a pole for the broad seed state $χ_{c1}(2P)$, and -- in the easiest scenario -- a virtual pole for the $X(3872)$. Thus, our approach describes both the seed state and the dynamically generated $X(3872)$ simultaneously. In particular, it explains the most prominent, both molecular-like and quarkonium-like, features of the $X(3872)$: its very small width (the decay into $D^{0}D^{\ast0}$ is predicted to be about 0.5 MeV), the enhanced radiative decay into $ψ(2S)γ$ w.r.t. $ψ(1S)γ$, and the isospin breaking decay into $J/ψρ$ (thanks to $DD^{\ast}$ loops mediating this decay channel). At the same time, we aim to determine the pole position and the properties of the charmonium seed state: quite interestingly, even if a pole is always present, it is possible that there is no peak corresponding to this state in the spectral function, thus potentially explaining why the corresponding resonance could not yet be seen in experiments.

hep-ph↗
arXiv2018

Radially excited $ψ$ mesons and the $Y$ enhancements

While many properties of the vector charmonium first excitations are yet to be measured, enhancements at unexpected energies are intriguing, alias the $Y$ states. In order to understand the naturally unquenched mesonic line-shapes, the influence of the most relevant hadronic decay channels must be taken into account. Within an unitary effective approach we present results where mesonic loops are included in an equivalent manner to coupled-channels. We show results for the $ψ(3770)$ and $ψ(4160)$ systems, where we find the nonperturbative effects of dynamical generation of poles and line-shape distortion.

hep-ph↗
arXiv2018

The $Y(4260)$ and $Y(4360)$ enhancements within coupled-channels

Puzzling structures have been observed in the charmonium energy region, namely the $Y(4260)$ and the $Y(4360)$, that cannot be easily accommodated within quark model frameworks. The proximity of nearby dominant hadronic thresholds suggests that they play an important role in the formation of the enhancements. We present results of an unitarized effective Lagrangian model, where mesonic loops, equivalent to coupled-channels, and charmonium vectors $ψ$ interplay to generate line-shapes and poles.

hep-ph↗
arXiv2018

Line-shape and poles of the $ψ(3770)$

We study the non-Breit-Wigner line-shape of the $ψ(3770)$ resonance, predominantly a $1\ ^{3}D_{1}$ $\bar{c}c$ state, using an unitarized effective Lagrangian approach, including the one-loop effects of the nearby thresholds $D^+D^-$ and $D^0\bar{D}^0$. A fit of the theoretical result to the total cross-section $e^{+}e^{-}\rightarrow D\bar{D}$ is performed, leading to a good description of data ($χ^{2}/d.o.f.\sim 1.03$). The partial cross sections $e^{+}e^{-}\rightarrow D^0\bar{D}^0$ and $e^{+}e^{-}\rightarrow D^+D^-$ turn out to be separately in good agreement with the experiment. We find a pole at $3777-i12$ MeV, that is within the Particle Data Group (PDG) mass and width estimation for this state. Quite remarkably, we find an additional, dynamically generated, companion pole at $3741-i19$ MeV, which is responsible for the deformation on the lower energy side of the line-shape. The width for the leptonic decay $ψ(3770)\rightarrow e^{+}e^{-}$ is 112 eV, hence smaller than the PDG fit of $262\pm 18$ eV, yet in agreement with a recent experimental study.

hep-ph↗
arXiv2018

Formation and Deformation of the $ψ(3770)$

The form of resonance line-shapes unveils information about its nonperturbative properties and formation mechanisms. Here, we study the non-Breit-Wigner energy distribution of the resonance $ψ(3770)$ using an unitarized effective Lagrangian approach, that includes the effect of the nearby threshold $D^{+}D^{-}$. Two poles are found in the second Riemann sheet near the resonance amplitude. We discuss the setting of the free parameters and possible effects contributing to the signal.

hep-ph↗
arXiv2017

Line-shape analysis of charmonium resonances

We discuss weather the new enhancements found by BES, alias the $Y(4220)$, $Y(4260)$, $Y(4360)$, and $Y(4390)$ are true resonances. We argue that the nearby thresholds $D_s^*\bar{D}_s^*$, $D\bar{D}_1+\bar{D}D_1$, $D_s\bar{D}_{s1}+\bar{D_s}D_{s1}$ and $D^*\bar{D}_1+\bar{D}^*D_1$, as well as the $ψ(4160)$ and $ψ(4415)$ states have a strong influence over the observed $ J/ψπ^+π^-$ and $h_c π^+π^-$ line-shapes. We propose an unitarized effective Lagrangian model to study the dynamical effect of the interaction between each known $ψ$ state and its closest thresholds. In addition, we present some of our recent motivating results, using the same model, for the $ψ(3770)$ resonance, where the distortion from a Breit-Wigner line-shape is shown to result not only from the kinematic interference, but also from the influence of the $D^0\bar{D}^0+D^+D^-$ one-loops. Moreover, two poles were found, at about 3.78 GeV and at 3.74 GeV, the second one generated dynamically, yet contributing to the distortion of the line-shape.

hep-ph↗
arXiv2016

Radially excited axial mesons and the enigmatic $Z_c$ and $Z_b$ in a coupled-channel model

The enigmatic charged states $Z_c(3900)$, $Z_c(4020)$, $Z_c(4050)$, $Z_b(10610)$, and $Z_b(10650)$ are studied within a coupled-channel Schrödinger model, where radially excited quark-antiquark pairs, with the same angular momenta and isospin as the $a_1(1260)$ and $b_1(1235)$, are strongly coupled to their Okubo-Zweig-Iizuka - allowed decay channels $D\bar{D}^*+\bar{D}D^*$ and $D^*\bar{D}^*$, or $B\bar{B}^*+\bar{B}B^*$ and $B^*\bar{B}^*$, in $S$ and $D$-wave. Poles, matching the experimental mass and width of all the above states, are found by varying only two free parameters. From the wave-function analysis of each resonance, the probability of each of the components contributing to the coupled system is estimated, and predictions can be made for the relative decay fractions among the coupled open-charm or open-bottom decay channels.

hep-ph↗
arXiv2016

Unquenching the meson spectrum: a model study of excited $ρ$ resonances

Quark models taking into account the dynamical effects of hadronic decay often produce very different predictions for mass shifts in the hadron spectrum. The consequences for meson spectroscopy can be dramatic and completely obscure the underlying confining force. Recent unquenched lattice calculations of mesonic resonances that also include meson-meson interpolators provide a touchstone for such models, despite the present limitations in applicability. On the experimental side, the $ρ(770)$ meson and its several observed radial recurrences are a fertile testing ground for both quark models and lattice computations. Here we apply a unitarised quark model that has been successful in the description of many enigmatic mesons to these vector $ρ$ resonances and the corresponding $P$-wave $ππ$ phase shifts. This work is in progress, with encouraging preliminary results.

hep-ph↗
arXiv2016

Using $X(3823)\to J/ψπ^+π^-$ to identify coupled-channel effects

Very recently, the Belle and BESIII experiments observed a new charmonium-like state $X(3823)$, which is a good candidate for the $D$-wave charmonium $ψ(1^3D_2)$. Because the $X(3823)$ is just near the $D\bar{D}^*$ threshold, the decay $X(3823)\to J/ψπ^+π^-$ can be a golden channel to test the significance of coupled-channel effects. In this work, this decay is considered including both the hidden-charm dipion and the usual quantum chromodynamics multipole expansion (QCDME) contributions. The partial decay width, the dipion invariant mass spectrum distribution $\mathrm{d}Γ[X(3823)\to J/ψπ^+π^-]/\mathrm{d}m_{π^+π^-}$, and the corresponding $\mathrm{d}Γ[X(3823)\to J/ψπ^+π^-]/\mathrm{d}\cosθ$ distribution are computed. Many parameters are determined from existing experimental data, so the results depend mainly only on one unknown phase between the QCDME and hidden-charm dipion amplitudes.

hep-ph↗
arXiv2015

Unquenched quark-model calculation of excited $ρ$ resonances and P-wave $ππ$ phase shifts

The $ρ(770)$ vector resonance, its radial recurrences, and the corresponding P-wave $ππ$ phase shifts are investigated in an unquenched quark model with all classes of relevant decay channels included, viz. pseudoscalar-pseudoscalar, vector-pseudoscalar, vector-vector, vector-scalar, axialvector-pseudoscalar, and axialvector-vector, totalling 26 channels. Two of the few model parameters are fixed at previously used values, whereas the other three are adjusted to the $ρ(770)$ resonance and the lower P-wave $ππ$ phases. Preliminary results indicate the model's capacity to reproduce these phases as well as the $ρ$ mass and width. However, at higher energies the phase shifts tend to rise too sharply. A possible remedy is an extension of the model so as to handle resonances in the final states for most of the included decay channels. Work in progress.

hep-ph↗
arXiv2015

No serious meson spectroscopy without scattering

The principal purpose of meson spectroscopy is to understand the confining force, which is generally assumed to be based on low-energy QCD. This is usually done in the context of quark models that ignore the dynamical effects of quark-pair creation and decay. Very recent lattice calculations confirm much earlier model results showing that neglecting such effects, in the so-called quenched approximation, may give rise to discrepancies of hundreds of MeV, and so distort the meson spectra resulting from quark confinement only. Models attempting to mimic unquenching through a redefinition of the constituent quark mass or screening of the confining potential at larger interquark separations are clearly incapable of accounting for the highly non-perturbative and non-linear effects on mesonic bound-state and resonance poles, as demonstrated with several published examples.

hep-ph↗
arXiv2015

Unquenching weak substructure

On assuming that Weak substructure has a dynamics which is similar to quantum chromodynamics but much stronger, we conclude that unquenching is indispensable for predictions on the spectrum of Weak-substructure resonances.

hep-ph↗
arXiv2014

On the existence of a superlight scalar boson

In this lecture we show that the study of hadronic resonances is severely hampered by the lack of accurate data and, moreover, that for similar reason the study of Weak substructure does not make sufficient progress. We furthermore report on an unexplained high statistics signal that may indicate the existence of a superlight scalar boson.

hep-ph↗
arXiv2014

Unquenched Meson Spectroscopy

Quantum chromodynamics (QCD), the quantum field theory of strong interactions, is highly nonpertubative in the low-energy sector, where confinement dominates and resonance phenomena are observed. Therefore, phenomenological unquenched models based on the old ideas of the $\mathcal{S}$-matrix theory give a fundamental contribution to understand the complex pattern of masses, widths and shapes of experimentally observed meson resonances. In the present thesis we employ the Resonance-Spectrum-Expansion coupled-channel model to study two enigmatic meson states, the isoscalar vector $ϕ(2170)$ and the charmonium-like axial-vector $X(3872)$. The same model is applied to describe the peculiar pattern of masses and widths of the open-charm axial-vectors - pseudovectors $D_1(2420)$ and $D_1(2430)$, and $D_{s1}(2460)$ and $D_{s1}(2536)$. Furthermore, a simplified Schrödinger model is used to study the dominant wave-function components of $X(3872)$ near its resonance mass. Both models successfully describe the whole variety of special features observed in experiment, which are not so easily explained in QCD-inspired quenched models.

hep-ph↗
arXiv2013

Substructures from weak interactions in light of possible threshold signals at LEP and LHC

We present indications of possible substructures from weak interactions, by inspecting LEP and LHC date and inferring threshold effects due to the production of pairs of composite heavy gauge bosons W+-, Z and their hypothetical partners with different spin. Thus, we find possible evidence of scalar or pseudoscalar partners of the W+- and the Z, viz. at 53 and 57 GeV, respectively. Additionally, data may indicate excited states of the Z at 210 and 240 GeV.

hep-ph↗
arXiv2013

X(3872) is not a true molecule

A solvable coordinate-space model is employed to study the $c\bar{c}$ component of the X(3872) wave function, by coupling a confined $^3P_1$ $c\bar{c}$ state to the almost unbound $S$-wave $D^0\bar{D}^{*0}$ channel via the $^3P_0$ mechanism. The two-component wave function is calculated for different values of the binding energy and the transition radius $a$, always resulting in a significant $c\bar{c}$ component. However, the long tail of the $D^0\bar{D}^{*0}$ wave function, in the case of small binding, strongly limits the $c\bar{c}$ probability, which roughly lies in the range 7-11%, for the average experimental binding energy of 0.16 MeV and $a$ between 2 and 3 GeV$^{-1}$. Furthermore, a reasonable value of 7.8 fm is obtained for the X(3872) r.m.s. radius at the latter binding energy, as well as an $S$-wave $D^0\bar{D}^{*0}$ scattering length of 11.6 fm. Finally, the $\mathcal{S}$-matrix pole trajectories as a function of coupling constant show that X(3872) can be generated either as a dynamical pole or as one connected to the bare $c\bar{c}$ confinement spectrum, depending on details of the model. From these results we conclude that X(3872) is not a genuine meson-meson molecule, nor actually any other mesonic system with non-exotic quantum numbers, due to inevitable mixing with the corresponding quark-antiquark states.

hep-ph↗