arXiv subjects

Susana Coito

Publications and source records attributed to Susana Coito.

26 records · Page 2Linked to original sources

arXiv2012

Meson spectroscopy: too much excitement and too few excitations

We briefly review the general status of meson spectroscopy, especially in light of the often made claim that there are too many observed resonances to be accounted for as $q\bar{q}$ states. Also, the adequacy of the usual Coulomb-plus-linear, alias "funnel", confining potential for reproducing the experimental spectra of light, heavy-light, and heavy mesons is critically analysed. Thus, many serious discrepancies are observed and discussed. As possible causes, we suggest the neglect of unitarisation and other coupled-channel effects, as well as the deficiency of the funnel potential itself. In order to illustrate our alternative, "unquenched" approach, we present some recent examples of successfully described puzzling mesonic enhancements and resonances, such as the charmonium states X(4260) and X(3872), as well as the axial-vector charmed mesons $D_1(2420)$, $D_1(2430)$, $D_{s1}(2536)$, and $D_{s1}(2460)$.

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arXiv2011

Towards Meson Spectroscopy Instead of Bump Hunting

Mesonic resonances are generally observed in data as narrow, moderately broad, or wide peaks in scattering or production processes. In the eyes of nearly all experimentalists, any suchlike bump is a true resonance as soon as its statistical significance exceeds certain minimal values. However, this simple point of view ignores possible effects from competing hadronic channels and the opening of the corresponding thresholds. On the other hand, most theoretical hadron-model builders consider mesons merely bound states of a quark and an antiquark, or of more exotic combinations sometimes involving valence gluons as well. Also the latter description is much too naive, since considerable mass shifts or even the dynamical generation of extra states due to unquenching are equally ignored. In the present paper, a largely empirical yet very successful approach to meson spectroscopy is revisited, in which all the above phenomena can be accounted for non-perturbatively, with concrete examples of some enigmatic mesonic states described in detail. First, the X(4260) charmonium enhancement is argued to be a non-resonant structure resulting from depletion effects due to competing channels and resonances. Then, the X(3872) charmonium-like meson is described as a unitarised $J^{PC}=1^{++}$ $c\bar{c}$ state. Also, the unusual pattern of masses and widths of the open-charm axial-vector mesons $D_1(2420)$, $D_1(2430)$, $D_{s1}(2536)$, and $D_{s1}(2460)$ is shown to follow from highly non-perturbative coupled-channel and mixing effects. Finally, first indications of a very light scalar boson are presented, on the basis of published BABAR data.

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arXiv2011

Quasi-bound states in the continuum: a dynamical coupled-channel calculation of axial-vector charmed mesons

Masses and widths of the charmed axial-vector mesons $D_1(2420)$, $D_1(2430)$, $D_{s1}(2536)$, and $D_{s1}(2460)$ are calculated nonperturbatively in the Resonance-Spectrum-Expansion model, by coupling various open and closed meson-meson channels to the bare $J^P=1^+$ $c\bar{q}$ ($q=u,d$) and $c\bar{s}$ states. The coupling to two-meson channels dynamically mixes and lifts the mass degeneracy of the spectroscopic $^3P_1$ and $^1P_1$ states, as an alternative to the usual spin-orbit splitting. Of the two resulting $S$-matrix poles in either case, one stays very close to the energy of the bare state, as a quasi-bound state in the continuum, whereas the other shifts considerably. This is in agreement with the experimental observation that the $D_1(2420)$ and $D_{s1}(2536)$ have much smaller widths than one would naively expect. The whole pattern of masses and widths of the axial-vector charmed mesons can thus be quite well reproduced with only two free parameters, one of which being already strongly constrained by previous model calculations. Finally, predictions for pole positions of radially excited axial-vector charmed mesons are presented.

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arXiv2011

Delicate interplay between the D0-D*0, rho0-J/psi, and omega-J/psi channels in the X(3872) resonance

The nature of the X(3872) enhancement is analysed in the framework of the Resonance-Spectrum Expansion, by studying it as a regular $J^{PC}=1^{++}$ charmonium state, though strongly influenced and shifted by open-charm decay channels. The observed but Okubo-Zweig-Iizuka-forbidden $ρ^0 J/ψ$ and $ωJ/ψ$ channels are coupled as well, but effectively smeared out by using complex $ρ^0$ and $ω$ masses, in order to account for their physical widths, followed by a rigorous algebraic procedure to restore unitarity. A very delicate interplay between the $D^0 D^{*0}$, $ρ^0 J/ψ$, and $ωJ/ψ$ channels is observed. The data clearly suggest that the X(3872) is a very narrow axial-vector $c\bar{c}$ resonance, with a pole at or slightly below the $D^0D^{*0}$ threshold.

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arXiv2010

Complex masses in the S-matrix

Most excited hadrons have multiparticle strong decay modes, which can often be described as resulting from intermediate states containing one or two resonances. In a theoretical approach, such a description in terms of quasi-two-particle initial and final states leads to unitarity violations, because of the complex masses of the involved resonances. In the present paper, an empirical algebraic procedure is presented to restore unitarity of the S-matrix while preserving its symmetry. Preliminary results are presented in a first application to S-wave pion-pion scattering, in the framework of the Resonance-Spectrum Expansion.

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arXiv2009

Multichannel calculation of excited vector $ϕ$ resonances and the $ϕ(2170)$

A multichannel calculation of excited $J^{PC}=1^{--}$ $ϕ$ states is carried out within a generalization of the Resonance-Spectrum Expansion, which may shed light on the classification of the $ϕ(2170)$ resonance, discovered by BABAR and originally denoted X(2175). In this framework, a complete spectrum of bare $s\bar{s}$ states is coupled to those OZI-allowed decay channels that should be most relevant for the considered energy range. The included $S$- and $P$-wave two-meson channels comprise the lowest pseudoscalar, vector, scalar, and axial-vector mesons, while in the $q\bar{q}$ sector both the $^{3}S_1$ and $^{3}D_1$ states are coupled. The only two free parameters are tuned so as to reproduce mass and width of the $ϕ(1020)$, but come out reasonably close to previously used values. Among the model's $T$-matrix poles, there are good candidates for observed resonances, as well other ones that should exist according to the quark model. Besides the expected resonances as unitarized confinement states, a dynamical resonance pole is found at $(2186-i246)$ MeV. The huge width makes its interpretation as the $ϕ(2170)$ somewhat dubious, but further improvements of the model may change this conclusion.

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arXiv2009

Light and Not So Light Scalar Mesons

A multichannel description of the light scalar mesons in the framework of the Resonance-Spectrum Expansion is generalised by including vector-vector and scalar-scalar channels, besides the usual pseudoscalar-pseudoscalar channels. Experimental data for the isoscalar, isodoublet and isovector cases are fitted up to energies well above 1 GeV. The resulting pole positions of the light and intermediate scalar mesons are compared to the listed resonances. Possible further improvements are discussed.

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arXiv2008

Multichannel Description of Light and Intermediate Scalar Mesons

The light scalar mesons in a variety of approaches are briefly reviewed, as well as their description in the Resonance-Spectrum Expansion and related coupled-channel formalisms. A recent multichannel modelling of the light scalars is extended to higher energies and with additional decay channels, allowing to make predictions for the intermediate scalar mesons as well. Prospects for further improvements are discussed. PACS numbers: 14.40.Cs, 14.40.Ev, 11.80.Gw, 11.55.Ds, 13.75.Lb

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