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Vinicius Rodrigues Debastiani
Publication date
2017
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English
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We have investigated $Omega_c$ states that are dynamically generated from the meson-baryon interaction. We use an extension of the local hidden gauge to obtain the interaction from the exchange of vector mesons. We show that the dominant terms come from the exchange of light vectors, where the heavy quarks are spectators. This has as a consequence that heavy quark symmetry is preserved for the dominant terms in the $(1/m_Q)$ counting, and also that the interaction in this case can be obtained from the $textrm{SU(3)}$ chiral Lagrangians. We show that for a standard value for the cutoff regulating the loop, we obtain two states with $J^{P}={1/2}^{-}$ and two more with $J^{P}={3/2}^{-}$, three of them in remarkable agreement with three experimental states in mass and width. We also make predictions at higher energies for states of vector-baryon nature.
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We have studied the meson-baryon interaction in coupled channels with the same quantum numbers of $Xi_{bc}$. The interaction is attractive in some channels and of sufficient intensity to lead to bound states or resonances. We use a model describing the meson-baryon interaction based on an extrapolation of the local hidden gauge approach to the heavy sector, which has been successfully used in predicting $Omega_c$ and hidden charm states. We obtain many states, some of them narrow or with zero width, as a consequence of the interaction, which qualify as molecular states in those channels. The success in related sectors of the picture used should encourage the experimental search for such states.
Stimulated by the new experimental LHCb findings associated with the $Omega_c$ states, some of which we have described in a previous work as being dynamically generated through meson-baryon interaction, we have extended this approach to make predictions for new $Xi_{cc}$ molecular states in the $C=2$, $S=0$ and $I=1/2$ sector. These states manifest themselves as poles in the solution of the Bethe-Salpeter equation in coupled channels. The kernels of this equation were obtained using the Lagrangians coming from the hidden local gauge symmetry, where the interactions are dominated by the exchange of light vector mesons. The extension of this approach to the heavy sector stems from the realization that the dominant interaction corresponds to having the heavy quarks as spectators, which implies the preservation of the heavy quark symmetry. As a result, we get several states: two states from the pseudoscalar meson-baryon interaction with $J^P=1/2^-$, and masses around $4080$ and $4090$ MeV, and one at $4150$ MeV for $J^P=3/2^-$. Furthermore, from the vector meson-baryon interaction we get three states degenerate with $J^P=1/2^-$ and $3/2^-$ from $4220$ MeV to $4330$ MeV, and two more states around $4280$ MeV and $4410$ MeV, degenerate with $J^P=1/2^-,, 3/2^-$ and $5/2^-$.
We observe that four peaks seen in the high energy part of the $Omega_b$ spectrum of the recent LHCb experiment are in remarkable agreement with predictions made for molecular $Omega_b$ states stemming from the meson-baryon interaction, with an approach that applied to the $Omega_c$ states gave rise to three states in good agreement with experiment in masses and widths. While the statistical significance of the peaks prevents us from claims of states at the present time, the agreement found should be an incentive to look at this experiment with increased statistics to give an answer to this suggestive idea.
We report a measurement of the lifetime of the $Omega_c^0$ baryon using proton-proton collision data at center-of-mass energies of 7 and 8~TeV, corresponding to an integrated luminosity of 3.0 fb$^{-1}$ collected by the LHCb experiment. The sample consists of about 1000 $Omega_b^-toOmega_c^0mu^-bar{ u}_{mu} X$ signal decays, where the $Omega_c^0$ baryon is detected in the $pK^-K^-pi^+$ final state and $X$ represents possible additional undetected particles in the decay. The $Omega_c^0$ lifetime is measured to be $tau_{Omega_c^0} = 268pm24pm10pm2$ fs, where the uncertainties are statistical, systematic, and from the uncertainty in the $D^+$ lifetime, respectively. This value is nearly four times larger than, and inconsistent with, the current world-average value.
We consider the ${bar D}^{(*)}Sigma_c^{(*)}$ states, together with $J/psi N$ and other coupled channels, and take an interaction consistent with heavy quark spin symmetry, with the dynamical input obtained from an extension of the local hidden gauge approach. By fitting only one parameter to the recent three pentaquark states reported by the LHCb collaboration, we can reproduce the three of them in base to the mass and the width, providing for them the quantum numbers and approximate molecular structure as $1/2^-$ $bar{D} Sigma_c$, $1/2^-$ $bar{D}^* Sigma_c$, and $3/2^-$ $bar{D}^* Sigma_c$, and isospin $I=1/2$. We find another state around 4374 MeV, of $3/2^-$ $bar{D} Sigma_c^*$ structure, for which indications appear in the experimental spectrum. Two other near degenerate states of $1/2^-$ $bar{D}^* Sigma_c^*$ and $3/2^-$ $bar{D}^* Sigma_c^*$ nature are also found around 4520 MeV, which although less clear, are not incompatible with the observed spectrum. In addition, a $5/2^-$ $bar D^* Sigma_c^*$ state at the same energy appears, which however does not couple to $J/psi p$ in $S-$wave, and hence it is not expected to show up in the LHCb experiment.
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