Study of the strong Σb→Λb π\Sigma_b\to \Lambda_b\, \pi and Σb∗→Λb π\Sigma_b^{*}\to \Lambda_b\, \pi in a non-relativistic quark model

We present results for the strong widths corresponding to the $\Sigma_b\to \Lambda_b\, \pi$ and $\Sigma_b^{*}\to \Lambda_b\, \pi$ decays. We apply our model in Ref. Phys. Rev. D 72, 094022 (2005) where we previously studied the corresponding transitions in the charmed sector. Our non-relativistic constituent quark model uses wave functions that take advantage of the constraints imposed by heavy quark symmetry. Partial conservation of axial current hypothesis allows us to determine the strong vertices from an analysis of the axial current matrix elements.Comment: 6 latex pages, 1 table, new references adde

Meson Resonances at large Nc: Complex Poles vs Breit-Wigner Masses

The rigorous quantum mechanical definition of a resonance requires determining the pole position in the second Riemann sheet of the analytically continued partial wave scattering amplitude in the complex Mandelstam s-variable plane. For meson resonances we investigate the alternative Breit-Wigner (BW) definition within the large Nc expansion. By assuming that the pole position is ${\cal O} (N_C^{0})$ and exploiting unitarity, we show that the BW determination of the resonance mass differs from the pole position by ${\cal O} (N_C^{-2})$ terms, which can be extracted from pi-pi scattering data. For the case of the f0(600) pole, the BW scalar mass is predicted to occur at about 700 MeV while the true value is located at about 800 MeV.Comment: 7 pages. No figures. (elsevier preprint

Couplings in coupled channels versus wave functions: application to the X(3872) resonance

We perform an analytical study of the scattering matrix and bound states in problems with many physical coupled channels. We establish the relationship of the couplings of the states to the different channels, obtained from the residues of the scattering matrix at the poles, with the wave functions for the different channels. The couplings basically reflect the value of the wave functions around the origin in coordinate space. In the concrete case of the X(3872) resonance, understood as a bound state of \ddn and \ddc (and $c.c.$), with the \ddn loosely bound, we find that the couplings to the two channels are essentially equal leading to a state of good isospin I=0 character. This is in spite of having a probability for finding the \ddn state much larger than for \ddc since the loosely bound channel extends further in space. The analytical results, obtained with exact solutions of the Schr\"odinger equation for the wave functions, can be useful in general to interpret results found numerically in the study of problems with unitary coupled channels methods.Comment: 14 pages, 4 figure

Isospin breaking effects in the dynamical generation of the X(3872)

We have studied isospin breaking effects in the X(3872) resonance and found a natural explanation for the branching fraction of the X decaying to $J/\psi$ with two and three pions being close to unit. Within our framework the X(3872) is a dynamically generated resonance in coupled channels. We also study the relationship between the couplings of the resonance to the coupled channels with its wave function, which further helps us to understand the isospin structure of the resonance.Comment: 5 pages, 1 figure. To appear in the Proceedings of XIII International Conference on Hadron Spectroscopy, November 29 - December 4, 2009, Florida State Universit

Thermal Field Theory in a wire: Applications of Thermal Field Theory methods to the propagation of photons in a one-dimensional plasma

We apply the Thermal Field Theory (TFT) methods to study the propagation of photons in a plasma wire, that is, a system in which the electrons are confined to a one-dimensional tube or wire, but are otherwise free. We find the appropriate expression for the photon \emph{free-field} propagator in such a medium, and write down the dispersion relation in terms of the free-field propagator and the photon self-energy. The self-energy is then calculated in the one-loop approximation and the corresponding dispersion relation is determined and studied in some detail. Our work differs from previous work on this subject in that we do not adopt any specific electronic wave functions in the coordinates that are transverse to the idealized wire, or rely on particular features of the electronic structure. We treat the electrons as a free gas of particles, constrained to move in one dimension, but otherwise in a model-independent way only following the rules of TFT adapted to the situation at hand. For the appropriate conditions of the plasma the \emph{static approximation} can be employed and the dispersion relation reduces to the results obtained in previous works, but the formula that we obtain is valid under more general conditions, including those in which the static approximation is not valid. In particular, the dispersion relation has several branches, which are not revealed if the static approximation is used. The dispersion relations obtained reproduce several unique features of these systems that have been observed in recent experiments.Comment: 17 pages Revised and extended discussion of the dispersion relation

The role of hyperfine mixing in b→cb\to c semileptonic decays of doubly-heavy baryons

We analyze the effects of hyperfine mixing in $b \to c$ semileptonic decays of doubly heavy baryons. We qualitatively confirm the results by W. Roberts and M. Pervin in Int. J. Mod. Phys. A, 2009, {\bf 24}: 2401-2413, finding that mixing has a great impact on those transitions. However, predictions without mixing differ by a factor of 2 and this discrepancy translates to the mixed case where large differences in decay widths are observed between the two calculations.Comment: 3 latex pages. Talk given at The 5-th International Conference on Quarks and Nuclear Physics (QNP09), Beijing, September 200

Coherent pion production off nuclei at T2K and MiniBooNE energies revisited

As a result of a new improved fit to old bubble chamber data of the dominant axial C_5^A nucleon-to-Delta form factor, and due to the relevance of this form factor for neutrino induced coherent pion production, we re-evaluate our model predictions in Phys. Rev. D79, 013002 (2009) for different observables of the latter reaction. Central values for the total cross sections increase by 20% to 30%, while differential cross sections do not change their shape appreciably. Furthermore, we also compute the uncertainties on total, differential and flux averaged cross sections induced by the errors in the determination of C_5^A. Our new results turn out to be compatible within about 1 sigma with the former ones. Finally, we stress the existing tension between the recent experimental determination of the sigma(CC coh \pi^+)/sigma(NC coh \pi^0) ratio by the SciBooNE Collaboration and the theoretical predictions.Comment: 6 latex pages, 1 table, 2 figures. Shortened versio

The role of hyperfine mixing in b→cb\to c semileptonic and electromagnetic decays of doubly-heavy baryons

We analyze the effects of hyperfine mixing in $b\to c\,$ semileptonic and electromagnetic decays of doubly heavy baryons.Comment: Talk given at the Chiral 10 Conferenc

Hyperfine mixing in b→cb\to c semileptonic decay of doubly heavy baryons

We qualitatively corroborate the results of W. Roberts and M. Pervin in Int. J. Mod. Phys. A 24, 2401 (2009) according to which hyperfine mixing greatly affects the decay widths of $b\to c$ semileptonic decays involving doubly heavy $bc$ baryons. However, our predictions for the decay widths of the unmixed states differ from those reported in the work of Roberts and Pervin by a factor of 2, and this discrepancy translates to the mixed case. We further show that the predictions of heavy quark spin symmetry, might be used in the future to experimentally extract information on the admixtures in the actual physical $bc$ baryons, in a model independent manner.Comment: 7 Latex pages, 4 Table

Heavy quark spin symmetric molecular states from Dˉ(∗)Σc(∗){\bar D}^{(*)}\Sigma_c^{(*)} and other coupled channels in the light of the recent LHCb pentaquarks

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.Comment: 8 page