arXiv subjects

R. Pavao

Publications and source records attributed to R. Pavao.

12 recordsLinked to original sources

arXiv2019

Charmed excited baryons with heavy-quark spin symmetry

Recently five $Ω_c$ excited states have been reported by the LHCb Collaboration, four of them corroborated by Belle. The Belle Collaboration has also discovered in 2017 one excited $Ξ_c$ with mass of 2930 MeV. In view of these recent detections, we analyze the possible molecular description of these states, using a unitarized baryon-meson model that incorporates both chiral and heavy-quark spin symmetries in a consistent manner. We pay a special attention to the renormalization procedure, so as to determine the robustness of our predictions. Within our model, we generate dynamically several $Ω_c$ and $Ξ_c$ states. We find that, at least, three of our $Ω_c$ states can be identified with the experimental ones and have spin-parity $J=1/2^-$ or $J=3/2^-$. Moreover, we find a plausible molecular description of not only the $Ξ_c(2930)$ state, but also $Ξ_c(2790)$, $Ξ_c(2815)$ and $Ξ_c(2970)$, reported in the PDG. Interestingly, we determine that $Ξ_c(2930)$ and $Ξ_c(2970)$ are heavy-quark spin partners with $J=1/2^-$ and $J=3/2^-$, respectively, while obtaining that $Ξ_c(2930)$, $Ω_c(3090)$ or the $Ω_c(3119)$, and $Σ_c(2800)$ would belong to the same SU(3) multiplet.

hep-ph↗

arXiv2019

$Ξ_c$ and $Ξ_b$ excited states within a ${\rm SU(6)}_{\rm lsf}\times$HQSS model

We study odd parity $J=1/2$ and $J=3/2$ $Ξ_c$ resonances using a unitarized coupled-channel framework based on a ${\rm SU(6)}_{\rm lsf}\times$HQSS-extended Weinberg-Tomozawa baryon-meson interaction, while paying a special attention to the renormalization procedure. We predict a large molecular $Λ_c \bar K$ component for the $Ξ_c(2790)$ with a dominant $0^-$ light-degree-of-freedom spin configuration. We discuss the differences between the $3/2^-$ $Λ_c(2625)$ and $Ξ_c(2815)$ states, and conclude that they cannot be SU(3) siblings, whereas we predict the existence of other $Ξ_c-$states, two of them related to the two-pole structure of the $Λ_c(2595)$. It is of particular interest a pair of $J=1/2$ and $J=3/2$ poles, which form a HQSS doublet and that we tentatively assign to the $Ξ_c(2930)$ and $Ξ_c(2970)$, respectively. Within this picture, the $Ξ_c(2930)$ would be part of a SU(3) sextet, containing either the $Ω_c(3090)$ or the $Ω_c(3119)$, and that would be completed by the $Σ_c(2800)$. Moreover, we identify a $J=1/2$ sextet with the $Ξ_b(6227)$ state and the recently discovered $Σ_b(6097)$. Assuming the equal spacing rule and to complete this multiplet, we predict the existence of a $J=1/2$ $Ω_b$ odd parity state, with a mass of 6360 MeV and that should be seen in the $Ξ_b \bar K$ channel.

hep-ph↗

arXiv2019

$Λ_b$ decays into $Λ_c^\ell\barν_\ell$ and $Λ_c^π^-$ $[Λ_c^*=Λ_c(2595)$ \& $Λ_c(2625)]$ and heavy quark spin symmetry

We study the implications for $Λ_b \to Λ_c^*\ell\barν_\ell$ and $Λ_b \to Λ_c^*π^-$ $[Λ_c^*=Λ_c(2595)$ and $Λ_c(2625)]$ decays that can be deduced from heavy quark spin symmetry (HQSS). Identifying the odd parity $Λ_c(2595)$ and $Λ_c(2625)$ resonances as HQSS partners, with total angular momentum--parity $j_q^P=1^-$ for the light degrees of freedom, we find that the ratios $Γ(Λ_b\rightarrowΛ_c(2595)π^-)/Γ(Λ_b\rightarrowΛ_c(2625)π^-)$ and $Γ(Λ_b\rightarrow Λ_c(2595) \ell \barν_\ell)/ Γ(Λ_b\rightarrowΛ_c(2625) \ell \barν_\ell)$ agree, within errors, with the experimental values given in the Review of Particle Physics. We discuss how future, and more precise, measurements of the above branching fractions could be used to shed light into the inner HQSS structure of the narrow $Λ_c(2595)$ odd-parity resonance. Namely, we show that such studies would constrain the existence of a sizable $j^P_q=0^-$ component in its wave-function, and/or of a two-pole pattern, in analogy to the case of the similar $Λ(1405)$ resonance in the strange sector, as suggested by most of the approaches that describe the $Λ_c(2595)$ as a hadron molecule. We also investigate the lepton flavor universality ratios $R[Λ_c^*] = {\cal B}(Λ_b \to Λ_c^* τ\,\barν_τ)/{\cal B}(Λ_b \to Λ_c^* μ\,\barν_μ)$, and discuss how $R[Λ_c(2595)]$ may be affected by a new source of potentially large systematic errors if there are two $Λ_c(2595)$ poles.

hep-ph↗

arXiv2018

Description of the $Ξ_c$ and $Ξ_b$ states as molecular states

In this work we study several $Ξ_c$ and $Ξ_b$ states dynamically generated from the meson-baryon interaction in coupled channels, using an extension of the local hidden gauge approach in the Bethe-Salpeter equation. These molecular states appear as poles of the scattering amplitudes, and several of them can be identified with the experimentally observed $Ξ_c$ states, including the $Ξ_c(2790)$, $Ξ_c(2930)$, $Ξ_c(2970)$, $Ξ_c(3055)$ and $Ξ_c(3080)$. Also, for the recently reported $Ξ_b(6227)$ state, we find two poles with masses and widths remarkably close to the experimental data, for both the $J^P=1/2^-$ and $J^P=3/2^-$ sectors.

hep-ph↗

arXiv2018

Coupled channels dynamics in the generation of the $Ω(2012)$ resonance

We look into the newly observed $Ω(2012)$ state from the molecular perspective in which the resonance is generated from the $\bar{K} Ξ^*$, $ηΩ$ and $\bar{K} Ξ$ channels. We find that this picture provides a natural explanation of the properties of the $Ω(2012)$ state. We stress that the molecular nature of the resonance is revealed with a large coupling of the $Ω(2012)$ to the $\bar{K} Ξ^*$ channel, that can be observed in the $Ω(2012) \rightarrow \bar{K} πΞ$ decay which is incorporated automatically in our chiral unitary approach via the use of the spectral function of $Ξ^*$ in the evaluation of the $\bar{K} Ξ^*$ loop function.

hep-ph↗

arXiv2018

$τ^- \to ν_τ M_1 M_2$, with $M_1, M_2$ pseudoscalar or vector mesons

We perform a calculation of the $τ^- \to ν_τ M_1 M_2$, with $M_1, M_2$ either pseudoscalar or vector mesons using the basic weak interaction and angular momentum algebra to relate the different processes. The formalism also leads to a different interpretation of the role played by $G$-parity in these decays. We also observe that, while $p$-wave $M_1 M_2$ production is compatible with chiral perturbation theory and experiment, $VP$ and $VV$ $p$-wave production is clearly incompatible with experiment and we develop the formalism also in this case. We compare our results with experiment and make predictions for unmeasured decays, and we show the value of these reactions, particularly if the $M_1 M_2$ mass distribution is measured, as a tool to learn about the meson-meson interaction and the nature of some resonances, coupling to two mesons, which are produced in such decays.

hep-ph↗

arXiv2018

Anomalous enhancement of the isospin-violating $Λ(1405)$ production by a triangle singularity in $Λ_c\rightarrowπ^+π^0π^0Σ^0$

The decay of $Λ_c^+$ into $π^+π^0Λ(1405)$ with the $Λ(1405)$ decay into $π^0Σ^0$ through a triangle diagram is studied. This process is initiated by $ Λ_c^+ \to π^+ \bar{K}^*N $, then the $\bar{K}^*$ decays into $\bar{K} π$ and $\bar{K} N$ produce the $Λ(1405)$ through a triangle loop containing $\bar{K}^*N\bar{K}$ which develops a singularity around $1890$ MeV. This process is prohibited by the isospin symmetry, but the decay into this channel is enhanced by the contribution of the triangle diagram, which is sensitive to the mass of the internal particles. We find a narrow peak in the $π^0Σ^0$ invariant mass distribution, which originates from the $Λ(1405)$ amplitude, but is tied to the mass differences between the charged and neutral $\bar{K}$ or $N$ states. The observation of the unavoidable peak of the triangle singularity in the isospin-violating $Λ(1405)$ production would provide further support for hadronic molecular picture of the $Λ(1405)$ and further information on the $\bar{K} N $ interaction.

hep-ph↗

arXiv2018

$Ω_c$ excited states within a ${\rm SU(6)}_{\rm lsf}\times$HQSS model

We have reviewed the renormalization procedure used in the unitarized coupled-channel model of Phys. Rev. D85 114032 (2012), and its impact in the $C=1$, $S=-2$, and $I=0$ sector, where five $Ω_c^{(*)}$ states have been recently observed by the LHCb Collaboration. The meson-baryon interactions used in the model are consistent with both chiral and heavy-quark spin symmetries and lead to a successful description of the observed lowest-lying odd parity resonances $Λ_c(2595)$ and $Λ_c(2625)$, and $Λ_b(5912)$ and $Λ_b(5920)$ resonances. We show that some (probably at least three) of the states observed by LHCb will also have odd parity and spins $J=1/2$ and $J=3/2$, belonging two of them to the same ${\rm SU(6)}_{\rm light-spin-flavor}\times$HQSS multiplets that the latter strangenessless heavy$-Λ$ baryons.

hep-ph↗

arXiv2018

Production of $N^(1535)$ and $N^(1650)$ in $Λ_c\rightarrow\bar{K}^0ηp$ $(πN)$ decay

In order to study the properties of the $N^*$(1535) and $N^*$(1650) we calculate the mass distributions of $M B$ in the $Λ_c \rightarrow \bar{K}^0 M B$ decay, with $MB=πN(I=1/2),ηp$ and $KΣ(I=1/2)$. We do this by calculating the tree-level and loop contributions, mixing pseudoscalar-baryon and vector-baryon channels using the local hidden gauge formalism. The loop contributions for each channel are calculated using the chiral unitary approach. We observe that for the $ηN$ mass distribution only the $N^*$(1535) is seen, with the $N^*$(1650) contributing to the width of the curve, but for the $πN$ mass distribution both resonances are clearly visible. In the case of $MB=KΣ$, we found that the strength of the $KΣ$ mass distribution is smaller than that of the mass distributions of the $πN$ and $ηp$ in the $Λ_c^+\rightarrow\bar{K}^0πN$ and $Λ_c^+\rightarrow\bar{K}^0ηp$ processes, in spite of this channel having a large coupling to the $N^*(1650)$. This is because the $KΣ$ pair production is suppressed in the primary production from the $Λ_c$ decay.

nucl-th↗

arXiv2017

Calculation of the ratios and absolute rates of the $Ξ_b^- \to π^- (D_s^- ) \ Ξ_c^0 (2790) \left(Ξ_c^0 (2815) \right)$ and $Ξ_b^- \to \barν_l l \ Ξ_c^0 (2790) \left(Ξ_c^0 (2815) \right)$ decays

In this work we calculate the ratios of rates of the $Ξ_b$ nonleptonic and semileptonic decays into the $Ξ_c$(2790) and $Ξ_c$(2815) ($Ξ_c^*$) resonances. These resonances are dynamically generated from the pseudoscalar-baryon and vector-baryon interactions, whose mixing is done using the chiral Weinberg-Tomozawa (WT) meson-baryon interaction extended to four flavors. The first part of the decay is a weak decay that we analyze through their quark constituents where it is noted that only the heavy quarks ($b$ and $c$) participate in the interaction, leaving the light pair ($ds$) as spectators. This first decay then produces a meson-baryon pair that creates the $Ξ_c^*$ through the WT interaction. We then proceed to calculate the decay rates to $Ξ_c$(2790) and $Ξ_c$(2815) for both the nonleptonic and semileptonic cases and then calculate the ratios between them. We do this calculation nonrelativistically and fully relativistically and notice that, even though both approaches yield somewhat different results in the rates, the ratios are very similar (difference on the order of 1\%) in both cases. The absolute values of the decay rates are also successfully calculated by obtaining the rates between our decays and $Λ_b \to π(D_s ) \ Λ_c (2595) \left(Λ_c (2625) \right)$ and $Λ_b \to \barν_l l \ Λ_c (2595) \left(Λ_c (2625) \right)$ for which there are experimental results, since the momentum transfer is similar such we can cancel out the influence of the quark wave functions in the ratios.

hep-ph↗

arXiv2017

The role of the triangle singularity in $Λ(1405)$ production in the $π^-p\rightarrow K^0πΣ$ and $pp\rightarrow pK^+πΣ$ processes

We have investigated the cross section for the $π^-p\rightarrow K^0πΣ$ and $pp\rightarrow pK^+πΣ$ reactions paying attention to a mechanism that develops a triangle singularity. The triangle diagram is realized by the decay of a $N^*$ to $K^*Σ$ and the $K^*$ decay into $πK$, and the $πΣ$ finally merges into $Λ(1405)$. The mechanism is expected to produce a peak around $2140$ MeV in the $KΛ(1405)$ invariant mass. We found that a clear peak appears around $2100$ MeV in the $KΛ(1405)$ invariant mass which is about $40$ MeV lower than the expectation, and that is due to the resonance peak of a $N^*$ resonance which plays a crucial role in the $K^*Σ$ production. The mechanism studied produces the peak of the $Λ(1405)$ around or below 1400 MeV, as is seen in the $pp\rightarrow pK^+πΣ$ HADES experiment.

hep-ph↗

arXiv2017

Triangle singularities in $B^-\rightarrow D^{0}π^-π^0η$ and $B^-\rightarrow D^{0}π^-π^+π^-$

The possible role of the triangle mechanism in the $B^-$ decay into $D^{*0}π^-π^0η$ and $D^{*0}π^-π^+π^-$ is investigated. In this process, the triangle singularity appears from the decay of $B^-$ into $D^{*0}K^-K^{*0}$ followed by the decay of $K^{*0}$ into $π^-K^+$ and the fusion of the $K^+K^-$ which forms the $a_0(980)$ or $f_0(980)$ which finally decay into $π^0η$ or $π^+π^-$ respectively. The triangle mechanism from the $\bar{K}^*K\bar{K}$ loop generates a peak around 1420 MeV in the invariant mass of $π^-a_0$ or $π^-f_0$, and gives sizable branching fractions $Br(B^-\rightarrow D^{*0}π^- a_0;a_0 \rightarrow π^0η)= 1.66 \times 10^{-6}$ and $Br(B^-\rightarrow D^{*0}π^- f_0 ; f_0 \rightarrow π^+π^-)= 2.82 \times 10^{-6}$.

hep-ph↗