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Register a new userVisible narrow cusp structure in $Lambda_c^+to p K^- pi^+$ enhanced by triangle singularity
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A resonance-like structure as narrow as 10 MeV is observed in the $K^-p$ invariant mass distributions in $Lambda_c^+to p K^- pi^+$ at Belle. Based on the large data sample of about 1.5 million events and the small bin width of just 1 MeV for the $K^-p$ invariant mass spectrum, the narrow peak is found precisely lying at the $Lambdaeta$ threshold. While lacking evidence for a quark model state with such a narrow width at this mass region, we find that this narrow structure can be naturally identified as a threshold cusp but enhanced by the nearby triangle singularity via the $Lambda$-$a_0(980)^+$ or $eta$-$Sigma(1660)^+$ rescatterings.
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We investigate $S=-1$ hyperon production from the $Lambda_c^+to K^-ppi^+$ and $Lambda_c^+to K^0_Sppi^0$ decays within the effective Lagrangian approach. We consider the $Sigma/Lambda$ ground states, $Lambda(1520)$, $Lambda(1670)(J^p=1/2^-)$, $Lambda(1890)(J^p=3/2^+)$; $Lambda/Sigma$-pole contributions from the combined resonances between 1800 MeV and 2100 MeV; and $N/Delta$-pole and $K^ast$-pole contributions, which include the proton, $Delta(1232)$, and $K(892)$. We calculate the Dalitz plot density $(d^2Gamma/dM_{K^-p}dM_{K^-pi^+}$) for the $Lambda_c^+to K^-ppi^+$ decay. The calculated result is in good agreement with experimental data from the Belle Collaboration. Using the parameters from the fit, we present the Dalitz plot density for the $Lambda_c^+to K^0_Sppi^0$ decay. In our calculation, a sharp peak-like structure near 1665 MeV is predicted in the $Lambda_c^+to K^-ppi^+$ decay because of the interference effects between the $Lambda(1670)$ resonance and $eta$-$Lambda$ loop channels. We also demonstrate that we can access direct information regarding the weak couplings of $Lambda(1670)$ and $Sigma(1670)$ from the $Lambda_c^+to K^0_Sppi^0$ decay. Finally, a possible interpretation for the 1665 MeV structure beyond our prediction is briefly discussed.
The LHCb collaboration reported the observation of a narrow peak in the $D^- K^+$ invariant mass distributions from the $B^+to D^+ D^- K^+$ decay. The peak is parameterized in terms of two resonances $X_0(2900)$ and $X_1(2900)$ with the quark contents $bar{c}bar{s}ud$, and their spin-parity quantum numbers are $0^+$ and $1^-$, respectively. We investigate the rescattering processes which may contribute to the $B^+to D^+ D^- K^+$ decays. It is shown that the $D^{*-}K^{*+}$ rescattering via the $chi_{c1}K^{*+}D^{*-}$ loop or the $bar{D}_{1}^{0}K^{0}$ rescattering via the $D_{sJ}^{+}bar{D}_{1}^{0}K^{0}$ loop simulate the $X_0(2900)$ and $X_1(2900)$ structures. Such phenomena are due to the analytical property of the scattering amplitudes with the triangle singularities located to the vicinity of the physical boundary.
Triangle mechanisms for $B^0to (J/psipi^+pi^-) K^+pi^-$ are studied. Experimentally, an $X(3872)$ peak has been observed in this process. When the final $(J/psipi^+pi^-)pi$ invariant mass is around the $D^*bar D^*$ threshold, one of the triangle mechanisms causes a triangle singularity and generates a sharp $X(3872)$-like peak in the $J/psipi^+pi^-$ invariant mass distribution. The Breit-Wigner mass and width fitted to the spectrum are 3871.68 MeV (a few keV above the $D^{*0}bar{D}^0$ threshold) and $sim$0.4 MeV, respectively.These Breit-Wigner parameters hardly depends on a choice of the model parameters. Comparing with the precisely measured $X(3872)$ mass and width, $3871.69pm 0.17$ MeV and $< 1.2$ MeV, the agreement is remarkable. When studying the $X(3872)$ signal from this process, this non-resonant contribution has to be understood in advance. We also study a charge analogous process $B^0to (J/psipi^0pi^-) K^+pi^0$. A similar triangle singularity exists and generates an $X^-(3876)$-like peak.
We have searched for the Cabibbo-suppressed decay $Lambda_c^+tophi ppi^0$ in $e^+e^-$ collisions using a data sample corresponding to an integrated luminosity of 915 $rm fb^{-1}$. The data were collected by the Belle experiment at the KEKB $e^+e^-$ asymmetric-energy collider running at or near the $Upsilon(4S)$ and $Upsilon(5S)$ resonances. No significant signal is observed, and we set an upper limit on the branching fraction of $mathcal{B}(Lambda_c^+to phi ppi^0) <15.3times10^{-5}$ at 90% confidence level. The contribution for nonresonant $Lambda_c^+to K^+K^- ppi^0$ decays is found to be consistent with zero and the corresponding upper limit on its branching fraction is set to be $mathcal{B}(Lambda_c^+to K^+K^-ppi^0)_{rm NR} <6.3times10^{-5} $ at 90% confidence level. We also measure the branching fraction for the Cabibbo-favored decay $Lambda_c^+to K^-pi^+ppi^0$; the result is $mathcal{B}(Lambda_c^+to K^-pi^+ppi^0)= (4.42pm0.05, (rm stat.) pm 0.12, (rm syst.) pm 0.16, (mathcal{B}_{rm Norm}))%$, which is the most precise measurement to date. Finally, we have searched for an intermediate hidden-strangeness pentaquark decay $P^+_stophi p$. We see no evidence for this intermediate decay and set an upper limit on the product branching fraction of ${cal B}(Lambda_c^+to P^+_s pi^0)times {cal B}(P^+_stophi p) <8.3times 10^{-5}$ at 90% confidence level.
Using a successful framework for describing S-wave hadronic decays of light hyperons induced by a subprocess $s to u (bar u d)$, we presented recently a model-independent calculation of the amplitude and branching ratio for $Xi^-_b to Lambda_b pi^-$ in agreement with a LHCb measurement. The same quark process contributes to $Xi^0_c to Lambda_c pi^-$, while a second term from the subprocess $cs to cd$ has been related by Voloshin to differences among total decay rates of charmed baryons. We calculate this term and find it to have a magnitude approximately equal to the $s to u (bar u d)$ term. We argue for a negligible relative phase between these two contributions, potentially due to final state interactions. However, we do not know whether they interfere destructively or constructively. For constructive interference one predicts ${cal B}(Xi_c^0 to Lambda_c pi^-) = (1.94 pm 0.70)times 10^{-3}$ and ${cal B}(Xi_c^+ to Lambda_c pi^0) = (3.86 pm 1.35)times 10^{-3}$. For destructive interference, the respective branching fractions are expected to be less than about $10^{-4}$ and $2 times 10^{-4}$.
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