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Register a new userThe $Z_c(3900)$ peak does not come from the triangle singularity
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We compare contributions from the triangle diagram and the $Dbar D^*$ bubble chain to the processes of $e^{+}e^{-}rightarrow J/psipi^{+}pi^{-}$ and $e^{+}e^{-}rightarrow (Dbar D^*)^mppi^{pm}$. By fitting the $J/psipi$ maximum spectrum and the $Dbar D^*$ spectrum, we find that the triangle diagram cannot explain the new data from BESIII Collaboration at center of mass at 4.23GeV and 4.26GeV, simultaneously. On the contrary, the molecular assignment of $Z_c(3900)$ gives a much better description.
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Assuming the newly observed $Z_c(3900)$ to be a molecular state of $Dbar D^*(D^{*} bar D)$, we calculate the partial widths of $Z_c(3900)to J/psi+pi;; psi+pi;; eta_c+rho$ and $Dbar D^*$ within the light front model (LFM). $Z_c(3900)to J/psi+pi$ is the channel by which $Z_c(3900)$ was observed, our calculation indicates that it is indeed one of the dominant modes whose width can be in the range of a few MeV depending on the model parameters. Similar to $Z_b$ and $Z_b$, Voloshin suggested that there should be a resonance $Z_c$ at 4030 MeV which can be a molecular state of $D^*bar D^*$. Then we go on calculating its decay rates to all the aforementioned final states and as well the $D^*bar D^*$. It is found that if $Z_c(3900)$ is a molecular state of ${1oversqrt 2}(Dbar D^*+D^*bar D)$, the partial width of $Z_c(3900)to Dbar D^*$ is rather small, but the rate of $Z_c(3900)topsi(2s)pi$ is even larger than $Z_c(3900)to J/psipi$. The implications are discussed and it is indicated that with the luminosity of BES and BELLE, the experiments may finally determine if $Z_c(3900)$ is a molecular state or a tetraquark.
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.
The COMPASS experiment recently discovered a new isovector resonance-like signal with axial-vector quantum numbers, the $a_1(1420)$, decaying to $f_0(980)pi$. With a mass too close to and a width smaller than the axial-vector ground state $a_1(1260)$, it was immediately interpreted as a new light exotic meson, similar to the $X$, $Y$, $Z$ states in the hidden-charm sector. We show that a resonance-like signal fully matching the experimental data is produced by the decay of the $a_1(1260)$ ground state into $K^ast(Kpi)bar{K}$ and subsequent rescattering through a triangle singularity into the coupled $f_0(980)pi$ channel. The amplitude for this process is calculated using a novel method based on partial-wave projections. For the first time, the triangle singularity model is fitted to the partial-wave data of the COMPASS experiment. Despite having less parameters, this fit shows a slightly better quality than the one using a resonance hypothesis and thus eliminates the need for an additional resonance in order to describe the data. We thereby demonstrate for the first time that a resonance-like structure in the experimental data can be described by rescattering through a triangle singularity, providing evidence for a genuine three-body effect.
The newly observed hidden-charm tetraquark state $Z_{cs}(3985)$, together with $Z_c(3900)$ and $X(4020)$, are studied in the combined theoretical framework of the effective range expansion, compositeness relation and the decay width saturation. The elastic effective-range-expansion approach leads to sensible results for the scattering lengths, effective ranges and the compositeness coefficients, $i.e.$, the probabilities to find the two-charm-meson molecule components in the tetraquark states. The coupled-channel formalism by including the $J/psipi$ and $Dbar{D}^*/bar{D}D^*$ to fulfill the constraints of the compositeness relation and the decay width, confirms the elastic effective-range-expansion results for the $Z_c(3900)$, by using the experimental inputs for the ratios of the decay widths between $Dbar{D}^*/bar{D}D^*$ and $J/psipi$. With the results from the elastic effective-range-expansion study as input for the compositeness, we generalize the discussions to the $Z_{cs}(3985)$ by including the $J/psi K^{-}$ and $D_s^{-}D^{*0}/D_s^{*-}D^{0}$, and predict the partial decay widths of the $J/psi K^{-}$. Similar calculations are also carried out for the $X(4020)$ by including the $h_cpi$ and $D^*bar{D}^*$, and the partial decay widths of the $h_cpi$ is predicted. Our results can provide useful guidelines for future experimental measurements.
The decay $Z_c(3900)^pmtoomegapi^pm$ is searched for using data samples collected with the BESIII detector operating at the BEPCII storage ring at center-of-mass energies $sqrt{s}=4.23$ and $4.26$~GeV. No significant signal for the $Z_c(3900)^pm$ is found, and upper limits at the 90% confidence level on the Born cross section for the process $e^+e^-to Z_c(3900)^pmpi^mptoomegapi^+pi^-$ are determined to be $0.26$ and $0.18$ pb at $sqrt{s}=4.23$ and 4.26 GeV, respectively.
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