Abstract
The radiative transitions from the neutral exotic $Z_c(4020)^0$ resonance to $X(3872)$ with emission of a photon and the pionic transitions from the charged $Z_c(4020)^\pm$ to $X(3872)$ and a charged pion $\pi^\pm$ are considered for both charmoniumlike states being charmed meson-antimeson molecules. The underlying processes for these transitions are the decays of the charmed vector meson to pseudoscalar plus a photon or a pion. It is found that the discussed transition rates typically amount to a branching fraction of several permil with a peak at the vector meson pair threshold.
Highlights
The radiative transitions from the neutral exotic Zcð4020Þ0 resonance to Xð3872Þ with emission of a photon and the pionic transitions from the charged Zcð4020ÞÆ to Xð3872Þ and a charged pion πÆ are considered for both charmoniumlike states being charmed meson-antimeson molecules
The four-quark component is likely dominated by a molecular JPC 1⁄4 1þþ state made from S-wave DÃ0D 0 pairs and their charge conjugates. (A discussion of the arguments for such description can be found e.g., in the review [5].) Most recently this picture of mostly exotic structure of the Xð3872Þ state has received an additional boost from the observation [6] of the decay Xð3872Þ → χc1π0, which transitions would be very small for a pure charmonium [7] and are expected at about the observed level for a four-quark system [7,8,9], in particular for a DÃ0D 0 þ D Ã0D0 molecule
The Zcð4020Þ isotopic triplet of resonances with the quantum numbers IGðJPÞ 1⁄4 1þð1þÞ observed [10,11] near the threshold of charmed vector meson pairs DÃD Ã likely presents another example of a molecular state
Summary
The radiative transitions from the neutral exotic Zcð4020Þ0 resonance to Xð3872Þ with emission of a photon and the pionic transitions from the charged Zcð4020ÞÆ to Xð3872Þ and a charged pion πÆ are considered for both charmoniumlike states being charmed meson-antimeson molecules. The charmoniumlike exotic resonance Xð3872Þ observed [1,2,3,4] within a fraction of MeV from the threshold of the charmed meson pairs DÃ0D0 commands a considerable interest due to its rather exotic properties that very strongly suggest that this state is dominantly a four-quark object with a relatively small admixture of a cccharmonium.
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