Recently the LHCb Collaboration announced the first observation of nontrivial structures in the double-
J
/
ψ
mass spectrum in the mass range 6.2–7.2 GeV, and a theoretical coupled-channel analysis ...of these data performed in Dong et al. (Phys Rev Lett 126:132001, 2021) evidenced the existence of a new state
X
(6200) close to the double-
J
/
ψ
threshold. Although its molecular interpretation seems the most plausible assumption, the present data do not exclude an admixture of a compact component in its wave function, for which a fully-charmed compact tetraquark is the most natural candidate. It is argued in this work that the QCD string model is compatible with the existence of a compact
c
c
c
¯
c
¯
state bound by QCD forces just below the double-
J
/
ψ
threshold. A nontrivial interplay of the quark dynamics associated with this compact state and the molecular dynamics provided by soft gluon exchanges between
J
/
ψ
mesons is discussed and the physical
X
(6200) is argued to be a shallow bound state, in agreement with the results of the aforementioned coupled-channel analysis of the LHCb data.
We propose a practical parametrization for the line shapes of near-threshold states compatible with all requirements of unitarity and analyticity. The coupled-channel system underlying the proposed ...parametrization includes bare poles and an arbitrary number of elastic and inelastic channels treated fully nonperturbatively. The resulting formulas are general enough to be used for a simultaneous analysis of the data in all available production and decay channels of the (system of) state(s) under consideration for a quite wide class of reactions. As an example, we fit the experimental data currently available for several decay channels for the charged Z super((')) sub(b) states in the spectrum of bottomonia and find a good overall description of the data. We find the present data to be consistent with the Z sub(b)(10610) as a virtual state and with the Z sub(b)(10650) as a resonance, both residing very close to the BB* and B*B* threshold, respectively.
Numerous quarkonium(like) states lying near S-wave thresholds are observed experimentally. We propose a self-consistent approach to these near-threshold states compatible with unitarity and ...analyticity. The underlying coupled-channel system includes a bare pole and an arbitrary number of elastic and inelastic channels treated fully nonperturbatively. The resulting analytical parametrization is ideally suited for a combined analysis of the data available in various channels that is exemplified by an excellent overall description of the data for the charged Z(b)(10610) and Z(b)(10650) states.
The possibility for a common effective field theory for hadronic molecules with different heavy-quark flavours is examined critically. It is argued that such a theory does not allow one to draw ...definite conclusions for doubly heavy molecules. In particular, it does not allow one to relate binding energies for the molecules in the
c
-quark and
b
-quark sectors with controlled uncertainties. Therefore, while this kind of reasoning does not preclude from employing heavy-quark spin symmetry for charmonium- and bottomonium-like states separately within a well established effective field theory framework, relations between different heavy-quark sectors can only be obtained using phenomenological approaches with uncontrolled uncertainties.
We discuss a model-independent effective-field-theory-based approach to bottomonium-like states
and
and a prediction of the properties of their spin partners with the quantum numbers
,
, and
.
In a recent paper Phys. Rev. D 98, 074023 (2018), the most up-to-date experimental data for all measured production and decay channels of the bottomoniumlike states Zb(10610) and Zb(10650) were ...analyzed in a field-theoretical coupled-channel approach which respects analyticity and unitarity and incorporates both the pion exchange and a short-ranged potential nonperturbatively. All parameters of the interaction were fixed directly from data, and pole positions for both Zb states were determined. In this work we employ the same approach to predict in a parameter-free way the pole positions and the line shapes in the elastic and inelastic channels of the (still to be discovered) spin partners of the Zb states. They are conventionally referred to as WbJ's with the quantum numbers JPC = J++ (J = 0, 1, 2). It is demonstrated that the results of our most advanced pionful fit, which gives the best χ2/d.o.f. for the data in the Zb channels, are consistent with all WbJ states being above-threshold resonances which manifest themselves as well-pronounced hump structures in the line shapes. On the contrary, in the pionless approach, all WbJ's are virtual states which can be seen as enhanced threshold cusps in the inelastic line shapes. Since the two above scenarios provide different imprints on the observables, the role of the one-pion exchange in the B(*)B¯(*) systems can be inferred from the once available experimental data directly.
Multiple lattice evidences support the existence of a confining but chirally symmetric regime of QCD above the chiral symmetry restoration crossover at Tch≃155 MeV. This regime is characterised by an ...approximate chiral spin symmetry of the partition function, which is a symmetry of the colour charge and the confining electric part of the QCD Lagrangian. It is traditionally believed that confinement should automatically induce spontaneous breaking of chiral symmetry, which would preclude the existence of a confining but chirally symmetric regime of QCD at high temperatures. We employ a well-known solvable quark model for QCD in 3+1 dimensions that is chirally symmetric and manifestly confining and argue that while confinement indeed induces dynamical breaking of chiral symmetry at T=0, a chiral restoration phase transition takes place at some critical temperature Tch. Above this temperature, the spectrum of the model consists of chirally symmetric hadrons with approximate chiral spin symmetry.
A
bstract
It has been proposed recently (
Phys. Rev. Lett.
115
(2015) 022001) that the charmoniumlike state named
X
(3915) and suggested to be a 0
++
scalar, is just the helicity-0 realisation of the ...2
++
tensor state
χ
c
2
(3930). This scenario would call for a helicity-0 dominance, which were at odds with the properties of a conventional tensor charmonium, but might be compatible with some exotic structure of the
χ
c
2
(3930). In this paper, we investigate, if such a scenario is compatible with the assumption that the
χ
c
2
(3930) is a
D
∗
D
¯
∗
molecular state — a spin partner of the
X
(3872) treated as a shallow bound state. We demonstrate that for a tensor molecule the helicity-0 component vanishes for vanishing binding energy and accordingly for a shallow bound state a helicity-2 dominance would be natural. However, for the
χ
c
2
(3930), residing about 100 MeV below the
D
∗
D
¯
∗
threshold, there is no a priori reason for a helicity-2 dominance and thus the proposal formulated in the above mentioned reference might indeed point at a molecular structure of the tensor state. Nevertheless, we find that the experimental data currently available favour a dominant contribution of the helicity-2 amplitude also in this scenario, if spin symmetry arguments are employed to relate properties of the molecular state to those of the
X
(3872). We also discuss what research is necessary to further constrain the analysis.
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