440 research outputs found
Can X(3915) be the tensor partner of the X(3872)?
It has been proposed recently (Phys. Rev. Lett. 115 (2015), 022001) that the
charmoniumlike state named X(3915) and suggested to be a scalar, is
just the helicity-0 realisation of the tensor state .
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 . In this paper, we
investigate, if such a scenario is compatible with the assumption that the
is a molecular state - a spin partner of the
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 , residing about 100 MeV below the 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.Comment: LaTeX2e, 23 pages, 2 figures, version to appear in JHE
Charge symmetry breaking as a probe for the real part of eta--nucleus scattering lengths
We demonstrate that one can use the occurrence of charge symmetry breaking as
a tool to explore the eta--nucleus interaction near the eta threshold. Based on
indications that the cross section ratio of pi+ and pi0 production on nuclei
deviates from the isotopic value in the vicinity of the eta production
threshold, due to, e.g., pi0-eta mixing, we argue that a systematic study of
this ratio as a function of the energy would allow to pin down the sign of the
real part of the eta-nucleus scattering length. This sign plays an important
role in the context of the possible existence of eta-nucleus bound states.Comment: 4 pages, 1 figur
Spin partners of the and revisited
We study the implications of the heavy-quark spin symmetry for the possible
spin partners of the exotic states and in the
spectrum of bottomonium. We formulate and solve numerically the coupled-channel
equations for the states that allow for a dynamical generation of these
states as hadronic molecules. The force includes short-range contact terms and
the one-pion exchange potential, both treated fully nonperturbatively. The
strength of the potential at leading order is fixed completely by the pole
positions of the states such that the mass and the most prominent
contributions to the width of the isovector heavy-quark spin partner states
with the quantum numbers () come out as predictions.
Since the accuracy of the present experimental data does not allow one to fix
the pole positions of the 's reliably enough, we also study the pole
trajectories of their spin partner states as functions of the binding
energies. It is shown that, once the heavy-quark spin symmetry is broken by
means of the physical and masses, especially the pion tensor force
has a significant impact on the location of the partner states clearly
demonstrating the need of a coupled-channel treatment of pion dynamics to
understand the spin multiplet pattern of hadronic molecules.Comment: 21 pages, 5 figures, 1 tabl
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