Speaker
Description
Whether an exotic hadron is elementary or composite is a statement about which fields appear in the effective Lagrangian. The Weinberg criterion has been invoked on the X(3872) to argue for a molecular nature, but it is applied to LHCb data fitted with a Flatté amplitude, which contains an elementary X field: the molecular conclusion is drawn from a fit that assumes the opposite --- and the fit works well.
We present an alternative parametrization from an auxiliary-field Lagrangian in which the X is dynamically generated, its coupling to the constituent hadrons fixed at the Landau value. It reproduces the deuteron effective range from np scattering with no free parameters. Applied to the X, a good fit at the corresponding Landau coupling is unavoidable if the X is indeed deuteron-like.
The analogy meets a limit in production. The X is copiously produced at high transverse momentum, where no deuteron is expected, and CMS reports an order-of-magnitude enhancement of prompt X over ψ(2S) in Pb–Pb. Reproducing the observed yields molecularly appears problematic.
These are symptoms of one fact: the X, like other exotic hadrons, has a compact tetraquark core while looking molecular at large distances. This is the conclusion reached in the Born–Oppenheimer approach.