X (3872) as a Universal and Composite State
X (3872) because of its mass that is just below D^0 D-^0* threshold and its quantum number j^pc=1^++ can be considered as a weakly bound molecule. It can be regarded as a non-relativistic system depicting universality and compositeness. Physical observables are called universal if they are insensitive to the range and other details of short-range interaction. Weinberg introduced compositeness criterion to classify a two-body composite system as the sum, with different weights, of a compact core and two distinct parts.
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