How the small hyperfine splitting of P-wave mesons evades large loop corrections

TL;DR

This paper explains why the hyperfine splitting of P-wave mesons remains small despite large loop corrections, due to the nature of the coupling driven by spin-one pair creation, aligning with recent experimental findings.

Contribution

It demonstrates that small hyperfine splitting is a generic feature in models with spin-one pair creation, explaining recent bottomonium and charmonium observations.

Findings

Hyperfine splitting remains small despite large mass shifts.

The effect is a generic feature of models with spin-one pair creation.

Recent bottomonium states confirm the theoretical prediction.

Abstract

The recent discoveries of the bottomonia states h_b(1P) and h_b(2P) confirm the quark model prediction, already verified in the charmonia sector, that the hyperfine splitting of P-wave mesons is very small. The striking agreement is somewhat surprising because the non-relativistic result, for which the splitting is zero, may be modified due to large mass shifts from coupling to open flavour meson pairs. This paper is based on the observation that in most models hyperfine splitting remains small despite what are in many cases large mass shifts. This effect is shown to be a generic feature of models in which the coupling is driven by the creation of a spin-one pair.