On enhanced corrections from quasi-degenerate states to heavy quarkonium observables

TL;DR

This paper investigates how quasi-degenerate states affect heavy quarkonium observables, showing that certain enhanced effects are suppressed to higher-order corrections due to the absence of specific matrix elements.

Contribution

It provides a formulation demonstrating that leading-order enhanced effects from quasi-degenerate states vanish, pushing their influence to higher-order corrections in heavy quarkonium.

Findings

Enhanced effects appear only at fifth-order in energy corrections.

Leptonic decay width corrections are affected at fourth-order.

Leading-order corrections vanish due to matrix element structure.

Abstract

It is well known that in perturbation theory existence of quasi-degenerate states can rearrange the order counting. For a heavy quarkonium system, naively, enhanced effects ($l$-changing mixing effects) could contribute already to the first-order and third-order corrections to the wave function and the energy level, respectively, in expansion in $\alpha_s$. We present a formulation and note that the corresponding (lowest-order) corrections vanish due to absence of the relevant off-diagonal matrix elements. As a result, in the quarkonium energy level and leptonic decay width, the enhanced effects are expected to appear, respectively, in the fifth- and fourth-order corrections and beyond.