Study of Di-Pion Transitions Among Upsilon(3S), Upsilon(2S), and Upsilon(1S) States

TL;DR

This paper measures decay matrix elements for hadronic transitions among Upsilon states, analyzing pion pair distributions to understand decay dynamics and account for anomalies, providing new insights into bottomonium transitions.

Contribution

It introduces detailed measurements of decay matrix elements for Upsilon transitions, revealing previously ignored amplitude terms and addressing the di-pion mass anomaly.

Findings

Observed and modeled the di-pion invariant mass anomaly.

Measured decay matrix elements for multiple Upsilon transitions.

Identified suppressed amplitude terms in decay processes.

Abstract

We present measurements of decay matrix elements for hadronic transitions of the form Upsilon(nS) -> pi pi Upsilon(mS) where (n, m) = (3, 1), (2, 1), and (3, 2). We reconstruct charged and neutral pion modes with the final state Upsilon decaying to either mu+mu- or e+e-. Dalitz plot distributions for the twelve decay modes are fit individually as well as jointly assuming isospin symmetry, thereby measuring the matrix elements of the decay amplitude. We observe and account for the anomaly previously noted in the di-pion invariant mass distribution for the Upsilon(3S) -> pi pi Upsilon(1S) transition and obtain good descriptions of the dynamics of the decay using the most general decay amplitude allowed by partial conservation of the axial-vector current (PCAC) considerations. The fits further indicate that the Upsilon(2S) -> pi pi Upsilon(1S) and Upsilon(3S) -> pi pi Upsilon(2S) transitions also show the presence of terms in the decay amplitude that were previously ignored, although at a relatively suppressed level.