First measurement of the $\pi^+\pi^-$ atom lifetime

TL;DR

This paper reports the first measurement of the $^+^-$ atom lifetime by the DIRAC experiment at CERN, providing a new estimate of the $$ scattering lengths with implications for low-energy QCD.

Contribution

It presents the initial measurement of the $^+^-$ atom lifetime, achieving a 10% precision, and derives the $$ scattering lengths from this measurement.

Findings

Measured lifetime: $ au=[2.91^{+0.49}_{-0.62}]\times 10^{-15}$ s

Derived scattering length difference: $|a_0 - a_2|=0.264^{+0.033}_{-0.020} m_{\pi}^{-1}$

Discussed systematic errors affecting the measurement.

Abstract

The goal of the DIRAC experiment at CERN (PS212) is to measure the $\pi^+\pi^-$ atom lifetime with 10% precision. Such a measurement would yield a precision of 5% on the value of the $S$-wave $\pi\pi$ scattering lengths combination $|a_0-a_2|$. Based on part of the collected data we present a first result on the lifetime, $\tau=[2.91 ^{+0.49}_{-0.62}]\times 10^{-15}$ s, and discuss the major systematic errors. This lifetime corresponds to $|a_0-a_2|=0.264 ^{+0.033}_{-0.020} m_{\pi}^{-1}$.