First observation of long-lived $\pi^+ \pi^-$ atoms

TL;DR

The paper reports the first observation of long-lived $^+$ atoms, enabling new measurements of pion interactions and scattering lengths, building on previous short-lived state observations.

Contribution

It presents the first detection of long-lived $^+$ pion atoms, expanding the understanding of exotic atomic states and their potential for scattering length measurements.

Findings

Detection of 436 ^+$ atom breakup events

Observation of states with lifetimes around 10^{-11} s or longer

Significant signal-to-noise ratio exceeding 7 standard deviations

Abstract

After observing and investigating the double-exotic $\pi^+\pi^-$ atom with the ground state lifetime $\tau$ of about $3 \times 10^{-15}$~s, the upgraded DIRAC experiment at the CERN PS accelerator observes for the first time long-lived states of the same atom with lifetimes of about $10^{-11}$~s and more. The number of characteristic pion pairs resulting from the breakup (ionisation) of long-lived $\pi^+\pi^-$ atoms amounts to $436\pm61$, corresponding to a signal-to-error ratio of better than 7 standard deviations. This observation opens a new possibility to measure energy differences between $p$ and $s$ atomic states and so to determine $\pi \pi$ scattering lengths.