Observation of $\pi^- K^+$ and $\pi^+ K^-$ atoms

TL;DR

This paper reports the first observation of hydrogen-like $$ mesonic atoms ($$ and $$), produced by high-energy proton interactions, with a significant detection of atomic pairs indicating atom breakup.

Contribution

The paper presents the first experimental observation of $$ mesonic atoms using the upgraded DIRAC setup at CERN, demonstrating the production and detection of these exotic atoms.

Findings

Observed 349  atomic pairs with 5.6 sigma significance

Produced  atoms using 24 GeV/c protons on platinum and nickel targets

Confirmed the existence of  mesonic atoms through characteristic atomic pair detection

Abstract

The observation of hydrogen-like $\pi K$ atoms, consisting of $\pi^- K^+$ or $\pi^+ K^-$ mesons, is presented. The atoms have been produced by 24 GeV/$c$ protons from the CERN PS accelerator, interacting with platinum or nickel foil targets. The breakup (ionisation) of $\pi K$ atoms in the same targets yields characteristic $\pi K$ pairs, called "atomic pairs", with small relative momenta in the pair centre-of-mass system. The upgraded DIRAC experiment has observed $349\pm62$ such atomic $\pi K$ pairs, corresponding to a signal of 5.6 standard deviations.