Observation of ${\pi }^{-}{K}^{+}$ and ${\pi }^{+}{K}^{-}$ Atoms

The observation of hydrogenlike $\pi K$ atoms, consisting of ${\pi }^{-}{K}^{+}$ or ${\pi }^{+}{K}^{-}$ mesons, is presented. The atoms are produced by $24\mathrm{GeV}/c$ protons from the CERN PS accelerator, interacting with platinum or nickel foil targets. The breakup (ionization) of $\pi K$ atoms in the same targets yields characteristic $\pi K$ pairs, called “atomic pairs,” with small relative momenta $Q$ in the pair center-of-mass system. The upgraded DIRAC experiment observed $349\pm 62$ such atomic $\pi K$ pairs, corresponding to a signal of 5.6 standard deviations. This is the first statistically significant observation of the strange dimesonic $\pi K$ atom.

Figure 1

Inclusive $\pi K$ production in the $24\mathrm{GeV}/c$ $p\text{−}\mathrm{Ni}$ interaction $p+\mathrm{Ni}\to {\pi }^{-}{K}^{+}+X$. The ionization or breakup of $A_{K\pi }$ leads to atomic pairs. (For more details, see the text in the section entitled “Bound and free $\pi K$ pair production.”)

Figure 2

General view of the DIRAC setup. 1: target station. 2: first shielding wall. 3: microdrift chambers. 4: scintillating fiber detector. 5: ionization hodoscope. 6: second shielding wall. 7: vacuum tube. 8: spectrometer magnet. 9: vacuum chamber. 10: drift chambers. 11: vertical hodoscope. 12: horizontal hodoscope. 13: aerogel Cherenkov counter. 14: heavy gas Cherenkov counter. 15: nitrogen Cherenkov counter. 16: preshower. 17: muon detector.

Figure 3

Difference of particle generation times for events with positively charged particle momenta $(4.4–4.5)\mathrm{GeV}/c$. Experimental data (histogram) are fitted by the event sum (black, solid): $K^{+}{\pi }^{-}$ (red, dashed), ${\pi }^{+}{\pi }^{-}$ (blue, dotted), $\mathit{p}{\pi }^{-}$ (magenta, dotted dashed), and accidentals (green, constant).

Figure 4

$Q_L$ distributions of potential ${\pi }^{-}{K}^{+}$ pairs after applying the selection as described in the text. Events with positive $Q_L$ are suppressed compared to those with negative $Q_L$ due to the lower acceptance and lower production cross section.

Figure 5

Top: $Q$ distribution of the experimental ${\pi }^{-}{K}^{+}$ and ${\pi }^{+}{K}^{-}$ pairs fitted by the sum of the simulated distributions of atomic (red, dotted dashed), Coulomb (blue, dashed), and non-Coulomb pairs (magenta, dotted). Free (Coulomb and non-Coulomb) pairs are shown as black and solid line. Bottom: difference distribution between the experimental and simulated free pair distributions compared with the simulated atomic pairs. The number of observed atomic pairs is denoted by $n_A$.