DIRAC

Low-energy QCD and specifically Chiral Perturbation Theory (ChPT) calculated $\pi\pi$ and $\pi K$ scattering lengths with per cent precision. For processes involving $u$- and $d$-quarks theoretical predictions have been experimentally checked by $\pi^+\pi^-$~atom lifetime measurement [1] and by analysis of $K$-decays [2,3]. Detection and lifetime measurement of $\pi K$~atom cast a look into processes which involve $s$-quark as well. We report evidence for $\pi K$ atoms production, using $24\:$GeV$/c$ proton beam from CERN PS interacting with a thin Ni target. We have identified $(178\pm 49)$ $\pi K$~pairs, which were produced and were subsequently broken-up (ionized) in the Ni target. Analysis yields a first measurement of the $\pi K$~atom lifetime $\left(2.5_{-1.8}^{+3.0}\right)$ fs [4]. This lifetime is connected in a model-independent way to the S-wave isospin-odd $\pi K$ scattering lengths difference $|a_0^-|=\frac{1}{3}|a_{1/2}-a_{3/2}|=\left(0.11_{-0.04}^{+0.09}\right)M_{\pi}^{-1}$ ($a_I$ for isospin~$I$). Through the measurement of the $\pi^+\pi^-$~atom (pionium) lifetime, the experiment obtained the S-wave $\pi^+\pi^-$ scattering lengths difference $|a_0-a_2|$ with 4% precision [1]. In 2011-2012 DIRAC collaboration collected data towards observation of long-lived (metastable) states of pionium. The observation of long-lived states opens the possibility to measure the energy difference between $ns$ and $np$ states and to determine the value of the combination $(2a_0 +a_2)$ of $S$-wave $\pi\pi$ scattering lengths. The experiment used two targets method: after production in the beryllium foil, atoms flied through a permanent magnetic field to reach the platinum ionization foil. The distance between foils is large enough for ns-states to vanish due to annihilation. Only $\pi^+\pi^-$ atoms in states with non-zero angular momentum can get into the second Pt target. We report unambiguous observation of long-lived (metastable) states of $\pi^+\pi^-$~atoms.

[1] B. Adeva, et al. (DIRAC Collaboration), Phys. Lett. B704 (2011) 24. [2] J.R. Batley, et al. (NA48/2 Collaboration), Eur. Phys. J. C64 (2009) 589. [3] J.R. Batley, et al. (NA48/2 Collaboration), Eur. Phys. J. C70 (2010) 635. [4] B. Adeva, et al. (DIRAC Collaboration), arXiv:1403.0845.