Evidence for πK-atoms with DIRAC

Abstract

We present evidence for the rst observation of electromagnetically bound π ± K ∓pairs (πK-atoms) with the DIRAC experiment at the CERN-PS. The πK-atoms are produced by the 24 GeV/c proton beam in a thin Pt-target and the π ± and K ∓mesons from the atom dissociation are analyzed in a two-arm magnetic spectrometer. The observed enhancement at low relative momentum corresponds to the production of 173 ± 54 πK-atoms. The mean life of πK-atoms is related to the s-wave πKscattering lengths, the measurement of which is the goal of the experiment. From these rst data we derive a lower limit for the mean life of 0.8 fs at 90% condence level.

Observation and investigation ofπKatoms

“…8. Numbers of π + K − and K + π − "atomic pairs" obtained with analysis of one-dimensional distributions over Q and |Q L | and two-dimensional (|Q L |, Q T ) distribution are presented in Table 1 to the previous investigation [12], the Pt data was analysed including the upstream detectors. The consequence is a decrease of the statistics, but on the other hand an increase of the Q T resolution.…”

“…In the present publication experimental data for πK atom investigation, collected in 2008-2010 and data collected in 2007 with Platinum (Pt) target [12] have been analysed with improved description of the setup response.…”

DIRAC experiment at CERN

“…The preshower detector (PSh) provide an additional electron/hadron separation to the N 2 Cerenkov, and his performance has been increasingly important after the cutting of the Ni Cerenkov in order to keep the level of electron rejection at the level of 98% [10]. With this modified detector and using only the downstream part of DIRAC for the tracking we have published the result of the Kπ analysis [11].…”

Experimental evidence for piK-atoms

“…[1] we use only detectors downstream of the dipole magnet. The trajectories are determined by the drift chambers, the pattern recognition starting from the coordinates in the last plane and extrapolating back to the target.…”

“…Electromagnetically bound π ∓ K ± -pairs (π ∓ K ± -atoms) have been observed for the first time in 2008 by our DIRAC-II Collaboration at CERN [1]. The π + K − -atom is unstable and decays through the strong force into π 0K0 (while π − K + -atoms decay into π 0 K 0 ).…”