First observation of π-K+ and π+K- atoms, their lifetime measurement and πK scattering lengths evaluation.

Abstract

The Low Energy QCD allows to calculate the ππ and πK scattering lengths with high precision. There are accurate relations between these scattering lengths and π + π − , π − K + , π + K − atoms lifetimes. The experiment on the first observation of π − K + and π + K − atoms is described. The atoms were generated in Nickel and Platinum targets hit by the PS CERN proton beam with momentum of 24 GeV/c. Moving in the target, part of atoms break up producing characteristic πK pairs (atomic pairs) with small relative momentum Q in their c.m.s. In the experiment, we detected n A = 349 ± 62 (5.6 standard deviations) π − K + and π + K − atomic pairs. The main part of πK pairs are produced in free state. The majority of such particles are generated directly or from short-lived sources as ρ, ω and similar resonances. The electromagnetic interactions in the final state create Coulomb pairs with a known sharp dependence on Q. This effect allows to evaluate the number of these Coulomb pairs. There is a precise ratio (∼1%) between the number of π − K + (π + K − ) Coulomb pairs with small Q and the number of produced π − K + (π + K − ) atoms. Using this ratio, we obtained the numbers of generated π − K + and π + K − atoms. The atom breakup probability in a target P br = n A /N A depends on the atom lifetime. Using such dependences for the Ni and Pt targets, known with a precision about 1%, the πK atom lifetime was measured and from this value the πK scattering lengths were evaluated. The presented analysis shows that the π − K + and π +