Detection of π⁺π⁻ atoms with the DIRAC spectrometer at CERN

Abstract

Abstract. The goal of the DIRAC experiment at CERN is to measure with high precision the lifetime of the π + π − atom (A 2π ), which is of order 3×10 −15 s, and thus to determine the s-wave ππ-scattering lengths difference |a 0 −a 2 |. A 2π atoms are detected through the characteristic features of π + π − pairs from the atom break-up (ionization) in the target. We report on a first high statistics atomic data sample obtained from p Ni interactions at 24 GeV/c proton momentum and present the methods to separate the signal from the background.

Radiative corrections in K→3π decays

“…In our opinion, this alternative way is best covered by dedicated measurements of the production and decay of pionium as performed e.g. in the DIRAC experiment [18,19]. Here, the theoretical background for the data analysis has already been provided with the necessary precision [40][41][42][43].…”

“…This article is devoted to an evaluation of the latter. Leaving aside for the moment the contributions from higher loops, we expect that K → 3π decays, with radiative corrections included, and combined with the information gained from K e4 decays [14][15][16][17] and the pionium lifetime [18,19], have the potential to test the very precise theoretical prediction of the scattering lengths [20,21].…”

Isospin symmetry breaking

“…The experiments on hadronic atoms [40][41][42] provide a beautiful possibility to directly extract the hadron-hadron scattering lengths from the experimental data without using the extrapolation to the threshold. However, the accuracy of the extraction of the scattering lengths critically depends on the possibility to gain a control on the isospin breaking corrections in the atomic observables.…”

Finite-size effect on two-particle production in continuous and discrete spectrum

“…The determination, on a percent level accuracy, of the breakup probability of the π + π − atoms produced by a high-energy beam in a thin target is of principle importance for a precise lifetime measurement of these atoms in the experiment DIRAC at CERN [1][2][3][4]. This experiment aims to measure the lifetime τ 10 of the π + π − atoms in the ground state with 10% precision.…”