After observing and investigating the double-exotic (adouble-exotic atom is a bound system, in which both oppositely charged components are unstable particles like mu, pi, K,...) pi(+)pi(-) atom with the ground state lifetime tau of about 3 x10(-15)s, the upgraded DIRAC experiment at the CERN PS accelerator observes for the first time long-lived states of the same atom with lifetimes of about 10(-11)s and more. The number of characteristic pion pairs resulting from the breakup (ionisation) of long-lived pi(+)pi(-) atoms amounts to 436 +/- 61, corresponding to a signal-to-error ratio of better than 7 standard deviations. This observation opens a new possibility to measure energy differences between p and s atomic states and so to determine pi pi scattering lengths. (C) 2015 CERN for the benefit of the DIRAC Collaboration. Published by Elsevier B.V.

The DIRAC experiment at CERN investigated in the reaction p(24 GeV/c)+Ni the particle pairs K+K−,π+π− and pp̄ with relative momentum Q in the pair system less than 100 MeV/c. Because of background influence studies, DIRAC explored three subsamples of K+K− pairs, obtained by subtracting – using time-of-flight (TOF) technique – background from initial Q distributions with K+K− sample fractions more than 70%, 50% and 30%. The corresponding pair distributions in Q and in its longitudinal projection QL were analyzed first in a Coulomb model, which takes into account only Coulomb final state interaction (FSI) and assuming point-like pair production. This Coulomb model analysis leads to a K+K− yield increase of about four at QL = 0.5 MeV/c compared to 100 MeV/c. In order to study contributions from strong interaction, a second more sophisticated model was applied, considering besides Coulomb FSI also strong FSI via the resonances f0(980) and a0(980) and a variable distance r∗ between the produced K mesons. This analysis was based on three different parameter sets for the pair production. For the 70% subsample and with best parameters, 3680± 370 K+K− pairs was found to be compared to 3900± 410 K+K− extracted by means of the Coulomb model. Knowing the efficiency of the TOF cut for background suppression, the total number of detected K+K− pairs was evaluated to be around 40000± 10%, which agrees with the result from the 30% subsample. The K+K− pair number in the 50% subsample differs from the two other values by about three standard deviations, confirming — as discussed in the paper — that experimental data in this subsample is less reliable. In summary, the upgraded DIRAC experiment observed increased K+K− production at small relative momentum Q. The pair distribution in Q is well described by Coulomb FSI, whereas a potential influence from strong interaction in this Q region is insignificant within experimental errors.

The proposed experiment is a further development of the DIRAC experiment already running at CERN PS. Up to now more than 21000 π+π− pairs originated from the π+π− atom (A2π) breakup were identified and the overall accuracy of the A2π lifetime is about 9% in accordance with the DIRAC proposal. This measurement allowed to extract the difference |a0 −a2| of s-wave ππ-scattering length with accuracy of 4.3%. The observation of long-lived (metastable) A2π states will be performed with the same setup. This observation opens a possibility to measure the energy difference between ns and np states and to determine the value of another combination 2a0 +a2 of ππ scattering length in a model-independent way. In combination with the first measurement it allows to get a0 and a2 separately. An additional measurement of the multiple scattering angles in different materials with accuracy better than 1% will be performed in parallel with the observation. This measurement is needed to improve the systematic accuracy in the A2π lifetime for the data already collected in 2008–2010.

The DIRAC experiment at CERN has achieved a sizeable production of pi(+)pi(-) atoms and has significantly improved the precision on its lifetime determination. From a sample of 21 227 atomic pairs, a 4% measurement of the S-wave pi pi scattering length difference |a(0) - a(2)| = (0.2533(-0.0078)(+0.0080)|stat(-0.0073)(+0.0078)|syst)M-pi+(-1) has been attained, providing an important test of Chiral Perturbation Theory. (C) 2011 Elsevier B.V. All rights reserved.

The DIRAC experiment aims to measure the lifetime of pi(+)pi(-) atoms in the ground state with 10% precision, using the 24 GeV/c proton beam of the CERN Proton Synchrotron. As the value of the above lifetime of order 10(-15)s is dictated by a strong interaction at low energy, the precise measurement of this quantity enables to determine a combination of S-wave pion scattering lengths to 5%. Pion scattering lengths have been calculated in the framework of chiral perturbation theory with high precision. Thus the accurate measurement of these values would submit the understanding of chiral symmetry breaking of QCD to a crucial test. Some preliminary results from the analysis of a 2000 data sample are presented.