The DIRAC spectrometer installed at CERN PS was upgraded in order to study simultaneously A(2 pi) and A(pi K) atoms, namely the bound states of two pi mesons, and of pi and K mesons, respectively. The detector system can now accept a high intensity beam of 2-6 x 10(11) primary protons per second. The electronics and the data-acquisition system can handle a very large amount of data to identify pi, K, p, e and mu, allowing the selection of pi pi and pi K pairs in the offline analysis. The resolution of the longitudinal and transverse components of the relative momentum Q of each meson pair in their center-of-mass system with respect to the direction of the pair was substantially improved. The analysis of their distributions allowed an reliable separation between the meson pairs originating from hadronic atoms and the backgrounds permitting the measurement of the lifetimes of hadronic atoms in the ground state and pi-pi, pi-K s-wave scattering lengths. The upgraded setup also allowed the study of the long-lived excited states of pi pi atoms. (C) 2016 Elsevier B.V. All rights reserved.

Experiment DIRAC at CERN PS detects 349 ± 62 pairs from π−K+ and π+K− atoms and makes observation of exotic atoms consist of pion and kaon. It allows to measure a difference of S-wave pion-kaon scattering length with isospin 1/2 and 3/2: |a01/2−a03/2|. Values of pion-kaon scattering lengths are predicted in a frame of ChPT and LQCD. Therefore investigation of π−K+ and π+K− atoms gives possibility to check these predictions for simplest hadronhadron system with s-quark.

B. Adeva a, L. Afanasyev b, A. Anania c, S. Aogaki d, A. Benelli e, V. Brekhovskikh f, T. Cechak e, M. Chiba g, P. Chliapnikov f, P. Doskarova e, D. Drijard h, A. Dudarev b, M. Duma d, D. Dumitriu d, D. Fluerasu d, A. Gorin f, O. Gorchakov b, K. Gritsay b, C. Guaraldo i, M. Gugiu d, M. Hansroul h, Z. Hons j, S. Horikawa k, Y. Iwashita l, V. Karpukhin b, J. Kluson e, M. Kobayashi m, V. Kruglov b, L. Kruglova b, A. Kulikov b, E. Kulish b, A. Kuptsov b, A. Lamberto c, A. Lanaro n, R. Lednicky o, C. Mariñas a, J. Martincik e, L. Nemenov b,h, M. Nikitin b, K. Okada p, V. Olchevskii b, V. Ovsiannikov q, M. Pentia d, A. Penzo r, M. Plo a, P. Prusa e, G. Rappazzo c, A. Romero Vidal a, A. Ryazantsev f, V. Rykalin f, J. Saborido a, J. Schacher s,∗, A. Sidorov f, J. Smolik e, F. Takeutchi p, L. Tauscher t, T. Trojek e, S. Trusov u, T. Urban e, T. Vrba e, V. Yazkov u, Y. Yoshimura m, M. Zhabitsky b, P. Zrelov b

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 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 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.

Pions, originated from pionium decay, are characterised by low relative momentum and cross upstream detectors at rather small relative distance. These atomic pairs cause double energy release by ionisation in a sensitive volume of the detector with respect to the ionisation loss by a single pion. Therefore, the uncertainties resulting from the inefficiency in detecting two tracks with relative distance approaching to zero can be significantly reduced. The previous dE/dx detector consisted of 2 planes by 16 vertically oriented 2 mm thick scintillator slabs [1]. Therefore it could resolve close tracks only in one projection. It had geometrical inefficiency due to approximately 0.5 mm wide gaps between slabs. New Ionisation Hodoscope consists of 4 planes: two planes with vertically and two planes with horizontally oriented slabs. Adjacent slabs are separated by thinner gaps (about 60 μm). So it allows to distinguish close tracks in both projections with better acceptance. We will demonstrate the feasibility of separation of double ionisation events from single ones with the help of the new dE/dx detector taking into account a system of corrections, which is different from [2].

We present evidence for the first observation of electromagnetically bound π±K∓-pairs (πK-atoms) with the DIRAC experiment at the CERN-PS. The πK-atoms are produced by the 24GeV/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 πK-scattering lengths, the measurement of which is the goal of the experiment. From these first data we derive a lower limit for the mean life of 0.8 fs at 90% confidence level.

A new threshold Cherenkov detector using C4F10 gas radiator was built and put into operation in the DIRAC-II experiment at CERN. Running on the C4F10 at room temperature and atmospheric pressure, the detector discriminates between pions and kaons in the momentum range of 4–8GeV/c. A compact radiator–gas recirculation system including a gas–liquid separation unit, hollow-fibre membranes and molecular sieves ensures gas purity for a long term of operation without a significant loss of the gas. The system is robust and stable and the pressure in the two detector vessels is regulated in the range of ±0.5mbar. We report on the design and the technical aspects of the detector and its response in the DIRAC 2007 run.

The goal of the DIRAC experiment at CERN (PS212) is to measure theππ atom lifetime with 10% precision. Such a measurement would yield a precision of 5% on the value of the S-waveππ scattering lengths combination |a0 − a2|. Based on part of the collected data we present a first result on the lifetime, τ = [

The goal of the DIRAC experiment at CERN is to measure with high precision the lifetime of the $\pi^+\pi^-$ atom ($A_{2\pi}$), which is of order $3\times10^{-15}$ s, and thus to determine the s-wave $\pi\pi$-scattering lengths difference $|a_{0}-a_{2}|$. $A_{2\pi}$ atoms are detected through the characteristic features of $\pi^+\pi^-$ 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.

The ionisation hodoscope (IH) of the DIRAC (PS 212) experiment at CERN is optimised to separate ionisation signals produced by otherwise unresolved double tracks from ionisation signals produced by a single particle. While its total thickness is only 1.5% of a radiation length, it consists of four planes and provides two measurements of ionisation loss in each of the two projections. It suppresses the background from single-track events by a factor 50 while keeping more than 80% of double-track events. The IH design and performance during data taking in 2001–2003 are described.

The DIRAC spectrometer has been commissioned at CERN with the aim of detecting $\pi^+ \pi^-$ atoms produced by a 24 GeV/$c$ high intensity proton beam in thin foil targets. A challenging apparatus is required to cope with the high interaction rates involved, the triggering of pion pairs with very low relative momentum, and the measurement of the latter with resolution around 0.6 MeV/$c$. The general characteristics of the apparatus are explained and each part is described in some detail. The main features of the trigger system, data-acquisition, monitoring and setup performances are also given.

Based on observation [L.G. Afanasyev et al., Phys. Lett. B 308 (1993) 200] of 272 -449 atoms formed by ~r + and ~ran estimation of the atom lifetime in the ground (1 S) state is obtained: ~'l = (2.9+~a) • 10-15 s or ~'t > 0.6.10-15 s at 90% confidence level. Atoms were produced in inclusive pTa interactions at 70 GeV.

We have designed, built and tested a gas threshold Cherenkov counter as prototype for a larger counter foreseen for use in the DIRAC experiment, at CERN. We describe the performances of the counter on a test beam.

Analysis is carried out of the experimental data on the inverse electroproduction of pions in /sup 7/Li nuclei (..pi../sup +/+/sup 7/Li..-->..e/sup +/+e/sup -/+X) at kinetic energy 380 MeV. On the basis of about 260 reaction events recorded previously the isovector Dirac nucleon form factor F/sup V//sub 1/ was determined at three values of the square of the time-like four-momentum transfer: k/sup 2/ = 0.09, 0.15, and 0.22 (GeV/c)/sup 2/.