In this work, the Coulomb effects (Coulomb correlations) in pi+pi- pairs produced in p + Ni collisions at 24 GeV=c, are studied using experimental pi+pi- pair distributions in Q, the relative momentum in the pair center-of-mass system (c.m.s.), and its projections Q(L) (longitudinal component) and Q(t) (transverse component) relative to the pair direction in the laboratory system (LS). The major part of the pion pairs ("Coulomb pairs") is produced in the decay of rho, omega and Delta resonances and other short-lived sources. In these pairs, the significant Coulomb interaction occurs at small Q, dominating the pi+pi- interaction in the final state. The minor part of the pairs ("non-Coulomb pairs") is produced if one or both pions arose from long-lived sources like eta, eta ' or from different interactions. In this case, the final state interaction is practically absent. The Q, Q(L), and Q(t) distributions of the Coulomb pairs in the c.m.s. have been simulated assuming they are described by the phase space modified by the known point-like Coulomb correlation function A(C)(Q), corrected for small effects due to the nonpointlike pair production and the strong two-pion interaction. The same distributions of non-Coulomb pairs have been simulated according to the phase space, but without A(C)(Q). In all Q(t) intervals, the experimental Q(L) spectrum shows a peak around Q(L) = 0 caused by the Coulomb final state interaction. The full width at half maximum increases with Q(t) from 3 MeV/c for 0 < Q(t) < 0.25 MeV/c to 11 MeV/c for 4.0 < Q(t) < 5.0 MeV/c. The experimental Q(L) distributions have been fitted with two free parameters: the fraction of Coulomb pairs and the normalization constant. The precision of the description of these distributions is better than 2% in Q(t) intervals 2-3, 3-4, and 4-5 MeV/c and better than 0.5% in the total Q(t) interval 0-5 MeV/c. It is shown that the number of Coulomb pairs in all Q(t) intervals, including the small Q(t) (small opening angles theta in the LS) is calculated with theoretical precision better than 2%. The comparison of the simulated and experimental numbers of Coulomb pairs at small Q(t) allows us to check and correct the detection efficiency for the pairs with small. (0.06 mrad and smaller). It is shown that Coulomb pairs can be used as a new physical tool to check and correct the quality of the simulated events. The special property of the Coulomb pairs is the possibility of checking and correcting the detection efficiency, especially for the pairs with small opening angles.

The DIRAC experiment at CERN investigated in the reaction p(24 GeV/c) + Ni the particle pairs K+K-, pi(+pi)-, and p p over line 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 the time-of-flight (TOF) technique-the 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 Q(L) were analyzed first in a Coulomb model, which takes into account only the Coulomb final -state interaction (FSI) and assuming pointlike pair production. This Coulomb model analysis leads to a K+K- yield increase of about four at Q(L) = 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 also strong FSI via the resonances f(0)(980) and a(0)(980) and a variable distance r* between the produced K mesons besides Coulomb FSI. This analysis was based on three different parameter sets for the pair production. For the 70% subsample and with the best parameters, 3680 +/- 370 K+K- pairs were 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 adapted DIRAC experiment at the CERN PS accelerator observed for the first time long-lived hydrogenlike π^{+}π^{-} atoms, produced by protons hitting a beryllium target. A part of these atoms crossed the gap of 96 mm between the target and a 2.1  μm thick platinum foil, in which most of them dissociated. Analyzing the observed number of atomic pairs, n_{A}^{L}=436_{-61}^{+157}|_{tot}, the lifetime of the 2p state is found to be τ_{2p}=(0.45_{-0.30}^{+1.08}|_{tot})×10^{-11}  s, not contradicting the corresponding QED 2p state lifetime τ_{2p}^{QED}=1.17×10^{-11}  s. This lifetime value is three orders of magnitude larger than our previously measured value of the π^{+}π^{-} atom ground state lifetime τ=(3.15_{-0.26}^{+0.28}|_{tot})×10^{-15}  s. Further studies of long-lived π^{+}π^{-} atoms will allow us to measure energy differences between p and s atomic states and so to discriminate between the isoscalar and isotensor ππ scattering lengths with the aim to check QCD predictions.

The adapted DIRAC experiment at the CERN PS accelerator observed for the first time long-lived hydrogen-like π+π− atoms, produced by protons hitting a beryllium target. A part of these atoms crossed the gap of 96 mm and got broken up in the 2.1 μm thick platinum foil. Analysing the observed number of atomic pairs, nA = 436 +157 −61 ∣∣∣ tot, the lifetime of the 2p state is found to be τ2p = (0.45+1.08 −0.30 ∣∣∣ tot) · 10 −11s, not contradicting the corresponding QED 2p state lifetime τ 2p = 1.17 · 10−11s. This lifetime value is three orders of magnitude larger than our previously measured value of the π+π− atom ground state lifetime τ = (3.15+0.28 −0.26 ∣∣∣ tot) · 10−15s. Further studies of long-lived π+π− atoms will allow to measure energy differences between p and s atomic states and so to determine ππ scattering lengths with the aim to check QCD predictions.

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 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.

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.

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 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.

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.

B. Adevap, L. Afanasyevl, Z. Berkab, V. Brekhovskikho, G. Caragheorgheopolm, T. Cechakb, M. Chibaj, S. Costantiniq, S. Constantinescum, C.C. Detraza, A. Doudarevl, D. Drijarda, I. Evangeloud, M. Ferro-Luzzi, M.V. Gallasa,p, J. Gerndtb, R. Giacomichf , P. Gianottie, M. Giardonie, D. Goldinq, F. Gomezp, A. Gorino, O. Gortchakovl , C. Guaraldoe, M. Hansroula, R. Hosekb, M. Iliescue,m, M. Jabitskil, N. Kalininan, V. Karpoukhinel, J. Klusonb, M. Kobayashig, P. Kokkasd,q, V. Komarovl, A. Koulikovl, A. Kouptsovl, V. Krouglovl, L. Krougloval, K.-I. Kurodak, A. Lambertof , A. Lanaroa,e, V. Lapshino, R. Lednickyc, P. Levi Sandrie, A. Lopez Aguerap, V. Lucherinie, T. Makii, I. Manuilovo, N. Manthosd, M. Medinskyo, L. Montaneta, L. Nemenova,l, M. Nikitinl, K. Okadah, V. Olchevskiil, M. Pentiam, A. Penzof , C. Petrascue,m, M. Plop, T. Pontam, D. Popm, Zh. Pustylnikl, G.F. Rappazzof , A. Riazantsevo, X.M. Rodriguezp, A. Rodriguez Fernandezp, V. Rykalino, C. Santamarinap, J. Saboridop, J. Schacherr, C. Schuetzq, A. Sidorovo, J. Smolikc, F. Takeutchih, A. Tarasovl, L. Tauscherq, M.J. Tobarp, F. Triantisd, S. Trousovn, S. Vlachosq, V. Yazkovn, Y. Yoshimurag, P. Zrelovl