DIRAC Experiment Research Articles

Investigation of K+K− pairs in the effective mass region near 2mK

The DIRAC experiment at CERN investigated in the reaction p(24 GeV/c)+Ni the particle pairs K+K−,π+π−, and pp¯ 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 QL 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 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 also strong FSI via the resonances f0(980) and a0(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.2 MoreReceived 12 May 2022Accepted 15 July 2022DOI:https://doi.org/10.1103/PhysRevD.106.032006Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by SCOAP3.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasBound statesQuantum chromodynamicsScattering theoryStrong interactionTechniquesParticle productionParticles & FieldsAtomic, Molecular & Optical

Maximal acceleration constant puzzle for DIRAC experiment in CERN

Maximal acceleration constant puzzle for DIRAC experiment in CERN

First Measurement of a Long-Lived π^{+}π^{-} Atom Lifetime.

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 96mm 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.

Study of Hadronic Hydrogen-like Atoms in the DIRAC Experiment at PS CERN

Study of Hadronic Hydrogen-like Atoms in the DIRAC Experiment at PS CERN

Observation of π^{-}K^{+} and π^{+}K^{-} Atoms.

The observation of hydrogenlike πK atoms, consisting of π^{-}K^{+} or π^{+}K^{-} mesons, is presented. The atoms are produced by 24 GeV/c protons from the CERN PS accelerator, interacting with platinum or nickel foil targets. The breakup (ionization) of πK atoms in the same targets yields characteristic πK pairs, called "atomic pairs," with small relative momenta Q in the pair center-of-mass system. The upgraded DIRAC experiment observed 349±62 such atomic πK pairs, corresponding to a signal of 5.6 standard deviations. This is the first statistically significant observation of the strange dimesonic πK atom.

The estimation of production rates of , and atoms in proton–nucleus interactions at 450 GeV c–1

Short-lived () , and atoms as well as long-lived ( s) atoms produced in proton–nucleus interactions at 24 GeV c–1 are observed and studied in the DIRAC experiment at the CERN Proton Synchroton. The purpose of this paper is to show that the yields of the short-lived , and atoms in proton–nucleus interactions at 450 GeV c–1 and are estimated to be, respectively 67 ± 13, 31 ± 6 and 15 ± 2 times higher. This may allow a significant improvement of the precision of their lifetime measurement and and scattering length combinations and . The yields of the long-lived , and atoms at 450 GeV c–1 are estimated to be 265 ± 53, 120 ± 24 and 60 ± 9 times higher per time unit than at 24 GeV c–1. This may allow the resonance method to be used for measuring the Lamb shift in the atom and a new scattering length combination to be obtained.

Last results of DIRAC experiment on study hadronic hydrogen-like atoms at PS CERN

Results on study the hydrogen-like atoms consisting of charged pions and Kaons are presented. The first measurement of K+π and Kπ+ atoms lifetime was fulfilled basing on identification of 178±49Kπ pairs from the atom breakup. The measured lifetime is τ=(2.5−1.8+3.0) fs. This value is dictated by properties of the strong πK-interaction at low energy, namely S-wave πK scattering length. The first experimental value of the isospin-odd combination of S-wave πK scattering length was obtained |a0−|=13|a/2−a3/2|=(0.11−0.04+0.09)Mπ−1 (ai for isospin I).A dedicated experiment with π+π atoms allows further study of these already observed atoms. The preliminary results on observation of the long-lived (metastable) states of π+π atoms are presented. The observation of long-lived states opens the possibility to measure the energy difference between ns and np states — the Lamb shift.

First observation of long-lived π+π− atoms

After observing and investigating the double-exotic (a double-exotic atom is a bound system, in which both oppositely charged components are unstable particles like μ,π,K,…) π+π− atom with the ground state lifetime τ of about 3×10−15 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 π+π− 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 ππ scattering lengths.

FirstπKatom lifetime measurement and recent results from the DIRAC experiment

We report evidence for πK atoms production, using 24 GeV/ c proton beam from CERN PS interacting with a thin Ni target. We have identified (178 ± 49) πK pairs, which were produced in a bound state — πK atom, which was subsequently broken-up (ionized) in the Ni target. Our analysis yields a first measurement of the πK atom lifetime (2.5 −1.8 +3.0 ) fs [1]. This lifetime is connected in a model-independent way to the S-wave isospin-odd πK scattering length | a 0 − | = 1/3| a 1/2 − a 3/2 | = (0.11 −0.04 +0.09 ) M π −1 ( a I for isospin I ).

Parameters of scalar resonances from the combined analysis ofππscattering and coupled-channel data

Peculiarities of obtaining parameters for broad multichannel resonances from data are discussed analyzing the experimental data on processes ππ → ππ , , in the scalar channel in a model-independent approach based on analyticity and unitarity and using an uniformization procedure. It is shown that one can obtain a good description of the ππ scattering data from the threshold up to 1.89 GeV with values of resonance parameters cited in the PDG tables as preferred. However, in this case the representation of the ππ background is not satisfactory and the data on the coupled process ππ → are not well described, above 1.15 GeV even qualitatively. The combined analysis of the considered coupled processes is needed, which is also carried out satisfactorily. Then both above-indicated deficiencies related to the one-channel analysis are removed. The most important change concerns parameters of the f 0 (600) which are now in some accordance with the prediction by Weinberg on the basis of mended symmetry. The obtained ππ - scattering length agrees well with the values extracted from the data on K e 4 decay and DIRAC experiment and also with results from ChPT.

DIRAC experiment at CERN

The precise measurements of π + π - and πK atom lifetime allow to check the predictions of Chiral Perturbation Theory for the pion-pion s-wave scattering lengths with isospin 0 and 2 and for the pion-kaon scattering lengths with isospin 1/2 and 3/2. The DIRAC experiment with the latest results is presented together with the proposal for future investigations.

Determination of ππ scattering lengths from measurement of [formula omitted] atom lifetime

The DIRAC experiment at CERN has achieved a sizeable production of π + π − 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 ππ scattering length difference | a 0 − a 2 | = ( 0.2533 − 0.0078 + 0.0080 | stat | syst − 0.0073 + 0.0078 ) M π + − 1 has been attained, providing an important test of Chiral Perturbation Theory.

Finite-size correction to the pionium lifetime due to ω and η′ contributions

It is shown that for pNi interactions at 24 GeV/c, the fractions of the π±ωπ∓ and pairs with one pion from the ω and η′ decays, respectively, at small values of the relative momentum Q ⩽ 4 MeV/c of pions in the c.m. system of the π+π− pair are fω = 0.150 ± 0.019 and . With these values of fω and , the uncertainty in the pionium breakup probability and the corresponding overestimation of the pionium lifetime in the DIRAC experiment at CERN are ΔPbr/Pbr = (3.0 ± 1.0)% and Δτ/τ = (7.8 ± 2.6)%.

Micro-Drift Chamber as a precise vertex detector for the DIRAC experiment

A possible use of the Micro-Drift Chamber (MDC) as a high-resolution vertex detector in the upstream part of the DIRAC spectrometer was investigated. Simulations of different MDC layouts were performed with the help of the GARFIELD package. On the basis of the simulation results the optimal chamber geometry and gas mixture were selected. A one-cell prototype was produced and its characteristics were measured at different particle fluxes, various gas pressures and electronics thresholds. The observed data together with the expected MDC coordinate resolution allow this detector to be used in various fields, especially in the DIRAC experiment.

Evidence for πK-atoms with DIRAC

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 24 GeV/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±54πK-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.

Direct calculation of the probability of pionium ionization in the target

The goal of the DIRAC experiment at CERN is the lifetime measurement of pionium (π + π − atom). Its lifetime is mainly defined by the charge-exchange process π + π − → π 0 π 0. The value of the lifetime in the ground state is predicted in the framework of chiral perturbation theory with high precision: τ 1S = (2.9 ± 0.1) × 10−15 s. The method used by DIRAC is based on analysis of π + π −-pair spectra with small relative momenta in their center-of-mass system in order to find out the signal from pionium ionization (breakup) in the target. Pioniums are produced in proton-nucleus collisions and have relativistic velocities (γ > 10). For fixed values of the pionium momentum and the target thickness, the probability of pionium ionization in the target depends on its lifetime in a unique way; thus, the pionium lifetime can be deduced from the experimentally defined probability of pionium ionization. On the basis of ionization cross sections of pionium with target atoms, we perform the first direct calculation of the pionium ionization probability in the target.

Control and monitoring of the trigger system of the DIRAC experiment

In experiments with a large amount of electronics, it is important to provide monitoring for operation, flexibility to changes of the system configuration, and convenience of its use by experimentalists who are not experts in electronics. This work describes the organization of the completely computer-controlled trigger system of the DIRAC experiment at CERN.

First measurement of the [formula omitted] atom lifetime

The goal of the DIRAC experiment at CERN (PS212) is to measure the π+π− atom lifetime with 10% precision. Such a measurement would yield a precision of 5% on the value of the S-wave ππ scattering lengths combination |a0−a2|. Based on part of the collected data we present a first result on the lifetime, τ=[2.91−0.62+0.49]×10−15 s, and discuss the major systematic errors. This lifetime corresponds to |a0−a2|=0.264−0.020+0.033mπ−1.

MEASURING LIFETIME OF THE PIONIUM ATOM WITH THE DIRAC EXPERIMENT

The DIRAC experiment at CERN has been measuring the lifetime of the pionium atom, a bound state of a π+ and π- meson. The isospin 0 and 2 scattering length difference will be determined unambiguously once the value of the lifetime is known. In what follows we describe the experimental goals and the first results.

Dynamics of the pionium with the density matrix formalism

The evolution of pionium, the π+π− hydrogen-like atom, passing through matter is described by the density matrix formalism in the first Born approximation. We compare the probability-based calculations with the more precise density matrix formalism which takes into account the interference effects, and consider their influence on the pionium break-up probability. We apply our general result to the specific conditions of the DIRAC experiment at CERN.