DIRAC Experiment Research Articles

The ionisation hodoscope of the DIRAC experiment

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.

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

The goal of the DIRAC experiment at CERN is to measure with high precision the lifetime of the π+π− atom (A2π), which is of order 3 × 10−15 s, and thus to determine the s-wave ππ-scattering lengths difference |a0 − a2|. A2π 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.

The ionisation hodoscope of the DIRAC experiment

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.

Dead time losses and their measurement in DIRAC

Different sources of the dead time in the DIRAC experiment are considered including the front-end electronics, the multilevel trigger scheme and the data acquisition system. Total dead time losses are measured using an admixture of test signals at the inputs of the front-end electronics. The influence of the dead time and of the electronic circuitry on the event selection at the data analysis is estimated as well.

A Monte Carlo calculation of the pionium break-up probability with different sets ofpionium target cross sections

Chiral perturbation theory predicts the lifetime of pionium, a hydrogen-likeπ+π− atom, to better than 3% precision. The goal of the DIRAC experiment at CERN is toobtain and check this value experimentally by measuring the break-up probability ofpionium in a target. In order to accurately measure the lifetime one needs to know therelationship between the break-up probability and the lifetime to 1% accuracy. We haveobtained this dependence by modelling the evolution of pionic atoms in the target usingMonte Carlo methods. The model relies on the computation of the pionium–target-atominteraction cross sections. Three different sets of pionium–target cross sections with varyingdegrees of complexity were used: from the simplest first-order Born approximationinvolving only the electrostatic interaction to a more advanced approach, taking intoaccount multiphoton exchanges and relativistic effects. We conclude that, in order toobtain the pionium lifetime to 1% accuracy from the break-up probability, thepionium–target cross sections must be known with the same accuracy for the lowexcited bound states of the pionic atom. This result has been achieved, for lowZ targets, with the two most precise cross section sets. For largeZ targets only the set accounting for multiphoton exchange satisfies the condition.

Readout logic and its hardware implementation in the DIRAC experiment

Readout logic and architecture of the readout hardware of the DIRAC experiment at CERN are described. The data collection system is configured from dedicated and commercial readout branches running in a parallel hardware-controlled mode. The readout process is controlled by the trigger processors which may decide to reject an event during its acquisition. The system design provides a small dead time resulting in a sufficiently high rate capability.

Measurement of π+π− atom lifetime at DIRAC

The DIRAC experiment aims to measure the lifetime of π + π − 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 −15s 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. The results on the π + π − atom detection and the preliminary estimation of their lifetime are presented.

Drift chamber readout system of the DIRAC experiment

A drift chamber readout system of the DIRAC experiment at CERN is presented. The system is intended to read out the signals from planar chambers operating in a high current mode. The sense wire signals are digitized in the 16-channel time-to-digital converter boards which are plugged in the signal plane connectors. This design results in a reduced number of modules, a small number of cables and high noise immunity. The system has been successfully operating in the experiment since 1999.

The multilevel trigger system of the DIRAC experiment

The multilevel trigger system of the DIRAC experiment at CERN is presented. It includes a fast first level trigger as well as various trigger processors to select events with a pair of pions having a low relative momentum typical of the physical process under study. One of these processors employs the drift chamber data, another one is based on a neural network algorithm and the others use various hit-map detector correlations. Two versions of the trigger system used at different stages of the experiment are described. The complete system reduces the event rate by a factor of 1000 with respect to detectors’ single rates, the detection efficiency of events in the relative momentum range of interest being ⩾95%.

The time-of-flight detector of the DIRAC experiment

The construction and performance of a large area time-of-flight detector for the DIRAC experiment at CERN is reported. With an average time resolution of 123ps per counter at rates up to 1MHz, it allows excellent separation of pπ− from π+π− pairs up to 4.6GeV/c momentum, as well as of Coulomb-correlated pion pairs from accidentals. The optimization of scintillator material, photomultiplier performance and readout electronics is described.

Third level trigger of the DIRAC experiment

A fast and complete programmable high level trigger processor for the DIRAC experiment at CERN was designed and arranged based on state-of-art Field Programmable Gate Array (FPGA) technology. The implemented logic was created from Monte Carlo simulation results and further checked with real experimental data. Correspondence between desired and implemented logic was proved previously by use of a complete digital pattern generator built also with FPGA technology. The resulting trigger processor provides a selection of charged particle pairs with a small relative momentum.

First level trigger of the DIRAC experiment

The first level trigger logic of the DIRAC experiment at CERN is described. A parallel running of different trigger modes with tagging of events and optional independent prescaling is realized. The CAMAC-based trigger system is completely computer controlled.

The neural network first level trigger for the DIRAC experiment

The neural network first level trigger for the DIRAC experiment at CERN is presented. Both the neural network algorithm used and its actual hardware implementation are described. The system uses the fast plastic scintillator information of the DIRAC spectrometer. In 210 ns it selects events with two particles having low relative momentum; such events are selected with an efficiency of more than 0.94. The corresponding rate reduction for background events is a factor of 2.5.

Data acquisition software for DIRAC experiment

The structure and basic processes of data acquisition software of the DIRAC experiment for the measurement of π +π − atom lifetime are described. The experiment is running on the PS accelerator of CERN. The developed software allows one to accept, record and distribute up to 3 Mbytes of data to consumers in one accelerator supercycle of 14.4 s duration. The described system is successfully in use in the experiment since its startup in 1998.

The influence of the hadronic interaction on the pionium wave functions

Abstract The influence of the hadronic ππ interaction on the wave functions of pionium (the Coulomb bound π + π − system) at distances ⩽10 fm is calculated for s-states with principal quantum numbers n ⩽4. Hadronic ππ potentials are used that reproduce the scattering phase shifts of two-loop chiral perturbation theory. The pionium wave functions ψ n ( r ) are obtained by integrating the coupled Schrodinger equations for the ( π + π − , π 0 π 0 ) system. We find that, for r ⩽10 fm, n 3/2 ψ n ( r ) is practically independent of n. From this we conclude that the production rates of the s-states of pionium are proportional to n −3 , a result needed for the interpretation of the DIRAC experiment currently running at CERN.

ππ scattering

We demonstrate that, together with the available experimental information, chiral symmetry determines the low energy behaviour of the $\pi\pi$ scattering amplitude to within very small uncertainties. In particular, the threshold parameters of the S-, P-, D- and F-waves are predicted, as well as the mass and width of the $\rho$ and of the broad bump in the S-wave. The implications for the coupling constants that occur in the effective Lagrangian beyond leading order and also show up in other processes, are discussed. Also, we analyze the dependence of various observables on the mass of the two lightest quarks in some detail, in view of the extrapolations required to reach the small physical masses on the lattice. The analysis relies on the standard hypothesis, according to which the quark condensate is the leading order parameter of the spontaneously broken symmetry. Our results provide the basis for an experimental test of this hypothesis, in particular in the framework of the ongoing DIRAC experiment: The prediction for the lifetime of the ground state of a $\pi^+\pi^-$ atom reads $\tau=(2.90\pm 0.10)10^{-15} sec$.

Dirac experiment

The main objective of DIRAC experiment is the measurement of the lifetime τ of the exotic hadronic atom consisting of π + and π − mesons. The lifetime of this atom is determined by the decay mode π + π − → π 0 π 0 due to the strong interaction. Through the precise relationship between the lifetime and the S-wave pion-pion scattering length difference | a 0 − a 2| for isospin 0 and 2 (respectively), a measurement of τ with an accuracy of 10% will allow a determination of | a 0 − a 2| at a 5% precision level. Pion-pion scattering lengths have been calculated in the framework of chiral perturbation theory with an accuracy below 5%. In this way DIRAC experiment will provide a crucial test of the chiral symmetry breaking scheme in QCD effective theories at low energies.

Coherent and incoherent atomic scattering: formalism and application to pionium interacting with matter

The experimental determination of the lifetime of pionium provides avery important test of chiral perturbation theory. This quantity isdetermined in the DIRAC experiment at CERN. In the analysis of thisexperiment, the breakup probabilities of pionium in matter are neededto high accuracy as a theoretical input. We study in detail theinfluence of the target electrons.They contribute through screening and incoherent effects. We useDirac-Hartree-Fock-Slaterwavefunctions in order to determine the corresponding form factors.We find that the inner-shell electrons contribute less thanthe weakly bound outer electrons. Furthermore, we establish a morerigorous estimate for the magnitude of the contribution from thetransverse current (magnetic terms thus far neglected in thecalculations).

On the lifetime of the π+π− atom

The π + π − atom decays – in the ground state – predominantly into two neutral pions. We present a general expression for the corresponding decay width in the framework of QCD (including photons). It contains all terms at leading and next-to-leading order in isospin breaking. The result allows one to evaluate the combination | a 0− a 2| of ππ S-wave scattering lengths from π + π − lifetime measurements, like the one presently performed by the DIRAC experiment at CERN.

A prototype threshold Cherenkov counter for DIRAC

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.