Search

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.

A pair of new hodoscopes made of 0.28 mm O scintillating fibre have been built and tested at T11 beam line of CERN PS. They consist of seven layers of fibres, and are read out using multi-anode photomultipliers. The average number of photoelectrons produced by a minimum ionizing particle is found to be about 11, which resulted in a very high detection efficiency (98.6%) at an appropriate threshold setting. The hit multiplicity for single-hit events was suppressed to 1.12 at this threshold setting, thanks to the Peak-Sensing Circuit used for the readout. A spatial resolution of ∼ 75 μ m and a time resolution of ∼ 0.56 ns (both in RMS) have been obtained with these hodoscopes. Further detailed study of the detector response such as double hit efficiency at very close distances, dependence on the incident angles has been performed.

The goal of the DIRAC experiment at CERN (PS212) is to measure the $\pi^+\pi^-$ atom lifetime with 10% precision. Such a measurement would yield a precision of 5% on the value of the $S$-wave $\pi\pi$ scattering lengths combination $|a_0-a_2|$. Based on part of the collected data we present a first result on the lifetime, $\tau=[2.91 ^{+0.49}_{-0.62}]\times 10^{-15}$ s, and discuss the major systematic errors. This lifetime corresponds to $|a_0-a_2|=0.264 ^{+0.033}_{-0.020} m_{\pi}^{-1}$.

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., Comment: 19 pages, 12 figures, 1 table

A hybrid-emulsion experiment has been carried out at KEK-PS to study S = −2 nuclei. With the analysis of about 35% of total emulsion, we have successfully found three events of sequential weak decay of a double-hypernucleus. The results of these E373 events are summarized together with those of past experiments. The first experimental study of the branching ratios of weak decay modes of double-hypernuclei has been performed.

We have developed a single-sided silicon strip detector with a sensitive area of 64×64 mm 2 as a real-time, fast and medium-segmented hodoscope for particle physics experiments in high beam background environment. The signal of each 2 mm wide readout strip was integrated in a hybrid preamplifier with a decay constant of 10 μs. The output was then amplified 20 times and shaped to a peaking time of ∼50 ns in a shaping amplifier. The performance was first examined on a test bench, and then with an intense π − /μ − /e − beam. The Equivalent Noise Charge (ENC, 1 σ ) at the preamplifier input was ∼2000 e (9% of the signal for single minimum ionizing particle ∼22 000 e ) at low beam rate, and increased by 10% (or 20%) at a high rate of 150 kHz (300 kHz) per readout-strip. As the cable length ( L ) from silicon board to preamplifier increased, the ENC increased from 2000 e for L =0.2 m to 2600 e for 0.5 m, 3500 e for 1 m, 3800 e for 1.5 m, and 4200 e for 2 m.

A hybrid emulsion experiment was carried out to study double-strangeness nuclei produced via Ξ− hyperon capture at rest with the expectation of ten times larger statistics than previous experiments. We have analyzed 5% of the total emulsion and found one “twin-hypernuclei” event involving the emission of two single-Λ hypernuclei and a nuclear fragment from a Ξ− hyperon stopping point. The event is interpreted as the decay of a Ξ − + 14 N atomic system to 5 Λ He + 5 Λ He + 4 He + neutron . The species of the Ξ−-atom and the fragmentation products are uniquely identified for the first time for twin-hypernuclei events. Combined with the results from a past hybrid-emulsion experiment, one double-Λ hypernucleus and three twin-hypernuclei events have been found from Ξ− captures on light emulsion nuclei. The ratio of the detected rate of double-Λ hypernuclei to that of twin-hypernuclei is compared with theoretical estimates.

In view of handling the anode signals from the position-sensitive photomultiplier a new front-end electronic circuit has been designed and realized, which allows one to suppress annoying effects of the cross-talk between adjacent channels. The circuit was tested in association with a scintillating fibre detector in a test beam at CERN. The basic performances, such as the detection efficiency and its uniformity, as well as the hit multiplicity for single tracks were compared to those of the conventional electronics based on the simple threshold discriminators. A great advantage of the peak-sensing circuit was experimentally demonstrated, in particular, in its real-time digitization of the track position without significant ambiguity of the track position due to the cross-talk.

A prototype of a scintillating fiber hodoscope was constructed with multi-cladding scintillating fibers and a multi anode photomultiplier (H6568, HAMAMTSU). The hodoscope has a seven structure of a scintillating fiber with 0.5 mm in diameter and stacked in closest packing in order to improve its efficiency and time resolution. The hodoscope has been tested using a proton beam. The time resolution of about 700 ps in FWHM has been obtained.

Fast and precise scintillating fiber detectors using KURARAY fibers and HAMAMATSU multianode H6568 tubes were constructed for new experiments (DIRAC and COMPASS) at CERN. Tests of the DIRAC detector and COMPASS prototype show high detection efficiency, good time and space resolution.

In view of implementing a topological trigger criterion in cosmic-ray detectors for efficient selection of required events a preliminary test has been done with a scintillating-fibre hodoscope using a position-sensitive photomultiplier. A space resolution of ∼0.6 mm in σ with a detection efficiency >97% has been obtained for charged particles of 5 GeV/c passing through 2×2 mm squared fibres. A custom-made discriminator, suppressing the cross-talk between adjacent fibres, allowed a better definition of track position even at high incident angles compared to the simple threshold discriminators. Simplicity as well as reliability of the prototype allow us to foresee a topological trigger device incorporated in different type of detectors, such as cosmic gamma or electron telescopes, solar neutron detectors and so on.

The H dibaryon search experiment using a scintillating-fiber target was performed at KEK-PS. The preliminary results are reported. No evidence of the H production via C(K−, K+) reaction has been found. The upper limit of the ratio of the H production to the quasi-free process, K−+(p)→K++Ξ− is 0.05–0.6% forM H⩽2200 MeV/c2. For 2200 MeV/c2⩽MH⩽2230 MeV/c2, the upper limit for the production of the H which decays into Σ−+p is 0.6–07%.

The invariant double-differential cross section, E1E2d6 delta/dp(1)3dp(2)3, and the double-spin asymmetry, A(LL), for inclusive multi-gamma pair production in which gamma-rays came from neutral mesons were measured with a 200 GeV/c longitudinally-polarized proton beam and a longitudinally-polarized proton target. Most of the multi-gamma pairs comes from two-jet type events which are sensitive to partonic interaction. The A(LL) values were found to be consistent with zero. The invariant double-differential cross section for inclusive pi0pi0 production was also measured. These measured cross sections are consistent with LUND Monte Carlo simulations. Using the LUND Monte Carlo simulation package with the Carlitz-Kaur model of spin dependent distribution functions of valence quarks, the A(LL) values have been compared with theoretical predictions of gluon polarization, DELTAG/G. The results put restrictions on the size of DELTAG/G in the region of 0.05 less-than-or-similar-to x less-than-or-similar-to 0.35.

The S= −2 hypernuclear states formed through (K − , K + ) reactions have been studied by using the 1.66 GeV/c K − beam provided by KEK Proton Synchrotron. Investigation has been done on the stars due to the capture of the ξ − hyperons at rest in nuclear emulsion, which hyperons were produced in (K − , K + ) reactions. We observed a clear case of an absorption of ξ − hyperon at rest by a nucleus followed by a back-to-back emission of two single-Λ hypernuclei

The new type of visual detector, scintillating fiber, has been used as an active target to search the H-dibaryon by using (K − ,K + ) interactions. The fiber target is readout by two set of image-intensifier tubes providing three dimensional information for production, decay and interaction of Ξ − 's. Several hundreds of visual images of stopped and inflight interactions of Ξ ' s are expected to be accumulated. Sensitivity for the H search as well as the feasibility to study Ξ − p , Ξ − C interactions are discussed.

The forward-angle cross sections of the (K − , K + ) reaction on nuclear targets of C, Al, Cu, Ag and Pb have been measured at the K − incident momentum of 1.65 GeV/ c in a side range K + momentum. The obtained K + momentum spectrum for each target nucleus is characterized by a peak in the high-momentum region ( p K + > 0.95 GeV/ c ) and a broad bump in the low-momentum region ( p K + c ). A comparison of the measured spectrum with a DWIA calculation indicates that the quasifree process, K − (p) → K + Ξ − , is dominant in the high-momentum region. The target mass-number dependence of the cross section is well fitted with A α , where α is 0.38 ± 0.03 for the high-momentum region and 0.56 ± 0.02 for the low-momentum region. The result indicates that the quasifree process takes place at the peripheral of the target nucleus, whereas the two-step processes are dominant in the low-momentum region.

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., Comment: 49 pages, 37 figures. Figures 1, 2, 5 and 28 are removed because of size limitations imposed by hep-ex. They don't offer essential information. Latex class file 'elsart.cls' also provided

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 efficiency $\geq$95% of detecting the events in the relative momentum range of interest., 21 pages, 11 figures

A double-hyperfragment event has been found in a hybrid-emulsion experiment. It is identified uniquely as the sequential decay of ( 6)(LambdaLambda)He emitted from a Xi(-) hyperon nuclear capture at rest. The mass of ( 6)(LambdaLambda)He and the Lambda-Lambda interaction energy DeltaB(LambdaLambda) have been measured for the first time devoid of the ambiguities due to the possibilities of excited states. The value of DeltaB(LambdaLambda) is 1.01+/-0.20(+0.18)(-0.11) MeV. This demonstrates that the Lambda-Lambda interaction is weakly attractive.

A fast and precise scintillating fiber (SciFi) hodoscope based on KURARAY SciFi and HAMAMATSU multianode H6568 tubes was constructed to be used in DIRAC experiment (PS-212) at CERN for tracking and topological triggering information. The front-end electronics was custom developed to allow on-line discrimination after built-in dynamic cross-talk rejection. Preliminary tests of the detector using more than 300 read-out channels have shown an excellent stability and optimum performance for this system: a uniform detection efficiency of 98%, a 125 μm single hit and ∼0.5 mm two-hit space resolution, 0.6 ns time resolution.

We present experimental results on measuring a single spin asymmetry in eta-meson production in the interaction of transversely polarized protons and antiprotons at p(lab) = 200 GeV/c with a proton target in the region 0.2 < x(F) < 0.7 for p up arrow p, 0.3 < x(F) < 0.7 for (p) over bar up arrow p and 0.7 < p(T) < 2.0 GeV/c. A comparison of single spin asymmetries in pi- and eta-meson production is made

The highest-energy measurement of ??L(pp) and the first ever measurement of ??L(p¯p), the differences between proton-proton and antiproton-proton total cross sections for pure longitudinal spin states, are described. Data were taken using 200-GeV/c polarized beams incident on a polarized-proton target. The results are measured to be ??L(pp)=-42±48(stat)±53(syst) ?b and ??L(p¯p)=-256±124(stat)±109(syst) ?b. Many tests of systematic effects were investigated and are described, and a comparison to theoretical predictions is also given. Measurements of parity nonconservation at 200 GeV/c in proton scattering and the first ever of antiproton scattering have also been derived from these data. The values are consistent with zero at the 10-5 level.

Surprisingly large polarizations in hyperon production by unpolarized protons have been known for a long time. The spin dynamics of the production process can be further investigated with polarized beams. Recently, a negative asymmetry A(N) was found in inclusive Lambda(0) production with a 200 GeV/c transversely polarized proton beam. The depolarization D-NN in p up arrow + p --> Lambda(0) + X has been measured with the same beam over a wide x(F) range and at moderate p(tau). D-NN reaches positive values of about 30% at high x(F) and p(T) similar to 1.0 Ge V/c. This result shows a sizable spin transfer from the incident polarized proton to the outgoing Lambda(0).

The pi(0) inclusive and semi-inclusive, single-spin asymmetries have been measured using transversely polarized, 200-GeV/c proton and antiproton beams colliding with an unpolarized hydrogen target. The measured asymmetries are consistent with zero within the experimental uncertainties for the kinematic region -0.153 GeV/c were not correct. These data indicate that PQCD expectations seem confirmed and the higher-twist contribution to the single-spin asymmetry in pi(0) production at x(F)=0 is not large. Additional evidence for such a conclusion comes from the measurement of a semi-inclusive pi(0) asymmetry, where associated charged particles are detected opposite to the pi(0) azimuthal direction. This experiment also provides high-statistics data on the inclusive pi(0) cross sections for pp and collisions at root s approximate to 19.4 GeV.

The two-spin parameter A(LL) in inclusive pi-0 production by longitudinally-polarized protons and antiprotons on a longitudinally-polarized proton target has been measured at the 200 GeV Fermilab spin physics facility, for pi-0,s at X(F) = 0 with 1 less-than-or-equal-to p(t) less-than-or-equal-to 3 GeV/c. The results exclude, at the 95% confidence level, values of A(LL) (pp) > 0.1 and < -0.1, for pi-0,s produced by protons, and values of A(LL) (antiproton-antiproton) > 0.1 and < -0.2 for incident antiprotons. The relevance of A(LL) (pp) for the gluon spin density is discussed. The data are in good agreement with "conventional", small or zero, gluon polarization.

The single-spin asymmetry A(N) (antiproton_proton) for inclusive pi-0 production at 0.5 < p(t) < 2 GeV/c by 200 GeV transversely-polarized antiprotons on protons has been measured at Fermilab over a wide range of X(F). We observed that A(N) (antiproton_proton) has the same sign, a similar X(F) dependence, and about half the magnitude as A(N) (pp) for pi-0 production by protons. We also present the ratio of the spin-averaged cross sections for pi-0 production by antiprotons and by protons.

States of 12 Λ C formed in the (K − , π − ) reaction have been studied for momentum transfers up to 260 MeV/ c , using an incident K − beam of 800 MeV/ c momentum. The angular distributions for the g.s. and for a peak at 11 meV have been measured between 0° and 19° in the laboratory. Limits on the splitting of the 11 MeV peak and on the formation of low-lying excited states are given.

A kinematically complete experiment on the 2 H(α, αp)n reaction has been done by using an α-beam of 165 MeV and a liquid deuterium target. The experimental differential cross sections are compared with those calculated in the frame of the impulse approximation. The data indicate the presence of the resonance state of 5 He.

The (${\ensuremath{\pi}}^{\ensuremath{-}},2n$) process has been studied in a kinematically complete experiment on three neighboring $1p$-shell nuclei. Distributions of excitation energy of the residual nuclei, of recoil momentum, of the angle between the emitted neutrons, and the angle between the sum and the difference of the neutron momenta have been measured. From the excitation spectra a striking analogy with ($p,2p$) data has been found in the region of $p$-shell nucleon removal. The possibility of $s$-shell nucleon removal has been investigated. The data are compared with theoretical predictions.

Results from ..pi../sup + -/ elastic and inelastic scattering from /sup 12/C and /sup 40/Ca are reported. The data were all taken at an incident momentum of 800 MeV/c over an angular range from 4/sup 0/ to 38/sup 0/. The elastic data are compared to first-order optical model calculations in momentum space; qualitative agreement is obtained. The inelastic data (from /sup 12/C only) are compared to distorted-wave Born approximation calculations, and reasonable agreement is found if realistic inelastic transition densities are used.

The (${\ensuremath{\pi}}^{\ensuremath{-}}$,$2n$) reaction on $^{6}\mathrm{Li}$ and $^{7}\mathrm{Li}$ nuclei has been studied in a kinematically complete experiment. Distributions of excitation energy of residual nuclei, of recoil momentum, and of angle spanned by relative and c.m. momenta of emitted neutrons have been extracted. The reaction mechanism has been investigated through a comparison with theoretical predictions.

Differential cross sections dSsigma/d dE have been measured with a germanium telescope for the ( , /sup +prime/) reaction on C, Ca, Sn, and Pb at incident energies of 67, 85, and 100 MeV. The yields at backward angles along with available information on total reaction cross sections are interpreted in terms of a simple classical model in which it is assumed that free-space pion nucleon interaction parameters prevail inside the nucleus. Among the experimental findings which can be accounted for in terms of the model (to within 10-15%) are the measured pion reaction cross sections and their dependence on target mass, the energy and mass dependences of the differential scattering cross section at backward angles, and the ratio of normal to charge exchange scattering cross sections. The model appears to overestimate the ratios to reaction and scattering cross sections but there appear to be some inconsistencies in the available data.

The (π −,2n) reaction on40Ca has been investigated via the determination of the momenta for the two neutrons emitted simultaneously following the absorption ofπ −. The38K excitation spectrum, opening-angle distributions, and recoil momentum distribution are determined. The direct mechanism is established for transitions to the lower part of the38K excitation spectrum.

The deuteron break-up reaction has been studied by bombarding liquid deuterium with 156 MeV protons. The kinematic conditions were completely determined by measuring the angles θ 1 , θ 2 of the two outgoing protons and the energy E 1 of one of them. The partial differential cross section d 3 σ/ d Ω 1 d Ω 2 d E 1 was measured for a range of kinematic conditions including large momentum transfers. The results are compared to calculations based on the simple impulse approximation (spectator model) and those including the effects of multiple scattering terms and final-state interactions.

A first study of the reaction 14N(π−, 2n)12C at rest is reported. The nucleon pair quantum numbers favoured in the absorption process are deduced. The excitation spectrum of the residual 12C nucleus differs from the one obtained through the (π+, 2p) reaction.

The cross sections for the reaction 2H(p, 2p)n are calculated by taking into account final state interactions including S, P and D waves for all pairs of outgoing nucleons. The D-state wave functions of the deuteron are also included in the calculation. The angular correlation distributions of the cross sections are compared with data at 156 MeV incident protons.

A first kinematically complete investigation of the reaction 16O(π−, 2n)14N with stopped pions has been performed at the CERN SC over a wide angular range. Measured distributions in excitation energy of the residual nucleus, recoil momentum, and angle between the two neutrons are presented. They are compared with theoretical predictions on two-hole states and with detailed calculations on the quasi-free two-nucleon absorption process.

The (..pi../sup +/,p) and (..pi../sup +/,d) reactions on 1p shell nuclei are studied between T/sub ..pi../ = 32 and 81 MeV. The cross sections to both the continuum and discrete two-body final states are given. The spectra and angular distributions of the (..pi../sup +/,p) continuum are interpreted in terms of a two-nucleon pion absorption mechanism. The /sup 13/C(..pi../sup +/,p) spectrum of discrete states is similar to the /sup 13/C(p,d) spectrum at the same momentum transfer. The two-neutron-pickup (..pi..,d) reaction is found to strongly favor transitions in the 1p shell of angular momentum transfer, L = 2. The relative strength of these transitions varies the same way as the corresponding (p,t) cross sections. No L = 0 transitions are clearly identifiable. There is an indication of the S = 1 /sup 7/Li(..pi../sup +/,d)/sup 5/Li(16.7 MeV) 3/2/sup +/ transition, contrary to the prediction that S = 1 transitions are suppressed. The (..pi../sup +/,d) angular distributions are compared to the calculations of Betz and Kerman. 2 tables.