Search

The PS185 experiment at LEAR/CERN has investigated strangeness production in antiproton-proton collisions with final states such as $\overline{\Lambda} \Lambda$, $\overline{\Sigma}^0 \Lambda + c.c$, $\overline{\Sigma^+} \Sigma^+$, $\overline{\Sigma^-} \Sigma^-$ and $K_S K_S$. Results are presented from a study of about 32,000 {$K_S K_S X$} events obtained at several antiproton momenta in the regions of 1.45 and 1.7 GeV/c. The $\overline{p} p \to K_S K_S \eta$ cross sections extracted at these momenta constitute the first measurement of this reaction in flight and are broadly consistent with expectations of a phase-space extrapolation of branching ratios from annihilation at rest.

The PS185 experiment at LEAR/CERN has studied strangeness production in antiproton-proton collisions. Exclusive final states of antihyperon-hyperon pairs have been measured in order to investigate strange-quark production dynamics. A comprehensive data set is available for the Λ Λ channel and observables include total and differential cross sections, polarisations and spin correlations. Data from other hyperon channels, such as Σ 0 Λ + c.c. , Σ + Σ + and Σ − Σ − , are also available. The analysis of a spin transfer measurement for the p p → Λ Λ reaction using a polarised proton target is in progress.

The new nucleon area (NA) at PSI is designed as a multi-purpose facility with nucleon beams. The area can be used for experiments with polarized or unpolarized protons or neutrons. For both kinds of nucleons the available energy ranges from about 100 MeV up to a maximum of about 590 MeV.

An intensive program of n-p measurement of spin observables in elastic and pion production reactions has been performed at the Paul Scherrer Institute taking advantage of the high intensity polarized neutron beam. In the elastic reaction 16 different spin observables have been measured for kinetic energies from 210 to 590 MeV with the centre of mass angular range from 60 to 180 degrees. This will allow to perform direct reconstruction in the model independent way of the isospin I=0 scattering amplitudes in elastic NN interaction. Three body final state process \(\vec np - pp\pi ^ - \) with polarized neutrons in exclusive experiment has been investigated.

The kinetic energy spectrum and the polarization of the PSI neutron beam produced in the reaction 12C(p,n)X at 0° with 590 MeV polarized protons were investigated. A strong energy dependence of the neutron beam polarization is observed which was not expected at the time the neutron beam was built.

The analyzing power, $A_{oono}$ , and the polarization transfer observables $K_{onno}$ , $K_{os''so}$ and $K_{os''ko}$ have been measured in neutron-proton elastic scattering at 260, 318, 386, 472 and 538 MeV for c.m. scattering angles between 64° and 160°. The data were obtained at PSI with a polarized neutron beam and a polarimeter analyzing the transverse polarization of the outgoing proton. They make a significant impact on the knowledge of the Isospin I = 0 nucleon-nucleon scattering.

A study has been performed of the reaction pbar p -> 4K using in-flight antiprotons from 1.1 to 2.0 GeV/c incident momentum interacting with a hydrogen jet target. The reaction is dominated by the production of a pair of phi mesons. The pbar p -> phi phi cross section rises sharply above threshold and then falls continuously as a function of increasing antiproton momentum. The overall magnitude of the cross section exceeds expectations from a simple application of the OZI rule by two orders of magnitude. In a fine scan around the xi/f_J(2230) resonance, no structure is observed. A limit is set for the double branching ratio B(xi -> pbar p) * B(xi -> phi phi) < 6e-5 for a spin 2 resonance of M = 2.235 GeV and Width = 15 MeV.

The pbar p -> Ks Ks -> 4pi+/- cross section was measured at incident antiproton momenta between 0.6 and 1.9 GeV/c using the CERN Low Energy Antiproton Ring (LEAR). This investigation was part of a systematic study of in-flight antiproton-proton annihilations into two-neutral-meson final states in a search for hadronic resonances. A coarse scan of the pbar p -> Ks Ks cross section as a function of center-of-mass energy between 1.964 and 2.395 GeV/c^2 and a fine scan of the region surrounding the Xi(2220) are presented. Upper limits on the product branching ratio BR(Xi -> pbar p)BR(Xi -> Ks Ks) are determined for a wide range of mass and width assumptions based on the non-observation of the Xi(2220). A rise in the pbar p -> Ks Ks cross section is observed near 2.15 GeV/c^2, which is consistent with the f2(2150) resonance.

The cross sections of the reactions pp → φφ, φK+K− and 4K± have been measured by the JETSET (PS202) experiment at CERN in the invariant mass range from 2.15 to 2.43 GeV/c2. A spin-parity analysis of the φφ system has been performed. The absolute value of the φφ cross section and the ratios σ(φφ)/σ(φK+K−), σ(φφ)/σ(4K±) are large, in marked violation of the OZI rule. Waves of quantum numbers JPC = 2++ give the dominant contribution near threshold.

In a search for gluonic hadrons, the formation channels pp → KsKs, pp → ηη, pp → π0η and pp → π0π0 were studied in the mass range from 2.1 to 2.4 GeV using the Jetset (PS202) detector and an internal molecular hydrogen cluster jet target installed in the Low Energy Antiproton Ring (LEAR) at CERN. Cross sections for pp → KsKs have been obtained and limits are set on the non-observation of the ξ(2230). Conversely, we find evidence for a narrow signal in a preliminary analysis of our pp → ηη data consistent with a narrow ξ(2230).

The PS 185 experiment at LEAR/CERN studies strangeness production in antiproton-proton collisions in flight. Exclusive final states containing pairs of antihyperons and hyperons have been analysed. The data have been used to investigate strange quark production dynamics. New data at the highest energies available at LEAR extend the existing data set for the Λ¯Λ channel. Preliminary data from a new threshold scan are available. For the Σ¯0Λ+cc channel first spin correlation data have been extracted. First preliminary differential cross section data from the Σ¯−Σ− channel are also presented.

The reactions p p → ∑ + ∑ + and p p → ∑ − ∑ − have been investigated at GeV/c beam momentum by the PS185 Collaboration at LEAR, CERN. Of both reaction types together 170 events could be identified. The cross section of the reaction p p → ∑ + ∑ + was determined to be σtot = 3.68 ± 0.43 μb at an excess energy of 23 MeV. Its angular distribution exhibits a pronounced forward peaking. The ratio of the cross sections σ( p p → ∑ + ∑ + ) σ( p p → ∑ − ∑ − ) = 2.4 −1.3 +3.0 was extracted with a confidence limit of 90%. It is the first time that experimental data of these reaction channels are available close to the threshold. We compare our results with those of earlier experiments and with theoretical predictions.

Interest in the production of hyperon-antihyperon pairs following antiproton-proton annihilation stems largely from attempts to understand the nature of flavor production. To date the major focus of both the experimental and the theoretical effort has been on the p¯p??¯? reaction. In this paper, we present data on the complementary channels p¯p?S¯ 0? and p¯p??¯S0. Events from the kinematically similar p¯p??¯? reaction were obtained in parallel. The procedure to distinguish these three separate reactions is described and results for all channels are presented. These include the total and differential cross sections, hyperon polarizations, and spin correlation coefficients. Data were obtained at incident antiproton lab momenta of 1.726 and 1.771 GeV/c which correspond to excess kinetic energies in the p¯p??¯S0+c.c. reaction of 26 and 40 MeV, respectively, above threshold. Comparisons are made to earlier work at similar excess energies in the p¯p??¯? channel. The low-energy regime has been highlighted in this experiment to reduce the complexity in the theoretical analysis.

Associated strangeness production has been studied in the --> --> p pi(-) reaction at the CERN antiproton facility LEAR using the experimental setup of PS185. Results from two high-statist ...

A strangelet search in Si+Pt and Au+Pt collisions at alternating-gradient synchrotron (AGS) energies, using a focusing spectrometer, sensitive to mass per charge of 3-14 GeV/${\mathit{c}}^{2}$ was conducted during the 1992 and 1993 heavy ion runs at the AGS. The null results thereof are presented as upper limits on the invariant production cross section, in the range of ${10}^{\ensuremath{-}5}$-${10}^{\ensuremath{-}4}$ mb ${\mathit{c}}^{3}$/${\mathrm{GeV}}^{2}$, and model dependent sensitivity limits in the range of ${10}^{\ensuremath{-}7}$-${10}^{\ensuremath{-}5}$ per collision. Measurements of the production cross sections of several nonstrange nuclear systems, from $p$ to $^{7}\mathrm{Be}$ and $^{8}\mathrm{Li}$, the background of the strangelet search, are also presented.

Recent estimates suggest that the production of strange exotic objects, such as the hypothetical {ital H} dibaryon ({ital H}), is surprisingly large in Au collisions at AGS energies. Two possible bound states of the {ital H}, namely {ital H}-{sup 3}He and {ital H}-{ital d}, would be well separated in mass from known nuclear isotopes and therefore could be uniquely identified by experiment. We report the null result of a first search for these objects in Au+Pt collisions at the AGS, and limit their production cross sections for stable systems to {similar_to}10{sup {minus}5} mb/(GeV){sup 2}, near {ital y}=0.6 and {ital p}{sub {ital t}}/{ital Z}=0.18 GeV/{ital c}.

During the 1993 AGS heavy ion run, experiment E886 conducted a strangelet search in Au + Pt reactions, with beam momentum of 10.8 GeV/c per nucleon. Presented here are the upper limit for strangelet production, invariant cross sections for p, t, d 3 He, 4 He, 6 He, 8 He, 6 Li, 7 Li, 8 Li and 7 Be, along with a demonstration of their agreement with a coalescence model fit to data collected by E886 during the 1992 engineering run, and upper limits for production cross sections for other rare objects, such as (H dibaryon)-nucleus systems. All results reported here are preliminary.

The JETSET (PS202) experiment at CERN-LEAR searches for hadronic resonances by means of in-flight antiproton-proton annihilations in the reaction p p → φφ . In order to obtain sufficient luminosity and good final-state mass resolution, this experiment uses an internal hydrogen-cluster jet target intersecting the LEAR antiproton beam. We report on the study of the reaction p p → 4K ± at 1.4 GeV / c incident p momentum, and we present the first experimental observation of a stro φφ signal in this reaction.

Neutron-proton total cross section differences with the beam and target polarizations orientated either transversally or longitudinally with respect to the beam direction, as well as 11 spin dependent elastic scattering observables measured at SATURNE II as a function of energy and angle are presented. A major part of results was measured using the quasimonoenergetic polarized neutron beam and/or the polarized proton target. A small part of data was obtained using a polarized deuteron beam considered as a beam of quasifree neutrons and protons. The present paper represents a review of measured observables. Several sets of present data are compared with results obtained in other laboratories below 0.8 GeV. Imaginary parts of spin dependent forward amplitudes for np scattering and for the isospin stateI=0 were determined. First direct reconstruction of the np scattering matrix at 0.84 GeV is shown.

We present data of several rescattering observables measured inn p elastic scattering between 0.80 and 1.10 GeV. The SATURNE II polarized beam of free neutrons obtained from the break-up of polarized deuterons was scattered on the Saclay polarized frozen-spin proton target. Three different configurations of beam and target polarization directions were used: the observablesDonon andKonno were measured with the normal-normal spin configuration at eight energies;Nonkk,Dos″ok andKos″ko were determined with the longitudinal-longitudinal configuration at six energies;Nonsk,Dos″ok andKos″so with the sideway-longitudinal configuration at six energies. Part of the data was obtained with an unpolarized CH2 target where only the two spin-index polarization transfer parametersKos″ko andKos″so were determined. Data are compared with phase shift analyses predictions and with the LAMPF results at 0.788 GeV. Present results are the first measurements of rescattering observables above 0.80 GeV. They provide an important contribution to any future theoretical or phenomenological analysis.

The reaction\(\bar pp \to \phi \phi \) is investigated in the JETSET (PS202) experiment using an internal target in LEAR. Data have been analysed at a variety of beam momenta from 1.2 GeV/c to 2.0 GeV/c, corresponding to centre-of-mass energies between 2.15 GeV and 2.43 GeV. A strong φφ-signal was observed over the whole, energy range, preliminary results giving φφ-yields of (1–2) μb. No evidence for narrow resonances was found within the accumulated statistics. The capability of the experiment to measure φ→K+K− decay angular distributions and thus put constraints on the spin-parity quantum numbers of a possible resonant intermediate state is demonstrated.

We present a total of 191 and 203 data points of the elastic neutron-protonspin correlation parameters A ookk and A oosk , respectively. Both observables were measured in a large angular interval. The observable A ookk was measured from 0.312 to 1.10 GeV and A oosk from 0.80 to 1.10 GeV. The SATURNE II polarized beam of free neutrons obtained from the break-up of polarized deuterons was scattered on the polarized Saclay frozen-spin proton target. The beam polarization was oriented either along the beam direction or sideways, the target polarization was oriented longitudinally. Data are compared with phase-shift analyses predictions and with the PSI, LAMPF and SATURNE II results. Present results provide an important contribution to any future theoretical or phenomenological analysis.

The reaction p p → Λ Λ → p π + pπ − is studied in the experiment PS185 at the CERN Low Energy Antiproton Ring (LEAR). A precise measurement of the excitation function in the immediate threshold region below 6 MeV excess energy was achieved. The total cross section shows an unexpected behaviour around 1 MeV excess energy.

We present results of the total cross section differenceΔσТ obtained in transmission measurements at the energies 0.86, 0.88, 0.91 and 0.94 GeV. The SATURNE II polarized beam of free neutrons obtained from the break-up of polarized deuterons was transmitted through the polarized Saclay frozen-spin proton target. The beam and target polarizations were oriented in the vertical direction. The present results agree with previous SATURNE measurements and improve the amplitude analysis in the forward direction.

We present a total of 273 independent data points of the analyzing powers A oono (nP) and A ooon (nP) in a large angular interval at four energies between 0.477 and 0.940 GeV. The SATURNE II polarized beam of free neutrons obtained from the break-up of polarized deuterons was scattered on the polarized Saclay frozen-spin proton target. Part of the data was obtained with a CH 2 target. A comparison of the two measured observables allows one to determine the polarization of the neutron beam. The present results provide an important contribution to any future theoretical or phenomenological analysis.

Measurements of the total and differential cross sections of the reaction p p → K s K s are presented for values of s in the region near 2230 MeV. The 18 energies of the scan were chosen to permit a sensitive search for resonant structure related to the ¢E(2230) state in a channel with a minimal non-resonant background. No such structure is observed. Stringent limits for the branching ratio are set based on various assumptions for the width and spin of the ¢E.

We present a total of 427 np analyzing power data points in a large angular interval at 12 energies between 0.312 and 1.10 GeV. The SATURNE II polarized beam of free monochromatic neutrons was scattered either on the Saclay frozen-spin polarized proton target or on CH2 and C targets. Present results are compared with existing elastic and quasieleastic data.

Measurements of the total and differential cross sections of the reaction pp → KsKs are presented for values of s in the region near 2230 MeV. The 18 energies of the scan were chosen to permit a sensitive search for resonant structure related to the ¢E(2230) state in a channel with a minimal non-resonant background. No such structure is observed. Stringent limits for the branching ratio are set based on various assumptions for the width and spin of the ¢E.

The main focus of the PS185 collaboration at LEAR is the study of exclusive antihyperon-hyperon production in antiproton-proton collisions. This has been done comprehensively for the reaction p p → Λ Λ . In recent years other channels like pp → -Λγ 0 + cc. or p p → ϵ/ +- ϵ/ +- have been included in the program. Results from the ongoing analysis for these final states will be presented.

A polarized neutron beam at PSI has been used to measure the spin-dependent neutron-proton total cross sections Δσ L and Δσ T from 140 to 590 MeV. The results are compared with recent phase-shift calculations.

Final results for total cross section differences ΔσT and ΔσL measured with a polarized neutron beam transmitted through a polarized proton target are presented. Measurements were carried out at SATURNE II, at 11 energies between 0.63 and 1.1 GeV for ΔσT and at 9 energies between 0.312 and 1.1 GeV for ΔσL. The results are compared with measurements at PSI and LAMPF as well as with ΔσL data points deduced from p-d and p-p transmission experiments at the ANL-ZGS. The present results together with the corresponding pp data allow to determine two of the three imaginary parts of forward scattering amplitudes for isospin I = 0.

The PS185 experiment at the CERN Low Energy Antiproton Ring (LEAR) has studied the reaction p p → \ gLΛ at several momenta. In this paper results from two runs with high statistics at 1.546 GeV/c and 1.695 GeV/c are described. Based on 4063 and 11362 analysed events, respectively, differential and integrated cross sections, polarizations and spin correlations are presented. The singlet fraction, extracted from the spin correlations, is consistent with zero at both momenta, showing that the \ gLΛ pairs are produced in a pure triplet state. A comparison of the decay asymmetry parameters of Λ and \ gL reduces the upper limits for the violation of the CP invariance for this system.

The reaction p p → Λ Σ 0 together with its charge conjugate channel (c.c.) has been measured at LEAR. The incident p momentum was 1.695 GeV/c, corresponding to an excess energy above threshold of 14.8 MeV. Results are given for the production cross section and the differential cross section as well as for the polarization. Comparisons are made with theoretical calculations and with the reaction p p → Λ Λ .

Inclusive cross sections for production of protons, deuterons and tritons by neutrons in the energy range of 300–580 MeV on copper and bismuth have been measured at five angles between 54° and 164°. The systematic dependence of the invariant cross sections on incident energy and emission angle are evaluated. For the study of the mass-number dependence earlier data on carbon are included. The results are discussed on the basis of different models, like quasi-two-body sealing or moving-source model.

At PSI a polarized neutron beam facility has been set up and was used to measure neutron-proton total cross section differences ΔσL and ΔσT in the energy range from 140 to 590 MeV.

For the reaction p p → Λ Λ from threshold up to 1.7 GeV c total and differential cross section data, polarisations and spin correlations have been obtained. The data indicate a remarkably strong p-wave contribution even close to the reaction threshold. The Λ Λ spin correlations are compatible with a pure triplet state.

A search for the H dibaryon has been conducted at the Brookhaven National Laboratory AGS, using a 1.8 GeV/c K{sup -} beam. {Xi}{sup -} hyperons were produced in a liquid-hydrogen target via the reaction K{sup -}+p{yields}K{sup +}+{Xi}{sup -}. The hyperons were slowed in degraders and those most likely to stop in an adjacent liquid-deuterium target were tagged by silicon detectors. Monoenergetic neutrons were sought as the signature for H formation in ({Xi}{sup -},d){sub atom}{yields}H+n. The experiment was designed for optimal sensitivity to a loosely-bound H, complementing recent (K{sup -},K{sup +}) measurements on nuclear targets. In addition, the experiment's sensitivity was independent of lifetime and of decay modes of the H. No statistically significant evidence for H formation was seen. Upper limits on the branching ratio for H formation in the above reaction have been set in a mass range extending from slightly above {Lambda}{Lambda} threshold to {approx}100 MeV of binding and are compared with a corresponding theoretical prediction.

We report on precision measurements of the cross sections for the reactions {bar p}p{yields}{bar {Lambda}}{Lambda}, {bar p}p{yields}{bar {Lambda}}{Sigma}{sup 0}, and {bar p}p{yields}{bar {Sigma}}{sup 0}{Lambda} in the region below 5 MeV excess kinetic energy above reaction threshold. For the {bar p}p{yields}{bar {Lambda}}{Lambda} measurement, the data represent a third and definitive scan through the threshold region by this experiment. Improvements in the detector system and analysis techniques, coupled with greatly increased statistics, have negated the question of the existence of a near-threshold resonance as had been indicated in our earlier, lower-statistics studies. The high statistics permit the first extraction of the polarization information at these low energies. We also report the first measurement of the cross section for the complementary reactions {bar p}p{yields}{bar {Lambda}}{Sigma}{sup 0}+c.c. at excess energies up to 2.5 MeV. This allows a comparison of the onset of {bar {Lambda}}{Lambda} production to that of {bar {Lambda}}{Sigma}{sup 0} and {bar {Sigma}}{sup 0}{Lambda}. The data show no evidence for near-threshold resonance structures in either reaction channel. Further, the {bar p}p{yields}{bar {Lambda}}{Sigma}{sup 0}+c.c. reactions exhibit a larger P-wave fraction close to threshold compared to {bar p}p{yields}{bar {Lambda}}{Lambda}.

Differential cross sections of elastic neutron-proton scattering have been measured for the energy range from 200 MeV to 580 MeV. The angular interval for the detection of the recoiling proton ranges up to 48 degree in the laboratory system. This corresponds to an interval of the scattering angle from 80 degree to 180 degree in the centre-of-mass system. For absolute normalization the simultaneously measured np --> d Pi^o reaction was used above 280 MeV. The charged Pi-NN coupling constant has been determined to f^2_{Pi-n-p} = 0.076+-0.001., 9 pages, 4 figures. accepted by Physica Scripta

A review is made of those past, present, and planned experiments at the AGS whose primary purpose is to determine whether the H‐dibaryon exists. Preliminary results are given for experiment E813, based on partially‐completed analysis.

At the Brookhaven AGS several experiments are searching for the unique strangeness S = −2 H-dibaryon with the quark composition (uuddss). The E813/E836 collaboration, in particular, is using a high-intensity, separated 1.8 GeV/c K − beam and two different target configurations. In E836 the reaction K − + 3He→K + + H + n is used to search for a relatively deeply-bound H. Complementary to E836 the reactions K − + p → Ξ − + K +, followed by (Ξ −, d)atom → H + n are used to search near twice the A mass. The status of these two experiments is summarized, and other H-dibaryon searches are briefly reviewed.

We describe here two experiments presently running at PSI using the NA2 polarized neutron beam. They are devoted to the measurement of 2- and 3-spin observables in np elastic scattering for kinetic energies from 230 to 590 MeV with a center of mass angular range from 60 to 180 degrees. The goal is to determine the five NN scattering amplitudes for isospin 0 in a model independent way. Preliminary results for KOSKO and KOSSO spin-transfers are presented.

Recently, highly relativistic Au beams have become available at the Brookhaven National Laboratory, Alternating Gradient Synchrotron. Inclusive production cross sections for composite particles, d, t, He3, and He4, in 11.5A GeV/c Au+Pt collisions have been measured using a beam line spectrometer. For comparison, composite particle production was also measured in Si+Pt and p+Pt collisions at similar beam momenta per nucleon (14.6A GeV/c and 12.9 GeV/c, respectively). The projectile dependence of the production cross section for each composite particle has been fitted to Aprojα. The parameter α can be described by a single function of the mass number and the momentum per nucleon of the produced particle. Additionally, the data are well described by momentum-space coalescence. Comparisons with similar analysis of Bevalac A+A data are made. The coalescence radii extracted from momentum-space coalescence fits are used to determine reaction volumes (‘‘source size’’) within the context of the Sato-Yazaki model.