Polarization of Σ0 hyperons in inclusive production from 28.5 GeV/c protons on beryllium

The polarization of Σ⁰ hyperons produced in an inclusive reactons hasbeen measured for the first time. From a sample of 11 000 events produced by 28.5 GeV/c ptorons in the reaction p+Be→Σ⁰+X, the Σ⁰ polarization has a value of +0.28±0.13 at p_t=1.01 GeV/c and x_f=0.60. The polarization of 53 000 Λ hyperons produced from 28.5 Gev/c protons inthe reaction p+Be→Λ+X has been measured in the kinematic range 0.64<p_t<1.14 GeV/c and 0.42<x_f<0.62. The average Λ polarization is found to be −0.188±0.024, consistent with previous results.     

Medium effects in the production and π0γ decay of ω-mesons in pA collisions in the GeV region

The ω resonance production and its π⁰γ decay in pA reactions close to threshold is considered within the Intranuclear Cascade (INC) model. The π⁰γ invariant-mass distribution shows two components which correspond to the ω decay “inside” and “outside” the nucleus, respectively. The “inside” component is distorted by medium effects, which introduce a mass shift as well as collisional broadening for the ω-meson and its decaying pion. The relative contribution of the “inside” component is analyzed in detail for different kinematical conditions and nuclear targets. It is demonstrated that a measurement of the correlation in azimuthal angle between the π⁰ and γ momenta allows to separate events related to the “inside”ω decay from different sources of background when uncorrelated π⁰'s and γ's are produced. Received: 2 April 2001 / Accepted: 5 June 2001     

Hypertriton and light nuclei production at Λ-production subthreshold energy in heavy-ion collisions

High-energy heavy-ion collisions produce abundant hyperons and nucleons. A dynamical coalescence model coupled with the ART model is employed to study the production probabilities of light clusters, deuteron (d), triton (t), helion (³He), and hypertriton (_Λ³H) at subthreshold energy of Λ production (≈ 1 GeV per nucleon). We study the dependence on the reaction system size of the coalescence penalty factor per additional nucleon and entropy per nucleon. The Strangeness Population Factor (S₃ / (³He × (Λ/p))) shows an extra suppression of hypertriton comparing to light clusters of the same mass number. This model predicts a hypertriton production cross-section of a few μb in ³⁶Ar+³⁶Ar, ⁴⁰Ca+⁴⁰Ca and ⁵⁶Ni+⁵⁶Ni in 1 A GeV reactions. The production rate is as high as a few hypertritons per million collisions, which shows that the fixed-target heavy-ion collisions at CSR (Lanzhou/China) at Λ subthreshold energy are suitable for breaking new ground in hypernuclear physics.     

Hypertriton and light nuclei production at Λ-production subthreshold energy in heavy-ion collisions

High-energy heavy-ion collisions produce abundant hyperons and nucleons. A dynamical coalescence model coupled with the ART model is employed to study the production probabilities of light clusters, deuteron (d), triton (t), helion (³He), and hypertriton (_Λ³H) at subthreshold energy of Λ production (≈ 1 GeV per nucleon). We study the dependence on the reaction system size of the coalescence penalty factor per additional nucleon and entropy per nucleon. The Strangeness Population Factor (S₃ / (³He × (Λ/p))) shows an extra suppression of hypertriton comparing to light clusters of the same mass number. This model predicts a hypertriton production cross-section of a few μb in ³⁶Ar+³⁶Ar, ⁴⁰Ca+⁴⁰Ca and ⁵⁶Ni+⁵⁶Ni in 1 A GeV reactions. The production rate is as high as a few hypertritons per million collisions, which shows that the fixed-target heavy-ion collisions at CSR (Lanzhou/China) at Λ subthreshold energy are suitable for breaking new ground in hypernuclear physics.     

Angular dependence of Bose-Einstein correlations in interactions of π+ andK + mesons with protons and nuclei at 250 GeV/c

The angular dependence of Bose-Einstein correlations is studied in interactions of ⁺ andK ⁺ mesons with protons and nuclei at 250 GeV/c. The pion source is found to be elongated along the interaction axis in the c.m.s., with the ratioa between transverse and longitudinal dimension equal to 0.55±0.06 for the case of proton, 0.53±0.13 for the case ofAl and 0.33±0.21 for the case ofAu target. For meson-proton interactions, indication for an increase of the elongation is found with increasing event multiplicity, pair momentum and particle transverse momentum.Now at UIA, Wilrijk, Belgium     

Short-range correlations in hadron pair production atp T ≧2 GeV/c

Production of oppositely charged particle pairs at largep _T is studied, where both particles, are in the central rapidity region within one unit of rapidity and nearby in azimuth. The data sample, which comes fromπ ^? Be interactions obtained using the CERN OMEGA spectrometer, contains events with a pair of oppositely charged particles havingp _T ≧2 GeV/c recoiling, against a third one withp _T ≧1 GeV/c. Our data provide new information on short range correlations involving the production of the nine possible pairs made from (π ⁺,K ⁺,p) and \((\pi ^ - ,K^ - ,\bar p)\) . Comparison with Lund Monte Carlo predictions is made. Apart from proton yields, the model reproduces correctly our data. The main discrepancies observed with protons can be attributed to hard scattering of diquarks from the target nucleons.     

Study of fermion pair production in \mathrm{e^+ e^-} collisions at 130–183 GeV

The cross sections and forward-backward asymmetries of hadronic and leptonic events produced in collisions at centre-of-mass energies from 130 to 183 GeV are presented. Results for , , , , and production show no significant deviation from the Standard Model predictions. This enables constraints to be set upon physics beyond the Standard Model such as four-fermion contact interactions, leptoquarks, bosons and R-parity violating squarks and sneutrinos. Limits on the energy scale of contact interactions are typically in the range from 2 to 10 TeV. Limits on R-parity violating sneutrinos reach masses of a few hundred GeV/ for large values of their Yukawa couplings. Received: 8 March 1999 / Published online: 8 December 1999     

Coherent production of (K + π 0) pairs on nuclei in a Charged-Kaon beam

The cross section for the coherent production of (K ⁺ π ⁰) pairs that is associated with a chiral anomaly and with diagrams involving intermediate K*(892), ρ, ω, and ?(1020) mesons is calculated for the Coulomb interaction of K ⁺ mesons with nuclei. The number of such events that is expected for the OKA detector at the Institute for High Energy Physics (IHEP, Protvino) is presented.     

Inclusive production from 10 GeV/c K+ on nuclei and comparison with the same production on hydrogen

Transverse and longitudinal distributions for the K⁰ inclusive production induced by 10 GeV/c K⁺ incident on complex nuclei are obtained. They are compared with the same distributions on hydrogen from K⁺ at comparable energies. Some differences are observed in the rapidity distribution of K⁰ when produced on nuclei and on free protons. These differences are correlated with the fast charged tracks multiplicity associated with the observed K⁰.     

K0πand K*0Σ+ photoproduction off the proton

The exclusive reactions γp → K ^*0Σ⁺(1189) and γp → K ⁰π⁰Σ⁺(1189) , leading to the p 4π⁰ final state, have been measured with a tagged photon beam for incident energies from threshold up to 2.5GeV. The experiment has been performed at the tagged photon facility of the ELSA accelerator (Bonn). The Crystal Barrel and TAPS detectors were combined to a photon detector system of almost 4π geometrical acceptance. Differential and total cross-sections are reported. At energies close to the threshold, a flat angular distribution has been observed for the reaction γp → K ⁰π⁰Σ⁺ suggesting dominant s -channel production. Σ^*(1385) and higher-lying hyperon states have been observed. An enhancement in the forward direction in the angular distributions of the reaction γp → K ^*0Σ⁺ indicates a t -channel exchange contribution to the reaction mechanism. The experimental data are in reasonable agreement with recent theoretical predictions.     

K0 production in e+e− annihilations at 30 GeV center of mass energy

Inclusive K⁰-production has been measured in e⁺e^- annihilation at a center of mass energy of about W = 30 GeV. The ratio of K⁰ + K⁰ production to μ⁺μ^- production is R_K0 = 5.6 ± 1.1 (statist. error) ± 0.8 (system.error) This value is about a factor of three higher than R_K0 at W = 7 GeV. The cross sections (s/β) dσ/dx is consistent with a scaling behaviour.     

Inclusive Δ++ production in K+p interactions at 8.2 and 16 GeV/c

The reaction K⁺p→Δ⁺⁺(1236)+ anything is studied at beam momenta 8.2 and 16 GeV/c and compared with other Δ⁺⁺ producing reactions. We find that the low-mass Δ⁺⁺π^? enhancement affects the shapes of Δ⁺⁺ inclusive distributions. The triple-Regge formula is found to be consistent with the data. Dual properties of this formula are successfully tested.     

Forward K1+ (892) production in K+p → π+Ks0p at 12.7 GeV/c

We have measured 618 K⁺p → π⁺K_S⁰p events at 12.7 GeV/c incident lab momentum, mass range 790 ? m_π⁺Ks⁰ ? 990 MeV and t range 0.01 ? ?t ? 0.60 (GeV/c)². The π⁺K_S⁰ mass spectrum is dominated by the K¹⁺ (892) resonance and a Breit-Wigner fit yields a mass m = 893.5 ± 1.1 MeV and a width Γ = 33.2 ± 4.1 MeV which is much narrower than measured hitherto. The t distribution of K¹⁺ (892) events shows a dip in the forward direction and an exponential fall off thereafter, consistent with dominance of helicity flip amplitudes. The spin density matrix is almost saturated by ρ₁₁ and ρ_1?1 which are very close to their maximum allowed value of $\text{1}\text{2}$ throughout the measured t range except in the very forward direction where ρ₀₀ and Re ρ₁₀ deviate from zero. Natural parity exchanges, therefore, dominate with unnatural parity exchanges being restricted to a small region in the forward direction. A Regge pole analysis of the differential cross sections of the present measurement in conjunction with previously measured total cross sections supports the f-coupled-pomeron hypothesis.     

Λ0 hyperon polarization in inclusive production by 24 GeV protons on platinum

We report a search for Λ⁰ polarization in inclusive production by 24 GeV protons on platinum made on data taken at CERN. These data show Λ⁰ polarization of the same direction and magnitude and with the same p_T dependence as found at 300 GeV at Fermilab.     

Production of π±, K±, p and p̄ by 200 GeV/c protons

Data are presented on the relative yields of π^±, K^±, p and p? with momenta between 6 and 37 GeV/c produced at 0° by 200 GeV/c protons on beryllium and aluminium.     

The K+π- elastic scattering cross section as observed in K+p→K+π-Δ++ from 3 to 13 GeV/c

The four-body reaction K⁺p→K⁺π^-Δ⁺⁺ is analysed in a sample of 77 300 bubble chamber events of K⁺p→K⁺π^-π⁺p for beam momenta from 2.5 to 12.6 GeV/c to study the K⁺π^- elastic scattering cross section. The relative merits of several analytic continuations to the pion pole are discussed. Maximum likelihood method is used and, for each experiment fits are performed simultaneously over the whole range of the M(pπ⁺), m(K⁺π^-) and t variables. The K⁺π^- cross section is parametrized in terms of the down solution for the s-wave, a p-wave resonating at the K¹(890) and a d-wave resonating at the K¹(1420). Fitted values are obtained for the four-body cross section, the s-wave contribution to the K⁺π^- cross section, and the masses and widths of the K¹(890) and K¹(1420).     

(e,e′K+) reaction on nuclei

We discuss the importance of the A(e, e′K⁺)_ΛA¹ reactin in nuclear physics. Cross section are given for exclusive and inclusive processes. The elementary p(e, e′K⁺)Λ, which is not well-known, is also discussed.