W. M. Gibson

Measurements of hyperon semileptonic decays at the CERN Super Proton Synchrotron

Results on five different hyperon semileptonic decays from the WA2 experiment, performed in the CERN SPS charged hyperon beam, have been analysed within the framework of the Cabibbo model. For the first time, the inconsistencies, which inevitably occur when the results from different experiments are combined, have been avoided in these comprehensive fits to high statistics data from a single hyperon decay experiment. Excellent agreement with the basic Cabibbo model has been obtained using the WA2 data either alone or together with neutron lifetime measurements. These results contrast strongly with other recent Cabibbo analyses which have indicated the presence ofSU(3) breaking effects. Including radiative corrections we have obtained the results,F=0.477±0.012,D=0.756±0.011 and sinθc=0.231±0.003. The ratio (D/D+F) is 0.613±0.009, which is close to theSU(6) prediction of 3/5. Using additional information onft values for superallowed nuclear Fermi transitions, an upper limit on the mixing parameter sinθ3 in the Kobayashi-Maskawa six quark scheme has been obtained: |sinθ3|<0.20.

Inclusive production of low-momentum charged pions at x = 0 at the CERN intersecting storage rings

Abstract Results are reported for the invariant differential cross-section of charged pions produced at x = 0 in proton-proton collisions at the CERN ISR. The range covered is 40 to 400 MeV/c in transverse momentum and 23 to 63 GeV in collision energy. The inclusive cross-section for π + and π − are increasing by 36 ± 2% and 41 ± 2%, respectively over the ISR energy range with a somewhat stronger increase at the lowest transverse momenta. The transverse momentum distribution is well described by an exponential in the transverse energy.

Particle and antiparticle production by 200 GeV/c protons in the charged hyperon beam at the CERN SPS

Abstract A charge hyperon beam has been brought into operation at the CERN SPS. Particles are identified by a DISC Cerenkov counter, and decay products are analysed by a magnetic spectrometer. Cross sections for the inclusive production of π + , K + , p , Σ + , Σ − , ζ − , d, and π − , K − , p , Σ + , Σ − , ζ − , ω − , d in the forward direction have been measured at laboratory momenta between 70 and 130 GeV/ c . This range of momenta corresponds to 0.35 ⩽ x ⩽ 0.66 for an incident proton momentum of 200 GeV/ c . Antihyperon ( Σ − , ζ − , Σ + ) and Σ + and ω − fluxes have been measured for the first time in a hyperon beam. Scaling behaviour is observed when the data are compared with the results from other experiments. The inclusive particle production cross sections have a (1 − x ) n dependence, where n is characteristics of the particle. A search has been made for new particles in the mass range 2–10 GeV/ c 2 . No evidence is found for such particles.

Observation of a narrow state at 2.46 GeV/c2 — a candidate for the charmed strange Baryon A+☆

Abstract A narrow state has been observed in the reaction Σ − + Be → ( Λ K − π + π + ) + X in an experiment at the CERN SPS hyperon beam. At 2.46 GeV/ c 2 the effective ( Λ K − π + π + ) mass distribution shows an excess of 82 events above a background estimated to be 147, corresponding to a statistical significance of more than 6 standard deviations. The positive charge of the observed final state, which has strangeness −2, suggests the interpretation as a Cabibbo favoured decay of the charmed strange baryon, A + [quark content (csu)]. The cross section times branching ratio is measured to be σ · B = (5.3 ± 2.0) μ b/ (Be nucleus) for x > 0.6. The invariant production cross section is described by E d 3 σ /d p 3 ∞ (1 - x ) (1.7±0.7) exp[−(1.1 −0.4 +0.7 ) p T 2 ].

Evidence for narrow states decaying into (Λp + pions) at 3.1GeV / c2 with charges + 1, 0 and −1☆

Abstract Evidence for narrow states produced by Σ− interactions in beryllium, and decaying into ( Λ p + pions ), has been obtained in an experiment at the CERN SPS hyperon beam. At 3.1 GeV/c2, the ( Λ p π + π + ), ( Λ p π + π + π − ) and ( Λ p π + π − ) effective mass distributions show an excess of 45, 19 and 62 events above a background of 50, 28 and 187, respectively. The possibility that these three signals are statistical fluctuations is remote. The observed widths of the signals are compatible with the mass resolution of the apparatus, 24 MeV/c2 FWHM. We have not been able to find an interpretation of the quantum numbers of the final states, which are baryon number 0, strangeness −1 and charge +1, 0 and −1.

Measurement of the lifetime of the charmed strange baryon A

Abstract The lifetime of the charmed strange baryon A+ has been determined in an experiment at the CERN SPS hyperon beam from the distribution of distances between production and decay of A+ candidates of known momentum. The result is: τ = (4.8 −1.8 +2.9 ) × 10 −13 s .

Measurements of the total cross sections of Σ− and Ξ− on protons and deuterons between 74 and 137 GeV/c

The Σ− p and Σ− d total cross sections have been measured to a statistical accuracy of ±1% and ±0.5%, respectively, at five momenta from 74.5 to 136.9 GeV/c, using the hyperon beam at the CERN SPS. The Ξ−p and Ξ− d total cross sections have also been measured to the same statistical accuracy at 101.5 and 133.8 GeV/c. The systematic uncertainty at each momentum is estimated to be of the order of ±0.5%. The hyperon-nucleon cross sections are shown to be rising with energy, and the data are compared with various phenomenological models.

First measurement of the Ω− decay branching ratios

Abstract In an experiment in the CERN-SPS charged-hyperon beam, the main Ω − decay branching ratios have been measured to be Γ(Ω − →Λ K − /Γ( all )=0.686±0.013 , Γ(Ω −1 →Ξ 0 π − )/Γ( all )=0.234±0.013 , Γ(Ω − →Ξ − π 0 )/Γ( all )=0.080±0.008 . The relative branching ratio of the two Ξπ modes provides a test of the ΔI= 1 2 rule in decuplet-octet transitions. A search has also been made for the rare decay modes Ω − →Λπ − , Ω − →Ξ − γ, Ω − →Ξ − π + π − and Ω − →Ξ 0 e − ν .

Measurement of Ω− decay properties in the CERN SPS hyperon beam

In an experiment in the CERN SPS charged hyperon beam, a sample of 16 000Ω− decays was collected at an Ω− momentum of 131 GeV/c the Ω− lifetime, the branching ratios and decay asymmetry parameters for the main decay modes were measured, giving: τΩ−=(0.823±0.015)×10−10s , Γ(Ω−→ΛK−)Γ(Ω−-→all)=0.675±0.008 , α(Ω−→ΛK−)= −0.025±0.028, Γ(Ω−→Ξ0π−)Γ(Ω−→all) = 0.236±0.008 , α(Ω−→Ξ0π−)=0.09±0.14, Γ(Ω−→Ξ−π0)Γ(Ω−→all)=0.089±0.005 , α(Ω−→Ξ−π0)=0.05±0.21. For the rare decay modes, the following results for the branching ratios were obtained: if Γ(Ω−→Ξ0e−ν)Γ(Ω−→all)=(0.56±0.28)×10−2 , if Γ(Ω−→Ξ−π+π−)(Γ(Ω→all)=(3.7−1.1+3.6)×10−4 , Γ(Ω−→Ξ−γ)(Γ(ω−→all)<2.2×10−3 at the 90% confidence level, Γ(Ω−→Λπ−)(Γ(Ω−→all)<1.9×10−4 at the 90% confidence level . A sample of 32 000 Ξ− decays collected under the same experimental conditions gave a new measurement of the Ξ− lifetime: τΞ−=(1.652±0.051)×10−10 s . Combining the measurements of the present experiment with those obtained in a previous experiment in the SPS hyperon beam gives: if τΩ−=(0.823±0.013)×10−10 s , Γ(Ω−→ΛK−)Γ(Ω−→all=0.678±0.007 , Γ(Ω−→Ξ0π−)Γ(Ω−→all)=0.236±0.007 , Γ(Ω−→Ξ−π0)Γ(Ω−→all=0.086±0.004 , Γ(Ω−→Ξ−π+π−)Γ(Ω−→all=(4.3−1.3+3.4)×10−4 . The experimental results are compared with theoretical calculations.

DCS for π−p elastic scattering from 1.2 to 3.0 GeV/c and phase shift analysis

Abstract Differential cross sections have been measured for π − p elastic scattering at laboratory momenta in the range 1.2 to 3.0 GeV/ c for the c.m. range 0.97 > cos θ ∗ > −0.98 . The corresponding mass range is 1.78 to 2.56 GeV/ c 2 . The data was obtained from a counter experiment in which the scattered pions and protons were detected in coincidence by arrays of scintillation counters. The results are discussed and compared with the CERN phase-shift analyses. The results of a further restricted phase-shift analysis are presented which give support to the evidence for the P 13 (1860) resonance and suggest a possible F 17 /G 17 doublet with a mass near 2000 MeV.