J.C. Gordon

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.

Production of hyperons and hyperon resonances inΞ−N interactions at 102 and 135 GeV/c*

AbstractΞ− interactions in hydrogen and deuterium are studied close to the forward direction using the CERN charged hyperon beam. The inclusive production of ∑*−(1385),Ξ−,Ξ*0(1530),Ξ*−(1700),Ξ*−(1830), and Ω− is observed, as well as an enhancement in theΞ−π+ channel at 1940 MeV/c2. The momentum distributions and the production cross sections are measured for ∑*−(1385),Ξ−,Ξ*0(1530), and Ω−.

Properties of the charmed strange baryonA + and evidence for the charmed doubly strange baryonT 0 at 2.74 GeV/c2

Results are presented from experiment WA62, which searched for charmed strange baryon states produced inΣ− interactions in the SPS charged hyperon beam at CERN. Properties of theA+ (csu) baryon at 2.46 GeV/c2 are summarized and upper limits are given for decay branching ratios into various channels. Three events observed at 2.74 GeV/c2 in theΞ−K−π+π+ mass spectrum are interpreted as the first evidence for theT0 baryon with quark content css. Results of a search for theA0 (csd), the isospin partner of theA+, are presented. The results are discussed in the context of current theoretical understanding, and a comparison with other experiments on hadroproduction of charmed baryons is made.

Measurement of theΞ - p elastic cross section at 102 and 135 GeV/c

TheΞ-p differential elastic cross section has been measured in the SPS hyperon beam at 102 and 135 GeV/c. In the range 0.01<−<0.42(GeV/c)2, thet distributions are found to be compatible with the formA exp(Bt) whereB is 7.7±0.4(GeV/c)−2 at 102 GeV/c and 8.2 ±0.5(GeV/c)−2 at 135 GeV/c. The corresponding total elastic cross sections areσel=4.9±0.7 mb andσel=5.6±0.9 mb, respectively. These results are compared with the predictions of phenomenological models.

A new upper limit for the branching ratio (Ξ− → nπ−)(Ξ− → Λπ−)

Abstract From a sample of 735 000 Ξ− decays, we have obtained a new upper limit for the branching ratio (Ξ − → n π − ) (Ξ − → Λπ − ) . The result is 1.9 × 10−5 at the 90% confidence level.

Determination of the α parameter for the decay Ξ− → Λπ−☆

Abstract The α parameter for Ξ− → Λπ− decay has been determined to be αΞ−=−0.462 ± 0.015, from a sample of 1.5 × 105 events recorded in the CERN SPS hyperon beam. This value is 2.5 standard deviations higher than the current world average.