D.V. Bugg

The Crystal Barrel spectrometer at LEAR

The crystal Barrel spectrometer used at LEAR, CERN to study the products of pd annihilations is described. A 1380 element array of Csl crystals measures photons from the decay of π0, η, η′ and ω mesons. A segmented drift chamber in a 1.5T magnetic field is used to identify and measure charged particles. A fast on-line trigger on charged and neutral multiplicities and on the invariant mass of secondary particles is available. The performance of the detector is discussed.

πp phase shifts from 88 to 310 MeV

Abstract Phase shift solutions are presented at pion kinetic energies from 88 to 292 MeV where new measurements of the differential elastic cross section are available, and at 310 MeV. The resulting phase shifts are fitted to effective-range formulae. The resonant P 33 phase shift is allowed to take different values for π + p and π − p scattering. There are differences in mass and width between these charge states of the resonance of M 0 − M ++ = 1.4 ± 0.4 MeV/ c 2 and Γ 0 − Γ ++ = 10.3 ± 1.3 MeV/ c 2 . The difference in width can be accounted for largely but not entirely by the Coulomb barrier, the channel π − p → γ n, and the difference in phase space between π − p → π 0 n and π + p → π + p.

Observation of two JPC=0++ isoscalar resonances at 1365 and 1520 MeV

From a simultaneous analysis of data on pp → π0π0π0 and pp → ηηπ0 at rest, two I = 0, JPC = 0++ resonances are identified above 1 GeV. The first has mass M = 1365−55+20 MeV and width г = 268 ± 70 MeV, close to the ƒ0(1400) of the Particle Data Group. The second has M = 1520 ± 25 MeV, г = 148−25+20 MeV.

The total cross-sections for pion-proton scattering between 70 mev and 290 mev

Abstract Measurements have been made of the π∓ proton total cross sections over the laboratory kinetic energy range 70 to 290 MeV. The absolute accuracy of the data is generally 0.5 %, but decreases to 1 % for some points where applied corrections are large or where low particle fluxes limit the statistical accuracy. The parameters of the Δ0, Δ++ resonance have been calculated, and a discussion is included on the limitations placed by Coulomb barrier effects on extracting the basic nuclear phase shifts from data in this energy region.

New values of pion-nucleon scattering lengths and f2

Abstract From fixed t dispersion relations we have obtained more accurate values for the pion nucleon coupling constant and s -wave scattering lengths, using our recent low energy πN angular distribution and total cross section data. The results are: f 2 = 0.0790 ± 0.0010, a 1 − a 3 = 0.262 ± 0.004 μ −1 , a 1 + 2 a 3 = − 0.014 ± 0.005 μ −1 .

Coupled channel analysis of pp annihilation into π0π0π0, π0ηη and π0π0η

We confirm the existence of the two IG(JPC) = 0+(0++) resonances f0(1370) and f0(1500) reported by us in earlier analyses. The analysis presented here couples the final states π0π0π0, π0π0η and π0ηη of pp annihilation at rest. It is based on a 3 × 3 K-matrix. We find masses and widths of M = (1390±30) MeV, Γ = (380±80) MeV; and M = (1500±10) MeV, Γ = (154 ± 30) MeV, respectively. The product branching ratios for the production and decay into π0π0 and ηη of the f0(1500) are (1.27 ± 0.33) · 10−3 and (0.60 ± 0.17) · 10−3, respectively.

Further amplitude analysis of JΨ → γ(π+π−π+π−)

Abstract Mark III data on J Ψ → γ(π + π − π + π − ) have been re-analysed including γσσ final states as well as γϱϱ. Here σ stands for the ππ S-wave amplitude. We fit with I = 0 resonances having JPC = 2++ at 1275 MeV, 0−+ at 1420 MeV, 0++ at 1505 MeV, 2++ at 1640 MeV, 0++ at 1750 MeV and 0++ at 2104 MeV. The 0++ resonances decay dominantly to σσ and the 2++ resonances to ϱϱ.

Observation of a new IG(JPC)=1−(0++) resonance at 1450 MeV

Abstract A new IG=1− JPC=0++πη resonance is observed in pp annihilation at rest into π0π0η. It has a mass M=(1450±40)MeV and a width Γ=(270±40)MeV.

πp elastic scattering from 88 to 292 MeV

Abstract Differential cross sections for π+p and π−p elastic scattering have been measured with an accuracy of typically ±2% at 10 and 9 energies respectively in the range 88 to 292 MeV of lab kinetic energy.

Reinterpreting several narrow ‘resonances’ as threshold cusps

Abstract The threshold p ¯ p peak in BES data for J / Ψ → γ p ¯ p may be fitted as a cusp. It arises from the well-known threshold peak in p ¯ p elastic scattering due to annihilation. Several similar examples are discussed. The PS185 data for p ¯ p → Λ ¯ Λ require an almost identical cusp at the Λ ¯ Λ threshold. There is likewise a cusp at the ΣN threshold in K − d → π − ( Λ p ) . Similar cusps are likely to arise at thresholds for all 2-body de-excitation processes, providing the interaction is attractive; likely examples are Λ p ¯ , Σ p ¯ , and K ¯ Λ . The narrow peak observed by Belle at 3872 MeV in π + π − J / Ψ may be a J P C = 1 + + cusp due to the D D ¯ * threshold. The narrow Ξ * ( 1862 ) observed by NA49 may be due to a threshold cusp in Σ ( 1385 ) K ¯ coupled to Ξπ and Σ K ¯ . The relation of cusps to known resonances such as f 0 ( 980 ) is discussed.