Analyzing power in the reaction pp→dπ for beam momenta from 1.17 to 1.96 GeV/c

The analyzing power A_y0 in the reaction p↑p→dπ⁺ has been measured using the polarized proton beam at Argonne National Laboratory's zero gradient synchrotron. Data were taken at beam momenta of 1.17, 1.47, 1.70, and 1.96 GeV/c and for pion center of mass angles from 8° to 163°.     

X → 2γ from π p → neutral meson + neuron at 3.8 GeV/c

We report on a measurement for the branching-ratio X⁰ → 2γ/X⁰ ar all. Our result is X⁰ → 2γ/X⁰ → all = (2.9 ± 0.9)%.     

Single and double dissociation in πp collisions at 11 and 16 GeV/c

The reactions π⁻p → 2π⁻π⁺p, π⁻p → 2π⁻π⁺π^op and π⁻p → 2π⁻2π⁺n are analysed at 11 and 16 GeV/c using longitudinal phase space (LPS) plots. The weighted LPS distributions for π⁻p → 2π⁻π⁺p is dominated by two well separated structures corresponding to single diffraction dissociation of the pion, π⁻p → (2π⁻π⁺)p, and of the proton, π⁻p → π⁻(π⁻π⁺p). The former is more abundant than the latter, and both are approximately constant with energy. In contrast, processes of type π⁻p → (2π)(πp) decrease with increasing energy.

In the five-body reactions the weighted LPS distribution reveals especially at 16 GeV/c a maximum for single dissociation of the proton into 3πp, namely π⁻p → π⁻(π⁻π⁺π^op); this process is likely to be diffractive. The neutron channel has a corresponding maximum displaced toward a multiperipheral configuration π⁻p → π⁻(π⁻2π⁺)n. Another strong maximum corresponds to the pion dissociation π⁻p → (2π⁻π⁺π^o)p. This is interpreted to be an ω-exchange process because no analogous structure occurs in π⁻p → (2π⁻2π⁺)n. Finally, a broad structure reveals double dissociation of both incident particles; it occurs in the two channels π⁻p → (2π⁻π⁺)(π^op) and π⁻p → (2π⁻π⁺)(π⁺n), being stronger in the latter. Further analysis of this process in terms of isospin exchange suggests that it is partially diffractive. Factorization is also discussed.

An appendix gives general aspects of the LPS analysis for the asymptotic study of n-body collisions at very high energy.     

A Partial Wave Analysis of the Reaction π+p→ (π+π−π0) Δ++ at 16 GeV/c

A partial wave analysis of the 3π-system has been performed for the reaction π⁺p→ (π⁺π^?π⁰) Δ⁺⁺ (1232) at 16 GeV/c. Beside the well-established A₂⁰ (1300), the resonant state ω* with isospin I = 0 and spin-parity J^P = 3^? decaying mainly into (?π) has been found. Its mass and width have been determined to be M = (1.71 ± 0.03) GeV and Γ = (0.22 ± 0.10) GeV. The cross section for the reaction π⁺p→ ω* (1700) Δ⁺⁺ (1232) is σ = (12 ± 6) μb.     

Rigorous pion-pion scattering lengths from threshold πN → ππN data

A new evaluation of the universal ππ scattering length relation is used to extract the ππ s-wave scattering lengths from threshold pion production data. Previous work has shown that the chiral perturbation series relating threshold pion production to ππ scattering lengths appears to converge well only for the isospin-2 case, giving a₂ = −0.031 ± 0.007m_π⁻¹. A model-independent and data-insensitive universal curve then implies a₀ = 0.235 ± 0.03m_π⁻¹ for the isospin-0 scattering length.     

Pion-pion amplitude from the πN→ππN reaction

The possibilities of extracting parameters of ππ interaction from the experimental data on the πN→ππN reaction are analyzed in detail. The interval 300<P_lab<500 MeV/c is shown to be the smoothness domain of the πN→ππN amplitude. The expression for the background part of the amplitude is developed model-independently. The ππ scattering is described by four parameters to be extracted from experimental data (along with 11 background parameters).     

Model amplitude analysis of πp→ππn

We present an amplitude analysis of the process π⁻p→π⁺π⁻n at 15 GeV/c assuming spin and phase coherence of the production amplitude. The bearing of the results on aspects of dipion production phenomenology is discussed.     

Collective sea-gull effect in πp interactions at 250 GeV/c

A collective “sea-gull” effect is observed for a system of fast hadrons produced in the fragmentation regions of nondiffractive π⁺p-interactions at 250 GeV/c. The effect is not reproduced by the FRITIOF fragmentation model in its details. It is demonstrated that hard-like processes, both in the collision phase and in the fragmentation phase, are not properly treated in the model.     

Note on the angular analysis of the decay J/ψ→f(→ππ)+γ

We derive the general (4-parameter) angular decay distribution for the cascade decay J/ψ→f(→ππ)+γ and determine its angular coefficients from the absorptive and dispersive contributions of the lowest order QCD diagram.     

A Multidimensional Study of Clustering in the Reactions π±p→pπ+π±π− at 16 GeV/c using the Yang Variables

The clustering of events in 7-dimensional phase space is studied in the reactions π^±p→pπ⁺π^±π^? at 16GeV/c. The Yang variables are used for locating events in the phase space. Clusters are defined and events are allocated by a novel iterative technique. This is based on a measure of the distance of each event from the cluster center, weighted by the covariance matrix of the event density. The clusters found are then related to reaction mechanisms such as diffraction dissociation of pion and proton, resonance production, etc. High mass enhancements in the (3π) system and in the (pπ⁺π^?) system are identified and separated from other mechanisms. Cross sections are given for individual sub-channels and compared to results obtained by other techniques.     

Anomalous J/ψ suppression in Pb---Pb interactions at 158 GeV/c per nucleon

The Drell-Yan and J/ψ cross-sections measured in Pb---Pb collisions are compared with the values extrapolated from the results obtained in proton and light ion induced reactions. While the Drell-Yan production exhibits the normal expected behaviour, the yield of J/ψ in Pb---Pb interactions is abnormally low, as it lies 9 standard deviations below the expected value. Moreover, the departure from the expected behaviour increases significantly from peripheral to central collisions.     

J/ψ and Drell-Yan cross-sections in Pb---Pb interactions at 158 GeV/c per nucleon

First results are reported on J/ψ and Drell-Yan cross-sections in Pb---Pb reactions at 158 GeV/c per nucleon. The ratio of cross-sections σ_J/ψ/σ_DY is studied as a function of the impact parameter of the collision estimated from the measured transverse energy.     

Deux trajectoires de regge dans la reaction πp → πn

The possibility of describing the high-energy reaction π⁻p^→π⁰n with two Regge trajectories (t) and _'(t) is studied in more detail. These trajectories are assumed to be exactly linear in t, with (t) constrained to pass through (m²) = 1. We examine hypotheses in which (t) obeys either the Gell-Mann mechanism or the Chew mechanism, and in which '(t) is or is not a conspiring trajectory. The model is in good agreement with the data in practically all cases; greater experimental accuracy would be necessary to distinguish between the various hypothesis. Predictions are given for the neutron polarization at |t| < 1 GeV².     

Supersymmetric contributions to direct CP violation in K→ππγ decays

We analyze the supersymmetric contributions to direct–CP–violating observables in K→ππγ decays induced by gluino–mediated magnetic–penguin operators. We find that ε′_+−γ and the differential width asymmetry of K^±→π^±π⁰γ decays could be substantially enhanced with respect to their Standard Model values, especially in the scenario where ε′/ε is dominated by supersymmetric contributions. These observables could therefore provide a useful tool to search for New Physics effects in |ΔS|=1 transitions, complementary to ε′/ε and rare decays.     

An anomalous -ω interference in the reaction ee → 3π

It is shown that in the π⁺π⁻ meson mass-spectrum in the reaction e⁺e⁻ → π⁺π⁻π⁰ at 2E > 1 GeV one should expect the anomalously large effect of the -ω interference. It can be used as a tool for the elucidation of the details of the e⁺e⁻ → Vπ reaction dynamics and as the additional possibility for the determination of the electromagnetic -ω mixing parameters.     

Experimental study of the time-dependent rate of K → πππ

The time-dependence of the decay rate of initially pure K⁰ into the final state (π⁺π⁻π⁰) has been studied in search for the decay k_S⁰→π⁺π⁻π⁰. No evidence is found in a sample of 384 observed events. The ratio of the CP -violating K_S⁰ amplitude and the K_L⁰ amplitude is η₊₋₀ = (0.13_−0.20^+0.17) + i(0.17_−0.26^+0.27); the ratio of the CP-conserving K_S⁰ amplitude and the K_L⁰ amplitude is < 0.4. The energy dependence of the K⁰→π⁺π⁻π⁰ matrix element is found to be a₊₋₀ = −0.31 ± 0.03.     

Perturbative QCD correction to the B → π transition form factor

We report on the perturbative O(_s) correction to the light-cone QCD sum rule for the B → π transition form factor f⁺. The correction to the product f_Bf⁺ in leading twist approximation is found to be about 30%, that is similar in magnitude to the corresponding O(_s) correction in the two-point sum rule for f_B. The resulting cancellation of large QCD corrections in f⁺ eliminates one important uncertainty in the sum-rule prediction for this form factor.     

A limit on the lepton-family number violating process π → μe

A FNAL E799 Collaboration has carried out a search for the lepton-family number violating decay π⁰ → μ^±e using π⁰'s produced from K_L → π⁰π⁰π⁰ decays in flight. No events were observed. Assuming that lepton-family number violation is charge independent, the 90% confidence level upper limit on

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was determined to be 8.6 × 10⁻⁹.