e + e − annihilation to π 0 π 0 γ and π 0 η γ as a source of information on scalar and vector mesons

We present a general framework for the model-independent decomposition of the fully differential cross section of the reactions e ⁺ e ^?→γ ^?→π ⁰ π ⁰ γ and e ⁺ e ^?→γ ^?→π ⁰ η γ, which can provide important information on the properties of scalar mesons: f ₀(600), f ₀(980) and a ₀(980). For the model-dependent ingredients in the differential cross section, an approach is developed, which relies on Resonance Chiral Theory with vector and scalar mesons. Numerical results are compared to data. The framework is convenient for development of a Monte Carlo generator and can also be applied to the reaction e ⁺ e ^?→γ ^?→π ⁺ π ^? γ.     

πp phase shifts from 88 to 310 MeV

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 P₃₃ 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 M⁰ ? M⁺⁺ = 1.4 ± 0.4 MeV/c² and Γ⁰ ? Γ⁺⁺ = 10.3 ± 1.3 MeV/c². The difference in width can be accounted for largely but not entirely by the Coulomb barrier, the channel π^?p → γn, and the difference in phase space between π^?p → π⁰n and π⁺p → π⁺p.     

Photoproduction of πΔ(1.232) from hydrogen in the energy range 2.4–4.8 GeV

We present differential cross sections andΔ ⁺⁺ spin density matrix elements for the photoproduction processγp→π ^? Δ ⁺⁺ and differential cross sections for the processγp→π ⁺ Δ ⁰. The incident photon energy dependence is studied and a comparison is made with previous experiments and with the predictions of a theoretical model.     

A search for anomalous contribution ine + e − →π 0/η γ annihilations

Data one ⁺ e ^?→η/π ⁰ γ collected by the Neutral Detector at Novosibirsk in a c.m. energy range up to 1.04 GeV have been studied with a purpose to single out of the scattering amplitude the triangle anomaly contribution predicted byqcd, pcac and Chiral Theories. We show that the cross section fore ⁺ e ^?→π ⁰ γ exhibits no sensitivity to theπ ⁰ triangle anomaly; instead, the cross section fore ⁺ e ^?→ηγ shows a clear sensitivity to the value of theη triangle anomaly and the best fit corresponds to a value for theη width in good correspondence with the world average value. It is also shown that the value of theρ→ηγ branching fraction is noticeably influenced by the existence of the anomaly. Finally, we give for the first time the absolute value for the hadronic phases of the vector mesons for the decay modesηγ andπ ⁰ γ.     

Measurement of elastic ω photoproduction at HERA ZEUS Collaboration

The reaction γp →ωp(ω → π⁺π^?π⁰ and π⁰ → γ γ) has been studied in ep interactions using the ZEUS detector at photon-proton centre-of-mass energies between 70 and 90GeV and ∣t∣ < 0.6GeV², where t is the squared four momentum transferred at the proton vertex. The elastic ω photoproduction cross section has been measured to be σ_{γp → ωp} = 1.21 ±0.12 ±0.23 μb. The differential cross section dσ_γp→ωp/d¦t¦ has an exponential shape e^?b∣t∣ with a slope b =10.0 ± 1.2 ± 1.3 GeV^?2. The angular distributions of the decay pions are consistent with s-channel helicity conservation. When compared to low energy data, the features of ω photoproduction as measured at HERA energies are in agreement with those of a soft diffractive process. Previous measurements of the ρ⁰ and ? photoproduction cross sections at HERA show a similar behaviour.     

The process e + e − → ωπ0 near the Φ resonance

The reaction e ⁺ e ^?→ωπ⁰ near a Φ resonance was studied with a spherical neutral detector at VEPP-2M e ⁺ e ^? collider. Both main modes of decay of a ω meson were investigated: ω→π⁺π^?π⁰ and ω→π ⁰γ. The probability of decay Φ→ωπ⁰ was obtained from the magnitude of the interference wave in the cross section for the reaction e ⁺ e ^?→ωπ⁰→ π⁺π^?π⁰π⁰: B(Φ→ωπ⁰) = (5.5 _?1.4 ^+1.6 ±0.3) ×10^?5. The ratio of the partial widths of the ω meson was obtained from the ratio of the cross sections for the two modes: Γ(ω→π⁰γ )/Γ(ω→π⁺π^?π⁰) = 0.0994 ± 0.0036 ± 0.0038.     

Nuclear levels in228Th populated in the decay of228Pa

The electron-capture decay of²²⁸Pa to levels in²²⁸Th has been studied using mass-separated sources and high-resolutionγ-ray and conversion-electron spectroscopy. A level at 979.5 keV is assigned as 2⁺ member of a second excited K_π=0⁺ band, with the 0⁺ band head at 938.6 keV. The 2⁺ and 3⁺ members of a second excited K_π=2⁺ band at 1153.5 and 1200.5 keV, which decay by strongE0 transitions to the 969 keVγ-vibrational band, are confirmed. In addition we tentatively propose a K_π=1⁺ band at 944 keV. The K_π=0^?, 1^? and 2^? members of the octupole quadruplet are confirmed, and theγ decay of these levels is analysed in an approach, in which the mixing of the octupole bands by the Coriolis interaction is taken into account. It is suggested that octupole correlations might be important for theE1 transition moments. A total of 29 levels is observed between ～1.4 and ～2.0 MeV, for which the nuclear structure, and the possible assignment to rotational bands, is unclear.     

Experimental tests of the factorisation of Regge trajectories in the proton fragmentation region

We present double differential cross sections for the reactions K^? + p → π^± + anything at 8.25 GeV/c. Using published data for reactions K⁺ + p→ π^? + anything and p + p → π⁺ anything we predict the differential cross sections for γ + p → π^? + anything, π^? + p → π^? + anything and π^? + p → π⁺ + anything in the target fragmentation region based on factorisation of the pomeron and Regge trajectories. Our predictions agree with the experimental data and support the hypothesis of factorisation of the Regge trajectories in the target fragmentation region.     

Radiative tau decay into pion

The possibility of the experimental investigation of the radiative tau decay into pion is discussed including background conditions. The decay width is estimated to be Br(τ→v _τ π γ)=0.15% (at the minimal registered photon energy 50 MeV). The contribution of the structural component is 10% of the total radiation. The main background is produced by photons from the τ→v _τ π ^? π ⁰ decay. We investigate the angular and energy spectra of the decay products and analyse the background conditions. The kinematic region was found where the structural radiation dominates over background.     

Measurement of the J/ψ radiative decay into f0(1270)γ

The J/ψ decay modes f⁰(1270)γ and ρπ have been studied in e⁺e^? collision at 3.1 GeV center of mass energy leading to the π⁺π^?γ and π⁺π^?π⁰ final states. The f⁰γ decay branching ratio is measured to be (0.20 ± 0.07) × 10^?2, a value comparable to the J/ψ radiative decay rates into the ηγ and η′γ channels. The J/ψ decay branching ratio into ?π is measured to be (1.6 ± 0.4) × 10^?2.     

3π resonances in 2− partial waves

The J^PC = 2^?+ partial wave intensities and their large phase changes prove the resonant nature of the A₃ meson (mass ≈ 1670 MeV, width ≈ 210 MeV). The decay modes are f⁰π, ?⁰π, and ?⁰π. Evidence is found for a further 2^? enhancement.     

Phenomenological model for describing pion-pair production on a proton by virtual photons in the energy region of nucleon-resonance excitation

A phenomenological model is developed for describing the production of π ⁺ π ^? pairs on a proton by virtual photons in the energy region of nucleon-resonance excitation.The cross sections are calculated for the channels γp → π ^?Δ⁺⁺, γp → π ⁺Δ⁰, and γp → ρp, which make a dominant contribution to pion-pair production, and the results are compared with available experimental data.The contributions of nucleon resonances to the cross section for the reaction γp → π ^?Δ⁺⁺ are predicted within the developed approach.     

Regge pole model for invariant functions. IV

TheNN→NN andγN→πN differential cross sections for polarized targets are discussed in terms of invariant functions. In former fits of high energy experimental data certain of these invariant functions came out to be small. Based on these fits thepn→np,p¯p→ n¯n, γp→ nπ ⁺ andγn→ pπ ^? differential cross sections are predicted to be independent of the target polarization.     

Measurement of radiative Σ±→nπ±γ decay

The radiativeΣ ⁺ → nπ⁺ γ decay mode was analysed in the 81 cm SACLAY hydrogen bubble chamber exposed to a stoppingK ^? beam at the CERN PS. The pion spectrum was measured up to momenta of 150 MeV/c in theΣ rest frame. We obtain the following branching ratios based on 180 Σ⁺→nπ⁺γ and 292Σ ^?→ nπ^?γ events:Γ(Σ⁺ → nπ⁺ γ)/Γ(Σ⁺→nπ⁺)=(0.93±0.10)×10^?3 Γ(Σ^?→nπ^? γ)/Γ(Σ^?→nπ^?)=(0.46±0.06)×10^?3. An unambiguous assignment ofS-wave to the Σ^?→nπ^? andP-wave toΣ ⁺→nπ⁺ was obtained. The absolute and differential decay rates as well as the parity assignments are in good agreement with theoretical predictions.     

Study of process e + e − → π+π−π0π0 at energies √s < 1 GeV with the spherical neutral detector

The cross section for the process e ⁺ e ^? → π⁺π^?π⁰π⁰ has been measured in an experiment with a spherical neutral detector, with the cross section at energies √s < 920 MeV having been measured for the first time. The model of vector dominance describes well the data obtained if the ρ, ρ′, and ρ″ resonances are taken into account. The probability of the decay ρ → π⁺π^?π⁰π⁰ is B(ρ → π⁺π^?π⁰π⁰) = (1.60 ± 0.74 ± 0.18) × 10^?5. The upper limit for the decay ω → π⁺π^?π⁰π⁰ has been improved by two orders of magnitude compared to previous measurements and is B(ω → π⁺π^?π⁰π⁰) < 2 × 10^?4 at a 90% confidence level.     

Measurement of the differential cross sections for π − p charge-exchange scattering in the region of the low-lying P 11, S 11, and D 13 resonances

The differential cross sections for the reaction π ^? p → π ⁰ n were measured for scattering angles in the backward hemisphere. The experiment was performed by using the pion channel of the synchrocyclotron installed at the Petersburg Nuclear Physics Institute (PNPI, Gatchina), the momenta of incident pions being varied in the range between 456 and 710 MeV/c. The measurements were performed by recording the recoil neutron in coincidence with one of the photons from the decay process π ⁰ → 2γ. The experimental facility used is described, and the results of the measurements are presented. These new results are characterized by a statistical accuracy higher than that of all results published previously and are in better agreement with the predictions of the partial-wave analysis (SM-95 version) performed by a group from Virginia Polytechnic Institute.     

A measurement of collinear and nearly collinear photon pairs produced by e+e− annihilation at the 3100 MeV resonance

We have measured collinear and nearly collinear photon pairs from e⁺e^? annihilation at total energies covering the resonance at 3100 MeV. The observed cross section is in good agreement with the e⁺e^? → γγ cross section expected from quantum electrodynamics. Upper limits for the decay modes (3100) → γγ, π^Oγ and other modes are derived.     

Forward photoproduction of neutral pions on protons in the third resonance region

Four angular distributions of the differential cross section of the reaction γ + p → π⁰ + p have been measured at the Bonn 2.5 GeV Synchrotron at pion c.m. angles between 3° and 63° in the photon energy range from 0.9 GeV to 1.2 GeV. The π⁰ mesons were detected by their two decay photons. The data are compared with data of other laboratories and predictions of two recent partial-wave analyses.     

The e + e −→π0π0γ process below 1.0 GeV

The e ⁺ e ^?→π⁰π⁰ γreaction was studied in the energy range 0.36–0.97 GeV on a VEPP-2M e ⁺ e ^? collider with an SND detector. The decay probabilities were found to be B(ω→ π⁰π⁰γ)= (7.8 ±2.7±2.0)×10^?5 and B(π→ π⁰π⁰γ)= (4.8 _?1.8 ^+3.4 ±0.2)×10^?5.     

Photoproduction ofN = (1236) by polarized γ-Quanta

The one-pion-exchange model with gauge invariant extension is used to calculate cross sections and decay distributions for the reactionγp→N⁼⁺⁺ π ^? where theγ-quanta are linearly polarized. We show that measurements with polarizedγ-quanta would provide a sensitive check of the model. In particular, the relevance of the terms added to the one-pion-exchange part to guarantee gauge invariance could be tested.