Isospin symmetry violation, meson production and η-nucleus interaction studies

We have studied isospin symmetry violation in nuclear reactions by measuring simultaneously the cross-section of the following two reactionsp +d →³H π⁺ andp +d →³He π⁰. The experiment was perfomed at the cooler synchrotron accelerator COSY, Jülich at several beam energies close to the correspondingη production threshold. We also have ongoing programmes onη-nucleus final-state interaction studies viap+⁶ Li →⁷ Be +η reactions, high resolution search for dibaryonic resonances and lambda-proton final state interaction studies. The experimental details and results obtained so far are presented here     

ωN final state interactions and ω-meson production from heavy-ion collisions

We calculate the elastic and inelastic ωN→ωN, →πN, →ρN, →ρπN, →ππN, →σN reactions within a boson exchange approximation where the ωρπ coupling constant and form factor are fixed by the reaction πN→ωN in comparison to the experimental data. We find rather large ωN cross sections at low relative momenta of the ω-meson which leads to a substantial broadening of the ω-meson width in nuclear matter. The implications of the ωN final state interactions are studied for ω production in ¹²C +¹²C, ⁴⁰Ca +⁴⁰Ca and ⁵⁸Ni +⁵⁸Ni reactions at about 2 · A GeV within the HSD transport approach; the drastic changes of the transverse mass spectra relative to a general m _T-scaling (for π⁰ and η mesons) might be controlled experimentally by the TAPS Collaboration. Received: 28 April 1999 / Revised version: 7 June 1999     

Electromagnetic corrections in <Emphasis Type="Italic">η</Emphasis>→3<Emphasis Type="Italic">π</Emphasis> decays

We re-evaluate the electromagnetic corrections to η→3π decays at next-to-leading order in the chiral expansion, arguing that effects of order e ²(m _u −m _d ) disregarded so far are not negligible compared to other contributions of order e ² times a light-quark mass. Despite the appearance of the Coulomb pole in η→π ⁺ π ⁻ π ⁰ and cusps in η→3π ⁰, the overall corrections remain small.     

Single-pion production in pp collisions at 0.95 GeV/c (I)

The single-pion production reactions pp → dπ⁺, pp → npπ⁺ and pp → ppπ⁰ were measured at a beam momentum of 0.95GeV/c ( T _p ≈ 400MeV) using the short version of the COSY-TOF spectrometer. The implementation of a central calorimeter provided particle identification, energy determination and neutron detection in addition to time-of-flight and angle measurements. Thus, all pion production channels were recorded with 1-4 overconstraints. The total and differential cross-sections obtained are compared to previous data and theoretical calculations. Main emphasis is put on the discussion of the ppπ⁰ channel, where we obtain angular distributions different from previous experimental results, however, partly in good agreement with recent phenomenological and theoretical predictions. In particular, we observe very large anisotropies for the π⁰ angular distributions in the kinematical region of small relative proton momenta revealing there a dominance of proton spinflip transitions associated with π⁰ s and d partial waves and emphasizing the important role of π⁰ d-waves.     

On rescattering effects in the reaction π?d → π?d

Rescattering corrections to the impulse approximation for the processes γd → π ⁰ d and π ⁻ d → π ⁻ d are discussed. It is shown that the rescattering effects give a nonnegligible contribution to the real part of these amplitudes. At the same time, the contributions from the imaginary parts of impulse and rescattering corrections drastically cancel each other. This cancellation means that the processes π ⁻ d → π ⁰ nn and γd → π ⁺ nn/π ⁻ pp, when the nucleon pair is in the spin triplet state, are strongly suppressed near threshold as required by the Pauli principle. From Yadernaya Fizika, Vol. 67, No. 4, 2004, pp. 764–768. Original English Text Copyright ? 2004 by Baru, Kudryavtsev, Tarasov. This article was submitted by the authors in English.     

T-violation and neutron optics experiments

It is shown that a two-step model of the reaction pd→³HeX (X=η, η′ eω, φ), involving the subprocesses pp) →dπ⁺ and π⁺ n→Xp, can account for the form of the energy dependence of experimental cross sections above the thresholds under the assumption that the singlet part of the pp→dπ⁺ amplitude dominates. The spin-spin asymmetry for the reaction dp→³HeX has been found to be ∼;−1 in the forward-backward approximation. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 1, 3–7 (10 January 1996) Published in English in the original Russian journal. Edited by Steve Torstveit.     

a 0 + (980)-resonance production in the reaction pp → dπ + η close to the K\bar K threshold

The reaction pp → dπ ⁺ η has been measured at a beam energy of T _p = 2.65 GeV (p _p = 3.46 GeV/c) using the ANKE spectrometer at COSY-Julich. The missing-mass distribution of the detected dπ ⁺ pairs exhibits a peak around the η mass on top of a strong background of multipion pp → dπ ⁺(nπ) events. The differential cross section d ⁴ σ/dΩ_d dΩ _π ⁺ dp _d dp _π ⁺ for the reaction pp → dπ ⁺ η has been determined model independently for two regions of phase space. Employing a dynamical model for the a ₀ ⁺ production allows one then to deduce a total cross section of σ(pp → da ₀ ⁺ → dπ ⁺ η) = 1.1 ± 0.3_stat ± 0.7_syst μb for the production of π ⁺ η via the scalar a ₀ ⁺ (980) resonance and σ(pp → dπ ⁺ η) = 3.5 ± 0.3_stat ± 1.0_syst μb for the nonresonant production. Using the same model as for the interpretation of recent results from ANKE for the reaction , the ratio of the total cross sections is , which is in agreement with branching ratios in the literature. The text was submitted by the authors in English.     

Mechanism of the production of the <Emphasis Type="Italic">a</Emphasis><Subscript>0</Subscript>(980) resonance in the γγ → π<Superscript>0</Superscript>η reaction

High-statistics Belle data on the γγ → π⁰η reaction have been analyzed in order to reveal the mechanism of two-photon production and the nature of the a ₀(980) resonance. The solution obtained for the γγ → π⁰η amplitude is in agreement with the predictions of the chiral theory for the πη-scattering length; with the strong coupling of the a ₀(980) resonance with the πη, K $ \bar K $ \bar K , and πη′ channels; and with the key role of the a ₀(980) → (K $ \bar K $ \bar K + π⁰η + π⁰η′) → γγ rescattering mechanisms in the a ₀(980) → γγ decay. This picture is much in favor of the q ² $ \bar q $ \bar q ² nature of a ₀(980) resonance and is consistent with the properties of its partners, σ₀(600) and f ₀(980) resonances, in particular, with those manifested in the γγ → ππ reactions. The important role of vector exchanges in the formation of the nonresonant background in the γγ → π⁰η reaction has been revealed. Preliminary information on the π⁰η → π⁰η reaction has been obtained.     

Bs,d→ππ,πK,KK: status and prospects

Several years ago, it was pointed out that the U-spin-related decays B_d→π⁺π^-, B_s→K⁺K^- and B_d→π^∓K^±, B_s→π^±K^∓ offer interesting strategies for the extraction of the angle γ of the unitarity triangle. Using the first results from the Tevatron on the B_s decays and the B-factory data on B_u,d modes, we compare the determinations of γ from both strategies, study the sensitivity on U-spin-breaking effects, discuss the resolution of discrete ambiguities, predict observables that were not yet measured but will be accessible at LHCb, explore the extraction of the width difference ΔΓ_s from untagged B_s→K⁺K^- rates, and address the impact of new physics. The data for the B_d→π⁺π^-, B_s→K⁺K^- system favour the BaBar measurement of direct CP violation in B_d→π⁺π^-, which will be used in the numerical analysis and result in a fortunate situation, yielding γ=(66.6^+4.3+4.0 _-5.0-3.0)^°, where the latter errors correspond to a generous estimate of U-spin-breaking effects. On the other hand, the B_d→π^∓K^±, B_s→π^±K^∓ analysis leaves us with 26°≤γ≤70°, and points to a value of the B_s→π^±K^∓ branching ratio that is larger than the current Tevatron result. An important further step will be the measurement of mixing-induced CP violation in B_s→K⁺K^-, which will also allow us to extract the B⁰ _s–B̄⁰ _s mixing phase unambiguously with the help of B_s→J/ψφ at the LHC. Finally, the measurement of direct CP violation in B_s→K⁺K^- will make the full exploitation of the physics potential of the B_s,d→ππ,πK,KK modes possible.     

Di-pion emission in heavy quarkonia decays

The dipion spectrum for the ϒ(nS) → ϒ(n′S) transition with n < 4 has the form dw/dq ∼ (phase space) |η − x|², with x = q ² − 4m _π² / (ΔM)² − 4m _π² < q ² ≡ M _ππ², and ΔM = M(nS) − M(n′S). The parameter η is calculated and the spectrum is shown to reproduce the experimental data for all three types of decays: 3 → 1, 2 → 1, and 3 → 2 with η ≈ 0.5, 0, and −3, respectively. The text was submitted by the author in English.     

Comments on preliminary data on the reactions e + e −→φ→γ f 0(980)→γπ0π0

Preliminary data on the reactions e ⁺ e ⁻→φ→γ f ₀(980)→γπ⁰π⁰ together with data on ππ scattering and the reactions J/ψ→φπ⁺π⁻ and K ⁻ p→π ⁺π⁻(Λ,Σ) are analyzed. The analysis shows that the mass of the f ₀(980) meson is = 950 MeV, and BR(φ→γf ₀→γπ⁰π⁰)≃ 1× 10⁻⁴, indicating that the f ₀(980) resonance is of a four-quark nature. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 7, 445–449 (10 April 1998)     

Pion production by protons on nuclei near the threshold

We propose including in the calculation of the cross section of the process p + N → N + N + π the process of pion production and absorption that proceeds through creation of a Δ-resonance. By taking into account the energy dependence of Δ-resonance width, the contribution of deuteron channel is included in the cross section of π⁰-, π⁺-meson production.     

Electromagnetic polarizabilities of the nucleon and properties of the σ-meson pole contribution

The t-channel contribution to the difference of electromagnetic polarizabilities of the nucleon, (α - β)^t, can be quantitatively understood in terms of a σ-meson pole in the complex t-plane of the invariant scattering amplitude A ₁(s, t) with properties of the σ-meson as given by the quark-level Nambu-Jona-Lasinio model (NJL). Equivalently, this quantity may be understood in terms of a cut in the complex t-plane where the properties of the σ-meson are taken from the ππ → σ → ππ, γγ → σ → ππ and Nˉ → σ → ππ reactions. This equivalence may be understood as a sum rule where the properties of the σ-meson as predicted by the NJL model are related to the f ₀(600) particle observed in the three reactions. In the following, we describe details of the derivation of (α - β)^t making use of predictions of the quark-level NJL model for the σ-meson mass. An erratum to this article is available at .     

Effective Lagrangians and IG(JPC)=1?(1?+) exotic resonance state in the VP and the PP channel

For I ^G (J ^PC)=1⁻(1⁻⁺) exotic waves in the reactions ρπ → ρπ, ρπ → ηπ, ρπ → η′π, and ρπ → and in allied reactions, a model is constructed that satisfies the conditions of unitarity and analyticity and which employs, as an input, an “anomalous” nondiagonal VPPP interaction relating the ρπ, ηπ, η′π, and channels. The possibility of obtaining, within this simple field-theoretical model, a resonance behavior of the I ^G(J ^PC)=1⁻(1⁻⁺) amplitudes corresponding to the {10} − and {8} representations of the SU(3) group and their mixing is demonstrated explicitly in the mass range 1.3–1.6 GeV, where data of present-day experiments suggest a rich exoticism. __________ Translated from Yadernaya Fizika, Vol. 65, No. 3, 2002, pp. 579–588. Original Russian Text Copyright ? 2002 by Achasov, Shestakov.     

Studying the ω mass in-medium in γ + A →π0γ + X reactions

Simulations based on a coupled-channel transport model have been performed to analyze the feasibility to study the in-medium ω mass exploiting the process γ + A→π⁰γ + X for C, Ca and Nb nuclei. The distortions due to final-state interactions of the π⁰ and background contributions from the reaction γ + A→π⁰π⁰ + X are found to be small in the mass range of interest ( 0.6 < M _π0γ < 0.8 GeV). Furthermore, the effect of the detector resolution on the π⁰γ mass determination is discussed. Received: 5 December 2000 / Accepted: 18 May 2001     

Cross-sections for low-energy πgamma; reactions

We review the cross-sections for low-energy π^-γ reactions in the framework of chiral perturbation theory. Charged-pion Compton scattering, π^-γ → π^-γ , is considered up to one-loop order where the pion's internal structure enters through the difference of the electric and magnetic pion polarizability, α_π - β_π . The ongoing COMPASS experiment aims at measuring this important structure constant with high statistics using the Primakoff effect. In the same way, the reaction π^-γ → π^-π⁰ serves as a test of the QCD chiral anomaly (via the γ3π -coupling constant F _γ3π . Furthermore, we calculate the total cross-sections for neutral and charged pion-pair production, π^-γ → π^-π⁰π⁰ and π^-γ → π^-π⁺π^- , which are governed by the chiral ππ -interaction. Finally, we investigate the radiative (correction) process π^-γ → π^-γγ and calculate the corresponding two-photon mass spectrum. This information will be useful for analyzing the π^- Primakoff scattering events with photons in the final state.     

np → ηd near threshold and the s -wave ηN amplitude

We calculate the process np → ηd near threshold using a separable potential model of the coupled ηN - πN - ππN subystems, and a relativistic three-body calculation for the ηd scattering amplitude. The ππN channels are represented by an effective σN channel, and we compare the case where the σ and π masses are related by m _σ = 2m _π and no width is considered, to another where the mass and width of the σ -meson are taken from ππ scattering data. The np → ηd cross-section can be well described up to about 60MeV by models where the real part of the ηN scattering length lies between 0.4≤Re(a _ηN)≤0.6 fm which allows us to determine the s -wave ηN scattering amplitude for -60≤E≤60 MeV.     

Dynamics of low-energy nuclear forces for electromagnetic and weak reactions with the deuteron in the Nambu-Jona-Lasinio model of light nuclei

A dynamics of low-energy nuclear forces is investigated for low-energy electromagnetic and weak nuclear reactions with the deuteron in the Nambu-Jona-Lasinio model of light nuclei by example of the neutron-proton radiative capture (M1-capture) n + p → D + γ, the photomagnetic disintegration of the deuteron γ + D → n + p and weak reactions of astrophysical interest. These are the solar proton burning p + p → D + e⁺ + ν _e, the pep-process p + e⁻ + p → D + ν _e and the neutrino and antineutrino disintegration of the deuteron caused by charged ν _e + D → e⁻ + p + p, + D → e⁺ + n + n and neutral ν _e() + D → ν _e() + n + p weak currents.     

Electromagnetic decays of radially excited mesons π0′, ρ0′, ω0′, and production of π0′ at lepton colliders

Radiative decays π ⁰(π ^0′) → γ + γ, π ^0′ → ρ ⁰(ω) + γ, ρ ^0′(ω′) → π ⁰ + γ, ρ ^0′(ω′) → π ^0′ + γ, and some processes of π ^0′ production at lepton colliders are considered in the framework of the nonlocal SU(2) × SU(2) Nambu-Jona-Lasinio model. Mixing of the radially excited and the ground meson states is taken into account. Numerical results for the decay and production processes are presented.