Coherent and incoherent η-photoproduction from nuclei

Elastic and inelastic η-photoproduction from complex nuclei is studied in a distorted-wave impulse approximation (DWIA) framework. The elementary operator is obtained by using a dynamical model which employs the reactions πN→πN, πN→ππN and π⁻p→ηn to fix the hadronic vertex as well as the isobar propagators and the process γN→πN to constrain the electromagnetic vertex. The nuclear structure input for the inelastic transitions has been extracted from electron-scattering form factors. The η final-state interaction has been included via a simple optical potential using the ηN t-matrix as an input. We find that coherent η-production is dominated by the D₁₃(1520) isobar while spin-flip transitions to excited nuclear states are sensitive to the S₁₁(1535) resonance. Predictions are given for coherent production on ⁴He, ¹²C and ⁴⁰Ca, as well as incoherent production on ⁶Li, ¹⁰B and ¹²C.     

Double pion photoproduction on the nucleon: study of the isospin channels

A model for the γp → π⁺π⁻p reaction developed earlier is extended to account for all isospin channels. The model includes N, Δ(1232), N^*(1440) and N^*(1520) as intermediate baryonic states and the meson as an intermediate 2π resonance. Although many terms contribute to the cross section, some channels exhibit particular sensitivity to certain mechanisms of resonance excitation or decay and the reactions provide novel information on such mechanisms. In particular the γN → N^*(1520) → Δπ process affects all the channels and is a key ingredient in the interpretation of the data. Comparison is made with all available data and the agreement is good in some channels. The remaining discrepancies in some other channels are discussed.     

Charged pion photoproduction in the quark model

In terms of a quark model an explanation is given of charged pion photoproduction ratios, at small momentum transfer, in the processes γN → π^±N, γN → π^±Δ. The model is also shown to be consistent with π⁰ photoproduction data.     

Regge-pole description of γN→πN backward-scattering data

Baryon Regge-pole contributions to u-channel helicity amplitudes for the γN→πN processes are derived, with attention to kinematic singularities and threshold conditions. An N, Nγ and Δδ Regge-exchange model is proposed that describes the backward scattering data on γp→π^op and γp→π⁺n at high energy. The N and Nγ trajectories are found to be nearly degenerate, with residues in the ratio β(Nγ)/β(N)≈0.6. Structure in the differential cross sections is explained as dominance at small u giving way to dominance at large u. An isoscaar-isovector admixture for the γ-coupling to is required by the fits. The solution extrapolates through the mean γp→π^op 180^o differential cross section at intermediate energies, as required by duality.     

The nature of 0 excitations in Er

Lifetimes of excited 0⁺ states in ¹⁶⁶Er have been measured with the (n,n′γ) reaction. The first and second excited 0⁺ states have no significant collective enhancements of their decays, and their properties are suggestive of pair-type excitations rather than those of a β vibration or phonon excitation built on the γ vibration. The third excited 0⁺ state at 1934 keV has an enhanced decay to the ground state band with B(E2; 0₄⁺ → 2_gsb⁺) = 8.8 ± 0.9 W.u., consistent with that expected for a β vibration.     

Observation of backward η production

The production of η mesons has been observed at high momentum transfer in the reaction π⁻p → nη. The ratio between this cross-section and that for backward charge exchange yields a value for the relative couplings g²_ηNN/g²_πNN.     

Neutrino masses, muon g−2, and lepton-flavour violation in the supersymmetric see-saw model

In the light of the recent muon (g_μ−2) result by the E821 experiment at the Brookhaven National Laboratory, we study the event rates of the charged lepton-flavour-violating (LFV) processes in the supersymmetric standard model (SUSY SM) with the heavy right-handed neutrinos (SUSY see-saw model). Since the left-handed sleptons get the LFV masses via the neutrino Yukawa interaction in this model, the event rate of μ→eγ and the SUSY-SM correction to (g_μ−2)/2 (δa_μ^SUSY) are strongly correlated. When the left-handed sleptons have a LFV mass between the first and second generations ( ) in the mass matrix, it should be suppressed by 10⁻³ (10⁻⁹/δa_μ^SUSY) compared with the diagonal components (m_SUSY²), from the current experimental bound on μ→eγ. The recent (g_μ−2) result indicates δa_μ^SUSY10⁻⁹. The future charged LFV experiments could cover . These experiments will give a significant impact on the flavour models and the SUSY-breaking models. In the SUSY see-saw model is proportional to square of the tau-neutrino Yukawa-coupling constant. In the typical models where the neutrino-oscillation results are explained and the top-quark and tau-neutrino Yukawa couplings are unified at the GUT scale, a large LFV mass of is generated, and the large LFV event rates are predicted. We impose a so-called no-scale condition for the SUSY-breaking parameters at the GUT scale, which suppresses the FCNC processes, and derive the conservative lower bound on μ→eγ. The predicted Br(μ→eγ) could be covered at the future LFV experiments.     

On the pp → ppη(η′) reactions near threshold

The production rate for η′ in pp → ppη′ at rest is calculated in a covariant one boson exchange model, previously applied to study π⁰ and η production in NN collisions. The transition amplitudes for the elementary BN → η′N processes with B being the meson exchanged (B = π, σ, η, , ω and a₀) are taken to be the sum of s- and u-channels with a nucleon in the intermediate states, and an a₀ meson pole in a t-channel. The couplings of the η′ to hadrons are a factor 0.4 weaker than the respective η-hadron couplings, as suggested by a quark model and a singlet-octet mixing angle θ = −23°. The model reproduces near threshold cross sections for the quasielastic processes π⁻p → nη(η′) and pp → ppη(η′) reactions.     

Photoproduction of positive pions from hydrogen with PHOENICS at ELSA

The differential cross section of the reaction γp→π⁺n has been measured with the PHOENICS detector at ELSA in Bonn. For the first time this cross section has been determined simultaneously over a large range of photon energies (E_γ = 220−900 MeV) and pion angles (Θ^c.m._pi = 35°−135°) with a tagged photon facility. The experimental set-up allowed a considerable kinematic overdetermination of the investigated reaction. Accordingly, the background contributions have been suppressed to below 1%. The measured differential cross section is in good agreement with existing data. The comparison with different model calculations is presented.     

The reaction Kp→Λπ in the c.m. energy range 1915 to 2168 MeV

A formationstudy of the reaction K⁻p→Λπ^o at 13 c.m. energies between 1915 and 2168 MeV is presented. Experimental differential cross sections and polarizations are given as expansions of Legendre polynomials and associated Legendre polynomials respectively. Partial wave analysis confirms the assignment of the Σ(2030) and the assignment of the Σ(1915). The parameters of these resonances are determined. The possible existence of other resonances coupled to this channel is investigated.     

Simple surface interaction mechanism for magnetic substate excitation by inelastic scattering

A simple model is presented to explain the marked behaviour of the de-excitation γ-pattern in certain recent inelastic -particle scattering experiments from even nuclei (0⁺→2⁺ excitation). The model is a slight generalization of the well-known ring-locus model in that it is based on a ring locus of finite angular width. The dependence of the inelastic angular distribution and the reaction-plane γ-pattern on the parameters of the ring locus is discussed.     

Patterns of lepton-flavour violation motivated by decoupling and sneutrino inflation

We present predictions for flavour-violating charged-lepton decays induced by the seesaw mechanism implemented within the constrained minimal supersymmetric standard model (CMSSM) with universal input soft supersymmetry breaking terms. We assume that one heavy singlet neutrino almost decouples from the seesaw mechanism, as suggested by the pattern of light neutrino masses and mixing angles. This is suggested independently by sneutrino inflation with a low reheating temperature, T_RH10⁷ GeV, so as to avoid overproducing gravitinos. This requirement further fixes the mass of the weakly-coupled sneutrino, whose decays may lead to leptogenesis. We find that BR(μ→eγ)10⁻¹³ but BR(τ→μγ)10⁻⁹ in the bulk of the acceptable parameter space, apart from a few isolated points. The ratio BR(μ→eγ)/BR(τ→eγ) depends on only one complex parameter, and is particularly interesting to compare with experiment.     

Heavy ion interactions of (14.0 MeV/u) Pb with U

The interactions of ²⁰⁸Pb projectiles with ^natU target have been studied at 14.0 MeV/u beam energy using two different threshold detectors; mica and lexan. The elastic scattering data have been separated from the binary events and used for the determination of quarter-point angle θ_1/4 (≡ grazing angle θ_gr). The values of maximum angular momentum , radius of interaction R_int and reaction cross-section σ_R^exp(el.) were then deduced by making use of the value of θ_1/4. The data of inelastic events of different multiplicities have been used to determine partial reaction cross-sections. The sum of the partial cross-sections, therefore, yielded another independent value of reaction cross-section σ_R^exp(inel.) that in turn was used to derive the alternative values of , θ_gr, and R_int independently. The average of these quantities provided the experimental values of the reaction parameters that are comparable to the theoretical predictions.     

Magnetic moment of the Δ (1232) resonance

The reaction γp → π^°γ′p has been measured with the TAPS BaF₂ calorimeter at the Mainz Microtron accelerator facility MAMI for energies between √2 = 1221–1331 MeV. Cross sections differential in angle and energy have been determined for the photon γ′ in three bins of the excitation energy. This reaction channel provides access to the magnetic dipole moment of the Δ⁺(1232) resonance and, for the first time, a value of μ_Δ+ = (2.7^+1.0_−1.3(stat) ± 1.5(syst) ± (theor)) π_N has been extracted.     

Tests of second-class currents

We argue that the presence of second-class neutral currents can be tested from the observations of and electron polarisation in η→π⁰e⁺e^-. Consequences of this model in η→π^±eν_e,τ^±→ηπ^±ν_τ and τ^±→ωπ^±ν_τ decays which would establish second-class charged currents are also discussed.     

Simultaneous measurements of the p+d→(A=3)+π reactions

A stack of annular detectors made of high-purity germanium was used to measure simultaneously pd → ³H π⁺ and pd → ³He π⁰ differential cross sections at beam momenta of 750 MeV/c, 800 MeV/c, and 850 MeV/c over a large angular range. The extracted total cross sections for the pd → ³He π⁰ reactions bridge a gap between near threshold data and those in the resonance region. The ratio of the cross sections for the two reaction channels taken at the same η=p^cm_π/m_π yields 2.11±0.08 indicating that a deviation from isospin symmetry is very small.     

Symmetry breaking and the decays ψ′ → J/ψπ(η) and J/ψ → η(η′) γ

The theoretical mechanisms for the decays ψ′ → ψπ⁰ and ψ′ → ψη, which violate SU₂ and SU₃, respectively discussed. It is argued that symmetry breaking in the decay amplitudes may be as important as π⁰ −η −η′ mixing. The π⁰ −η mechanism ψ′ → ψν → ψπ⁰ leads to Γ(ψ′ → ψπ⁰) = (3.3±1.0) × 10⁻³ Γ(ψ′ → ψν), but this number may be enhanced by a factor as large as 12 by π⁰−η′ mixing and isospin violation in the decay amplitude. The related decays ψ → ηγ and ψ → η′γ are also discussed.     

Study of impulse model in reaction Kd → pΛπ+ neutrals

In an analysis of 1114 events of the type K⁻d → pΛ⁰π⁻+ neutrals with ‘spectator’ proton momentum in excess of 0.11 GeV/c we have found that the number of protons having a momentum in the range 0.25–1.2 GeV/c exceeds that predicted on the basis of the impulse model using the Hulthén distribution by 3 : 1. Final-state interactions in which the proton does not form resonances with the other final-state particles are mainly held responsible for this effect.     

Non-1/mb power suppressed contributions to inclusive B → Xsll decays

We compute non-perturbative contributions to B → X_sl⁺l⁻ that are not explicitly suppressed by powers of the b-quark mass. They are proportional to and arise from an interference between the free-quark amplitude and high order terms in the matrix element of the four-quark operator s⁻γ^μ(1 − γ₅)cc^−βγ_μ(1 − γ₅)b_β. This correction is found to be small over most of the dalitz plot except near the charm threshold. Unfortunately, the perturbative computation we have performed is invalid near charm threshold and we do not except to see the structure found at lowest order reproduced in the data. We conclude that these non-perturbative contributions do not significantly modify the previous analysis of B → X_sl⁺l⁻.     

Effects of non-standard tri-linear couplings in photon-photon collisions (I) γγ → WW

The effect of anomalous couplings in γγ → W⁺W^t- is studied for different energies of the γγ mode of the next linear collider. The analysis based on the maximum likelihood method exploits the variables in the four-fermion semi-leptonic final state. Polarized differential cross sections based on the complete set of diagrams for these channels with the inclusion of anomalous couplings are used and compared to an approximation based on γγ → W⁺W^t- with full spin correlations. To critically compare these results with those obtained in e⁺e^t- we perform an analysis based on the complete calculation of the four-fermion semi-leptonic final state. The anomalous couplings that we consider are derived from the next-to-leading order operators in the non-linear realization of symmetry breaking.