Observation of inclusive η production in ee annihilation at CM energies of 34 GeV

The inclusive production of η mesons at CM energies around 34 GeV has been studied for e⁺e⁻ annihilation into hadrons. The average number of η-mesons per event is found to be 0.72 ± 0.10 (stat.) ± 0.18 (syst.). The abundance of η's is studied as a function of event shape parameters and it is compared to the corresponding π⁰ rates.     

Vortices and phase transitions in two-dimensional non-abelian spin models

We modify O(n) models (n>2) in two dimensions so as to comparedifferent thoeries with identical local properties and different global ones. Our O′ (3) model with a particular interaction has vortex-like configuration (π₁(P₂) = Z₂) though it is locally equivalent to an O (3) model (π₁(S₂) = 0). Our results have been obtained by means of strong coupling methods. The Padé extrapolants show a critical value x_c = 11.8.     

Selfconsistent description of vector-mesons in matter

We study the influence of the virtual pion cloud in nuclear matter at finite densities and temperatures on the structure of the ρ- and ω-mesons. The in-matter spectral function of the pion is obtained within a selfconsistent scheme of coupled Dyson equations where the coupling to the nucleon and the Δ(1232)-isobar resonance is taken into account. The selfenergies are determined using a two-particle irreducible (2PI) truncation scheme (Φ-derivable approximation) supplemented by Migdal's short range correlations for the particle–hole excitations. The so obtained spectral function of the pion is then used to calculate the in-medium changes of the vector-meson spectral functions. With increasing density and temperature a strong interplay of both vector-meson modes is observed. The four-transversality of the polarisation tensors of the vector-mesons is achieved by a projector technique. The resulting spectral functions of both vector-mesons and, through vector dominance, the implications of our results on the dilepton spectra are studied in their dependence on density and temperature.     

A study of the Li(π, pp)He reaction within the three-body problem

The differential cross section for the ⁶Li(π⁺, pp)⁴He reaction for symmetrical coplanar kinematics has been calculated at the incident pion energy E_π = 70 MeV within the three-body problem + 2N taking into account the Pauli exclusion principle in each N subsystem. Three-body distortions in the final state have been taken into account in the eikonal approximation. The treatment is based on the superposition of the single-particle and two-particle mechanisms of absorption leading to the rescattering of the π- and ρ-mesons through the Δ-isobar. The analytical character of the adopted wave function for the ⁶Li nucleus (the multidimensional gaussian basis) makes it possible to ascertain clearly that the reaction amplitude of the type under study is insensitive to short-range NN correlations both in the initial and final states. The calculated momentum distribution of recoil -particles is in good agreement with the experimental data available, but the experimental accuracy should be improved in order to see the contribution of a specific three-body part of the ⁶Li wave function orthogonal to the ad channel. Some problems to be solved in future are discussed.     

The relativistic stripping model of (p, π) reactions in the Δ resonance region

The relativistic stripping model is presented for the (p, π⁺) reaction in the Δ-resonance region. The Dirac impulse approximation and a Dirac-Hartree calculation are used for the proton nucleus interaction and neutron bound state wave function respectively. The Δ-hole model is employed for the pion-nucleus interaction. This model is applied to the reaction ¹⁶O(p, π⁺)¹⁷O(g.s.).     

Chiral symmetry and the A1 contribution to the nucleon-nucleon interaction

The contribution of A₁ exchange to the nucleon-nucleon potential is studied in a broken chiral symmetric model. The A₁ is treated as a finite-width resonance in the πρ s-wave. Connections between pseudoscalar and pseudovector pion-nucleon coupling in the underlying model lagrangian are studied in detail. It is found that large terms in the NN interaction arising from πρ exchange with pseudoscalar coupling are suppressed by interference with a₁ exchange. With pseudovector coupling there is a suppression of the A₁ exchange by the so-called “seagull” terms in πρ exchange which arise from gauge invariance. The suppression becomes an exact cancellation in the limit of infinite ρ and a₁ masses and exact chiral symmetry. We found that inclusion of the a₁ decay into the πρ state strongly modifies the a₁] exchange potential, suppressing the tensor part but leaving the spin-spin part almost unchanged.     

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.     

Radiative decay of ρ and φ mesons in a chiral unitary approach

We study the ρ⁰ and φ decays into π⁺π⁻γ, π⁰π⁰γ and φ into π⁰ηγ using a chiral unitary approach to deal with the final state interaction of the MM system. The final state interaction modifies only moderately the large momenta tail of the photon spectrum of the ρ⁰→π⁺π⁻γ decay. In the case of φ decay the contribution to π⁺π⁻γ and π⁰π⁰γ decay proceeds via kaonic loops and gives a distribution of ππ invariant masses in which the f₀(980) resonance shows up with a very distinct peak. The spectrum found for φ→π⁰π⁰γ decay agrees with the recent experimental results obtained at Novosibirsk. The branching ratio for φ→π⁰ηγ, dominated by the a₀(980), is also in agreement with recent Novosibirsk results.     

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).     

Threshold enhancement in η photoproduction from 2H and 4He

The photoproduction of η-mesons from ²H and ⁴He has been studied for energies close to the production thresholds. The experiments were carried out with the tagged photon beam of the Mainz MAMI accelerator. The η-mesons were detected via their two photon decays with the electromagnetic calorimeter TAPS. Total cross-sections, angular and momentum distributions of the η-mesons have been determined for both reactions. The total cross-sections in the threshold region show a large enhancement over the predictions of a participant-spectator model, indicating significant final-state interaction effects. The results are compared to recent model calculations taking into account nucleon-nucleon and nucleon-η final-state interaction effects on different levels of sophistication. Received: 23 November 2001 / Accepted: 11 February 2002     

Particle tracks and the mechanism of decoherence in a model bubble chamber

We put forward a toy model for a “bubble chamber” and study its interaction with an incoming object particle. We discuss the notion of particle “tracks” inside the bubble chamber and analyze the mechanisms that provoke a loss of quantum mechanical coherence (decoherence). The model is solvable and provides interesting insights into some of the most salient features of the interaction between a microscopic particle and a macroscopic device.     

Low-temperature phase and magnetic interactions in FCC Fe-Cr-Ni alloys

The low-temperature (5 K < T < 300 K) magnetic properties of a set of nine isostructural fcc Fe-Cr-Ni (Fe ≈ 68 at %, Cr ≈ 20 at%, Ni ≈ 9 at%) alloys were studied by SQUID magnetometry, neutron diffraction and ultrasonic techniques. Type-1 antiferromagnetic (AF) ordering was observed below the Néel temperature, T_N. The dc susceptibility, χ(T), did not exhibit a simple Curie-Weiss dependence. Above T_N, atemperature independent component χ₀ was observed, i.e., χ(T) = χ₀ + C / (itT + θ. T_N was systematically influenced by the lattice parameter, a, decreasing from (47.9 ± 0.5) K to (35.0 ± 0.5) K as a increased by only 0.25%. The average magnetic moment of ≈ 0.6μ_B obtained from neutron scattering was lower than the ≈ 1 μ_B obtained from the SQUID data. Mean field estimates of antiferromagnetic nearest-neighbors exchange interaction (J₁) and ferromagnetic second-nearest-neighbors interaction (J₂) indicate that |J₂/J₁|≈ 1.5. We believe that this is evidence of the RKKY interaction, and self-consistently argue that only the external d electrons are responsible for the localized average moment. This may mean that s-d hybridization of the external electrons is weak in these alloys.     

Excitation energy and angular momentum dependence of nuclear level densities and spin cut-off factor in SPA and SPA + RPA approaches

We investigate the excitation energy (E^*) and angular momentum (J) dependence of nuclear level density and spin cut-off factor (σ) within microscopic approaches based on SPA and its extension SPA + RPA representation of the grand partition function for quadrupole-quadrupole interaction model Hamiltonian. For ¹¹⁰Sn, we find that excitation energy dependence of the total level density obtained within these approaches is significantly different. On the other hand, these approaches yield similar behaviour for J-dependence of the level density at fixed values of E^*. Values of σ_{SPA + RPA} at low E^* are found to be slightly smaller than σ_SPA but they tend to become almost the same at higher E^* (> 30 MeV). We also find that Bethe's formula for fixed-J level density based on the spin cut-off approximation can be used to compute (E^*, J) near the yrast line provided one uses an appropriate value of the spin cut-off factor.     

Critical dynamics of a stochastic n-vector model below Tc

The dynamic properties of a stochastic n-vector model are investigated for T < T_c in d=4−ε dimensions. Besides the non-conserved order parameter the model involves also the conserved densities of generators of the symmetry group O(n). We calculate the excitation spectra of those conserved densities and the transverse fluctuations of the order parameter to linear order in ε in the hydrodynamic region kξ1. The propagating modes have linear dispersion and quadratic damping in accordance with the phenomenological theory. The relaxing modes, however, exhibit non-hydrodynamic wavenumber dependence with a relaxation rate ω_k ∞ k^d/2.     

Electroproduction of π from C leading to low-lying residual states in B

Energy distributions of π⁺ produced from ¹²C by electrons of total energy 195 MeV were measured at various angles. The results show large contributions from transitions leaving the residual nucleus in the ground (1⁺), first (2⁺) excited state and states at around 4.5 MeV. The angular distributions of ¹²C(γ, π⁺)¹²B leading to these residual states are deduced from the energy distributions by the unfolding method with the virtual photon theory. Theoretical results with the Helm model and the shell model are compared with the experimental results. Their relative shapes are in good agreement. A better agreement in the absolute value is found for the theoretical results which include the final-state interaction estimated with a pion optical potential. The surface production model shows better agreement with the experimental (γ, π⁺) cross sections than the volume production model.     

Microhydrodynamical modeling of transverse susceptibility of magnetic inks (Theory of ‘DIMAG’)

The time and field dependence of the transverse susceptibility, χ_t, of a suspension of single domain ferromagnetic Brownian particles in a viscous fluid is calculated. Magnetic interactions are taken into account through the mean field model. The model is used to interpret (so far qualitatively) the so-called DIMAG experiment, in which a DC field is applied to the ink sample for a short period of time and a small perpendicular AC field probes χ_t before, during and after the DC field impact. The relative increase in χ_t, after the DC field is switched on was used earlier to characterize the dispersion quality of inks. We show that in the systems with negative interactions (acicular particles) this increase has a maximum when plotted as a function of the DC field. The maximum shifts to higher fields as the interaction increases, which corresponds to what happens in the DIMAG testing of less dispersed inks.     

Interpolating between Ising, XY-, and non-linear σ-models

The β-functions of O(N)-symmetric non-linear σ-models on the lattice were recently discovered to be non-monotonic for N 3. We explain the non-monotonic behaviour as a non-perturbative lattice effect by relating it to the Kosterlitz-Thouless transition of the XY-model. We also relate the latter transition to the phase transition of the Ising model. These relationships are established by interpolating between the O(N)- and the O(N − 1)-symmetric non-linear σ-models by suppression of the Nth component of the N-vector field with a mass term. A critical line in the coupling-mass plane connects the critical point of the Ising model (N = 1) with the critical point of the XY-model (N = 2). This line extends towards the region of non-monotonic behaviour of the β-function of the O(3)-symmetric model. The nature of the transition lines is also investigated.     

Probing the EOS and nn cross section by analyzing the collective flow at high and intermediate energy in the RVUU approach

We have studied the collective flow at high and intermediate energy in a relativistic Vlasov-Uehling-Uhlenbeck (RVUU) approach based on Walecka's QHD-I model, with the aim to probe the nuclear-matter equation of state (EOS) and the in-medium nucleon-nucleon cross section σ. At high energy (1.2 GeV/u), the out-of-plane azimuthal correlation function C(Ψ) is only sensitive to the effective mass m^* and insensitive to the nuclear compressibility K and the effective nucleon-nucleon cross section σ within a reasonable range. We have found that the preferred value of m^* is about 0.85 m. With this value of m^*, from the in-plane mean transverse momentum P_x(Y) which is sensitive to both m^* and σ we have drawn an effective nn cross section σ, namely σ 0.8σ_f where σ_f is the free nucleon-nucleon cross section in Cugnon's parametrization. Taking advantage of the fact that the energy of vanishing flow (EVF) at intermediate energy (around 100 MeV/u) is only sensitive to the nucleon-nucleon cross section σ, we have drawn some information on the nucleon-nucleon cross section σ, namely σ = (1.4±0.2)σ_f.     

Charge susceptibility and charge correlation function in the d---p model

The properties of the charge fluctuation are investigated in the d---p model with the repulsion U_pd between holes on the nearest-neighbor Cu and O sites and the infinite on-site repulsion U_d at the Cu site. We calculate the charge susceptibility χ^c(q, iω_n) and the charge correlation function S^c(q) = TΣ_ωn χ^c(q, iω_n). It is found that S^c(q) has a peak at the Γ point and a maximum in a ring around the Γ point. The former is due to Tχ^c(q, 0). Its intensity is proportional to temperature T and strongly enhanced by U_pd. The latter is due to TΣ_{ωn ≠ 0} χ^c(q, iω_n) and shows a weak T and U_pd dependence. The intensity of the diffuse X-ray scattering on taking the charge fluctuation into account is also calculated. The result is consistent with the experiments in La_2−δSr_δCuO₄.     

Theoretical aspects of nuclear parity violation

We make use of the topological chiral soliton model of the nucleon to predict the form factors related to the parity-violating meson-nucleon vertices. We find that these are of monopole type at low q² with cut-offs similar to the equivalent strong meson-nucleon form factors. We furthermore investigate the parity-violating πΔN vertex. We find that while the isoscalar coupling vanishes identically, the isovector parity-violating πΔN vertex has a coupling stant of about 2 × 10⁻⁸ being of comparable magnitude as the equivalent weak pion-nucleon vertex strength. We also discuss the parity-violating ωΔN and ρΔN vertices in the model and furthermore comment on the pv meson ΔΔ vertices. In the appendix we will correct the formula for h_ω⁰ of ref. ⁵), where some terms have been omitted.