Pion condensation at finite temperature: (II). Simple models including thermal excitations of the pion field

We calculate, in several increasingly complex models, the threshold for charged pion condensation in neutron-rich matter at finite temperature, including effects of thermal excitations of the pion field. The thermal pion excitations have two important effects: first, to modify the phase diagram qualitatively from that predicted by mean field theory, and second, to make the phase transition to a nonuniform condensed state at finite temperature always first, rather than second, order.     

Hadroid and skyrmion

The interplay between the hedgehog pion field and the rho field is discussed in a hidden gauge symmetry model of rho-pion interactions. The importance of the non-perturbative excitation of the rho field, called a hadroid, is investigated. The hadroid combines with the hedgehog pion field to yield the nucleon and its excitations; the solutions are constructed numerically. The problem of matching the hedgehog pion with the hadroid for non-spherical configurations is briefly considered.     

Coupled-channels calculations of intermediate energy π−C inelastic scattering and charge exchange

Coupled-channels calculations have been carried out for pion inelastic scattering and charge exchange on 1p-shell nuclei at pion energies between 120 and 226 MeV. A momentum space technique is used, with the 1p-shell shell-model wavefunctions of Cohen and Kurath. Results are presented here for A=13. Channel-coupling effects are negligible in the strong 3/2^- and 5/2^- inelastic excitations of ¹³C, but enhance the analog charge-exchange cross sections by 7 to 20%.     

Pion capture in hydrogen: chemical aspects

Measurements of the probability of pion capture by hydrogen provide information on the properties of the chemical bond of hydrogen in molecules. This is demonstrated in the examples of the temperature dependence of pion capture in hydrogen-bonded liquids and of the influence of the position of the hydrogen atom in molecules. For the proper analysis and interpretation of the pion capture data we studied the effects of pion transfer from p^– to heavier atoms. Measurements were made in H₂ + D₂ mixtures and in alcohols. Together with the information available on the muon capture and cascade processes, the pion transfer results could be used for understanding muon transfer and, in particular, to estimate the parameterq _1s in muon catalysed fusion.     

The Collective Excitation Spectra of σ, ω and π Mesons in Nuclear Matter

The recent progress on the study of the collective excitation in relativistic nuclear matter is reviewed. The collective excitation modes are derived by meson propagators in nuclear matter. The mesons we studied are σ, ω, γ and π mesons. For pion, we derived not only the relativistic particle-hole, delta-hole excitations but also antiparticle excitations, suchas particle-antiparticle, antidelta-particle, delta-antiparticle excitations. By calculating the dispersion relation and the spin-isospin-dependent response function, the effects of all these excitation are studied.     

Energy spectra of neutrons emitted following muon capture in O and C

The energy spectra of neutrons emitted following muon capture in ¹⁶O and ¹²C are investigated using the continuum shell model. Nuclear wave functions, which have been shown by Ohtsubo and the author to describe the radiative pion capture reaction in the above nuclei well, are adopted. The calculated neutron energy spectra explain well the observed main peaks, at 5 MeV for ¹⁶O and 4 MeV for ¹²C, which are considered to be the giant resonances excited in the muon capture reaction. These peaks are interpreted as the 2⁻ state at 20.3 MeV for ¹⁶O and the 1⁻ state at 22.5 MeV for ¹²C. Comparisons with photon spectra in radiative pion capture reactions are also made. The calculated total capture rates exceed the experimental values by a factor of 2.5 for ¹⁶O and by 30–40% for ¹²C.     

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.     

Chiral invariant phase space event generator

An event generator based on the CHIPS model is implemented in the GEANT4 simulation toolkit. Nuclear fragment production in the process of pion capture on nuclei is used to tune the parameters of the CHIPS model describing clusterization of nucleons in nuclei. The spectra of nucleons and nuclear fragments in pion capture and in photonuclear reactions at 60 MeV are compared with experimental data. Received: 30 May 2000 / Accepted: 2 November 2000     

Search for A γ-branch from shape isomers in 236U and 238Np

It is pointed out that for excitations involving magnetic transitions, pion scattering from nuclei should in principle allow a separation of the orbital and spin contributions. This is not possible in electron scattering.     

Importance of momentum dependent terms in radiative pion capture

The contribution of the momentum dependent terms in radiative pion capture from bound atomic orbits is investigated. A shell model calculation for ⁶Li shows that the momentum dependent terms represent ≈ 2% of the 1s capture rate and up to 40% of 2p capture rates.     

The (π, 2π) reaction and spin-isospin strength distribution in nuclei

We study pion production on nuclei within the distorted wave impulse approximation (DWIA) and using the local density approximation (LDA) for the treatment of possible nuclear spin-isospin mode enhancements. The scattering amplitude input is based on a one-nucleon mechanism since two-nucleon processes are here estimatedito be relatively unimportant. For discrete nuclear levels, we study differential and total (angle-integrated) cross sections for various pion-like levels (J^P = 0^?, 1⁺, 2^?,…; T = 1); these are also compared with non-pion-like excitations. The cross sections for discrete excitations are quite small. The inclusive cross sections, achieved by summing over all nuclear states, are considerably larger, of the order of 0.1–10 mb. We use the Fermi gas model to test the importance of the quasi-free peak for various incident pion momenta k, and to study the closure approximation for summing over nuclear levels when calculating inclusive cross sections. It is found that for k of the order of about twice the Fermi momentum most of the quasi-free peak region contributes to the nuclear excitations, and the use of closure is justified and practical. All cases are studied as a function of g', the Migdal spin-isospin parameter, and it is found that the dependence upon this parameter may range from changes of an order of magnitude to a factor of two over the range 0.4 ? g' ? 0.7.     

Partial transitions in radiative pion capture on light atomic nuclei

The partial transitions in radiative pion capture on light nuclei are studied within the shell model with intermediate coupling. The probabilities of capture from s- and p-states of a mesoatom and the total yield of γ-quanta have been calculated and compared with experimental data.     

Branching ratio for radiative pion capture in 6Li and 12C

The radiative pion capture rate in ¹²C and ⁶Li leading to bound final states is calculated using the impulse approximation. The T-matrix is obtained from the time-reversed pion photo-production amplitude and initial-state distortion of the bound pion is taken into account. Using recently published capture schedules for pionic atoms the branching ratio R is calculated and compared to experimental values. The agreement is excellent if those experimental values are used that have been obtained by direct observation of the emitted γ-ray and if an optical-model value for the total 2P absorption width in ¹²C is used.     

Quarksonic matter at high isospin density

Analogous to the quarkyonic matter at high baryon density in which the quark Fermi seas and the baryonic excitations coexist,it is argued that a "quarksonic matter" phase appears at high isospin density where the quark(antiquark) Fermi seas and the mesonic excitations coexist.We explore this phase in detail in both large Nc and asymptotically free limits.In the large Nc limit,we sketch a phase diagram for the quarksonic matter.In the asymptotically free limit,we study the pion superfluidity and thermodynamics of the quarksonic matter by using both perturbative calculations and an effective model.     

核物质中π分子引起的粒子－空穴和△－空穴激发

以π介子为例给出了正确计算核物质中粒子－空穴激发的相对论方法，指出了它与通常计算方法的区别及通常计算方法中所作近似的不合理性．并与非相对论的粒子－空穴激发的色散关系进行了比较．我们还用这一方法计算并给出了△－空穴激发的表达式．     

Possibility of a phase transition to a pion condensate in neutron stars

We consider the possibility that a pion condensate may arise in an infinite nuclear medium as a consequence of the modification of the pion propagator due to isobar-hole and nucleon-hole excitations, This does not seem likely in a system with N = Z at densities of less than 0.17 fm^?3, but does seem likely to occur at the densities encountered in the interior of neutron stars. We estimate a necessary neutron density of ? 0.38 fm^?3 and a condensate energy density of ? ? 0.25 MeV fm^?3.     

Investigation of zero-frequency solutions to the pion dispersion equation

The instability of nuclear matter is considered for the case where it is generated by the vanishing of the frequencies of collective excitations belonging to specific types (specifically, excitations that have the pion quantum numbers J ^π = 0^?). The behavior of zero-frequency solutions to the pion dispersion equation is analyzed versus the strength G′ of spin—isospin particle—hole interaction. It is shown that there exists a strength value G′_tr (|G′_tr| ? 1) such that, for G′ < G′_tr, zero-frequency solutions are excitations of the ω _P type, while, for G′ ≥ G′_tr, such solutions are excitations of the ω _c type. Excitations of the ω _P type for G′ < ?1 describe the instability of nuclear matter against small density fluctuations (Pomeranchuk’s instability), while excitations of the ω _c type are responsible for the instability associated with pion condensation at G′ ≈ 2. For stable nuclear matter, the solutions ω _P(κ) and ω _c (κ) lie on unphysical sheets of the complex plane of frequency.