Pion Interactions Research Articles

ϱ-meson production and decay in proton-nucleus collision

We analyze the production of ϱ-mesons in p + A reactions including both the production by proton-nucleon as well as pion-nucleon collisions within a coupled channel transport approach. The final state interactions of the ϱ-meson with nucleons are evaluated from a resonance model which allows to extract elastic and inelastic cross sections. We include the latter final state interactions, the ϱ-meson decay into two pions as well as the final pion-nucleon interactions within the transport approach. We find the invariant mass distribution of pion pairs to be sensitive to the ϱ-meson properties in the nuclear medium. However, due to the strong final state interactions of pions only light targets like 12C might be suited to extract a ϱ-signal from the uncorrelated two pion background which carries information about the in-medium properties of the ϱ-meson.

Chiral symmetry: Pion-nucleon interactions in constituent quark models

We study the interactions of an elementary pion with a nucleon made of constituent quarks and show that the enforcement of chiral symmetry requires the use of a two-body operator, whose form does not depend on the choice of the pion-quark coupling. The coordinate space NN effective potential in the pion exchange channel is given as a sum of terms involving two gradients, which operate on both the usual Yukawa function and the confining potential. We also consider an application to the case of quarks bound by a harmonic potential and show that corrections due to the symmetry are important.

Pion-nucleus elastic scattering on12C,40Ca,90Zr,and208Pbat 400 and 500 MeV

Pion-nucleus elastic scattering at energies above the Delta(1232) resonance is studied using both pi+ and pi- beams on 12C, 40Ca, 90Zr, and 208Pb. The present data provide an opportunity to study the interaction of pions with nuclei at energies where second-order corrections to impulse approximation calculations should be small. The results are compared with other data sets at similar energies, and with four different first-order impulse approximation calculations. Significant disagreement exists between the calculations and the data from this experiment.

The two-pion exchange NN potential in nuclear matter and nuclear stability

A meson exchange model of the ππ interaction which fits free ππ scattering data is used to calculate the interactions of pions in nuclear matter as a function of nuclear density. Polarization of the nuclear medium by the pions results in a marked increase in the s-wave ππ attraction at low energy. The influence of this effect on the nucleon-nucleon interaction is a corresponding increase with density of the NN central potential due to the exchange of two correlated pions, resulting in an NN interaction which fails to saturate. A possible mechanism for restoring the theoretical stability of nuclear matter is explored and found to be effective.

RareK decays

The rare decays of the $K$ meson have had a long tradition as a laboratory for testing the symmetry properties of the weak interactions, and the manner in which these symmetries are broken by higher order effects. Present--day interest is focussed on decays that are suppressed by $CP$--symmetry or GIM symmetry. Such decays, in the standard theory, are sensitive to effects of the virtual top quark, and could also reveal new interactions transcending the standard model. In addition, the radiative decays of the $K$ meson have become a useful testing--ground for effective Lagrangians describing the low energy interactions of pions, kaons and photons.

The CHAOS spectrometer for pion physics at TRIUMF

The Canadian high acceptance orbit spectrometer (CHAOS) is a unique magnetic spectrometer system recently commissioned for studies of pion induced reactions at TRIUMF. It is based on a cylindrical dipole magnet producing vertical magnetic fields up to 1.6 T. The scattering target is located in the center of the magnet. Charged particle tracks produced by pion interactions there are identified using four concentric cylindrical wire chambers surrounding the target. Particle identification and track multiplicity are determined by cylindrical layers of scintillation counters and lead-glass Cherenkov counters, which also provide a first level trigger. A sophisticated second level trigger system permits pion fluxes in excess of 5 MHz to be employed. The detector subtends 360° in the horizontal plane, and ±7° out of this plane for a solid angle coverage approximately 10% of 4π sr. The momentum resolution delivered by the detector system is 1% (σ).

Neutron-to-proton ratio in intermediate-energy nuclear reactions.

The ratio of inclusive spectra of neutrons and protons in antiproton-, pion-, and proton-nucleus interactions at intermediate energies has been considered in the framework of the intranuclear cascade (INC) model. The ratio predicted by the standard INC model is consistent with the experimental data for pion- and proton-induced reactions, but contradicts the ratio obtained in antiproton-nucleus annihilation measurements. It has been shown that for pion-nucleus interactions the n/p ratio is much affected by isotopic effects in pion absorption and mainly depends on pion charge. Nevertheless, due to the presence of positive, negative, and neutral pions in antiproton-nucleon annihilation the calculated n/p ratio becomes close to the N/Z ratio of the target nucleus. The coalescence of nucleons, preequilibrium emission of particles, interaction of annihilation pions with the nuclear Coulomb field, neutron halo effects, and many body annihilation have been considered as possible candidates which would affect the n/p ratio in antiproton-nucleus annihilation. A simple quark counting estimation for the n/p ratio in many body annihilation has been made.

Short-range correlations in the pion s-wave self-energy of pionic atoms

We evaluate the contribution of second-order terms to the pion-nucleus s-wave optical potential of pionic atoms generated by short-range nuclear correlations. The corrections are sizeable because they involve the isoscalar s-wave πN amplitude for half off-shell situations where the amplitude is considerably larger than the on-shell one. In addition, the s-wave optical potential is reanalyzed by looking at all the different conventional contributions together: lowest-order, Pauli corrected rescattering term, second-order absorptive effects, terms from the interaction of pions with the virtual pion cloud (chiral corrections) and correlation effects. Different off-shell extrapolations for the πN amplitude are used and it is found that, although some individual terms are sensitive to the extrapolation, the sum of them is rather insensitive. The results are compared with empirical values from best fits to the data and are found to be compatible, within theoretical and empirical uncertainties. The results do not rule out further contributions but they put stringent constraints on their strength.

FINAL STATE INTERACTIONS AND PREASYMPTOTIC EFFECTS IN NONLEPTONIC KAON DECAYS

Nonleptonic decay rates of [Formula: see text] and K− have been estimated using the two independent theoretical schemes. With final state interactions of emitted pions included, both schemes yield qualitatively a correct and consistent description of decay rates. Various contributions to decay rates have also been estimated. It turns out that contributions due to the quark annihilation mechanisms are much larger than the decay term, contrary to the usual beliefs based on helicity arguments.

Containment and Neutron Production by Charged Pions in Antiproton Microfission Experiments

Containment and interaction of charged pions in a solid linear implosion system are simulated. Pions are generated from annihilation of antiprotons at the surface of a compressed target. A three-dimensional Monte Carlo code has been developed to simulate the interaction of charged pions with the system. Neutron yields are presented for several 27-g uranium targets compressed under different initial plasma conditions. Effects on neutron yields from the diffused magnetic field and density profiles at peak compression are discussed. Results show that the magnetic field at peak compression significantly increases overall neutron yields.

Measurements of 525 GeV pion interactions in emulsion.

Measurements have been made of inclusive 525 GeV [pi][sup [minus]] interactions in emulsion. The results are compared to proton-emulsion and lower energy pion-emulsion data. Average multiplicities of relativistic shower particles increase with increasing energy, although with a somewhat steeper slope above 60 GeV than at lower energies. The ratio [l angle][ital n][sub [ital s]][r angle][sub [ital p]]/[l angle][ital n][sub [ital s]][r angle][sub [pi]][similar to]1.1 over the energy range 60--525 GeV. The ratio of the dispersion in the multiplicity distribution to the average multiplicity is the same for proton and pion collisions in emulsion, and is independent of projectile energy. The shape of the shower particle multiplicity distribution does not vary significantly with energy, and KNO scaling appears to hold over the energy range 60--525 GeV. The shower particle pseudorapidity distributions are independent of the beam energy in the target and projectile fragmentation regions, and both the pseudorapidity and multiplicity distributions agree reasonably well with the FRITIOF model predictions for 525 GeV pions. The dependence of the shower particle multiplicity [l angle][ital n][sub [ital s]][r angle] on the number of heavy tracks [ital N][sub [ital h]] appraoches saturation as the total shower particle energy becomes a significant fraction of [radical][ital s]more » , and the pseudorapidity distributions shift toward smaller [l angle][eta][r angle] with increasing numbers of grey and black tracks at 525 GeV. Neither the average number [l angle][ital N][sub [ital h]][r angle] nor the multiplicity distributions of the heavily ionizing tracks vary significantly with energy, and the normalized angular distributions of grey and black tracks are independent of the type of projectile or projectile energy.« less

The s-wave π-nucleaus interaction and the (p,n) puzzle

The expansion of the off-shell s-wave isospin-symmetriic πN amplitude suggests that the s-wave interaction of pions with the nuclear medium has complex behaviour. It is appreciably repulsive for photon-like or space-like pions but it becomes small for on-shell pions. This has consequences for a number of physical quantities such as the quark condensate in nuclei and the effective pion mass. It may also offer an explanation for the (p,n) puzzle, where the attraction from the pion exchange force has failed to appear.

Possible way out of the (p,n) puzzle.

The off-shell expansion of the [ital s]-wave [pi][ital N] amplitude suggests that the [ital s]-wave interaction of space-like pions with the nuclear medium is appreciably repulsive. This has consequences for the experiments aimed at the exploration of the spin longitudinal response or of the pion excess in nuclei. It may offer in particular an explanation for the ([ital p],[ital n]) puzzle where the attraction from the pion exchange force has failed to appear.

Final state interactions inK→ππ decays and ΔS=2 matrix element for $$K^0 - \bar K^0 $$ mixing

It is shown that a quark model for mesons and the final state interactions of emitted pions lead to the acceptable theoretical description ofK→2π decays. The same approach, with the same parametrization, predicts for\(K^0 - \bar K^0 \) transition theB-parameter valueB(μ 2 B )∼0.84.

The experiment 705 electromagnetic shower calorimeter

Experiment 705 at Fermi National Accelerator Laboratory has designed, built, and operated a large acceptance, highly segmented electromagnetic shower calorimeter using SF5 lead glass, SCG1-C scintillating glass, and two types of gas-based fine-grained hodoscopes. The calorimeter was used to reconstruct photons and electrons with energies ranging from a few GeV/ c 2 to over 100 GeV/ c 2 in 300 GeV/ c pion and proton interactions on a lithium target at instantaneous interaction rates approaching several MHz. Construction details of the calorimeter are given. The readout electronics, calibration, and algorithms used to reconstruct the positions and energies of showering particles are discussed. Energy resolution, position resolution, and reconstruction efficiency are assessed using both calibration electron beams and electrons and π 0 mesons reconstructed in 300 GeV/ c interactions.

Contributions of the C-odd axial-vector resonances to eta --> pi 0 gamma gamma and gamma gamma --> pi 0 pi 0.

We consider the contributions of the $C$-odd axial-vector resonances (${b}_{1}$, ${h}_{1}$) to $\ensuremath{\eta}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{0}\ensuremath{\gamma}\ensuremath{\gamma}$, as well as those of the usual $C$-odd vector resonances ($\ensuremath{\rho}$, $\ensuremath{\omega}$) and chiral loops. Inclusion of ${b}_{1}(1235)$, ${h}_{1}(1170)$, ${h}_{1}(1380)$ increases the decay width of $\ensuremath{\eta}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{0}\ensuremath{\gamma}\ensuremath{\gamma}$, improving the theoretical prediction. The measurement of the $\ensuremath{\gamma}\ensuremath{\gamma}$ spectrum can test our model. We also discuss the effects of these $C$-odd axial-vector resonances on electromagnetic interactions of neutral pions: $\ensuremath{\gamma}\ensuremath{\gamma}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{0}{\ensuremath{\pi}}^{0}$ and electric and magnetic polarizabilities of ${\ensuremath{\pi}}^{0}$.

Sigma --nucleus interactions in emulsion at 350 GeV.

Experimental data on multiplicities and angular distributions of heavy ionizing and shower particles in inelastic interactions of 350 GeV ${\ensuremath{\Sigma}}^{\ensuremath{-}}$ hyperons in nuclear emulsion are presented. The data are compared with the results of other experiments on proton and pion interactions in emulsion at energies of 60-800 GeV. We observe no significant differences in the global characteristics of strange hyperon interactions relative to nonstrange baryon interactions at equivalent energies, other than those attributable to the differing cross sections.

CHIRAL INVARIANCE AND ELECTROWEAK INTERACTIONS OF PIONS AND KAONS

Electroweak interactions of pions and kaons are studied in the context of chiral perturbation theory. We demonstrate that the effective Lagrangian framework allows a consistent and predictive picture of such processes as well as a succinct way in which to represent the many previous studies of these reactions which have been performed using chiral models.

Chiral symmetry and the nucleon-nucleon interaction: Tensor decomposition of Feynman diagrams

A class of Lagrangians that describe the interaction of nucleons and pions and that provide a nonlinear representation of chiral symmetry is considered. We simplify the form of these Lagrangians by making an expansion in inverse powers of ${\mathit{f}}_{\mathrm{\ensuremath{\pi}}}$ and calculate the irreducible fermion-fermion scattering amplitude to order ${\mathit{f}}_{\mathrm{\ensuremath{\pi}}}^{\mathrm{\ensuremath{-}}4}$. Some of the integrals encountered in these calculations are divergent and are regulated with a (Euclidean) momentum-space cutoff, \ensuremath{\Lambda}, where \ensuremath{\Lambda}\ensuremath{\simeq}1 GeV. While elements of the S matrix are independent of the form of the Lagrangian used, somewhat different results are obtained for the irreducible amplitudes calculated with Lagrangians that have either pseudoscalar or pseudovector coupling of the pion field to the nucleon. We compare our results for the isoscalar irreducible amplitudes to the potentials used in the one-boson-exchange model of the nucleon-nucleon force. In the case of pseudovector coupling, there is only a single relevant diagram of order ${\mathit{g}}^{4}$, a ``crossed-box'' diagram. We find that this crossed-box diagram is well represented by the exchange of a ``pseudo-eta'' particle, that is, an isoscalar-pseudoscalar meson with an imaginary coupling constant. There is also a relatively small scalar attraction seen, while tensor, vector, and axial vector terms are quite small. We also consider a Lagrangian with pseudoscalar coupling. In this case, there are four diagrams of order ${\mathit{g}}^{4}$ in the irreducible amplitude. Again, we find a significant attractive pseudoscalar exchange term (``pseudo-eta''). Relatively small repulsive interactions of vector and scalar type are also found in this case. Further analysis of the isoscalar amplitude requires that we extend our model to reproduce the dynamics of correlated two-pion exchange.

Theoretical Nuclear Physics: Nuclear Reactions

Multiple scattering formal theory of nuclear reactions elastic and inelastic scattering transfer reactions multistep reactions heavy ions high energy nuclear phenomena pion and kaon interactions with nuclei.