Pion Interactions Research Articles

Anomalous C-violating three photon decay of the neutral pion in noncommutative quantum electrodynamics

We show that a simple and reasonable generalization of the anomalous interaction between the neutral pion and two photons can induce the C-violating three photon decay of the neutral pion in noncommutative quantum electrodynamics. We find that it is mandatory for consistency reasons to include simultaneously the normal neutral pion and photon interaction in which the neutral pion transforms under U (1) in a similar way as in the adjoint representation of a non-Abelian gauge theory. We demonstrate that the decay has a characteristic distribution although its rate still seems too small to be experimentally reachable in the near future. We also describe how to manipulate phase space integration correctly when Lorentz invariance is lost.

Test of Lorentz invariance through observation of the longitudinal development of ultrahigh-energy extensive air showers

An idea that Lorentz invariance can be violated was proposed by Coleman and Glashow to overcome the astrophysical problems of air showers of ultrahigh energies E>1020 eV. This statement can be tested by analyzing experimental data on these showers. The longitudinal development of showers and the distribution of the depths of shower maxima were calculated in the model of quark-gluon strings with allowance made for the Landau-Pomeranchuk-Migdal effect and the interactions of ultrahigh-energy neutral pions. Comparison of the calculations with available experimental data provides a new bound |cγ−cπ°| <0−20 for the possible difference between the speeds of photons and neutral pions. This bound becomes |cγ−cπ°|<10−22 when one takes the upper limiting value for the observed depth of maximum.

Inelasticity fluctuations and the efficiency of emulsion chambers for hadrons

Emulsion chamber experiments register high energy hadrons in cosmic rays indirectly, via gamma rays produced by nuclear reactions in the chamber materials (usually carbon and lead). If hadrons fail to produce any gamma rays above the detection threshold of typically a few TeV they will escape detection even in an infinitely thick chamber. The hadron energy is determined by dividing the energy sum of the secondary gamma rays by a mean inelasticity. Here the questions arise whether this conversion factor is the same for mesons and baryons and how large the fluctuations of this inelasticity are. These problems are investigated by simulations of the interactions of charged pions and protons with carbon and lead nuclei using the VENUS interaction model. Our calculations indicate considerable deviations of the detection probability from 1 in the energy region one to two orders of magnitude above the detection threshold for gamma rays, and clear differences between mesons and baryons, also as far as inelasticities and their fluctuations are concerned.

Pion interaction with light nuclei at energies below the Δ resonance

The annealing of diamond particles (d≈1000 nm) at 1800–2000 K leads to the formation of nanometric closed curved graphitic structures (CCGS) with tubular or conical forms as well as to defective extended graphitic structures. The intermediate structures that occur during the diamond transformation to curved graphitic structures have been observed. At the interface between diamond and evolving graphite the formation of two curved graphite sheets from three diamond planes (111) were detected. The CCGS formation is probably a self-assembling process. CCGSs are proposed to enhance electron emission from diamond surfaces.

Coulomb interaction of pions in central Pb+Pb collisions at 158 A GeV

Coulomb interaction of pions in central Pb+Pb collisions at 158 A GeV

Transition from hadronic to partonic interactions for a composite spin-1/2 model of a nucleon

A simple model of a composite nucleon is developed in which a fermion and a boson, representing quark and diquark constituents of the nucleon, form a bound state owing to a contact interaction. Photon and pion couplings to the quark provide vertex functions for the photon and pion interactions with the composite nucleon. By a suitable choice of cutoff parameters of the model, realistic electromagnetic form factors are obtained for the proton. When a pseudoscalar pion-quark coupling is used, the pion-nucleon coupling is predominantly pseudovector. A virtual photopion amplitude is considered in which there are two types of contributions: hadronic contributions where the photon and pion interactions have an intervening propagator of the nucleon or its excited states, and contactlike contributions where the photon and pion interactions occur within a single vertex. At large Q, the contactlike contributions are dominant. The model nucleon exhibits scaling behavior in deep-inelastic scattering and the normalization of the parton distribution provides a rough normalization of the contactlike contributions. Calculations for the virtual photopion amplitude are performed using kinematics appropriate to its occurrence as a meson-exchange current in electron-deuteron scattering. The results suggest that the contactlike terms can dominate the meson-exchange current for $Q&gt;1\mathrm{GeV}/c.$ There is a direct connection of the contactlike terms to the off-forward parton distributions of the model nucleon.

Emission pattern of high-energy pions: A new probe for the early phase of heavy-Ion collisions

The emission pattern of charged pions has been measured in Au+Au collisions at 1 GeV/nucleon incident energy. In peripheral collisions and at target rapidities, high-energy pions are emitted preferentially towards the target spectator matter. In contrast, low-energy pions are emitted predominantly in the opposite direction. The corresponding azimuthal anisotropy is explained by the interaction of pions with projectile and target spectator matter. This interaction with the spectator matter causes an effective shadowing which varies with time during the reaction. Our observations show that high-energy pions stem from the early stage of the collision whereas low-energy pions freeze out later.

Pion production excitation functions in proton-nucleus collisions from the absolute threshold to 500 MeV

Pion excitation functions in proton-nucleus collisions, from the absolute threshold to 500 MeV, have been measured with 1 MeV beam energy resolution at the CELSIUS storage ring, operating in slow ramping mode. Total yields, angular distributions, and target mass dependence of π+ production are generally reproduced well by QMD calculations. The π+/π− ratios are not reproduced equally well, obviously because the detailed description of the pion interaction with the Coulomb field is very delicate.Received 31 March 1999DOI:https://doi.org/10.1103/PhysRevC.62.014610©2000 American Physical Society

Chiral symmetry and properties of hadron correlators in matter

The constraints imposed by chiral symmetry on hadron correlation functions in nuclear medium are discussed. It is shown that these constraints imply a certain structure for the in-medium hadron correlators and lead to the cancelation of the order ρm π term in the in-medium nucleon correlator. We also consider the effect of mixing of the chiral partners correlation functions arising from the interaction of nuclear pions with corresponding interpolating currents. It reflects the phenomena of partial restoration of chiral symmetry. The different scenarios of such restoration are briefly discussed.

Chiral SU(2)×SU(2) model with infrared quark confinement

An SU(2)×SU(2) chiral quark model describing the properties and interaction of pions and scalar and vector mesons is considered. The confinement of quarks is introduced in the model by means of an infrared cut-off in the one-loop quark diagrams. This cutoff gives rise to the elimination of the unphysical thresholds of the quark-antiquark pair production. The π-a1 transitions are taken into account. The model conserves all low-energy theorems. The masses of mesons and the widths of the decays ρ → 2π and σ → 2π are calculated.

Classical pion fields in the presence of a source

A classical pion field that is similar to a disoriented chiral condensate is considered in the presence of an external source. The field is similar to the condensate in that the isotopic orientation of the field in the whole space is determined by a single vector. Within the nonlinear sigma model, classical solutions are considered both in the chiral limit, where the pions are massless, and in the case of a finite pion mass. In either case, the classical filed is similar to the Coulomb field of a charged particle; however, the nonlinear pion interaction results in the existence of several solutions. In the massless case and in the case where the source is sufficiently small, there are a great number of classical solutions characterized by finite discrete energies. In the more realistic case of heavy ions, there are no stable solutions of this type; however, long-lived quasistationary states, which slowly decay, emitting very soft pions, can be formed. The structure and the energies of these solutions are studied numerically.

Pion-induced transport of π mesons in nuclei

A large body of data for pion-induced neutral pion continuum spectra spanning outgoing energies near 180 MeV shows no dip there that might be ascribed to internal strong absorption processes involving the formation of {delta}'s. This is the same observation previously made for the charged pion continuum spectra. Calculations in an intranuclear cascade model or a cascade exciton model with free-space parameters predict such a dip for both neutral and charged pions. We explore several medium modifications to the interactions of pions with internal nucleons that are able to reproduce the data for nuclei from {sup 7}Li through Bi. (c) 2000 The American Physical Society.

Quark loop calculation of theγ→3πform factor

The presently experimentally interesting form factor for the anomalous process gamma -> pi^+ pi^0 pi^- is calculated as the quark "box"-amplitude where the intermediate fermion loop is the one of constituent quarks with the pseudoscalar coupling to pions. This also corresponds to the form factor, in the lowest order in pion interactions, of the sigma-model and of the chiral quark model. We give the analytic expression for the form factor in terms of an expansion in the pion momenta up to the order O(p^8) relative to the soft point result, and also perform its exact numerical evaluation. We compare our predictions with those of the vector meson dominance and chiral perturbation theory, as well as with the scarce data available so far.

Novye dannye o pion-pionnom vzaimodeistvii pri nizkikh energiyakh

Novye dannye o pion-pionnom vzaimodeistvii pri nizkikh energiyakh

Neutrino-nucleon reaction rates in the supernova core in the relativistic random phase approximation

We calculate neutrino reaction rates with nucleons via the neutral and charged currents in the supernova core in the relativistic random phase approximation (RPA) and study their effects on the opacity of the supernova core. The formulation is based on the Lagrangian employed in the calculation of the nuclear equation of state (EOS) in the relativistic mean field theory (RMF). The nonlinear meson terms are treated appropriately so that the consistency of the density correlation derived in RPA with the thermodynamic derivative obtained from the EOS by the RMF is satisfied in the static and long wavelength limit. We employ pion and rho meson exchange interactions together with the phenomenological Landau-Migdal parameters for the isospin-dependent nuclear interactions. We find that both the charged and neutral current reaction rates are suppressed from Bruenn's standard approximate formula considerably in the high density regime $({\ensuremath{\rho}}_{b}\ensuremath{\gtrsim}{10}^{14}{\mathrm{g}/\mathrm{c}\mathrm{m}}^{3}$ with ${\ensuremath{\rho}}_{b}$ the baryonic density). In the low density regime $({\ensuremath{\rho}}_{b}\ensuremath{\lesssim}{10}^{14}{\mathrm{g}/\mathrm{c}\mathrm{m}}^{3}),$ on the other hand, the vector current contribution to the neutrino-nucleon scattering rate is enhanced in the vicinity of the boundary of the liquid-gas phase transition, while the other contributions are moderately suppressed there also. In the high temperature regime $(T\ensuremath{\gtrsim}40\mathrm{MeV}$ with T the temperature) or in the regime where electrons have a large chemical potential, the latter of which is important only for the electron capture process and its inverse process, the recoil of nucleons cannot be neglected and further reduces the reaction rates with respect to the standard approximate formula which discards any energy transfer in the processes. These issues could have a great impact on the neutrino heating mechanism of collapse-driven supernovae.

Charged pion production in fixed-target Pb + Pb collisions at 158 GeV/nucleon

Changes in pion production as a function of the impact parameter of the collision or the incident energy, may reveal characteristics of a possible first-order phase transition from nuclear to quark matter, as predicted by lattice quantum chromodynamics. In this paper we investigate charged pion production in Pb+Pb collisions at 158 GeV/nucleon near 0° production angle and at forward rapidity (4.3 y6.3). The centrality dependence of pion production is shown in the impact parameter range ~ 2-12 fm at the rapidities y = 5.7 and 6.3. An enhancement in the -/+ ratio has been measured near beam rapidity, indicating Coulomb interaction of charged pions with the spectator protons. The charged pion yield per nucleon participating in the collision (Np) at y = 5.7 increases faster than linearly with Np, up to Np~100 and then it saturates, while at y = 6.3 it does not exhibit any sudden change as a function of Np.

Pion interaction in nuclear matter and chiral symmetry restoration

This paper is devoted to the interplay between p-wave, s-wave pion-nucleon/nucleus interaction and in-medium pion-pion interaction with special emphasis on the role of the nuclear pionic scalar density driving a large amount of chiral symmetry restoration. In particular we show that the πNN coupling constant and the Goldberger-Treiman relation are preserved in the nuclear medium under certain conditions. We also discuss the related problem of the in-medium pion-pion strength function.

Nonrelativistic pion interactions and the pionium lifetime

We construct an effective Lagrangian for interacting pions with non-relativistic energies. The coupling constants can be expressed in terms of the different scattering lengths and slopes. When used in the calculation of the pionium decay rate, the scattering slope contribution gives a correction of about 8% compared with the lowest order contribution coming from the scattering lengths alone.

Chiral symmetry and mixing of axial and vector correlators in matter

The effect of mixing of the vector and axial vector correlation functions in the nuclear medium arising from the interaction of nuclear pions with corresponding interpolating currents is considered. It is shown that the difference between ρ and a1 meson correlators gets smaller with increasing nuclear density. This reflects partial restoration of chiral symmetry whereas the individual correlators could either decrease or increase with density depending on the model used for the phenomenological spectral function.

Evidence for different freeze-out radii of high- and low-energy pions emitted in Au+Au collisions at 1 A·GeV

Abstract Double differential production cross sections of π − and π + mesons and the number of participating protons have been measured in central Au+Au collisions at 1 A·GeV. At low pion energies the π − yield is strongly enhanced over the π + yield. The energy dependence of the π − / π + ratio is assigned to the Coulomb interaction of the charged pions with the protons in the reaction zone. The deduced Coulomb potential increases with increasing pion c.m. energy. This behavior indicates different freeze-out radii for different pion energies in the c.m. frame.