Showing papers on "Pion published in 1993"
TL;DR: In this paper, the authors studied the equilibration of hot hadronic matter in the framework of relativistic kinetic theory and derived non-equilibrium properties of a mixture comprised of pions, kaons and nucleons.
247 citations
TL;DR: In this paper, a set of consistency conditions for the pion-baryon coupling constants in the large-Nc limit of QCD was derived, and the symmetry breaking corrections to the baryon-pion couplings vanish to firts order in 1 N c.
181 citations
TL;DR: In this article, the properties of ϱ-mesons in vacuum and nuclear matter were studied in a hadronic model which satisfies unitarity and current conservation, including the coupling of the ϱmeson to two pions which, in matter, are strongly mixed with delta-particle-nucleon-hole states.
178 citations
TL;DR: In this article, the authors consider quenching in the O(4) linear sigma model and show that in the period immediately following quench long wavelength modes of the pion field are amplified.
157 citations
TL;DR: In this article, a new formulation of QCD sum rules at finite temperature (T) is developed, where all the soft dynamics including the effect of finite T in the hadronic phase is taken into account in the thermal average of local operators.
150 citations
TL;DR: The strange axial-vector mesons are reanalyzed in the light of the updated experimental information and compared with the recent result on the [ital K] production in [tau] decay.
Abstract: The strange axial-vector mesons ${K}_{1} (1270)$ and ${K}_{1} (1400)$ are reanalyzed in the light of the updated experimental information and compared with the recent result on the $K\ensuremath{\pi}\ensuremath{\pi}$ production in $\ensuremath{\tau}$ decay. The mixing angle between the strange mesons of $^{3}P_{1}$ and $^{1}P_{1}$ is determined by the partial decay rates, and, independently, by the masses. They lead to ${\ensuremath{\theta}}_{K}\ensuremath{\approx}33\ifmmode^\circ\else\textdegree\fi{} or 57\ifmmode^\circ\else\textdegree\fi{}$. The observed ${K}_{1} (1400)$ production dominance in the $\ensuremath{\tau}$ decay favors ${\ensuremath{\theta}}_{K}\ensuremath{\approx}33\ifmmode^\circ\else\textdegree\fi{}$. Flavor-SU(3) breaking of 20% or so in the production amplitudes can explain quantitatively the observed production ratio.
118 citations
TL;DR: In this article, a modified quark-quark interaction is applied to study the nucleon-nucleon interaction, which is suggested by instanton models and includes pion ( pi ) and sigma ( sigma ) exchanges as non-perturbative components and the one-gluon exchange as a perturbative one.
Abstract: A modified quark-quark interaction is applied to study the nucleon-nucleon interaction. The quark potential is suggested by instanton models and includes pion ( pi ) and sigma ( sigma ) exchanges as non-perturbative components and the one-gluon exchange as a perturbative one. The nucleon-nucleon potential derived from this model presents short-range repulsion and medium-range attraction besides the usual pion tail. Using this interaction the nucleon-nucleon phase shifts within the resonating group method have been calculated. They agree reasonably well with experimental values.
110 citations
CERN1
TL;DR: In this paper, the perturbative contribution to the pion form factor in the few-geV region was shown to be self-consistent for momentum transfers as low as 1 to 2 GeV.
109 citations
TL;DR: The Bethe-Salpeter equation is solved in order to calculate the spectrum of pseudoscalar, vector, and scalar meson bound states for light as well as heavy quarks and obeys known relations derived on the basis of current algebra for light quark and a nonrelativistic analysis for heavy quark.
Abstract: We solve the Bethe-Salpeter equation in order to calculate the spectrum of pseudoscalar, vector, and scalar meson bound states for light as well as heavy quarks, extending our previous calculation of pseudoscalar mesons. The fermion propagators appearing in the equation are obtained by solving the Schwinger-Dyson equation consistently with the Bethe-Salpeter equation, a procedure necessary for demonstrating the Goldstone nature of the pion in the chiral limit. We probe in our calculations a model for the gluon propagator which leads to the expected three-dimensional potential in configuration space, while maintaining its known ultraviolet behavior. Our procedure provides a unified description of both the light and heavy mesons and obeys known relations derived on the basis of current algebra for light quarks and a nonrelativistic analysis for heavy quarks. The results are in good agreement with experiment for the mass spectrum and for the pseudoscalar mesons' decay constants. We also calculate electromagnetic and isospin mass splittings for these mesons.
105 citations
TL;DR: The connection between calculations based on dispersion relations and on chiral perturbation theory is established by matching the low-energy chiral amplitude with that given by a full dispersive treatment.
Abstract: Recent attempts to detect the pion polarizability via analysis of $\ensuremath{\gamma}\ensuremath{\gamma}\ensuremath{\rightarrow}\ensuremath{\pi}\ensuremath{\pi}$ measurements are examined The connection between calculations based on dispersion relations and on chiral perturbation theory is established by matching the low-energy chiral amplitude with that given by a full dispersive treatment Using the values for the polarizability required by chiral symmetry, predicted and experimental cross sections are shown to be in agreement
100 citations
TL;DR: In this article, the chiral corrections to Weinberg's prediction for the S-wave πN scattering lengths up to and including order O (M π 3 ) making use of heavy baryon chiral perturbation theory.
01 Nov 1993-Nuclear Instruments & Methods in Physics Research Section A-accelerators Spectrometers Detectors and Associated Equipment
TL;DR: In this article, a method to estimate the displacement damage induced by low energy pions in silicon was presented, which indicated that a major part of radiation damage at an LHC inner tracker comes from pions with energies below 1 GeV.
Abstract: We present a method to estimate the displacement damage induced by low energy pions in silicon. Our results indicate that a major part of radiation damage at an LHC inner tracker comes from pions with energies below 1 GeV. We put our main emphasis on the necessity to measure the pion damage constant at energies around 200 MeV.
TL;DR: Using a cross-section distribution for pionic projectiles on nucleons consistent with the data on the moments of the distribution, the probability for a pion to be in a pointlike configuration is calculated and predictions based on a combination of perturbative QCD and QCD sum rules are compared.
Abstract: The scattering of hadrons at high energies can be described by a distribution function of cross sections, which incorporates the phenomena of color transparency and color opacity. We infer here a cross-section distribution for pionic projectiles on nucleons consistent with the data on the moments of the distribution. Using this distribution we calculate the probability for a pion to be in a pointlike configuration and compare with predictions based on a combination of perturbative QCD and QCD sum rules. Comparison with the corresponding distribution for nucleon-nucleon scattering shows that color transparency effects should be more pronounced for a meson beam.
TL;DR: In this paper, the most general Lagrangian for parity-violating pion-nucleon interactions was constructed to all orders in the pion field and to one power of pion momentum.
TL;DR: It is shown how the pole shift of the pion can be expressed in terms of these scattering amplitudes and the size of corrections from three-particle collisions is discussed.
Abstract: We calculate the pole shift of the pion up to second order in the density, considering the low-temperature expansion of the axial two-point function. To this order both absorptive and dispersive properties of pions are completely determined by two- and three-particle [ital S]-matrix elements. In particular, we show how the pole shift can be expressed in terms of these scattering amplitudes and discuss the size of corrections from three-particle collisions.
TL;DR: In this article, a technique is presented which allows rapid calculation of multi-particle Bose-Einstein interference to all orders for a finite system, which is used to illustrate how multiparticle interference can enhance the emission of pions in a heavy-ion collision.
TL;DR: In this paper, the results for the pion-nucleus optical potential were extrapolated for low-energy pions (T π = 0-50 MeV) using microscopic many-body techniques.
TL;DR: In this paper, the physics of pion exchange in forming hadronic molecules is discussed and the notion of critical mass is introduced, which is the minimal reduced mass required for molecule formation.
TL;DR: Using this formalism, it is shown how the nucleon’s transversity distribution can be measured in single pion inclusive electroproduction.
Abstract: We define twist-two and twist-three quark fragmentation functions in quantum chromodynamics and study their physical implications. Using this formalism we show how the nucleon's transversity distribution can be measured in single pion inclusive electroproduction.
TL;DR: In this paper, a model for electro-and photo-pion production is presented which is applicable at threshold and through the first and second resonance regions, and a detailed comparison with experiment is found with data on the proton target than with the neutron target.
TL;DR: It is demonstrated that pionic double charge exchange reaction features find their natural explanation by a narrow resonance in the [pi][ital NN] subsystem with [ital J][sup [ital P]]=0[sup [minus]] and a mass of 2.065 GeV.
Abstract: All currently available data on the pionic double charge exchange reaction ([pi][sup +],[pi][sup [minus]]) on nuclei exhibit a very peculiar energy dependence near [ital T][sub [pi]]=50 MeV, while the angular distributions behave quite regularly. We demonstrate that these features find their natural explanation by a narrow resonance in the [pi][ital NN] subsystem with [ital J][sup [ital P]]=0[sup [minus]] and a mass of 2.065 GeV.
TL;DR: The complete development of this model, which gives a very good fit to all the data up to 600 MeV, is presented.
Abstract: Pion nucleon scattering up to 600 MeV lab kinetic energy is described by a manifestly covariant wave equation in which the pion is restricted to its mass-shell.The kernel of the equation includes nucleon (N), Roper (N*), delta, and D(sub)13 poles, with their corresponding crossed pole terms approximated by contact interactions, and contact sigma and rho-like exchange terms.The pi-NN vertex is treated as a mixture of gamma^5 and gamma^mu gamma^5 coupling, with a mixingparameter lambda chosen so that the dressed nucleon pole will be unshifted by the interaction.Chiral symmetry is maintained at threshold.The resonance contributions are fully unitarized by the equation, with their widths determined by the dynamics included in the model.The delta and D(sub)13 are treated as a pure spin 3/2 particles, with no spin 1/2 amplitude in the S-channel.The complete development of this model, which gives a very good fit to all the data up to 600 MeV, is presented.
TL;DR: In this paper, the authors measured the emission pattern of nucleons with respect to the reaction plane in relativistic nucleus nucleus collisions and showed that newly created hadrons, predominantly pions, can be observed in the collisions.
Abstract: In several experiments the emission pattern of nucleons with respect to the reaction plane has been measured. A sidewards deflection of the spectator fragments (”bounce off”) was found as well as directed flow of nucleons from the overlap region between the colliding nuclei (participants) in the reaction plane (”side-splash”) 2, 3, 4, 5. Additionally, an azimuthally anisotropic emission peaked perpendicular to the event plane is observed for protons, neutrons and light nuclei (”squeeze-out”)6, 7. In addition to the participant and spectator nucleons, newly created hadrons, predominantly pions, are observed in relativistic nucleus nucleus collisions. In contrast to case of the nucleons, no data were available so far for the emission pattern of pions in symmetric heavy mass systems. We studied therefore the correlation between the momentum vectors of pions and the reaction plane in 197Au+197Au collisions at a beam energy of 1 GeV/u8, 9.
TL;DR: In this article, the pion structure function is studied in the Nambu and Jona-Lasinio (NJL) model, and the forward scattering amplitude of a virtual photon from a pion target in the Bjorken limit is calculated.
TL;DR: An exploratory calculation of the I=2 $\pi-\pi$ scattering amplitude at threshold using Wilson fermions in the quenched approximation, including all the required contractions finds good agreement with the predictions of chiral perturbation theory even for pions of mass 560-700 MeV.
Abstract: We present an exploratory calculation of the $I=2$ $\pi-\pi$ scattering amplitude at threshold using Wilson fermions in the quenched approximation, including all the required contractions. We find good agreement with the
predictions of chiral perturbation theory even for pions of mass 560-700 MeV. Within the 10% errors, we do not see the onset of the bad chiral behavior expected for Wilson fermions. We also derive rigorous inequalities that apply to 2-particle correlators and as a consequence show that the interaction in the antisymmetric state of two pions has to be attractive.
TL;DR: In this paper, the authors propose a scenario in which domains areannealed by a dynamically evolving effective potential in the heavy ion system, and domains of sizes exceeding 3~fm are possible in this scenario.
Abstract: Relativistic heavy ion collisions can generate metastable domains in which the chiral condensate is disoriented. Nucleus-sized domains can yield measurable fluctuations in the number of neutral and charged pions. We propose a scenario in which domains are `annealed' by a dynamically evolving effective potential in the heavy ion system. Domains of sizes exceeding 3~fm are possible in this scenario.
TL;DR: Bose-Einstein correlations between charged pion and [ital K][sup +] pairs are calculated using the space-time and four-momentum distributions from relativistic quantum molecular dynamics and compared to pion correlation measurements from two CERN experiments good agreement is found.
Abstract: Bose-Einstein correlations between charged pion and ${\mathit{K}}^{+}$ pairs are calculated using the space-time and four-momentum distributions from relativistic quantum molecular dynamics for 200 GeV/nucleon $^{32}$S+Pb and compared to pion correlation measurements from two CERN experiments. Good agreement is found. The predicted apparent radii from the ${\mathit{K}}^{+}$ correlation functions are smaller than the corresponding pion radii.
TL;DR: It is found that as the nuclear density increases the rho meson peak in the spectral function shifts to smaller invariant masses and its width becomes smaller.
Abstract: The property of a rho meson in dense nuclear matter is studied using the QCD sum rule. The spectral function appearing on the hadronic side of the sum is evaluated in the vector dominance model that takes into account the interaction between the rho meson and the pion. Including pion modification by the delta-hole polarization in the nuclear medium, we find that as the nuclear density increases the rho meson peak in the spectral function shifts to smaller invariant masses and its width becomes smaller. We discuss the possibility of studying the rho meson property in dense matter via the dilepton invariant mass spectrum from heavy-ion collisions.
TL;DR: It is shown that quark-antiquark annihilation processes in single-spin inclusive production experiments can yield useful information on hadron spin structure in general, and provide crucial tests for the existence of orbiting valence quarks in particular.
Abstract: We show that quark-antiquark annihilation processes in single-spin inclusive production experiments can yield useful information on hadron spin structure in general, and provide crucial tests for the existence of orbiting valence quarks in particular. There are several experimental indications and theoretical arguments for the existence of such orbital motion inside polarized protons or antiprotons. Simple relations between quark-spin distributions and left-right asymmetries in such production processes can be given, and quantitative predictions can be made.
TL;DR: In this article, the authors consider two scenarios for the expansion of a quark-gluon plasma and compare hadron production rates in the two approaches and show that for a hadronisation temperatureT≈200 MeV and baryonic chemical potential μ B ≥ 500 MeV, their predictions essentially coincide.
Abstract: We consider two scenarios for the expansion of a quark-gluon plasma. If the evolution is slow enough, the system can remain in equilibrium throughout its entire history up to the freeze-out of a hadron gas; for a very rapid expansion, it may break up into hadrons before or at the confinement transition, without ever going through an equilibrium hadron phase. We compare hadron production rates in the two approaches and show that for a hadronisation temperatureT≏200 MeV and baryonic chemical potential μ
B
≲500 MeV, their predictions essentially coincide. Present data on strange particle production lead to values in this range and hence cannot provide a distinction between the two scenarios. Pion, nucleon and non-strange meson production seem to require a considerably lower freeze-out temperature and baryonic chemical potential. In the hadron gas picture, this is in accord with the difference in mean free path of the different hadrons in the medium; it suggests a sequential freeze-out, in which strange hadrons stop interacting earlier than non-strange hadrons. In the quark-gluon plasma break-up, the hadronic final state fails to provide the high entropy per baryon observed in non-strange hadron production. The break-up moreover leads to a decrease of the entropy per baryon; hence it must be conceptually modified before it can be considered as a viable hadronisation mechanism.