Showing papers on "Pion published in 1997"

Neutrino fluxes from active galaxies: A Model independent estimate

Abstract: There are tantalizing hints that jets, powered by supermassive black holes at the center of active galaxies, are true cosmic proton accelerators. They produce photons of TeV energy, possibly higher, and may be the enigmatic source of the highest energy cosmic rays. Photoproduction of neutral pions by accelerated protons on UV light may be the source of the highest energy photons in which most of the bolometric luminosity of some galaxies is emitted. The case that proton beams power active galaxies is, however, far from conclusive. Neutrinos from the decay of charged pions represent an incontrovertible signature for the proton-induced cascades. We show that their flux can be estimated by model-independent methods, based on dimensional analysis and textbook particle physics. Our calculations also demonstrate why different models for the proton blazar yield very similar results for the neutrino flux that are consistent with the ones obtained here. As regards astrophysics, they illustrate that proton beams are required to generate TeV photons without fine-tuning.

Measurement of the spectral functions of vector current hadronic tau decays

Abstract: A measurement of the spectral functions of non-strange tau vector current final states is presented, using 124 358 tau pairs recorded by the ALEPH detector at LEP during the years 1991 to 1994. The spectral functions of the dominant two-and four-pion tau decay channels are compared to published results of ece-annihilation experiments via isospin rotation. A combined fit of the pion form factor from tau decays and e(+)e(-)data is performed using different parameaizations. The mass and the width of the rho(+/-)(770) and the p(0)(770) are separately determined in order to extract possible isospin violating effects. The mass and width differences are measured to be M

Pion and Rho Structure Functions from Lattice QCD

Abstract: We calculate the lower moments of the deep-inelastic structure functions of the $\ensuremath{\pi}$ and the $\ensuremath{\rho}$ meson on the lattice. Of particular interest to us are the spin-dependent structure functions of the $\ensuremath{\rho}$. The calculations are done with Wilson fermions and for three values of the quark mass, so that we can perform an extrapolation to the chiral limit.

Improved Determination of the Hadronic Contribution to the Muon (g-2) and to alpha(M_Z**2) Using new Data from Hadronic Tau Decays

Abstract: We have reevaluated the hadronic contribution to the anomalous magnetic moment of the muon (g-2) and to the running of the QED fine structure constant alpha(s) at s=M_Z**2. We incorporated new data from hadronic tau decays, recently published by the ALEPH Collaboration. In addition, compared to previous analyses, we use more extensive e+e- annihilation data sets. The integration over the total hadronic cross section is performed using experimental data up to 40 GeV and results from perturbative QCD above 40 GeV. The improvement from tau data concerns mainly the pion form factor, where the uncertainty in the corresponding integral could be reduced by more than a factor of two. We obtain for the lowest order hadronic vacuum polarization graph a_mu(had) = (695.0 +/- 15.0) x 10^{-10} and delta(alpha(M_Z**2))(had) = (280.9 +/- 6.3) x 10^{-4} using e+e- data only. The corresponding results for combined e+e- and tau data are a_mu(had) = (701.1 +/- 9.4) x 10^{-10} and delta(alpha(M_Z**2))(had) = (281.7 +/- 6.2) x 10^{-4}, where the latter is calculated using the contribution from the five lightest quarks.

Unpolarized and polarized quark distributions in the large N(c) limit

Abstract: The isosinglet unpolarized and isovector polarized twist-2 quark distributions of the nucleon at low normalization point are calculated in the large- N{sub c} limit The nucleon is described as a soliton of the effective chiral theory We derive the expressions for the distribution functions in the large-N{sub c} limit starting from their definition as numbers of partons carrying a momentum fraction x in the infinite momentum frame We develop a numerical method for computation of the quark and antiquark distributions as sums over the quark single-particle levels in the pion field of the soliton The contributions of the discrete bound-state level as well as the Dirac continuum are taken into account The quark and antiquark distributions obtained explicitly satisfy all general requirements The results are in reasonable agreement with parametrizations of the data at a low normalization point {copyright} {ital 1997} {ital The American Physical Society}

Many Body approach to the inclusive $(e,e')$ reaction from the quasielastic to the $\Delta$ excitation region

Abstract: We have performed a many body calculation of the inclusive (e,e') cross section which runs over the three traditional regions at intermediate energies: the quasielastic peak, the dip region and the delta region. The longitudinal and transverse response functions in the quasielastic peak have also been evaluated. Traditional effects like polarization, meson exchange currents, final state interaction and delta renormalization in the nuclear medium have been included. Meson exchange currents are generated from a model of pion electroproduction on the nucleon which reproduces accurately the experimental data. The inclusive cross section accounts for 1N,2N,3N mechanisms of virtual photon absorption and one pion production. Meson exchange currents associated to the (\gamma^*,2\pi) reaction are also accounted for. We obtain good results for the (e,e') cross sections in the whole energy range and for different nuclei. The response functions are also in good agreement with the latest experimental analysis. On the other hand, the method provides the separation of the contribution to the inclusive cross section from different physical channels which is a necessary input to evaluate cross sections like (e,e' N), (e,e' NN), (e,e' \pi) etc.

Enhanced chiral logarithms in partially quenched QCD

Abstract: I discuss the properties of pions in {open_quotes}partially quenched{close_quotes} theories, i.e., those in which the valence and sea quark masses m{sub V} and m{sub S} are different. I point out that for lattice fermions which retain some chiral symmetry on the lattice, e.g., staggered fermions, the leading order prediction of the chiral expansion is that the mass of the pion depends only on m{sub V}, and is independent of m{sub S}. This surprising result is shown to receive corrections from loop effects which are of relative size m{sub S}lnm{sub V}, and which thus diverge when the valence quark mass vanishes. Using partially quenched chiral perturbation theory, I calculate the full one-loop correction to the mass and decay constant of pions composed of two nondegenerate quarks, and suggest various combinations for which the prediction is independent of the unknown coefficients of the analytic terms in the chiral Lagrangian. These results can also be tested with Wilson fermions if one uses a nonperturbative definition of the quark mass. {copyright} {ital 1997} {ital The American Physical Society}

Gauge invariant theory of pion photoproduction with dressed hadrons

Abstract: Based on an effective field theory of hadrons in which quantum chromodynamics is assumed to provide the necessary bare cutoff functions, a gauge-invariant theory of pion photoproduction with fully dressed nucleons is developed. The formalism provides consistent dynamical descriptions of {pi}N{r_arrow}{pi}N scattering and {gamma}N{r_arrow}{pi}N production mechanisms in terms of nonlinear integral equations for fully dressed hadrons. Defining electromagnetic currents via the gauging of hadronic n-point Green{close_quote}s functions, dynamically detailed currents for dressed nucleons are introduced. The dressed hadron currents and the pion photoproduction current are explicitly shown to satisfy gauge invariance in a self-consistent manner. Approximations are discussed that make the nonlinear formalism manageable in practice and yet preserve gauge invariance. This is achieved by recasting the gauge conditions for all contributing interaction currents as continuity equations with {open_quotes}surface{close_quotes} terms for the individual particle legs coming into or going out of the hadronic interaction region. General procedures are given that approximate any type of (global) interaction current in a gauge-invariance-preserving manner as a sum of single-particle {open_quotes}surface{close_quotes} currents. It is argued that these prescriptions carry over to other reactions, irrespective of the number or type of contributing hadrons or hadronic systems. {copyright} {ital 1997} {ital The American Physical Society}

Dominance of pion exchange in R-parity violating supersymmetry contributions to neutrinoless double beta decay

Abstract: We present a new contribution of the $R$-parity-violating $(R{}_{p})$ supersymmetry (SUSY) to neutrinoless double beta decay $(0\ensuremath{ u}\ensuremath{\beta}\ensuremath{\beta})$ via the pion exchange between decaying neutrons. The pion coupling to the final state electrons is induced by the $R{}_{p}$ SUSY interactions. We have found this pion-exchange mechanism to dominate over the conventional two-nucleon one. The latter corresponds to direct interaction between quarks from two decaying neutrons without any light hadronic mediator like $\ensuremath{\pi}$ meson. The constraints on the certain $R{}_{p}$ SUSY parameters are extracted from the current experimental $0\ensuremath{ u}\ensuremath{\beta}\ensuremath{\beta}$-decay half-life limit. These constraints are significantly stronger than those previously known or expected from the ongoing accelerator experiments.

Proton and pion production relative to the reaction plane in Au + Au collisions at 11A GeV/c

Abstract: Results are presented of an analysis of proton and charged pion azimuthal distributions measured with respect to the reaction plane in Au + Au collisions at a beam momentum of about 11A GeV/c. The azimuthal anisotropy is studied as a function of particle rapidity and transverse momentum for different centralities of the collisions. The triple differential (in rapidity, transverse momentum, and azimuthal angle) distributions are reconstructed. A comparison of the results with a previous analysis of charged-particle and transverse energy flow as well as with model predictions are presented. {copyright} {ital 1997} {ital The American Physical Society}

Energy and charged particle flow in 10.8A GeV/c Au+Au collisions

Abstract: Experimental results and a detailed analysis are presented of the transverse energy and charged particle azimuthal distributions measured by the E877 Collaboration for different centralities of Au+Au collisions at a beam momentum of 10.8A GeV/c. The anisotropy of these distributions is studied with respect to the reaction plane reconstructed on an event-by-event basis using the transverse energy distribution measured by calorimeters. Results are corrected for the reaction plane resolution. For semicentral events we observe directed flow signals of up to 10{percent}. We observe a stronger anisotropy for slow charged particles. For both the charged particle and transverse energy distributions we observe a small but nonzero elliptic anisotropy with the major axis pointing into the reaction plane. Combining the information on transverse energy and charged particle flow we obtain information on the flow of nucleons and pions. The data are compared to event generators and the need to introduce a mean field or nucleon-nucleon potential is discussed. {copyright} {ital 1997} {ital The American Physical Society}

Virtual Photons in Low Energy pi-pi Scattering

Abstract: The low energy scattering of pions is investigated in the presence of electromagnetic interactions at leading order and at next-to-leading order for the amplitudes involving at most one pair of charged pions. The size of the electromagnetic corrections to the S-wave scattering lengths is found to be comparable to the size of the two loop strong interaction corrections.

Weak decay of hypernuclei

Abstract: The nonmesonic weak decay of $\ensuremath{\Lambda}$ hypernuclei is studied in a shell model framework. A complete strangeness-changing weak $\ensuremath{\Lambda}N\ensuremath{\rightarrow}\mathrm{NN}$ transition potential, based on one boson exchange, is constructed by including the exchange of the pseudoscalar mesons $\ensuremath{\pi}$, $K$, $\ensuremath{\eta}$ as well as the vector mesons $\ensuremath{\rho},\ensuremath{\omega}$, and ${K}^{*}$, whose weak-coupling constants are obtained from soft meson theorems and SU(6)${}_{w}.$ General expressions for nucleons in arbitrary shells are obtained. The transition matrix elements include realistic $\ensuremath{\Lambda}N$ short-range correlations and $\mathrm{NN}$ final state interactions based on the Nijmegen baryon-baryon potential. The decay rates are found to be especially sensitive to the inclusion of the strange mesons $K$ and ${K}^{*}$ even though the role of kaon exchange is found to be reduced with recent couplings obtained from one-loop corrections to the leading order in chiral perturbation theory. With the weak couplings used in this study the rates remain dominated by the pion-exchange mechanism since the contributions of heavier mesons either cancel each other or are suppressed by form factors and short-range correlations. The total decay rate therefore remains in agreement with present measurements. However, the partial rates which are even more sensitive to the inclusion of heavier mesons cannot be reconciled with the data. The proton asymmetry changes by 50% once heavier mesons are included and agrees with the available data.

Improving flavor symmetry in the Kogut-Susskind hadron spectrum

Abstract: We study the effect of modifying the coupling of Kogut-Susskind quarks to the gauge field by replacing the link matrix in the quark action by a ``fat link,'' or sum of link plus three-link paths. Flavor symmetry breaking, determined by the mass difference between the Goldstone and non-Goldstone local pions, is reduced by approximately a factor of 2 by this modification.