299,945 research outputs found

    Threshold Two-Pion Photo- and Electroproduction: More neutrals than expected

    Full text link
    We present an exploratory study of two pion photo-- and electroproduction off the nucleon in the threshold region. To calculate the pertinent amplitudes, we make use of heavy baryon chiral perturbation theory. We show that due to finite chiral loops the production cross section for final states with two neutral pions is considerably enhanced. The experimental implications are briefly discussed.Comment: 23pp, plain TeX, 11 figures available upon request, CRN 94/1

    SU(3) Flavor Breaking in Hadronic Matrix Elements for BBˉB - \bar B Oscillations

    Full text link
    We present an analysis, using quenched configurations at 6/g2=6/g^2=5.7, 5.85, 6.0, and 6.3 of the matrix element \MP\equiv\langle \bar P_{hl}|\bar h \gamma_\mu (1-\gamma_5)l \bar h \gamma_\mu(1-\gamma_5)l|P_{hl}\rangle for heavy-light pseudoscalar mesons. The results are extrapolated to the physical BB meson states, \Bd and \Bs. We directly compute the ratio \MS/\MB, and obtain the preliminary result \MS/\MB=1.54(13)(32). A precise value of this SU(3) breaking ratio is important for determining VtdV_{td} once the mixing parameter xsx_s for \Bs-\bar\Bs is measured experimentally. We also determine values for the corresponding B parameters, Bbs(2GeV)=Bbd(2GeV)=1.02(13)B_{bs}(2 \rm{GeV})=B_{bd}(2 \rm{GeV})=1.02(13), which we cannot distinguish in the present analysis.Comment: Poster presented at LATTICE96(heavy quarks). LaTeX, uses espcrc2.sty and epsf, 4 pages, 4 postscript figures include

    On KππK\to\pi\pi Decays in Quenched and Unquenched Chiral Perturbation Theory

    Get PDF
    We calculate the logarithmic corrections to the matrix elements for K+π+K^+\to\pi^+ and KK\tovacuum (which are used on the lattice to determine KππK\to\pi\pi amplitudes), in one-loop quenched and unquenched Chiral Perturbation Theory. We find that these corrections can be large. We also discuss, and present some results for, the direct determination of KππK\to\pi\pi amplitudes. In particular, we address effects from choosing ms=mdm_s=m_d and vanishing external spatial momenta, finite volume and quenching. In the quenched octet case, we find enhanced finite-volume contributions which may make numerical estimates of this matrix element unreliable for large volumes.Comment: LATTICE99(Matrix Elements), LaTeX(espcrc2.sty), 3p, 1 ps figur

    Low--Energy Theorems for Weak Pion Production

    Full text link
    We derive novel low--energy theorems for single pion production off nucleons through the isovector axial current. We find that the k2k^2-dependence of the multipole L0+(+)L_{0+}^{(+)} at threshold is given by the nucleon scalar form factor, namely σ(k2Mπ2)/(3πMπFπ)\sigma(k^2-M_\pi^2 ) /(3 \pi M_\pi F_\pi ). The relation to PCAC results for soft pions including electroweak form factors is also clarified.Comment: 9 pp, TeX, 2 figures available as ps files, CRN 93-5

    Double neutral pion photoproduction at threshold

    Full text link
    We consider the chiral expansion of the threshold amplitude for the reaction γpπ0π0p\gamma p \to \pi^0 \pi^0 p to order O(Mπ2){\cal O}(M_\pi^2). We substantiate a claim that this photoproduction channel is significantly enhanced close to threshold due to pion loops. A precise measurement of the corresponding cross sections is called for which allows to test chiral perturbation theory.Comment: 8 pp, LaTe

    On the analysis of the pion--nucleon σ\sigma--term: The size of the remainder at the Cheng--Dashen point

    Full text link
    We calculate the one--loop contributions of order Mπ4M_\pi^4 to the difference ΔR\Delta_R between the on--shell pion--nucleon scattering amplitude Dˉ+(0,2Mπ2)\bar{D}^+(0,2M_\pi^2) at the Cheng--Dashen point ν=0\nu =0, t=2Mπ2t=2M_\pi^2 and the scalar form factor σ(2Mπ2)\sigma (2M_\pi^2) in the framework of heavy baryon chiral perturbation theory. We proof that to this order ΔR\Delta_R contains nono chiral logarithms and therefore it vanishes simply as Mπ4M_\pi^4 in the chiral limit. Numerically, we find as an upper limit ΔR2\Delta_R \simeq 2\,MeV.Comment: 7 pp, LaTeX, uses epsf, 1 figure in separate PS fil

    Pion-nucleon scattering in an effective chiral field theory with explicit spin-3/2 fields

    Get PDF
    We analyze elastic-pion nucleon scattering to third order in the so-called small scale expansion. It is based on an effective Lagrangian including pions, nucleons and deltas as active degrees of freedom and counting external momenta, the pion mass and the nucleon-delta mass splitting as small parameters. The fermion fields are considered as very heavy. We present results for phase shifts, threshold parameters and the sigma term. We discuss the convergence of the approach. A detailed comparison with results obtained in heavy baryon chiral perturbation theory to third and fourth order is also given.Comment: 44 pp, LaTeX2e, uses epsf, 6 fig

    Towards the continuum limit with quenched staggered quarks

    Get PDF
    We extend previous work on finite-size effects with dynamical staggered quarks to the quenched approximation. We again emphasize the large volume limit that is of interest for spectrum calculations which may hope to approach the experimental values. Relying on new calculations at 6/g2=5.76/g^2=5.7 and recent work with weaker couplings, we extrapolate to the continuum limit and find a nucleon to rho mass ratio in close agreement with the experimental value and the value obtained by extrapolations from calculations with Wilson quarks. Additional calculations that should be done to improve the reliability of the extrapolation are discussed.Comment: 3 pages, PostScript, Contribution to Lattice '9
    corecore