arXiv subjects
Publications and source records attributed to N. Kaiser.
We present a detailed analysis of resonance contributions in the context of higher twist effects in the moments of the proton spin structure function g_1. For each of these moments, it is found that there exists a characteristic Q^2 region in which (perturbative) higher twist corrections coexist with (non-perturbative) resonance contribution of comparable magnitude.
We analyze the total cross section data for $pp \to ppω$ near threshold measured recently at SATURNE. Using an effective range approximation for the on-shell $pp$ S-wave final state interaction we extract from these data the modulus $|Ω| = 0.53$ fm$^4$ of the threshold transition amplitude $Ω$. We present a calculation of various (tree-level) meson exchange diagrams contributing to $Ω$. It is essential that $ω$-emission from the anomalous $ωρπ$-vertex interferes destructively with $ω$-emission from the proton lines. The contribution of scalar $σ$-meson exchange to $Ω$ turns out to be negligibly small. Without introducing off-shell meson-nucleon form factors the experimental value $|Ω|=0.53$ fm$^4$ can be reproduced with an $ωN$-coupling constant of $g_{ωN}=10.7$. The results of the present approach agree qualitatively with the Jülich model. We also perform a combined analysis of the reactions $pp\to ppπ^0, pnπ^+, ppη, ppω$ and $pn\to pnη$ near threshold.
We describe in detail the processing of a set of images of the z = 0.42 supercluster MS0302 taken with the UH8K camera at CFHT. The result of this is a pair of seamless combined V- and I-band images of the field, along with a characterization of the noise properties and of the point spread function (PSF), and catalogs of about 30,000 faint galaxies. The analysis involves the following steps: image preparation; detection of stars and registration to find the transformation from detector to sky coordinates; correction for extinction and/or gain variations; modeling of the PSF; generation of images with a circular PSF; image warping and averaging; modeling of the noise auto-correlation function; faint object detection, aperture photometry, and shape measurement. The shear analysis is described elsewhere.
We describe the 2dF Galaxy Redshift Survey (2dFGRS), and the current status of the observations. In this exploratory paper, we apply a Principal Component Analysis to a preliminary sample of 5869 galaxy spectra and use the two most significant components to split the sample into five spectral classes. These classes are defined by considering visual classifications of a subset of the 2dF spectra, and also by comparing to high quality spectra of local galaxies. We calculate a luminosity function for each of the different classes and find that later-type galaxies have a fainter characteristic magnitude, and a steeper faint-end slope. For the whole sample we find M*=-19.7 (for Omega=1, H_0=100 km/sec/Mpc), alpha=-1.3, phi*=0.017. For class 1 (`early-type') we find M*=-19.6, alpha=-0.7, while for class 5 (`late-type') we find M*=-19.0, alpha=-1.7. The derived 2dF luminosity functions agree well with other recent luminosity function estimates.
We analyze the recent total cross section data for $pp \to pΛK^+$ near threshold measured at COSY. Using an effective range approximation for the on-shell $pΛ$ S-wave final state interaction we extract from these data the combination ${\cal K} = \sqrt{2|K_s|^2+|K_t|^2} = 0.38$ fm$^4$ of the singlet ($K_s$) and triplet ($K_t$) threshold transition amplitudes. We present an exploratory calculation of various (tree-level) vector and pseudoscalar meson exchange diagrams. Pointlike $ω$-exchange alone and the combined ($ρ^0,ω,K^{*+} $)-exchange can explain the experimental value of $\cal K$. The pseudoscalar meson exchanges based on a SU(3) chiral Lagrangian turn out to be too large. However, when adding $π^0$-exchange in combination with the resonant $πN \to S_{11}(1650)\to K Λ$ transition and introducing monopole form factors with a cut-off $Λ_c= 1.5$ GeV one is again able to reproduce the experimental value of $\cal K$. More exclusive measurements are necessary to reveal the details of the $pp\to pΛK^+$ production mechanism.
We evaluate the threshold matrix-element for the reaction $pp \to ppπ^0$ in a fully relativistic Feynman diagrammatic approach. We employ a simple effective range approximation to take care of the S-wave $pp$ final-state interaction. The experimental value for the threshold amplitude ${\cal A} = (2.7 - i 0.3)$ fm$^4$ can be reproduced by contributions from tree level chiral (long-range) pion exchange and short-range effects related to vector meson exchanges, with $ω$ exchange giving the largest individual contribution. Pion loop effects appear to be small. We stress that the commonly used heavy baryon formalism is not applicable in the NN-system above the pion production threshold due to the large external momentum, $|\vec p | \simeq \sqrt {Mm_π}$, with $M$ and $m_π$ the nucleon and the pion mass, respectively. We furthermore investigate the reaction $pp\to p n π^+$ near threshold within the same approach. We extract from the data the triplet threshold amplitude as ${\cal B}= (2.8 -i 1.5)$ fm$^4$. Its real part can be well understood from (relativistic) tree level meson-exchange diagrams. In addition, we investigate the process $pp \to pp η$ near threshold. We use a simple factorization ansatz for the $ppη$ final-state interaction and extract from the data the modulus of the threshold amplitude, $|{\cal C}| = 1.32 $fm$^4$. With $g_{ηN}=5.3$, this value can be reproduced by (relativistic) tree level meson-exchange diagrams and $η$-rescattering, whose strength is fixed by the $ηN$ scattering length. We also comment on the recent near threshold data for $η'$-production.
We perform a weak lensing and photometric study of the z=0.42 supercluster MS0302+17 using deep I and V band images taken with the UH8K CCD mosaic camera at the CFHT. We use archival ROSAT HRI data to estimate fluxes, gas masses and, in one case, the binding mass of the three major clusters. We then use our CCD data to determine the optical richness and luminosities of the clusters and to map out the spatial distribution of the early type galaxies in the supercluster and in other foreground and background structures. We measure the gravitational shear from a sample of \simeq 30,000 faint background galaxies in the range 22 < m_I < 26 and find this correlates strongly with that predicted from the early type galaxies if they trace the mass with M/L_B \simeq 250 h. We make 2-dimensional reconstructions of the mass surface density. These recover all of the major concentrations of galaxies and indicate that most of the supercluster mass, like the early type galaxies, is concentrated in the three X-ray clusters, and we obtain mean mass-to-light ratios for the clusters of M/L_B \simeq 260 h. Cross-correlation of the measured mass surface density with that predicted from the early type galaxy distribution shows a strong peak at zero lag (significant at the \simeq 9-sigma level), and that at separations \gsim 200 h^{-1}$kpc the early galaxies trace the mass very accurately. This conclusion is supported by cross-correlation in Fourier space; we see little evidence for any variation of M/L or `bias' with scale, and from the longest wavelength modes with λ= 1.5-6 h^{-1}Mpc we find M/L \simeq (280 \pm 40)h, quite similar to that obtained for the cluster centers. We discuss the implication of these results for the cosmological density parameter.
We generalize the sum rules for the nucleon electric plus magnetic polarizability $Σ=α+β$ and for the nucleon spin-polarizability $γ$, to virtual photons with $Q^2>0$. The dominant low energy cross sections are represented in our calculation by one-pion-loop graphs of relativistic baryon chiral perturbation theory and the $Δ(1232)$-resonance excitation. For the proton we find good agreement of the calculated $Σ_p(Q^2)$ with empirical values obtained from integrating up electroproduction data for $Q^2<0.4 GeV^2$. The proton spin-polarizability $γ_p(Q^2)$ switches sign around $Q^2= 0.4 GeV^2$ and it joins smoothly the "partonic" curve, extracted from polarized deep-inelastic scattering, around $Q^2=0.7 GeV^2$. For the neutron our predictions of $Σ_n(Q^2)$ and $γ_n(Q^2)$ agree reasonably well at $Q^2=0$ with existing determinations. Upcoming (polarized) electroproduction experiments will be able to test the generalized polarizability sum rules investigated here.
We evaluate, within one-loop chiral perturbation theory, the two-pion exchange diagrams with single and double delta-isobar excitation contributing to elastic NN-scattering. We find that virtual $Δ$-excitation processes (in the static limit) produce the correct amount of isoscalar central attraction as needed in the peripheral partial waves with $L\geq 3$. Furthermore we compute the two-loop diagrams involving the $ππ$-interaction (so-called correlated $2π$-exchange). Contrary to common believe these processes lead to negligibly small and repulsive corrections to the NN-potential. The exchange of vector mesons ($ρ, ω$) turns out to be important for the F-wave phase shifts above $T_{lab}=150$ MeV. Without adjustable parameters we are able to reproduce the empirical NN phase shifts up to 350 MeV for $L\geq 3$ and up to about (50--80) MeV for the D-waves. This is therefore the characteristic window in which the NN-interaction is basically governed by chiral symmetry. Not surprisingly, the lower partial waves require non-perturbative methods and additional short-distance parametrizations of the NN-dynamics.
We report the detection of weak gravitational lensing of distant background galaxies by two rich, X-ray luminous clusters of galaxies, MS 1137+66 at z=0.783 and RXJ 1716+67 at z=0.813. We detect a shear signal in each cluster over the radial range 100 ~0.8.
Within the one-loop approximation of baryon chiral perturbation theory we calculate all one-pion and two-pion exchange contributions to the nucleon-nucleon interaction. In fact we construct the elastic NN-scattering amplitude up to and including third order in small momenta. The phase shifts with orbital angular momentum $L\geq2 $ and the mixing angles with $J\geq2$ are given parameterfree and thus allow for a detailed test of chiral symmetry in the two-nucleon system. We find that for the D-waves the $2π$-exchange corrections are too large as compared with empirical phase shifts, signaling the increasing importance of shorter range effects in lower partial waves. For higher partial waves, especially for G-waves, the model independent $2π$-exchange corrections bring the chiral prediction close to empirical NN phase shifts. We propose to use the chiral NN phase shifts with $L\geq 3$ as input in a future phase shift analysis. Furthermore, we compute the irreducible two-pion exchange NN-potentials in coordinate space. They turn out to be of van-der-Waals type, with exponential screening of two-pion mass range.
Based on an effective Lagrangian which combines chiral SU(3) dynamics with vector meson dominance, we have developed a model for the forward vector meson-nucleon scattering amplitudes. We use this as an input to calculate the low energy part of the current-current correlation function in nuclear matter. Its spectrum enters directly in the ``left hand side'' of QCD sum rules. For the isovector channel we find a significant enhancement of the in-medium spectral density below the rho resonance while the rho meson mass itself changes only slightly. The situation is different in the isoscalar channel, where the mass and peak position of the omega meson move downward while its width increases less drastically than in the rho meson case. For the phi meson we find almost no mass shift; the width of the peak broadens moderately. We observe a remarkable degree of consistency with the operator product expansion of QCD sum rules in all three channels. We point out, however, that these results cannot simply be interpreted, as commonly done, in terms of a universal rescaling of vector meson masses in matter.
Single pion production off nucleons is studied in the framework of relativistic baryon chiral perturbation theory at tree level with the inclusion of the terms from the dimension two effective pion-nucleon Lagrangian. The five appearing low-energy constants are fixed from pion-nucleon scattering data. Despite the simplicity of the approach, most of the existing data for total and differential cross sections as well as for the angular correlation functions for incoming pion kinetic energies up to 400 MeV can be satisfactorily described.
The next-to-leading order chiral pion-nucleon Lagrangian contains seven finite low-energy constants. Two can be fixed from the nucleon anomalous magnetic moments and another one from the quark mass contribution to the neutron-proton mass splitting. We find a set of nine observables, which to one loop order do only depend on the remaining four dimension two couplings. These are then determined from a best fit. We also show that their values can be understood in terms of resonance exchange related to $Δ$ excitation as well as vector and scalar meson exchange. In particular, we discuss the role of the fictitious scalar-isoscalar meson. We also investigate the chiral expansion of the two P-wave scattering volumes $P_1^-$ and $P_2^+$ as well as the isovector S-wave effective range parameter $b^-$. The one-loop calculation is in good agreement with the data. The difference $P_1^- - P_2^+$ signals chiral loop effects in the $πN$ P-waves. The calculated D- and F-wave threshold parameters compare well with the empirical values.
We identify the leading s-wave amplitudes of the SU(3) chiral meson-baryon Lagrangian with an effective coupled-channel potential which is iterated in a Lippmann-Schwinger equation. The strangeness $S=-1$ resonance $Λ(1405)$ and the $S_{11}(1535)$ nucleon resonance emerge as quasi-bound states of anti-kaon/nucleon and kaon/$Σ$-hyperon. Our approach to meson photoproduction introduces no new parameters. By adjusting a few finite range parameters we are able to simultaneously describe a large amount of low energy data. These include the cross sections of $K^-p$ elastic and inelastic scattering, the cross sections of eta meson and kaon photoproduction from nucleons as well as those of pion induced production of etas and kaons (16 different reaction channels altogether).
An improved update of the structure and decays of $ρ^0$, $ω$ and $ϕ$ mesons based on a chiral SU(3) Lagrangian, including anomaly terms is presented. We demonstrate that a consistent and quantitatively successful description of both pion and kaon electromagnetic form factors can be achieved. We also discuss the $e^+e^- \to π^+ π^0 π^-$ cross section, the Dalitz decay $ω\to π^0 μ^+ μ^-$ and aspects of $ρ^0 ω$ and $ωϕ$ mixing. Relations to previous versions of the Vector Meson Dominance model will be examined.
We investigate the imaginary parts of the nucleon electromagnetic and axial form factors close to threshold in the framework of heavy baryon chiral perturbation theory. For the isovector electromagnetic form factors, we recover the well known strong enhancement near threshold. For the isoscalar ones, we show that there is no visible enhancement due to the three--pion continuum. This justifies the use of vector meson poles only in dispersion--theoretical calculations. We also calculate the imaginary part of the nucleon isovector axial form factor and show that it is small in the threshold region.
We calculate the one--loop contributions of order $M_π^4$ to the difference $Δ_R$ between the on--shell pion--nucleon scattering amplitude $\bar{D}^+(0,2M_π^2)$ at the Cheng--Dashen point $ν=0$, $t=2M_π^2$ and the scalar form factor $σ(2M_π^2)$ in the framework of heavy baryon chiral perturbation theory. We proof that to this order $Δ_R$ contains $no$ chiral logarithms and therefore it vanishes simply as $M_π^4$ in the chiral limit. Numerically, we find as an upper limit $Δ_R \simeq 2\,$MeV.