arXiv subjects
Publications and source records attributed to N. Kaiser.
We consider the chiral expansion of the threshold amplitude for the reaction $γp \to π^0 π^0 p$ to order ${\cal O}(M_π^2)$. 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.
We calculate the one--loop corrections to the Kroll--Ruderman low--energy theorems for charged pion photoproduction in the framework of heavy baryon chiral perturbation theory. We predict the threshold S--wave multipole $E_{0+}$ to be $E_{0+}^{\rm thr}(γp \to π^+ n) = ( 28.2 \pm 0.6 )\cdot 10^{-3}/M_π$ and $E_{0+}^{\rm thr} (γn \toπ^- p) = ( -32.7 \pm 0.6 )\cdot 10^{-3}/M_π$, respectively, for a fixed pion--nucleon coupling constant, $g_{πN} = 13.4$. A comparison to the existing data is also given.
We report on the detection of dark matter in the cluster of galaxies Abell~2163 using the weak gravitational distortion of background galaxies, and an analysis of the cluster X-ray emission. We find that while the qualitative distributions of the cluster light and the dark matter are similar -- shallow and extended, with significant substructure -- the X-ray morphology shows a more regular overall appearance. We interpret the joint lensing and X-ray observations as a signature of a merger event in the cluster. We present new ROSAT/HRI data and reanalyze ROSAT/PSPC data, accounting for the effect of a varying background to determine the best fit parameters in the $β$-model formalism. We combine the surface brightness fits with two determinations of the radial temperature profile to determine the total mass. Although there are slight variations in the total mass determinations introduced by the uncertainties in the $β$-fit, the main contributor to the error arises from the uncertainties in the temperature determinations. Even though the morphologies of the dark matter/light and X-ray gas are quite different, we find that the total mass determined from the X-ray and weak lensing estimates are consistent with each other within the $2σ$ error bars, with the X-ray inferred mass a factor of $\simeq 2$ larger. However, as the lensing mass estimates are differential (the surface density at any point is determined relative to the mean in a control annulus), the shallow, extended nature of the mass profile biases the lensing inferred mass downwards. We estimate the correction for this effect and find very good agreement between the corrected lensing and X-ray results. We determine the gas mass fraction and find $f_g \simeq 0.07h^{-3/2}$ at all radii and a constant mass-to-light ratio of $M/L_V
We report on the detection of dark matter in the cluster Abell 2390 using the weak gravitational distortion of background galaxies. We find that the cluster light and total mass distributions are quite similar over an angular scale of $\simeq 7^\prime \;(1 \Mpc$). The cluster galaxy and mass distributions are centered on the cluster cD galaxy and exhibit elliptical isocontours in the central $\simeq 2^\prime \; (280 \kpc)$. The major axis of the ellipticity is aligned with the direction defined by the cluster cD and a ``straight arc'' located $\simeq 38^{\prime\prime}$ to the northwest. We determined the radial mass-to-light profile for this cluster and found a constant value of $(320 \pm 90) h\; M_\odot/L_{\odot V}$, which is consistent with other published determinations. We also compared our weak lensing azimuthally averaged radial mass profile with a spherical mass model proposed by the CNOC group on the basis of their detailed dynamical study of the cluster. We find good agreement between the two profiles, although there are weak indications that the CNOC density profile may be falling more steeply for $θ\geq 3^\prime$ $(420\kpc)$.
In the framework of heavy baryon chiral perturbation theory, we give thIn the framework of heavy baryon chiral perturbation theory, we give the chiral expansion for the $πN \to ππN$ threshold amplitudes $D_1$ and $D_2$ to quadratic order in the pion mass. The theoretical results agree within one standard deviation with the empirical values. We also derive a relation between the two threshold amplitudes of the reaction $πN \to ππN$ and the $ππ$ S--wave scattering lengths, $a_0^0$ and $a_0^2$, respectively, to order ${\cal O}(M_π^2)$. We show that there are uncertainties mostly related to resonance excitation which make an accurate determination of the $ππ$ scattering length $a_0^0$ from the $ππN$ threshold amplitudes at present very difficult. The situation is different in the $ππ$ isospin two final state. Here, the chiral series converges and one finds $a_0^2 = -0.031 \pm 0.007$ consistent with the one--loop chiral perturbation theory prediction.
We investigate neutral pion electroproduction off protons in the framework of heavy baryon chiral perturbation theory. The chiral expansion of the S--wave multipoles $E_{0+}$ and $L_{0+}$ is carried out to three orders. There appear several undetermined low--energy constants. Three are taken from a recent study of the new TAPS threshold $π^0$ photoproduction data, one is fixed from the proton Dirac form factor and the novel S--wave constants appearing at orders $q^4$ and $q^5$ are determined from a best fit (constrained by resonance exchange) to the recent NIKHEF and MAMI data at photon momentum transfer squared $k^2 = -0.1 \, \, {\rm GeV}^2$. The inclusion of a particular set of dimension five operators is forced upon the fit by a soft--pion theorem which severely constrains the momentum dependence of the longitudinal S--wave multipole $L_{0+}$ at order $q^4$. We give predictions for lower photon virtualities for the various multipoles and differential cross sections. Further improvements are briefly touched upon.
We analyze the new threshold data for neutral pion photoproduction off protons in the framework of heavy baryon chiral perturbation theory. We show that large loop corrections are needed to understand the S--wave multipole $E_{0+}$ and that all pertinent low--energy constants can be understood within the framework of resonance exchange saturation. Previous inconsistencies in the description of this reaction in the threshold region are resolved.
We apply a likelihood analysis to the data of \markcite{Lauer \& Postman 1994} With $P(k)$ parametrized by $(σ_8, Γ)$, the likelihood function peaks at $σ_8\simeq0.3$, $Γ\lesssim0.025$, indicating at face value very strong large-scale power, though at a level incompatible with COBE\@. There is, however, a ridge of likelihood such that more conventional power spectra do not seem strongly disfavored. The likelihood calculated using as data only the components of the bulk flow solution peaks at higher $σ_8$, in agreement with other analyses, but is rather broad. The likelihood incorporating both bulk flow and shear gives a different picture. The components of the shear are all low, and this pulls the peak to lower amplitudes as a compromise. The Lauer \& Postman velocity data alone are therefore {\em consistent}\/ with models with very strong large scale power which generates a large bulk flow, but the small shear (which also probes fairly large scales) requires that the power would have to be at {\em very}\/ large scales, which is strongly disfavored by COBE\@. The velocity data also seem compatible with more conventional $P(k)$ with $0.2\lesssimΓ\lesssim0.5$, and the likelihood is peaked around $σ_8\sim1$, in which case the bulk flow is a moderate, but not extreme, statistical fluctuation. Applying the same techniques to the data of \markcite{Riess, Press, \& Kirshner 1995}, the results are quite different. The flow is not inconsistent with the microwave dipole and we derive only an upper limit to the amplitude of the power spectrum: $σ_8\lesssim1.5$ at roughly 99\%.
The $SU(3)$ chiral effective lagrangian at next-to-leading order is applied to the S-wave meson-baryon interaction in the energy range around the $ηN$ threshold. Potentials are derived from this lagrangian and used in a coupled channel calculation of the $πN$, $ηN$, $K Λ$, $K Σ$ system in the isospin-$1/2$, $l=0$ partial wave. Using the same parameters as obtained from a fit to the low energy $\overline{K} N$ data it is found that a quasi-bound $K Σ$-state is formed, with properties remarkably similar to the $S_{11}(1535)$ nucleon resonance. In particular, we find a large partial decay width into $ηN$ consistent with the empirical data.
We report on the detection of dark matter in the cluster Abell 2218 using the weak gravitational distortion of background galaxies. We find a highly significant, coherent detection of the distortion in the images of the background galaxies. The inferred 2D mass distribution has a peak that is coincident with the optical and X-ray centroid. The qualitative distributions of the cluster light, the X-ray emission and the dark matter are similar and the projected total mass, gas, and light surface densities are consistent with a $r^{-1}$ profile at distance of $r > 180^{\prime\prime}$ from the cluster cD galaxy. Using the weak lensing technique, we determine a lower bound for the total mass in A2218 of $(3.9 \pm 0.7) \times 10^{14}$~h$^{-1}$~M$_\odot$ within a fiducial aperture of radius 0.4~h$^{-1}$Mpc. The associated cluster mass-to-light ratio is $(440 \pm 80)$~h~$M_\odot/L_{\odot B}$. The mass estimated by the weak lensing method is consistent with that inferred from the X-ray data under the assumption of hydrostatic equilibrium and we derive an upper bound for the gas-to-total mass ratio at 400~h$^{-1}$kpc of $M_{gas}/M_{tot} = (0.04 \pm 0.02)$h$^{-3/2}$.
We evaluate the S-wave pion--nucleon scattering length $a^-$ in the framework of heavy baryon chiral perturbation theory up--to--and--including terms of order $M_π^4$. We show that the order $M_π^4$ piece of the isovector amplitude at threshold, $T^-_{\rm thr}$, vanishes exactly. We predict for the isovector scattering length, $0.088 \, M_{π^+}^{-1} \le a^- \le 0.096 \, M_{π^+}^{-1}$.
We examine the meson-baryon interaction in the strangeness S=-1 sector using an effective chiral Lagrangian. Potentials are derived from this Lagrangian and used in a coupled-channel calculation of the low energy observables. The potentials are constructed such that in the Born approximation the s-wave scattering amplitude is the same as that given by the effective chiral Lagrangian, up to order $q^2$. Comparison is made with the available low energy hadronic data of the coupled $K^-p, Σπ, Λπ$ system, which includes the $Λ(1405)$ resonance, $K^-p$ elastic and inelastic scattering, and the threshold branching ratios of the $K^-p$ decay. Good fits to the experimental data and estimates of previously unknown Lagrangian parameters are obtained.
Near threshold pion production experiments have been recently carried out and used to extract S--wave $ππ$ scattering lengths. We emphasize here that at present these processes are related only at the tree level (and its first correction) in chiral perturbation theory. Higher order corrections (including loops) must be evaluated before rigorous claims concerning S--wave $ππ$ scattering lengths can be made.
We review the implications of the spontaneous chiral symmetry breaking in QCD for processes involving one, two or more nucleons.
We present novel low--energy theorems for the P--wave multipoles $2M_{1+} + M_{1-}$, $M_{1+} -M_{1-}$, $E_{1+}$ and $L_{1\pm}$ for neutral pion electroproduction off protons. These should be very useful for the analysis of existing or future threshold data.
We investigate threshold pion photoproduction in the framework of heavy baryon chiral perturbation theory. We give the expansion of the electric dipole amplitude $E_{0+}$ to three orders in $μ$, the ratio of the pion to nucleon mass, and show that it is slowly converging. We argue that this observable is not a good testing ground for the chiral dynamics of QCD. In contrast, we exhibit new and fastly converging low--energy theorems in the P--waves which should be used to constrain the data analysis. We also discuss the importance of polarization observables to accurately pin down certain multipoles and give predictions for the reaction $γn \to π^0 n$.
We consider the chiral expansion for the reaction $πN \to ππN$ in heavy baryon chiral perturbation theory. To order $M_π$ we derive novel low--energy theorems that compare favorably with recent determinations of the total cross sections for $π^+ p \to π^+ π^+ n$ and $π^- p \to π^0 π^0 n$.
Using chiral Ward identities of QCD, we derive a relation for the induced pseudoscalar coupling constant which is accurate within a few percent, $g_P = 8.44 \pm 0.16$.