5,801 research outputs found

    Resources and student achievement – evidence from a Swedish policy reform

    Get PDF
    This paper utilizes a policy change to estimate the effect of teacher density on student performance. We find that an increase in teacher density has a positive effect on student achievement. The baseline estimate – obtained by using the grade point average as the outcome variable – implies that resource increases corresponding to the class-size reduction in the STAR-experiment (i.e., a reduction of 7 students) improves performance by 2.6 percentile ranks (or 0.08 standard deviations). When we use test score data for men, potentially a more objective measure of student performance, the effect of resources appears to be twice the size of the baseline estimate.Student performance; teacher/student ratio; policy reform; differences-in-differences

    Exact 1/N and Optimized Perturbative Evaluation of mu_c for Homogeneous Interacting Bose Gases

    Full text link
    In the framework of the O(N) three-dimensional effective scalar field model for homogeneous dilute weakly interacting Bose gases we use the 1/N expansion to evaluate, within the large N limit, the parameter r_c which is directly related to the critical chemical potential mu_c. This quantity enters the order-a^2 n^{2/3} coefficient contributing to the critical temperature shift Delta T_c where a represents the s-wave scattering length and n represents the density. Compared to the recent precise numerical lattice simulation results, our calculation suggests that the large N approximation performs rather well even for the physical case N=2. We then calculate the same quantity but using different forms of the optimized perturbative (variational) method, showing that these produce excellent results both for the finite N and large-N cases.Comment: 12 pages, 2 figures. We have performed a refined and extended numerical analysis to take into account the very recent results of Ref. [15

    A Study of the N=2N=2 Kazakov-Migdal Model

    Full text link
    We study numerically the SU(2) Kazakov-Migdal model of `induced QCD'. In contrast to our earlier work on the subject we have chosen here {\it not} to integrate out the gauge fields but to keep them in the Monte Carlo simulation. This allows us to measure observables associated with the gauge fields and thereby address the problem of the local Z2Z_2 symmetry present in the model. We confirm our previous result that the model has a line of first order phase transitions terminating in a critical point. The adjoint plaquette has a clear discontinuity across the phase transition, whereas the plaquette in the fundamental representation is always zero in accordance with Elitzur's theorem. The density of small Z2Z_2 monopoles shows very little variation and is always large. We also find that the model has extra local U(1) symmetries which do not exist in the case of the standard adjoint theory. As a result, we are able to show that two of the angles parameterizing the gauge field completely decouple from the theory and the continuum limit defined around the critical point can therefore not be `QCD'.Comment: 11 pages, UTHEP-24

    Observing the build-up of the colour-magnitude relation at redshift ~0.8

    Full text link
    We analyse the rest-frame (U-V) colour-magnitude relation for 2 clusters at redshift 0.7 and 0.8, drawn from the ESO Distant Cluster Survey. By comparing with the population of red galaxies in the Coma cluster, we show that the high redshift clusters exhibit a deficit of passive faint red galaxies. Our results show that the red-sequence population cannot be explained in terms of a monolithic and synchronous formation scenario. A large fraction of faint passive galaxies in clusters today has moved onto the red sequence relatively recently as a consequence of the fact that their star formation activity has come to an end at z<0.8.Comment: 5 pages, 2 figures, to appear in Proc. of IAU Colloq. 195: "Outskirts of Galaxy Clusters: Intense Life in the Suburbs" -- minor typos correcte

    Weak lensing mass reconstructions of the ESO Distant Cluster Survey

    Full text link
    We present weak lensing mass reconstructions for the 20 high-redshift clusters i n the ESO Distant Cluster Survey. The weak lensing analysis was performed on deep, 3-color optical images taken with VLT/FORS2, using a composite galaxy catalog with separate shape estimators measured in each passband. We find that the EDisCS sample is composed primarily of clusters that are less massive than t hose in current X-ray selected samples at similar redshifts, but that all of the fields are likely to contain massive clusters rather than superpositions of low mass groups. We find that 7 of the 20 fields have additional massive structures which are not associated with the clusters and which can affect the weak lensing mass determination. We compare the mass measurements of the remaining 13 clusters with luminosity measurements from cluster galaxies selected using photometric redshifts and find evidence of a dependence of the cluster mass-to-light ratio with redshift. Finally we determine the noise level in the shear measurements for the fields as a function of exposure time and seeing and demonstrate that future ground-based surveys which plan to perform deep optical imaging for use in weak lensing measurements must achieve point-spread functions smaller than a median of 0.6" FWHM.Comment: 35 pages, 24 figures, accepted to A&A, a version with better figure resolution can be found at http://www.mpa-garching.mpg.de/ediscs/papers.htm

    Convergence of the Optimized Delta Expansion for the Connected Vacuum Amplitude: Zero Dimensions

    Full text link
    Recent proofs of the convergence of the linear delta expansion in zero and in one dimensions have been limited to the analogue of the vacuum generating functional in field theory. In zero dimensions it was shown that with an appropriate, NN-dependent, choice of an optimizing parameter \l, which is an important feature of the method, the sequence of approximants ZNZ_N tends to ZZ with an error proportional to ecN{\rm e}^{-cN}. In the present paper we establish the convergence of the linear delta expansion for the connected vacuum function W=lnZW=\ln Z. We show that with the same choice of \l the corresponding sequence WNW_N tends to WW with an error proportional to ecN{\rm e}^{-c\sqrt N}. The rate of convergence of the latter sequence is governed by the positions of the zeros of ZNZ_N.Comment: 20 pages, LaTeX, Imperial/TP/92-93/5
    corecore