897 research outputs found
Resources and student achievement – evidence from a Swedish policy reform
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
Light-Front Approach for Pentaquark Strong Decays
Assuming the two diquark structure for the pentaquark state as advocated in
the Jaffe-Wilczek model, we study the strong decays of light and heavy
parity-even pentaquark states using the light-front quark model in conjunction
with the spectator approximation. The narrowness of the Theta width is ascribed
to the p-wave configuration of the diquark pair. Taking the Theta width as a
benchmark, we estimate the rates of the strong decays Xi_{3/2}-- to Xi- pi-,
Sigma- K-, Sigma_{5c}0 to D_s- p, D_{s0}*- p and Xi_{5c}0 to D_s- Sigma+,
D_{s0}^{*-} Sigma+ with Sigma_{5c} Xi_{5c} being antisextet charmed pentaquarks
and D_{s0}* a scalar strange charmed meson. The ratio of Gamma(P_c to Baryon
D_{s0}*)/Gamma(P_c to Baryon D_s) is very useful for verifying the parity of
the antisextet charmed pentaquark P_c. It is expected to be of order unity for
an even parity P_c and much less than one for an odd parity pentaquark.Comment: 24 pages, 2 figure
Implications of Recent Measurements of Hadronic Charmless B Decays
Implications of recent CLEO measurements of hadronic charmless B decays are
discussed. (i) Employing the Bauer-Stech-Wirbel (BSW) model for form factors as
a benchmark, the data indicate that the form factor
is smaller than that predicted by the BSW model, whereas the
data of imply that the form factors are greater than the BSW model's values. (ii) The tree-dominated
modes imply that the effective
number of colors N_c(LL) for (V-A)(V-A) operators is preferred to be smaller,
while the current limit on shows that N_c(LR)>3. The data of and clearly indicate that . (iii) In
order to understand the observed suppression of and
non-suppression of modes, both being governed by the form factor
, the unitarity angle is preferred to be greater than
. By contrast, the new measurement of no
longer strongly favors . (iv) The observed pattern K^-\pi^+\sim
\ov K^0\pi^-\sim {2\over 3}K^-\pi^0 is consistent with the theoretical
expectation: The constructive interference between electroweak and QCD penguin
diagrams in the mode explains why {\cal B}(B^-\to K^-\pi^0)>{1\over
2}{\cal B}(\ov B^0\to K^-\pi^+). (v) The observation \nc(LL)<3<\nc(LR) and
our preference for \nc(LL)\sim 2 and \nc(LR)\sim 6 are justified by a
recent perturbative QCD calculation of hadronic rare B decays in the heavy
quark limit.Comment: 21 pages; CLEO measurements of several charmless B decay modes are
updated. Discussion of the unitarity angle gamma in the \rho\pi mode is
revise
Updated Analysis of a_1 and a_2 in Hadronic Two-body Decays of B Mesons
Using the recent experimental data of , , and various model calculations on form
factors, we re-analyze the effective coefficients a_1 and a_2 and their ratio.
QCD and electroweak penguin corrections to a_1 from and
a_2 from are estimated. In addition to the
model-dependent determination, the effective coefficient a_1 is also extracted
in a model-independent way as the decay modes are related by
factorization to the measured semileptonic distribution of at . Moreover, this enables us to extract model-independent
heavy-to-heavy form factors, for example,
and
. The determination of the magnitude of
a_2 from depends on the form factors ,
and at . By requiring that a_2 be
process insensitive (i.e., the value of a_2 extracted from and
states should be similar), as implied by the factorization
hypothesis, we find that form factors are severely constrained;
they respect the relation . Form factors and at
inferred from the measurements of the longitudinal
polarization fraction and the P-wave component in are
obtained. A stringent upper limit on a_2 is derived from the current bound on
\ov B^0\to D^0\pi^0 and it is sensitive to final-state interactions.Comment: 33 pages, 2 figures. Typos in Tables I and IX are corrected. To
appear in Phys. Rev.
Topologically Stable Electroweak Flux Tube
We show that for a large range of parameters in a
electroweak theory with two Higgs doublets there may exist classically stable
flux tubes of Z boson magnetic field. In a limit of an extra global symmetry, these flux-tubes become topologically stable. These results are
automatically valid even if is gauged.Comment: 10 pages, LATE
Decays in QCD Factorization
The hadronic decays are interesting because
experimentally they are the only color-suppressed modes which have been
measured, and theoretically they are calculable by QCD factorization even the
emitted meson is heavy. We analyze the decay within the
framework of QCD factorization in the heavy quark limit. We show explicitly the
scale and -scheme independence of decay amplitudes and infrared
safety of nonfactorizable corrections at twist-2 order. Leading-twist
contributions from the light-cone distribution amplitudes (LCDAs) of the mesons
are too small to accommodate the data; the nonfactorizable corrections to naive
factorization are small and not significant. We study the twist-3 effects due
to the kaon and find that the coefficient is largely enhanced by
the nonfactorizable spectator interactions arising from the twist-3 kaon LCDA
, which are formally power-suppressed but chirally,
logarithmically and kinematically enhanced. Therefore, factorization breaks
down at twist-3 order. Higher-twist effects of are briefly discussed.
Our result also resolves the long-standing sign ambiguity of ,
which turns out to be positive for its real part.Comment: 18 pages, 2 figures. Typos in Eqs.(3.4), (3.5), and (3.6) are
correcte
Applicability of perturbative QCD to decays
We develop perturbative QCD factorization theorem for the semileptonic heavy
baryon decay , whose form factors are
expressed as the convolutions of hard quark decay amplitudes with universal
and baryon wave functions. Large logarithmic
corrections are organized to all orders by the Sudakov resummation, which
renders perturbative expansions more reliable. It is observed that perturbative
QCD is applicable to decays for velocity transfer
greater than 1.2. Under requirement of heavy quark symmetry, we predict the
branching ratio , and determine
the and baryon wave functions.Comment: 12 pages in Latex file, 3 figures in postscript files, some results
are changed, but the conclusion is the sam
Helicity conservation in inclusive nonleptonic decay B to VX: Test of long-distance final state interaction
The polarization measurement in the inclusive B decay provides us with a
simple test of how much the long-distance final-state interaction takes place
as the energy of the observed meson varies in the final state. We give the
expectation of the perturbative QCD for the energy dependence of the helicity
fractions in a semiquantitative form. Experiment will tell us for which decay
processes the perturbative calculation should be applicable.Comment: 15 pages in Revtex with 3 figures embedde
Final-state interaction and s-quark helicity conservation in B -> J/psi K*
The Section of charm quark spin conservation is deleted since it involves
more dynamical assumptions than previously stated. A few comments are added in
view of new experimental results.Comment: To replace the earlier version of hep-ph/0106354. Minor additions and
one deletion with no change in the main argument nor the conclusio
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