3,768 research outputs found
A terrestrial search for dark contents of the vacuum, such as dark energy, using atom interferometry
We describe the theory and first experimental work on our concept for
searching on earth for the presence of dark content of the vacuum (DCV) using
atom interferometry. Specifically, we have in mind any DCV that has not yet
been detected on a laboratory scale, but might manifest itself as dark energy
on the cosmological scale. The experimental method uses two atom
interferometers to cancel the effect of earth's gravity and diverse noise
sources. It depends upon two assumptions: first, that the DCV possesses some
space inhomogeneity in density, and second that it exerts a sufficiently strong
non-gravitational force on matter. The motion of the apparatus through the DCV
should then lead to an irregular variation in the detected matter-wave phase
shift. We discuss the nature of this signal and note the problem of
distinguishing it from instrumental noise. We also discuss the relation of our
experiment to what might be learned by studying the noise in gravitational wave
detectors such as LIGO.The paper concludes with a projection that a future
search of this nature might be carried out using an atom interferometer in an
orbiting satellite. The apparatus is now being constructed
Is the Top Quark Really Heavier than the Boson?
Scalar induced top decays may drastically suppress
and still hide the top below . The collider experiments should
enlarge the scope and study the plane. Specific model
signatures such as (multiple high -jets) and
, (with $B(t\to b\tau\nu) \
\raisebox{-.5ex}{\rlap{}} \raisebox{.4ex}{}\ 1/3t^\primeb^\prime$ quark,
while top quark and toponium physics could still turn up at LEP-II.Comment: 11 pages (RevTex), 3 figures (not included), NTUTH-93-0
Narrowing the window for millicharged particles by CMB anisotropy
We calculate the cosmic microwave background (CMB) anisotropy spectrum in
models with millicharged particles of electric charge q\sim 10^{-6}-10^{-1} in
units of electron charge. We find that a large region of the parameter space
for the millicharged particles exists where their effect on the CMB spectrum is
similar to the effect of baryons. Using WMAP data on the CMB anisotropy and
assuming Big Bang nucleosynthesis value for the baryon abundance we find that
only a small fraction of cold dark matter, Omega_{mcp}h_0^2 < 0.007 (at 95%
CL), may consists of millicharged particles with the parameters (charge and
mass) from this region. This bound significantly narrows the allowed range of
the parameters of millicharged particles. In models without paraphoton
millicharged particles are now excluded as a dark matter candidate. We also
speculate that recent observation of 511 keV gamma-rays from the Galactic bulge
may be an indication that a (small) fraction of CDM is comprised of the
millicharged particles.Comment: 10 pages, 3 figures; v2: journal version, references adde
Big-Bang Nucleosynthesis and Hadronic Decay of Long-Lived Massive Particles
We study the big-bang nucleosynthesis (BBN) with the long-lived exotic
particle, called X. If the lifetime of X is longer than \sim 0.1 sec, its decay
may cause non-thermal nuclear reactions during or after the BBN, altering the
predictions of the standard BBN scenario. We pay particular attention to its
hadronic decay modes and calculate the primordial abundances of the light
elements. Using the result, we derive constraints on the primordial abundance
of X. Compared to the previous studies, we have improved the following points
in our analysis: The JETSET 7.4 Monte Carlo event generator is used to
calculate the spectrum of hadrons produced by the decay of X; The evolution of
the hadronic shower is studied taking account of the details of the energy-loss
processes of the nuclei in the thermal bath; We have used the most recent
observational constraints on the primordial abundances of the light elements;
In order to estimate the uncertainties, we have performed the Monte Carlo
simulation which includes the experimental errors of the cross sections and
transfered energies. We will see that the non-thermal productions of D, He3,
He4 and Li6 provide stringent upper bounds on the primordial abundance of
late-decaying particle, in particular when the hadronic branching ratio of X is
sizable. We apply our results to the gravitino problem, and obtain upper bound
on the reheating temperature after inflation.Comment: 94 pages, 49 figures, to appear in Phys. Rev. D. This is a full
length paper of the preprint astro-ph/040249
Bremsstrahlung Suppression due to the LPM and Dielectric Effects in a Variety of Materials
The cross section for bremsstrahlung from highly relativistic particles is
suppressed due to interference caused by multiple scattering in dense media,
and due to photon interactions with the electrons in all materials. We present
here a detailed study of bremsstrahlung production of 200 keV to 500 MeV
photons from 8 and 25 GeV electrons traversing a variety of target materials.
For most targets, we observe the expected suppressions to a good accuracy. We
observe that finite thickness effects are important for thin targets.Comment: 52 pages, 13 figures (incorporated in the revtex LaTeX file
Low Energy Pion-Hyperon Interaction
We study the low energy pion-hyperon interaction considering effective
non-linear chiral invariant Lagrangians including pions, rho mesons, hyperons
and corresponding resonances. Then we calculate the S- and P-wave phase-shifts,
total cross sections, angular distributions and polarizations for the momentum
in the center-of-mass frame up to k=400 MeV. With these results we discuss the
CP violation in the csi-> pi-lambda and omega-> pi-csi weak decays.Comment: 10 pages, 10 figure
Regression of left ventricular mass following conversion from conventional hemodialysis to thrice weekly in-centre nocturnal hemodialysis
<p>Abstract</p> <p>Background</p> <p>Increased left ventricular mass (LVM) is associated with adverse outcomes in patients receiving chronic hemodialysis. Among patients receiving conventional hemodialysis (CHD, 3×/week, 4 hrs/session), we evaluated whether dialysis intensification with in-centre nocturnal hemodialysis (INHD, 3×/week, 7-8 hrs/session in the dialysis unit) was associated with regression of LVM.</p> <p>Methods</p> <p>We conducted a retrospective cohort study of CHD recipients who converted to INHD and received INHD for at least 6 months. LVM on the first echocardiogram performed at least 6 months post-conversion was compared to LVM pre-conversion. In a secondary analysis, we examined echocardiograms performed at least 12 months after starting INHD. The effect of conversion to INHD on LVM over time was also evaluated using a longitudinal analysis that incorporated all LVM data on patients with 2 or more echocardiograms.</p> <p>Results</p> <p>Thirty-seven patients were eligible for the primary analysis. Mean age at conversion was 49 ± 12 yrs and 30% were women. Mean pre-conversion LVM was 219 ± 66 g and following conversion, LVM declined by 32 ± 58 g (p = 0.002). Among patients whose follow-up echocardiogram occurred at least 12 months following conversion, LVM declined by 40 ± 56 g (p = 0.0004). The rate of change of LVM decreased significantly from 0.4 g/yr before conversion, to -11.7 g/yr following conversion to INHD (p < 0.0001).</p> <p>Conclusion</p> <p>Conversion to INHD is associated with a significant regression in LVM, which may portend a more favourable cardiovascular outcome. Our preliminary findings support the need for randomized controlled trials to definitively evaluate the cardiovascular effects of INHD.</p
Charged Scalar Particles and Leptonic Decay
Charged scalar particles introduced in some extensions of the standard model
can induce leptonic decay at tree level. We find that with some charged
SU(2)-singlet scalar particles, like ones introduced in Zee-type models,
leptonic decay width is always smaller than what is predicted by the standard
model, therefore they may offer a natural solution to decay puzzle. To
be more specific, we examine some Zee-type models in detail to see if at the
same time they are acceptable in particle physics, cosmology and astrophysics.
It is shown that decay data do put some constrains on these models.Comment: ICTP Report No. IC/93/31, 12 pages, Latex, one figure is not
included, it is available upon deman
Teachers as writers: a systematic review
This paper is a critical literature review of empirical work from 1990-2015 on teachers as writers. It interrogates the evidence on teachers’ attitudes to writing, their sense of themselves as writers and the potential impact of teacher writing on pedagogy or student outcomes in writing. The methodology was carried out in four stages. Firstly, educational databases keyword searches located 438 papers. Secondly, initial screening identified 159 for further scrutiny, 43 of which were found to specifically address teachers’ writing identities and practices. Thirdly, these sources were screened further using inclusion/exclusion criteria. Fourthly, the 22 papers judged to satisfy the criteria were subject to in-depth analysis and synthesis. The findings reveal that the evidence base in relation to teachers as writers is not strong, particularly with regard to the impact of teachers’ writing on student outcomes. The review indicates that teachers have narrow conceptions of what counts as writing and being a writer and that multiple tensions exist, relating to low self-confidence, negative writing histories, and the challenge of composing and enacting teacher and writer positions in school. However, initial training and professional development programmes do appear to afford opportunities for reformulation of attitudes and sense of self as writer
Measurement of Dielectric Suppression of Bremsstrahlung
In 1953, Ter-Mikaelian predicted that the bremsstrahlung of low energy
photons in a medium is suppressed because of interactions between the produced
photon and the electrons in the medium. This suppression occurs because the
emission takes place over on a long distance scale, allowing for destructive
interference between different instantaneous photon emission amplitudes. We
present here measurements of bremsstrahlung cross sections of 200 keV to 20 MeV
photons produced by 8 and 25 GeV electrons in carbon and gold targets. Our data
shows that dielectric suppression occurs at the predicted level, reducing the
cross section up to 75 percent in our data.Comment: 11 pages, format is postscript file, gzip-ed, uuencode-e
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