247 research outputs found
Surface Superconductivity in Niobium for Superconducting RF Cavities
A systematic study is presented on the superconductivity (sc) parameters of
the ultrapure niobium used for the fabrication of the nine-cell 1.3 GHz
cavities for the linear collider project TESLA. Cylindrical Nb samples have
been subjected to the same surface treatments that are applied to the TESLA
cavities: buffered chemical polishing (BCP), electrolytic polishing (EP),
low-temperature bakeout (LTB). The magnetization curves and the complex
magnetic susceptibility have been measured over a wide range of temperatures
and dc magnetic fields, and also for di erent frequencies of the applied ac
magnetic field. The bulk superconductivity parameters such as the critical
temperature Tc = 9.26 K and the upper critical field Bc2(0) = 410 mT are found
to be in good agreement with previous data. Evidence for surface
superconductivity at fields above Bc2 is found in all samples. The critical
surface field exceeds the Ginzburg-Landau field Bc3 = 1.695Bc2 by about 10% in
BCP-treated samples and increases even further if EP or LTB are applied. From
the field dependence of the susceptibility and a power-law analysis of the
complex ac conductivity and resistivity the existence of two different phases
of surface superconductivity can be established which resemble the Meissner and
Abrikosov phases in the bulk: (1) coherent surface superconductivity, allowing
sc shielding currents flowing around the entire cylindrical sample, for
external fields B in the range between Bc2 and Bcohc3, and (2) incoherent
surface superconductivity with disconnected sc domains between Bcohc3 and Bc3.
The coherent critical surface field separating the two phases is found to be
Bcoh c3 = 0.81Bc3 for all samples. The exponents in the power law analysis are
different for BCP and EP samples, pointing to different surface topologies.Comment: 15 pages, 21 figures, DESY-Report 2004-02
Pairs Emission in a Uniform Background Field: an Algebraic Approach
A fully algebraic general approach is developed to treat the pairs emission
and absorption in the presence of some uniform external background field. In
particular, it is shown that the pairs production and annihilation operators,
together with the pairs number operator, do actually fulfill the SU(2)
functional Lie algebra. As an example of application, the celebrated Schwinger
formula is consistently and nicely recovered, within this novel approach, for a
Dirac spinor field in the presence of a constant and homogeneous electric field
in four spacetime dimensions.Comment: 26 pages, no figure
Baryogenesis vs. proton stability in theories with extra dimensions
We propose a new scenario of baryogenesis in the context of theories with
large extra dimensions. The baryon number is almost conserved at zero
temperature by means of a localization mechanism recently analyzed by
Arkani-Hamed and Schmaltz: leptons and quarks are located at two slightly
displaced positions in the extra space, and this naturally suppresses the
interactions which ``convert'' the latter in the former. We show that this is
expected to be no longer true when finite temperature effects are taken into
account. The whole scenario is first presented in its generality, without
referring to the bulk geometry or to the specific mechanism which may generate
the baryon asymmetry. As an example, we then focus on a baryogenesis model
reminiscent of GUT baryogenesis. The Sakharov out of equilibrium condition is
satisfied by assuming nonthermal production of the bosons that induce baryon
number violation.Comment: 10 pages, 1 figur
Spin polarization of electrons by ultraintense lasers
Electrons in plasmas produced by next-generation ultraintense lasers (I>5×1022W/cm2) can be spin polarized to a high degree (10%-70%) by the laser pulses on a femtosecond time scale. This is due to electrons undergoing spin-flip transitions as they radiate γ-ray photons, preferentially spin polarizing in one direction. Spin polarization can modify the radiation reaction force on the electrons, which differs by up to 30% for opposite spin polarizations. Consequently, the polarization of the radiated γ-ray photons is also modified: the relative power radiated in the σ and π components increases and decreases by up to 30%, respectively, potentially reducing the rate of pair production in the plasma by up to 30%
What does inflation really predict?
If the inflaton potential has multiple minima, as may be expected in, e.g.,
the string theory "landscape", inflation predicts a probability distribution
for the cosmological parameters describing spatial curvature (Omega_tot), dark
energy (rho_Lambda, w, etc.), the primordial density fluctuations (Omega_tot,
dark energy (rho_Lambda, w, etc.). We compute this multivariate probability
distribution for various classes of single-field slow-roll models, exploring
its dependence on the characteristic inflationary energy scales, the shape of
the potential V and and the choice of measure underlying the calculation. We
find that unless the characteristic scale Delta-phi on which V varies happens
to be near the Planck scale, the only aspect of V that matters observationally
is the statistical distribution of its peaks and troughs. For all energy scales
and plausible measures considered, we obtain the predictions Omega_tot ~
1+-0.00001, w=-1 and rho_Lambda in the observed ballpark but uncomfortably
high. The high energy limit predicts n_s ~ 0.96, dn_s/dlnk ~ -0.0006, r ~ 0.15
and n_t ~ -0.02, consistent with observational data and indistinguishable from
eternal phi^2-inflation. The low-energy limit predicts 5 parameters but prefers
larger Q and redder n_s than observed. We discuss the coolness problem, the
smoothness problem and the pothole paradox, which severely limit the viable
class of models and measures. Our findings bode well for detecting an
inflationary gravitational wave signature with future CMB polarization
experiments, with the arguably best-motivated single-field models favoring the
detectable level r ~ 0.03. (Abridged)Comment: Replaced to match accepted JCAP version. Improved discussion,
references. 42 pages, 17 fig
Tocilizumab in Hospitalized Patients with Severe Covid-19 Pneumonia
BACKGROUND
Coronavirus disease 2019 (Covid-19) is associated with immune dysregulation and hyperinflammation, including elevated interleukin-6 levels. The use of tocilizu- mab, a monoclonal antibody against the interleukin-6 receptor, has resulted in better outcomes in patients with severe Covid-19 pneumonia in case reports and retrospective observational cohort studies. Data are needed from randomized, placebo-controlled trials.
METHODS
In this phase 3 trial, we randomly assigned patients who were hospitalized with severe Covid-19 pneumonia in a 2:1 ratio receive a single intravenous infusion of tocilizumab (at a dose of 8 mg per kilogram of body weight) or placebo. Approxi- mately one quarter of the participants received a second dose of tocilizumab or placebo 8 to 24 hours after the first dose. The primary outcome was clinical status at day 28 on an ordinal scale ranging from 1 (discharged or ready for discharge) to 7 (death) in the modified intention-to-treat population, which included all the patients who had received at least one dose of tocilizumab or placebo.
RESULTS
Of the 452 patients who underwent randomization, 438 (294 in the tocilizumab group and 144 in the placebo group) were included in the primary and secondary analyses. The median value for clinical status on the ordinal scale at day 28 was 1.0 (95% confidence interval [CI], 1.0 to 1.0) in the tocilizumab group and 2.0 (non-ICU hospitalization without supplemental oxygen) (95% CI, 1.0 to 4.0) in the placebo group (between-group difference, −1.0; 95% CI, −2.5 to 0; P=0.31 by the van Elteren test). In the safety population, serious adverse events occurred in 103 of 295 patients (34.9%) in the tocilizumab group and in 55 of 143 patients (38.5%) in the placebo group. Mortality at day 28 was 19.7% in the tocilizumab group and 19.4% in the placebo group (weighted difference, 0.3 percentage points (95% CI, –7.6 to 8.2; nominal P=0.94).
CONCLUSIONS
In this randomized trial involving hospitalized patients with severe Covid-19 pneu- monia, the use of tocilizumab did not result in significantly better clinical status or lower mortality than placebo at 28 days. (Funded by F. Hoffmann–La Roche and the Department of Health and Human Services; COVACTA ClinicalTrials.gov num- ber, NCT04320615.
CMB-S4 Science Book, First Edition
This book lays out the scientific goals to be addressed by the
next-generation ground-based cosmic microwave background experiment, CMB-S4,
envisioned to consist of dedicated telescopes at the South Pole, the high
Chilean Atacama plateau and possibly a northern hemisphere site, all equipped
with new superconducting cameras. CMB-S4 will dramatically advance cosmological
studies by crossing critical thresholds in the search for the B-mode
polarization signature of primordial gravitational waves, in the determination
of the number and masses of the neutrinos, in the search for evidence of new
light relics, in constraining the nature of dark energy, and in testing general
relativity on large scales
An Ignoble Approach to Large Field Inflation
We study an inflationary model developed by Kaloper and Sorbo, in which the
inflaton is an axion with a sub-Planckian decay constant, whose potential is
generated by mixing with a topological 4-form field strength. This gives a 4d
construction of "axion monodromy inflation": the axion winds many times over
the course of inflation and draws energy from the 4-form. The classical theory
is equivalent to chaotic inflation with a quadratic inflaton potential. Such
models can produce "high scale" inflation driven by energy densities of the
order of , which produces primordial gravitational waves
potentially accessible to CMB polarization experiments. We analyze the possible
corrections to this scenario from the standpoint of 4d effective field theory,
identifying the physics which potentially suppresses dangerous corrections to
the slow-roll potential. This yields a constraint relation between the axion
decay constant, the inflaton mass, and the 4-form charge. We show how these
models can evade the fundamental constraints which typically make high-scale
inflation difficult to realize. Specifically, the moduli coupling to the
axion-four-form sector must have masses higher than the inflationary Hubble
scale (\la\ 10^{14}\ GeV). There are also constraints from states that become
light due to multiple windings of the axion, as happens in explicit string
theory constructions of this scenario. Further, such models generally have a
quantum-mechanical "tunneling mode" in which the axion jumps between windings,
which must be suppressed. Finally, we outline possible observational
signatures.Comment: 39 pages LaTeX with bibtex, 5 .eps figure
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