376 research outputs found
Numerical simulations of a field emitter-based extractor gauge for pressure measurements in cryogenic vacuum systems
Development of a field emitter-based extractor gauge for pressure measurement in cryogenic vacuum systems
Observation of tunable single-atom Yu-Shiba-Rusinov states
The coupling of a spin to an underlying substrate is the basis for a plethora
of phenomena. In the case of a metallic substrate, Kondo screening of the
adatom magnetic moment can occur. As the substrate turns superconducting, an
intriguing situation emerges where the pair breaking due to the adatom spins
leads to Yu-Shiba-Rusinov bound states, but also intertwines with Kondo
phenomena. Through scanning tunneling spectroscopy, we analyze the
interdependence of Kondo screening and superconductivity. Our data obtained on
single Fe adatoms on Nb(110) show that the coupling and the resulting YSR
states are strongly adsorption site-dependent and reveal a quantum phase
transition at a Kondo temperature comparable to the superconducting gap. The
experimental signatures are rationalized by combined density functional theory
and continuous-time quantum Monte-Carlo calculations to rigorously treat
magnetic and hybridization effects on equal footing.Comment: 6 pages, 4 figure
Exploring the use of underground gravity monitoring to evaluate radar estimates of heavy rainfall
The radar-based estimation of intense
precipitation produced by convective storms is a challenging task and the
verification through comparison with gauges is questionable due to the very
high spatial variability of such types of precipitation. In this study, we explore the potential
benefit of using a superconducting gravimeter as a new source of in situ
observations for the evaluation of radar-based precipitation estimates. The
superconducting gravimeter used in this study is installed in Membach (BE),
48 m underneath the surface, at 85 km distance from a C-band weather radar
located in Wideumont (BE). The 15-year observation record 2003–2017 is
available for both gravimeter and radar with 1 and 5 min time steps,
respectively. Water mass increase at ground due to precipitation results in a
decrease in underground measured gravity. The gravimeter integrates soil
water in a radius of about 400 m around the instrument. This allows capture
of rainfall at a larger spatial scale than traditional rain gauges. The
precision of the gravimeter is a few tenths of nm s−2, 1 nm s−2
corresponding to 2.6 mm of water. The comparison of reflectivity and gravity
time series shows that short-duration intense rainfall events produce a rapid
decrease in the underground measured gravity. A remarkable correspondence
between radar and gravimeter time series is found. The precipitation amounts
derived from gravity measurements and from radar observations are further
compared for 505 rainfall events. A correlation coefficient of 0.58, a mean
bias (radar–gravimeter)/gravimeter of 0.24 and a mean absolute difference
(MAD) of 3.19 mm are obtained. A better agreement is reached when applying a
hail correction by truncating reflectivity values to a given threshold. No
bias, a correlation coefficient of 0.64 and a MAD of 2.3 mm are reached
using a 48 dBZ threshold. The added value of underground gravity
measurements as a verification dataset is discussed. The two main benefits
are the spatial scale at which precipitation is captured and the interesting
property that gravity measurements are directly influenced by water mass at
ground no matter the type of precipitation: hail or rain.</p
Design of the HIV Prevention Trials Network (HPTN) Protocol 054: A cluster randomized crossover trial to evaluate combined access to Nevirapine in developing countries
HPTN054 is a cluster randomized trial designed to compare two approaches to providing single dose nevirapine to HIV-seropositive mothers and their infants to prevent mother-to-child transmission of HIV in resource limited settings. A number of challenging issues arose during the design of this trial. Most importantly, the need to achieve high participation rates among pregnant, HIV-seropositive women in selected prenatal care clinics led us to develop a method of collecting anonymous and unlinked information on a key surrogate endpoint instead of pursuing linked and identified information on a clinical endpoint. In addition, since group counseling is the standard model for prenatal care in sub-Saharan Africa, the prenatal care clinic serves as the unit of randomization. However, constraints on the number of suitable clinics and other logistical difficulties necessitated a unique type of hybrid parallel/stepped wedge cluster randomized design in which some clinics cross over between the two treatment modalities and some do not. We describe the design for the HPTN054 trial with an emphasis on the logistic and statistical features that allowed us to address these issues. We also provide some general statistical results that are useful for computing power in parallel, crossover, stepped wedge or mixed designs of cluster randomized trials
Collins and Sivers asymmetries in muonproduction of pions and kaons off transversely polarised proton
Measurements of the Collins and Sivers asymmetries for charged pions and
charged and neutral kaons produced in semi-inclusive deep-inelastic scattering
of high energy muons off transversely polarised protons are presented. The
results were obtained using all the available COMPASS proton data, which were
taken in the years 2007 and 2010. The Collins asymmetries exhibit in the
valence region a non-zero signal for pions and there are hints of non-zero
signal also for kaons. The Sivers asymmetries are found to be positive for
positive pions and kaons and compatible with zero otherwise.Comment: 15 pages, 13 figures and 1 tabl
Measurement of the charged-pion polarisability
The COMPASS collaboration at CERN has investigated pion Compton scattering,
, at centre-of-mass energy below 3.5 pion
masses. The process is embedded in the reaction
, which is initiated by
190\,GeV pions impinging on a nickel target. The exchange of quasi-real photons
is selected by isolating the sharp Coulomb peak observed at smallest momentum
transfers, \,(GeV/). From a sample of 63\,000 events the
pion electric polarisability is determined to be $\alpha_\pi\ =\ (\,2.0\ \pm\
0.6_{\mbox{\scriptsize stat}}\ \pm\ 0.7_{\mbox{\scriptsize syst}}\,) \times
10^{-4}\,\mbox{fm}^3\alpha_\pi=-\beta_\pi$, which
relates the electric and magnetic dipole polarisabilities. It is the most
precise measurement of this fundamental low-energy parameter of strong
interaction, that has been addressed since long by various methods with
conflicting outcomes. While this result is in tension with previous dedicated
measurements, it is found in agreement with the expectation from chiral
perturbation theory. An additional measurement replacing pions by muons, for
which the cross-section behavior is unambigiously known, was performed for an
independent estimate of the systematic uncertainty.Comment: Published version: 9 pages, 3 figures, 1 tabl
Search for exclusive photoproduction of Z(3900) at COMPASS
A search for the exclusive production of the hadron by
virtual photons has been performed in the channel . The data cover the range from 7 GeV to 19 GeV in the
centre-of-mass energy of the photon-nucleon system. The full set of the COMPASS
data set collected with a muon beam between 2002 and 2011 has been used. An
upper limit for the ratio of has been established at the confidence
level of 90%.Comment: 8 pages, 3 figures, 1 tabl
Spin alignment and violation of the OZI rule in exclusive and production in pp collisions
Exclusive production of the isoscalar vector mesons and is
measured with a 190 GeV proton beam impinging on a liquid hydrogen target.
Cross section ratios are determined in three intervals of the Feynman variable
of the fast proton. A significant violation of the OZI rule is found,
confirming earlier findings. Its kinematic dependence on and on the
invariant mass of the system formed by fast proton
and vector meson is discussed in terms of diffractive
production of resonances in competition with central
production. The measurement of the spin density matrix element of
the vector mesons in different selected reference frames provides another
handle to distinguish the contributions of these two major reaction types.
Again, dependences of the alignment on and on are
found. Most of the observations can be traced back to the existence of several
excited baryon states contributing to production which are absent in
the case of the meson. Removing the low-mass resonant
region, the OZI rule is found to be violated by a factor of eight,
independently of .Comment: 23 pages, 13 figures and 5 table
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