52,172 research outputs found
Are there S=-2 Pentaquarks?
Recent evidence for pentaquark baryons in the channels ,
and their anti-particles claimed by the NA49 collaboration is
critically confronted with the vast amount of existing data on
spectroscopy which was accumulated over the past decades. It is shown that the
claim is at least partially inconsistent with these data. In addition two
further exotic channels of the pentaquark type available in the NA49 data are
investigated. It is argued that this study leads to internal inconsistency with
the purported signals
Improved approach to Fowler-Nordheim plot analysis
This article introduces an improved approach to Fowler-Nordheim (FN) plot
analysis, based on a new type of intercept correction factor. This factor is
more cleanly defined than the factor previously used. General enabling theory
is given that applies to any type of FN plot of data that can be fitted using a
FN-type equation. Practical use is limited to emission situations where slope
correction factors can be reliably predicted. By making a series of
well-defined assumptions and approximations, it is shown how the general
formulas reduce to provide an improved theory of orthodox FN-plot data
analysis. This applies to situations where the circuit current is fully
controlled by the emitter characteristics, and tunneling can be treated as
taking place through a Schottky-Nordheim (SN) barrier. For orthodox emission,
good working formulas make numerical evaluation of the slope correction factor
and the new intercept correction factor quick and straightforward. A numerical
illustration, using simulated emission data, shows how to use this improved
approach to derive values for parameters in the full FN-type equation for the
SN barrier. Good self-consistency is demonstrated. The general enabling
formulas also pave the way for research aimed at developing analogous
data-analysis procedures for non-orthodox emission situations.Comment: Paper is extended version of poster presented at the 25th
International Vacuum Nanoelectronics Conference, Jeju island, South Korea,
July 2012. Third version includes small changes made at proof correction
stag
Comments concerning the paper "Measurement of negatively charged pion spectra in inelastic p+p interactions at 20, 31, 40, 80 and 158 GeV/c" by the NA61 collaboration
New data from the NA61 collaboration on the production of negative pions in
p+p interactions at beam momenta between 20 and 158 GeV/c are critically
compared to available results in the same energy range. It is concluded that
the NA61 data show some discrepancies with the previous results. This concerns
in particular the total yields, the integrated rapidity distributions and
the double differential cross sections.Comment: 16 pages, 13 figure
Effects of ursodeoxycholic acid on synthesis of cholesterol and bile acids in healthy subjects
Background/Aims: Ursodeoxycholic acid ( UDCA) decreases biliary secretion of cholesterol and is therefore used for the dissolution of cholesterol gallstones. It remains unclear whether these changes in biliary cholesterol excretion are associated with changes in cholesterol synthesis and bile acid synthesis. We therefore studied the activities of rate-limiting enzymes of cholesterol synthesis and bile acid synthesis, 3-hydroxy-3-methyl-glutarylcoenzyme A reductase and cholesterol 7alpha-hydroxylase, respectively, in normal subjects during UDCA feeding. Methods: UDCA was given to 8 healthy volunteers ( 5 men, 3 women; age 24-44 years) in a single dose of 10-15 mg/kg body weight for 40 days. Before and during ( days 3, 5, 10, 20, 30 and 40) UDCA treatment, urinary excretion of mevalonic acid and serum concentrations of 7alpha-hydroxy-4-cholesten-3-one (7alpha-HCO) were determined as markers of cholesterol and bile acid synthesis, respectively. The Wilcoxon signed rank test and Spearman's rank correlation coefficient were used for statistical analysis. Results: Cholesterol synthesis and serum lipid concentrations remained unchanged during UDCA treatment for 40 days. However, synthesis of bile acids increased during long-term treatment with UDCA as reflected by an increase in 7alpha-HCO serum concentrations from 39.7 +/- 21.3 ng/ml (median 32.8 ng/ml) before treatment to 64.0 +/- 30.4 ng/ml (median 77.5 ng/ml) at days 30-40 of UDCA treatment ( p < 0.05). Conclusions: UDCA treatment does not affect cholesterol synthesis in the liver, but does increase bile acid synthesis after prolonged treatment. This may represent a compensatory change following decreased absorption of endogenous bile acids as observed with UDCA therapy
High-Precision Optical Measurement of the 2S Hyperfine Interval in Atomic Hydrogen
We have applied an optical method to the measurement of the 2S hyperfine
interval in atomic hydrogen. The interval has been measured by means of
two-photon spectroscopy of the 1S-2S transition on a hydrogen atomic beam
shielded from external magnetic fields. The measured value of the 2S hyperfine
interval is equal to 177 556 860(15) Hz and represents the most precise
measurement of this interval to date. The theoretical evaluation of the
specific combination of 1S and 2S hyperfine intervals D_21 is in moderately
good agreement with the value for D_21 deduced from our measurement
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