7,583 research outputs found
Model-independent constraints on the shape parameters of dilepton angular distributions
The coefficients determining the dilepton decay angular distribution of
vector particles obey certain positivity constraints and a rotation-invariant
identity. These relations are a direct consequence of the covariance properties
of angular momentum eigenstates and are independent of the production
mechanism. The Lam-Tung relation can be derived as a particular case, simply
recognizing that the Drell-Yan dilepton is always produced transversely
polarized with respect to one or more quantization axes. The dilepton angular
distribution continues to be characterized by a frame-independent identity also
when the Lam-Tung relation is violated. Moreover, the violation can be easily
characterized by measuring a one-dimensional distribution depending on one
shape coefficient.Comment: 7 page
Pressure tuning of light-induced superconductivity in K3C60
Optical excitation at terahertz frequencies has emerged as an effective means
to manipulate complex solids dynamically. In the molecular solid K3C60,
coherent excitation of intramolecular vibrations was shown to transform the
high temperature metal into a non-equilibrium state with the optical
conductivity of a superconductor. Here we tune this effect with hydrostatic
pressure, and we find it to disappear around 0.3 GPa. Reduction with pressure
underscores the similarity with the equilibrium superconducting phase of K3C60,
in which a larger electronic bandwidth is detrimental for pairing. Crucially,
our observation excludes alternative interpretations based on a high-mobility
metallic phase. The pressure dependence also suggests that transient, incipient
superconductivity occurs far above the 150 K hypothesised previously, and
rather extends all the way to room temperature.Comment: 33 pages, 17 figures, 2 table
Line tensions, correlation lengths, and critical exponents in lipid membranes near critical points
Membranes containing a wide variety of ternary mixtures of high chain-melting
temperature lipids, low chain-melting temperature lipids, and cholesterol
undergo lateral phase separartion into coexisting liquid phases at a
miscibility transition. When membranes are prepared from a ternary lipid
mixture at a critical composition, they pass through a miscibility critical
point at the transition temperature. Since the critical temperature is
typically on the order of room temperature, membranes provide an unusual
opportunity in which to perform a quantitative study of biophysical systems
that exhibit critical phenomena in the two-dimensional Ising universality
class. As a critical point is approached from either high or low temperature,
the scale of fluctuations in lipid composition, set by the correlation length,
diverges. In addition, as a critical point is approached from low temperature,
the line tension between coexisting phases decreases to zero. Here we
quantitatively evaluate the temperature dependence of line tension between
liquid domains and of fluctuation correlation lengths in lipid membranes in
order to extract a critical exponent, nu. We obtain nu=1.2 plus or minus 0.2,
consistent with the Ising model prediction nu=1. We also evaluate the
probability distributions of pixel intensities in fluoresence images of
membranes. From the temperature dependence of these distributions above the
critical temperature, we extract an independent critical exponent beta=0.124
plus or minus 0.03 which is consistent with the Ising prediction of beta=1/8.Comment: 22 pages, 7 figure
Sub-story detection in Twitter with hierarchical Dirichlet processes
Social media has now become the de facto information source on real world events. The challenge, however, due to the high volume and velocity nature of social media streams, is in how to follow all posts pertaining to a given event over time – a task referred to as story detection. Moreover, there are often several different stories pertaining to a given event, which we refer to as sub-stories and the corresponding task of their automatic detection – as sub-story detection. This paper proposes hierarchical Dirichlet processes (HDP), a probabilistic topic model, as an effective method for automatic sub-story detection. HDP can learn sub-topics associated with sub-stories which enables it to handle subtle variations in sub-stories. It is compared with state-of-the-art story detection approaches based on locality sensitive hashing and spectral clustering. We demonstrate the superior performance of HDP for sub-story detection on real world Twitter data sets using various evaluation measures. The ability of HDP to learn sub-topics helps it to recall the sub-stories with high precision. This has resulted in an improvement of up to 60% in the F-score performance of HDP based sub-story detection approach compared to standard story detection approaches. A similar performance improvement is also seen using an information theoretic evaluation measure proposed for the sub-story detection task. Another contribution of this paper is in demonstrating that considering the conversational structures within the Twitter stream can bring up to 200% improvement in sub-story detection performance
Quantum Effects in Friedmann-Robertson-Walker Cosmologies
Electrodynamics for self-interacting scalar fields in spatially flat
Friedmann-Robertson-Walker space-times is studied. The corresponding one-loop
field equation for the expectation value of the complex scalar field in the
conformal vacuum is derived. For exponentially expanding universes, the
equations for the Bogoliubov coefficients describing the coupling of the scalar
field to gravity are solved numerically. They yield a non-local correction to
the Coleman-Weinberg effective potential which does not modify the pattern of
minima found in static de Sitter space. Such a correction contains a
dissipative term which, accounting for the decay of the classical configuration
in scalar field quanta, may be relevant for the reheating stage. The physical
meaning of the non-local term in the semiclassical field equation is
investigated by evaluating this contribution for various background field
configurations.Comment: 17 pages, plain TeX + 5 uuencoded figure
Fermat hypersurfaces and Subcanonical curves
We extend the classical Enriques-Petri Theorem to -subcanonical
projectively normal curves, proving that such a curve is -gonal if and
only if it is contained in a surface of minimal degree. Moreover, we show that
any Fermat hypersurface of degree is apolar to an -subcanonical
-gonal projectively normal curve, and vice versa.Comment: 18 pages; AMS-LaTe
Floristic and coenological data from the travertine substrates of the SAC “Travertini Acque Albule (Bagni di Tivoli)” (Lazio Region – Central Italy)
During a phytosociological field-work campaign on the vegetation of the travertine outcrops, included in the Special Area of Conservation (SAC) “Travertini Acque Albule (Bagni di Tivoli)” (Central Italy), several taxa of particular interest were identified. Carex vulpina, Lolium apenninum, Onosma echioides subsp. angustifolia, Typha domingensis, T. laxmannii and Vicia pannonica subsp. pannonica are new for the Lazio administrative Region, while Ophrys illyrica and Zannichellia peltata are confirmed for the flora of this Region. For each of these taxa phytosociological samples describing the plant communities in which they were found are provided. New records for rare species were also reported for cryptogams such as Algae, Lichens and Mosses
An Optimal Decision Procedure for MPNL over the Integers
Interval temporal logics provide a natural framework for qualitative and
quantitative temporal reason- ing over interval structures, where the truth of
formulae is defined over intervals rather than points. In this paper, we study
the complexity of the satisfiability problem for Metric Propositional Neigh-
borhood Logic (MPNL). MPNL features two modalities to access intervals "to the
left" and "to the right" of the current one, respectively, plus an infinite set
of length constraints. MPNL, interpreted over the naturals, has been recently
shown to be decidable by a doubly exponential procedure. We improve such a
result by proving that MPNL is actually EXPSPACE-complete (even when length
constraints are encoded in binary), when interpreted over finite structures,
the naturals, and the in- tegers, by developing an EXPSPACE decision procedure
for MPNL over the integers, which can be easily tailored to finite linear
orders and the naturals (EXPSPACE-hardness was already known).Comment: In Proceedings GandALF 2011, arXiv:1106.081
Qualitative Properties of the Dirac Equation in a Central Potential
The Dirac equation for a massive spin-1/2 field in a central potential V in
three dimensions is studied without fixing a priori the functional form of V.
The second-order equations for the radial parts of the spinor wave function are
shown to involve a squared Dirac operator for the free case, whose essential
self-adjointness is proved by using the Weyl limit point-limit circle
criterion, and a `perturbation' resulting from the potential. One then finds
that a potential of Coulomb type in the Dirac equation leads to a potential
term in the above second-order equations which is not even infinitesimally
form-bounded with respect to the free operator. Moreover, the conditions
ensuring essential self-adjointness of the second-order operators in the
interacting case are changed with respect to the free case, i.e. they are
expressed by a majorization involving the parameter in the Coulomb potential
and the angular momentum quantum number. The same methods are applied to the
analysis of coupled eigenvalue equations when the anomalous magnetic moment of
the electron is not neglected.Comment: 22 pages, plain Tex. In the final version, a section has been added,
and the presentation has been improve
Pancreatic cancer-associated diabetes mellitus: an open field for proteomic applications.
Background: Diabetes mellitus is associated with pancreatic cancer in more than 80% of the cases. Clinical, epidemiological, and experimental data indicate that pancreatic cancer causes diabetes mellitus by releasing soluble mediators which interfere with both beta-cell function and liver and muscle glucose metabolism. Methods: We analysed, by matrix-assisted laser desorption ionization time of flight (MALDI-TOF), a series of pancreatic cancer cell lines conditioned media, pancreatic cancer patients' peripheral and portal sera, comparing them with controls and chronic pancreatitis patients' sera. Results: MALDI-TOF analysis of pancreatic cancer cells conditioned media and patients' sera indicated a low molecular weight peptide to be the putative pancreatic cancer-associated diabetogenic factor. The sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) analysis of tumor samples from diabetic and non-diabetic patients revealed the presence of a 1500 Da peptide only in diabetic patients. The amino acid sequence of this peptide corresponded to the N-terminal of an S-100 calcium binding protein, which was therefore suggested to be the pancreatic cancer-associated diabetogenic factor. Conclusions: We identified a tumor-derived peptide of 14 amino acids sharing a 100% homology with an S-100 calcium binding protein, which is probably the pancreatic cancer-associated diabetogenic facto
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