41,927 research outputs found
New contributions to heavy quark sum rules
We analyse new contributions to the theoretical input in heavy quark sum
rules and we show that the general theory of singularities of perturbation
theory amplitudes yields the method to handle these specific features. In
particular we study the inclusion of heavy quark radiation by light quarks at
O(alpha_s^2) and non-symmetric correlators at O(alpha_s^3). Closely related, we
also propose a solution to the construction of moments of the spectral
densities at O(alpha_s^3) where the presence of massless contributions
invalidates the standard approach. We circumvent this problem through a new
definition of the moments, providing an infrared safe and consistent procedure.Comment: 15 pages, 3 figures. Version accepted for publication in The European
Journal of Physics C; several new comments and references added, conclusions
unchange
The Schur-Horn theorem for operators and frames with prescribed norms and frame operator
Let be a Hilbert space. Given a bounded positive definite
operator on , and a bounded sequence of non negative real numbers, the pair is
frame admissible, if there exists a frame on
with frame operator , such that , . We relate the existence of such frames with the Schur-Horn
theorem of majorization, and give a reformulation of the extended version of
Schur-Horn theorem, due to A. Neumann. We use it to get necessary conditions
(and to generalize known sufficient conditions) for a pair ,
to be frame admissible.Comment: To appear in Illinois Journal of Mat
Comparing univariate and multivariate models to forecast portfolio value-at-risk
This article addresses the problem of forecasting portfolio value-at-risk (VaR) with multivariate GARCH models vis-Ã -vis univariate models. Existing literature has tried to answer this question by analyzing only small portfolios and using a testing framework not appropriate for ranking VaR models. In this work we provide a more comprehensive look at the problem of portfolio VaR forecasting by using more appropriate statistical tests of comparative predictive ability. Moreover, we compare univariate vs. multivariate VaR models in the context of diversified portfolios containing a large number of assets and also provide evidence based on Monte Carlo experiments. We conclude that, if the sample size is moderately large, multivariate models outperform univariate counterparts on an out-of-sample basis.Market risk, Backtesting, Conditional predictive ability, GARCH, Volatility, Capital requirements, Basel II
Electronic Raman Scattering in Twistronic Few-Layer Graphene
We study electronic contribution to the Raman scattering signals of two-,
three- and four-layer graphene with layers at one of the interfaces twisted by
a small angle with respect to each other. We find that the Raman spectra of
these systems feature two peaks produced by van Hove singularities in moir\'{e}
minibands of twistronic graphene, one related to direct hybridization of Dirac
states, and the other resulting from band folding caused by moir\'{e}
superlattice. The positions of both peaks strongly depend on the twist angle,
so that their detection can be used for non-invasive characterization of the
twist, even in hBN-encapsulated structures.Comment: 7 pages (including 4 figures) + 10 pages (3 figures) supplemen
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