5,227 research outputs found
Limit Cycles and Conformal Invariance
There is a widely held belief that conformal field theories (CFTs) require
zero beta functions. Nevertheless, the work of Jack and Osborn implies that the
beta functions are not actually the quantites that decide conformality, but
until recently no such behavior had been exhibited. Our recent work has led to
the discovery of CFTs with nonzero beta functions, more precisely CFTs that
live on recurrent trajectories, e.g., limit cycles, of the beta-function vector
field. To demonstrate this we study the S function of Jack and Osborn. We use
Weyl consistency conditions to show that it vanishes at fixed points and agrees
with the generator Q of limit cycles on them. Moreover, we compute S to third
order in perturbation theory, and explicitly verify that it agrees with our
previous determinations of Q. A byproduct of our analysis is that, in
perturbation theory, unitarity and scale invariance imply conformal invariance
in four-dimensional quantum field theories. Finally, we study some properties
of these new, "cyclic" CFTs, and point out that the a-theorem still governs the
asymptotic behavior of renormalization-group flows.Comment: 31 pages, 4 figures. Expanded introduction to make clear that cycles
discussed in this work are not associated with unitary theories that are
scale but not conformally invarian
Limit Cycles in Four Dimensions
We present an example of a limit cycle, i.e., a recurrent flow-line of the
beta-function vector field, in a unitary four-dimensional gauge theory. We thus
prove that beta functions of four-dimensional gauge theories do not produce
gradient flows. The limit cycle is established in perturbation theory with a
three-loop calculation which we describe in detail.Comment: 12 pages, 1 figure. Significant revision of the interpretation of our
result. Improved description of three-loop calculatio
Scale without Conformal Invariance at Three Loops
We carry out a three-loop computation that establishes the existence of scale
without conformal invariance in dimensional regularization with the MS scheme
in d=4-epsilon spacetime dimensions. We also comment on the effects of scheme
changes in theories with many couplings, as well as in theories that live on
non-conformal scale-invariant renormalization group trajectories. Stability
properties of such trajectories are analyzed, revealing both attractive and
repulsive directions in a specific example. We explain how our results are in
accord with those of Jack & Osborn on a c-theorem in d=4 (and d=4-epsilon)
dimensions. Finally, we point out that limit cycles with turning points are
unlike limit cycles with continuous scale invariance.Comment: 21 pages, 3 figures, Erratum adde
To imitate or differentiate: Cross-level identity work in an innovation network
Survival in global high-tech industries requires many organizations to participate in specialized innovation networks. However, sustained participation in these networks often proves more challenging than expected for organizations and their representatives, due to complex cross-level identity tensions that are indiscernible when only one level of analysis is considered. The purpose of this study is to analyze cross-level identity tensions at the interface of personal and organizational identities in an innovation network. We identify three key cross-level identity tensions related to intellectual property, communication and market definition, which together contribute to an overall organizational-personal identity tension opposing differentiation and imitation. These tensions are indicative of a complex process of âpartial isomorphismâ in identity work, which can facilitate collaboration while simultaneously fostering innovation among personal and organizational network members
Exchange rate forecasting and the performance of currency portfolios. IHS Economics Series 326
We examine the potential gains of using exchange rate forecast models and forecast combination methods in the management of currency portfolios for three exchange rates, the euro (EUR) versus the US dollar (USD), the British pound (GBP) and the Japanese yen (JPY). We use a battery of econometric specifications to evaluate whether optimal currency portfolios implied by trading strategies based on exchange rate forecasts outperform single-currency and the equally weighted portfolio. We assess the differences in profitability of optimal currency portfolios for different types of investor preferences, different trading strategies, different composite forecasts and different forecast horizons.
Our results indicate that the benefits of integrating exchange rate forecasts from state-of-the-art econometric models in currency portfolios are sensitive to the trading strategy under consideration and vary strongly across prediction horizons
Exchange rate forecasting and the performance of currency portfolios
We examine the potential gains of using exchange rate forecast models and forecast combination methods in the management of currency portfolios for three exchange rates: the euro versus the US dollar, the British pound, and the Japanese yen. We use a battery of econometric specifications to evaluate whether optimal currency portfolios implied by trading strategies based on exchange rate forecasts outperform single currencies and the equally weighted portfolio. We assess the differences in profitability of optimal currency portfolios for different types of investor preferences, two trading strategies, mean squared errorâbased composite forecasts, and different forecast horizons. Our results indicate that there are clear benefits of integrating exchange rate forecasts from stateâofâtheâart econometric models in currency portfolios. These benefits vary across investor preferences and prediction horizons but are rather similar across trading strategies
Onsager phase factor of quantum oscillations in the organic metal theta-(BEDT-TTF)4CoBr4(C6H4Cl2)
De Haas-van Alphen oscillations are studied for Fermi surfaces illustrating
the Pippard's model, commonly observed in multiband organic metals. Field- and
temperature-dependent amplitude of the various Fourier components, linked to
frequency combinations arising from magnetic breakdown between different bands,
are considered. Emphasis is put on the Onsager phase factor of these
components. It is demonstrated that, in addition to the usual Maslov index,
field-dependent phase factors must be considered to precisely account for the
data at high magnetic field. We present compelling evidence of the existence of
such contributions for the organic metal theta-(BEDT-TTF)4CoBr4(C6H4Cl2)
Recent developments in the determination of the amplitude and phase of quantum oscillations for the linear chain of coupled orbits
De Haas-van Alphen oscillations are studied for Fermi surfaces (FS)
illustrating the model proposed by Pippard in the early sixties, namely the
linear chain of orbits coupled by magnetic breakdown. This FS topology is
relevant for many multiband quasi-two dimensional (q-2D) organic metals such as
-(BEDT-TTF)Cu(NCS) and
-(BEDT-TTF)CoBr(CHCl) which are considered in
detail. Whereas the Lifshits-Kosevich model only involves a first order
development of field- and temperature-dependent damping factors, second order
terms may have significant contribution on the Fourier components amplitude for
such q-2D systems at high magnetic field and low temperature. The strength of
these second order terms depends on the relative value of the involved damping
factors, which are in turns strongly dependent on parameters such as the
magnetic breakdown field, effective masses and, most of all, effective
Land\'{e} factors. In addition, the influence of field-dependent Onsager phase
factors on the oscillation spectra is considered.Comment: arXiv admin note: text overlap with arXiv:1304.665
Analytical treatment of the dHvA frequency combinations due to chemical potential oscillations in an idealized two-band Fermi liquid
de Haas-van Alphen oscillation spectrum is studied for an idealized
two-dimensional Fermi liquid with two parabolic bands in the case of canonical
(fixed number of quasiparticles) and grand canonical (fixed chemical potential)
ensembles. As already reported in the literature, oscillations of the chemical
potential in magnetic field yield frequency combinations that are forbidden in
the framework of the semiclassical theory. Exact analytical calculation of the
Fourier components is derived at zero temperature and an asymptotic expansion
is given for the high temperature and low magnetic field range. A good
agreement is obtained between analytical formulae and numerical computations.Comment: 10 pages, 4 figure
Diatomées périphytiques comme indicateurs de stress combinés liés à l'historique minier et à la pression urbaine dans le bassin versant de Junction Creek (Ontario, Canada)
International audienceSudbury (Ontario, Canada) has a long mining history that has left the region with a distinctive legacy of environmental impacts. Several actions have been undertaken since the 1970s to rehabilitate this deteriorated environment, in both terrestrial and aquatic ecosystems. Despite a marked increase in environmental health, we show that the Junction Creek system remains under multiple stressors from present and past mining operations, and from urban-related pressures such as municipal wastewater treatment plants, golf courses and stormwater runoff. Water samples have elevated metal concentrations, with values reaching up to 1 mg·
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