24,666 research outputs found
Electron - positron cascades in multiple-laser optical traps
We present an analytical and numerical study of multiple-laser QED cascades
induced with linearly polarised laser pulses. We analyse different polarisation
orientations and propose a configuration that maximises the cascade
multiplicity and favours the laser absorption. We generalise the analytical
estimate for the cascade growth rate previously calculated in the field of two
colliding linearly polarised laser pulses and account for multiple laser
interaction. The estimate is verified by a comprehensive numerical study of
four-laser QED cascades across a range of different laser intensities with QED
PIC module of OSIRIS. We show that by using four linearly polarised 30 fs laser
pulses, one can convert more than 50 % of the total energy to gamma-rays
already at laser intensity . In this
configuration, the laser conversion efficiency is higher compared with the case
with two colliding lasers
On the integrated behaviour of non-stationary volatility in stock markets
This paper analyses the behaviour of volatility for several international
stock market indexes, namely the SP 500 (USA), the Nikkei (Japan), the PSI 20
(Portugal), the CAC 40 (France), the DAX 30 (Germany), the FTSE 100 (UK), the
IBEX 35 (Spain) and the MIB 30 (Italy), in the context of non-stationarity. Our
empirical results point to the evidence of the existence of integrated
behaviour among several of those stock market indexes of different dimensions.
It seems, therefore, that the behaviour of these markets tends to some
uniformity, which can be interpreted as the existence of a similar behaviour
facing to shocks that may affect the worldwide economy. Whether this is a cause
or a consequence of market globalization is an issue that may be stressed in
future work.Comment: 10 pages, 3 figures. Paper presented in the APFA 5 conferenc
Quantum Electrodynamics vacuum polarization solver
The self-consistent modeling of vacuum polarization due to virtual
electron-positron fluctuations is of relevance for many near term experiments
associated with high intensity radiation sources and represents a milestone in
describing scenarios of extreme energy density. We present a generalized
finite-difference time-domain solver that can incorporate the modifications to
Maxwell's equations due to vacuum polarization. Our multidimensional solver
reproduced in one dimensional configurations the results for which an analytic
treatment is possible, yielding vacuum harmonic generation and birefringence.
The solver has also been tested for two-dimensional scenarios where finite
laser beam spot sizes must be taken into account. We employ this solver to
explore different types of counter-propagating configurations that can be
relevant for future planned experiments aiming to detect quantum vacuum
dynamics at ultra-high electromagnetic field intensities
Improved tests for forecast comparisons in the presence of instabilities
Of interest is comparing the out-of-sample forecasting performance of two competing models in the presence of possible instabilities. To that effect, we suggest using simple structural change tests, sup-Wald and UDmax for changes in the mean of the loss differences. It is shown that Giacomini and Rossi (2010) tests have undesirable power properties, power that can be low and non-increasing as the alternative becomes further from the null hypothesis. On the contrary, our statistics are shown to have higher monotonic power, especially the UDmax version. We use their empirical examples to show the practical relevance of the issues raised
Electron trapping and acceleration by the plasma wakefield of a self-modulating proton beam
It is shown that co-linear injection of electrons or positrons into the
wakefield of the self-modulating particle beam is possible and ensures high
energy gain. The witness beam must co-propagate with the tail part of the
driver, since the plasma wave phase velocity there can exceed the light
velocity, which is necessary for efficient acceleration. If the witness beam is
many wakefield periods long, then the trapped charge is limited by beam loading
effects. The initial trapping is better for positrons, but at the acceleration
stage a considerable fraction of positrons is lost from the wave. For efficient
trapping of electrons, the plasma boundary must be sharp, with the density
transition region shorter than several centimeters. Positrons are not
susceptible to the initial plasma density gradient.Comment: 9 pages, 9 figures, 1 table, 44 reference
An econophysics approach to analyse uncertainty in financial markets: an application to the Portuguese stock market
In recent years there has been a closer interrelationship between several
scientific areas trying to obtain a more realistic and rich explanation of the
natural and social phenomena. Among these it should be emphasized the
increasing interrelationship between physics and financial theory. In this
field the analysis of uncertainty, which is crucial in financial analysis, can
be made using measures of physics statistics and information theory, namely the
Shannon entropy. One advantage of this approach is that the entropy is a more
general measure than the variance, since it accounts for higher order moments
of a probability distribution function. An empirical application was made using
data collected from the Portuguese Stock Market.Comment: 8 pages, 2 figures, presented in the conference Next Sigma-Phi 200
Buffer or amplifier? Longitudinal effects of social support for functional autonomy/dependence on older adults’ chronic-pain experiences
Objective: This longitudinal study aimed to investigate (a) the moderating role of formal social support for functional autonomy versus dependence on the relationship between pain intensity and pain-related disability among older adults with chronic pain and (b) the mediating role of pain-related self-efficacy and pain-related fear in this moderation. Method: One hundred and seventy older adults (Mage = 78.0; SD = 8.7) with chronic musculoskeletal pain participated in a 3-month prospective study, with 3 measurement moments. Participants filled out the Formal Social Support for Autonomy and Dependence in Pain Inventory, the Portuguese versions of the Brief Pain Inventory, the Pain Self-Efficacy Questionnaire, and the Tampa Scale of Kinesiophobia. Results: Using structural equation modeling, it was found that perceived promotion of autonomy, at Time 1, moderated the relationship between pain intensity (T1) and pain-related disability (T2); this moderation was fully mediated by pain-related self-efficacy (T2). Perceived promotion of dependence was not a significant moderator. Conclusions: These findings highlight the importance of social support for functional autonomy in buffering the impact of pain intensity on older adults’ pain-related disability. Also, they clarify the role of pain-related self-efficacy in this effect. Implications for the development of intervention programs, with formal caregivers, to reduce the impact of chronic pain on older adults’ healthy ageing process, are discussed.info:eu-repo/semantics/acceptedVersio
Spanish validation of the Shorter Version of the Workplace Incivility Scale: An employment status invariant measure
Workplace Incivility (WI) occurs worldwide and has negative consequences on individuals and organizations. Valid and comprehensive instruments have been used, specifically in English speaking countries, to measure sucadverse process at work, but it is not available a validated instrument for research carried out in Spanish speaking countries. In this study we aim to test the psychometric properties of the Matthews and Ritter’s four-item Workplace Incivility Scale (2016) with Spanish workers (N = 407) from different sectors. Participants’ mean age was 38.73 (SD = 10.45) years old and the percentage of female employees was 59.2%. Confirmatory factor analysis using AMOS 19.0 was carried out, presenting a good fit. The internal consistency, convergent and concurrent validity of the scale were examined. Results show good scale reliability and expected high correlations with social undermining. Moreover, the scale related to propensity to leave a job, job satisfaction, and psychological well-being in the expected way. After configural invariance across groups was established, testing for metric invariance and scalar invariance was performed. Considering Δχ2 and ΔCFI tests for two nested models, the 4-item scale was invariant when the employment status is considered (permanent vs. temporal, full-time vs. part-time, and supervisor vs. non-supervisors). Overall, our findings showed good psychometric properties of the shorter version of the WIS in Spain. Theoretical and practical implications of this study are discussed.Ministerio de Economía y Competitividad de España PSI2015-64894-PFundação para a Ciência e a Tecnologia (FCT) Portugal SFRH/BPD/121748/201
Towards entrepreneurial universities : barriers, facilitators, and best practices in Bulgarian and Portuguese universities
Purpose: Тhe main objective of this research is to investigate barriers, facilitators and best practices in the transformation of Bulgarian and Portuguese universities into entrepreneurial universities. Design/Methodology/Approach: This study relies on a survey among experts in the field to identify barriers, facilitators and best practices in the transformation of Bulgarian and Portuguese universities into entrepreneurial universities. Findings: The research findings demonstrate that there are both internal and external barriers and facilitators of the entrepreneurial transformation of universities in Bulgaria and Portugal and reveals the relative importance of the various internal and external factors. The study describes several best practices in the transformation towards an entrepreneurial university adopted at Sofia University “St. Kliment Ohridski” and ISCTE-IUL. Practical Implications: This research raises awareness of the internal challenges to making Bulgarian and Portuguese universities more entrepreneurial as well as of factors that may facilitate the process of transformation and the need to strengthen entrepreneurship ecosystem at these universities. Policy makers should devote special attention to external barriers to the transformation and especially to the need for more appropriate legal framework and more state funding. Originality/Value: The study highlights that the relative importance of the various internal and external factors is context specific.peer-reviewe
Long Memory and Volatility Clustering: is the empirical evidence consistent across stock markets?
Long memory and volatility clustering are two stylized facts frequently
related to financial markets. Traditionally, these phenomena have been studied
based on conditionally heteroscedastic models like ARCH, GARCH, IGARCH and
FIGARCH, inter alia. One advantage of these models is their ability to capture
nonlinear dynamics. Another interesting manner to study the volatility
phenomena is by using measures based on the concept of entropy. In this paper
we investigate the long memory and volatility clustering for the SP 500, NASDAQ
100 and Stoxx 50 indexes in order to compare the US and European Markets.
Additionally, we compare the results from conditionally heteroscedastic models
with those from the entropy measures. In the latter, we examine Shannon
entropy, Renyi entropy and Tsallis entropy. The results corroborate the
previous evidence of nonlinear dynamics in the time series considered.Comment: 8 pages; 2 figures; paper presented in APFA 6 conferenc
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