86 research outputs found
Conductivity of Coulomb interacting massless Dirac particles in graphene: Regularization-dependent parameters and symmetry constraints
We compute the Coulomb correction to the a. c. conductivity of
interacting massless Dirac particles in graphene in the collisionless limit
using the polarization tensor approach in a regularization independent
framework. Arbitrary parameters stemming from differences between
logarithmically divergent integrals are fixed on physical grounds exploiting
only spatial rotational invariance of the model which amounts to
transversality of the polarization tensor. Consequently is
unequivocally determined to be within this effective model. We
compare our result with explicit regularizations and discuss the origin of
others results for found in the literature
Two-loop renormalisation of gauge theories in Implicit Regularisation: transition rules to dimensional methods
We compute the two-loop -function of scalar and spinorial quantum
electrodynamics as well as pure Yang-Mills and quantum chromodynamics using the
background field method in a fully quadridimensional setup using Implicit
Regularization (IREG). Moreover, a thorough comparison with dimensional
approaches such as conventional dimensional regularization (CDR) and
dimensional reduction (DRED) is presented. Particularly, for our calculations
we show that the inclusion of evanescent -scalar particle
contributions needed in quasi-dimensional methods such as DRED and Four
Dimensional Helicity (FDH) cancel out in the determination of the ultraviolet
(UV) structure of the models we study. Subtleties related to Lorentz algebra
contractions/symmetric integrations inside divergent integrals as well as
renormalisation schemes are carefully discussed within IREG where the
renormalisation constants are fully defined as basic divergent integrals to
arbitrary loop order. Moreover we confirm the hypothesis that momentum routing
invariance in the loops of Feynman diagramas implemented via setting
well-defined surface terms to zero deliver non-abelian gauge invariant
amplitudes within IREG just as it has been proven for abelian theories
Dimensional regularization vs methods in fi xed dimension with and without y5
We study the Lorentz and Dirac algebra, including the antisymmetric e tensor
and the y
5 matrix, in implicit gauge-invariant regularization/renormalization methods
de ned in xed integer dimensions. They include constrained differential, implicit and
four-dimensional renormalization. We nd that these xed-dimension methods face the
same di culties as the different versions of dimensional regularization. We propose a consistent
procedure in these methods, similar to the consistent version of regularization by
dimensional reduction.The work of A.M.B. and M.P.V. has been supported by the Spanish MINECO project
FPA2016-78220-C3-1-P (Fondos FEDER) and the Junta de Andalucía grant FQM101. The
work of M.P.V. has also been supported by the European Commission, through the contract
PITN-GA-2012-316704 (HIGGSTOOLS). A.L.C. acknowledges nancial support from
CAPES (Coordenação de Aperfeiçoamento de Pessoal de Nível Superior), Brazil, as well
as networking support by the COST Action CA16201
Systematic Implementation of Implicit Regularization for Multi-Loop Feynman Diagrams
Implicit Regularization (IReg) is a candidate to become an invariant
framework in momentum space to perform Feynman diagram calculations to
arbitrary loop order. In this work we present a systematic implementation of
our method that automatically displays the terms to be subtracted by
Bogoliubov's recursion formula. Therefore, we achieve a twofold objective: we
show that the IReg program respects unitarity, locality and Lorentz invariance
and we show that our method is consistent since we are able to display the
divergent content of a multi-loop amplitude in a well defined set of basic
divergent integrals in one loop momentum only which is the essence of IReg.
Moreover, we conjecture that momentum routing invariance in the loops, which
has been shown to be connected with gauge symmetry, is a fundamental symmetry
of any Feynman diagram in a renormalizable quantum field theory
To , or not to : Recent developments and comparisons of regularization schemes
We give an introduction to several regularization schemes that deal with
ultraviolet and infrared singularities appearing in higher-order computations
in quantum field theories. Comparing the computation of simple quantities in
the various schemes, we point out similarities and differences between them.Comment: 61 pages, 12 figures; version sent to EPJC, references update
Vector-like Singlet Quarks: a Roadmap
We review the theory and phenomenology of isosinglet vector-like quarks
(VLQs). In recent years, interest in VLQs has been increasing, due to their
contributions to new physics effects that can be tested in experiments at LHC
and High-Luminosity LHC. The similarities of models with isosinglet VLQs and
the seesaw framework in the leptonic sector are pointed out. The existence of
VLQs leads to flavour-changing neutral currents at tree level and deviations
from unitarity of the CKM matrix, introducing rich phenomenological
implications. These new effects are naturally suppressed by the masses of the
new quarks, that are constrained to be above the electroweak scale. In
addition, striking new effects can be achieved with the inclusion of an extra
complex scalar singlet. Such a minimal extension of the SM can give rise to new
sources of CP violation with profound theoretical implications, allowing for a
solution to the strong CP problem and a possible explanation for the baryon
asymmetry of the Universe. We list and explain strong motivations to consider
this class of models. We also briefly review how models with VLQs can be
matched to the SM effective field theory (SMEFT). A detailed analysis of
flavour observables that can be affected by the presence of VLQs is presented.
Current bounds from collider searches of VLQs are summarized. We point out that
the discovery of VLQs can be within the reach of present or future colliders
being planned.Comment: 109 pages, 7 figures, 7 tables. Work intended as a review. Comments,
suggestions and missing references are welcome
Duration of temporary catheter use for hemodialysis: an observational, prospective evaluation of renal units in Brazil
<p>Abstract</p> <p>Background</p> <p>For chronic hemodialysis, the ideal permanent vascular access is the arteriovenous fistula (AVF). Temporary catheters should be reserved for acute dialysis needs. The AVF is associated with lower infection rates, better clinical results, and a higher quality of life and survival when compared to temporary catheters. In Brazil, the proportion of patients with temporary catheters for more than 3 months from the beginning of therapy is used as an evaluation of the quality of renal units. The aim of this study is to evaluate factors associated with the time between the beginning of hemodialysis with temporary catheters and the placement of the first arteriovenous fistula in Brazil.</p> <p>Methods</p> <p>This is an observational, prospective non-concurrent study using national administrative registries of all patients financed by the public health system who began renal replacement therapy (RRT) between 2000 and 2004 in Brazil. Incident patients were eligible who had hemodialysis for the first time. Patients were excluded who: had hemodialysis reportedly started after the date of death (inconsistent database); were younger than 18 years old; had HIV; had no record of the first dialysis unit; and were dialyzed in units with less than twenty patients. To evaluate individual and renal unit factors associated with the event of interest, the frailty model was used (N = 55,589).</p> <p>Results</p> <p>Among the 23,824 patients (42.9%) who underwent fistula placement in the period of the study, 18.2% maintained the temporary catheter for more than three months until the fistula creation. The analysis identified five statistically significant factors associated with longer time until first fistula: higher age (Hazard-risk - HR 0.99, 95% CI 0.99-1.00); having hypertension and cardiovascular diseases (HR 0.94, 95% CI 0.9-0.98) as the cause of chronic renal disease; residing in capitals cities (HR 0.92, 95% CI 0.9-0.95) and certain regions in Brazil - South (HR 0.83, 95% CI 0.8-0.87), Midwest (HR 0.88, 95% CI 0.83-0.94), Northeast (HR 0.91, 95% CI 0.88-0.94), or North (HR 0.88, 95% CI 0.83-0.94) and the type of renal unit (public or private).</p> <p>Conclusion</p> <p>Monitoring the provision of arteriovenous fistulas in renal units could improve the care given to patients with end stage renal disease.</p
To d , or not to d : recent developments and comparisons of regularization schemes
We give an introduction to several regularization schemes that deal with ultraviolet and infrared singularities appearing in higher-order computations in quantum field theories. Comparing the computation of simple quantities in the various schemes, we point out similarities and differences between them
A seven-year study about the negative cloud-to-ground lightning ash characteristics in Southeastern Brazil
Abstract A seven-year study of negative cloud-to-ground (CG) lightning ash characteristics in southeastern Brazil is presented. The study is based on approximately 10 million ashes recorded by a Lightning Position and tracking system lightning detection network from November 1988 to December 1995. The data set is the longest ever obtained in the tropics using an almost constant network conÿguration. It provides a unique opportunity to study the long-term annual, monthly, and local time distributions of the number, intensity (peak current) and multiplicity of negative CG ashes in the tropics. The annual distribution of the number of ashes has variations as large as 80%. The variations does not show any clear relationship with any meteorological parameter, possibly indicating the complex interactions of di erent processes responsible for the lightning activity. The monthly and local time distributions seem to follow closely the related distributions of air temperature. The annual distribution of peak current shows an average value of 40:4 kA and has a signiÿcant decrease from 1991 to 1994, apparently related to an El Niño seasonal e ect. The monthly distribution of peak current shows lower values in the winter, in contrast with the results recently reported for the United States, and seems to be related to the monthly distribution of the number of ashes. The local time distribution of the peak current seems also to be related to the local time distribution of the number of ashes; however, the dependency is less evident. The annual distribution of multiplicity has a systematic decrease during the period, with no apparent relationship with any of the variables studied. The monthly distribution of multiplicity is consistent with the hypothesis that the multiplicity is related to the average height of thunderstorms. The local time distribution of multiplicity, in turn, shows a sunrise/sunset e ect. The results are compared to similar ones obtained in other regions of the world
The interface between health sector reform and human resources in health
The relationship between health sector reform and the human resources issues raised in that process has been highlighted in several studies. These studies have focused on how the new processes have modified the ways in which health workers interact with their workplace, but few of them have paid enough attention to the ways in which the workers have influenced the reforms. The impact of health sector reform has modified critical aspects of the health workforce, including labor conditions, degree of decentralization of management, required skills and the entire system of wages and incentives. Human resources in health, crucial as they are in implementing changes in the delivery system, have had their voice heard in many subtle and open ways – reacting to transformations, supporting, blocking and distorting the proposed ways of action. This work intends to review the evidence on how the individual or collective actions of human resources are shaping the reforms, by spotlighting the reform process, the workforce reactions and the factors determining successful human resources participation. It attempts to provide a more powerful way of predicting the effects and interactions in which different "technical designs" operate when they interact with the human resources they affect. The article describes the dialectic nature of the relationship between the objectives and strategies of the reforms and the objectives and strategies of those who must implement them
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