Search results for "loop"
showing 10 items of 608 documents
Symmetries and Effective Vertices
2015
When facing the computation of more realistic processes, the calculations can become lengthy very fast as the number of Feynman diagrams grows. Before starting the calculation process the problem should be reduced to its minimal form. Here we will present an example of how to reduce the number of calculated diagrams for a given process (which in this case will be a Higgs-like scalar decay to two photons through a charged scalar loop) using gauge symmetry.
Does one need theO(ε)- andO(ε2)-terms of one-loop amplitudes in a next-to-next-to-leading order calculation ?
2011
This article discusses the occurrence of one-loop amplitudes within a next-to-next-to-leading-order calculation. In a next-to-next-to-leading-order calculation, the one-loop amplitude enters squared and one would therefore naively expect that the $\mathcal{O}(\ensuremath{\epsilon})$- and $\mathcal{O}({\ensuremath{\epsilon}}^{2})$-terms of the one-loop amplitudes are required. I show that the calculation of these terms can be avoided if a method is known, which computes the $\mathcal{O}({\ensuremath{\epsilon}}^{0})$-terms of the finite remainder function of the two-loop amplitude.
Non-perturbative chiral approach to S-wave interactions
1998
The s-wave meson-nucleon interaction in the $S = -1$ sector is studied by means of coupled-channel Lippmann Schwinger equations, using the lowest order chiral Lagrangian and a cut off to regularize the loop integrals. The method reproduces succesfully the $\Lambda (1405)$ resonance and the $K^- p \to K^- p, \bar{K}^0 n, \pi^0 \Lambda, \pi^0 \Sigma, \pi^+ \Sigma^-, \pi^- \Sigma^+$ cross sections at low energies. The inclusion of the $\eta \Lambda, \eta \Sigma^0$ channels in the coupled system is found very important and allows a solution in terms of only the lowest order Lagrangian.
Recent progress on the calculation of three-loop heavy flavor Wilson coefficients in deep-inelastic scattering
2014
We report on our latest results in the calculation of the three-loop heavy flavor contributions to the Wilson coefficients in deep-inelastic scattering in the asymptotic region $Q^2 \gg m^2$. We discuss the different methods used to compute the required operator matrix elements and the corresponding Feynman integrals. These methods very recently allowed us to obtain a series of new operator matrix elements and Wilson coefficients like the flavor non-singlet and pure singlet Wilson coefficients.
Efficient Pole Expansion of the Generalized Impedance Matrix Representation for Planar Waveguide Junctions
2006
This paper proposes a novel pole expansion of the generalized impedance matrix representation for planar waveguide junctions. Proceeding in this way, we have obtained a very efficient algorithm for the accurate wide-band modelling of such junctions, since the most expensive computations are performed outside the frequency loop. For verification purposes, several practical examples are shown in order to prove the numerical efficiency and accuracy provided by this new technique.
One-Loop Effective Lagrangian in QED
2020
Our main goal in this section is the derivation of an expression for the effective Lagrangian in one-loop approximation. So let’s start with the vacuum persistence amplitude in presence of an external field: $$\displaystyle \langle 0_+\vert 0_-\rangle ^A = e^{ iW^{(1)}[A]} = e^{i \int d^4x\mathcal {L}^{(1)}(x)} $$
Design of a Close Power Loop Test Bench for Contra-Rotating Propellers
2020
The aim of the research is to develop an azimuthing contra-rotating propeller for commercial applications with a power of 2000 kW. The thruster system is designed especially to be installed on high speed crafts (HSCs) for passenger transport with a cruising speed of about 35–40 knots. The topic is very useful because the azimuth thruster solutions currently do not find commercial applications in naval units for passenger transport. The latter are heavy, not very efficient from a hydrodynamic point of view and suitable for maximum cruising speed of about 18–20 knots. The study is interesting because among the advantages that these solutions provide are the possibility of transmitting very hi…
Effect of coronal loop structure on wave heating through phase mixing
2020
Context. The mechanism(s) behind coronal heating still elude(s) direct observation and modelling of viable theoretical processes and the subsequent effect on coronal structures is one of the key tools available to assess possible heating mechanisms. Wave heating via the phase mixing of magnetohydrodynamic (MHD) transverse waves has been proposed as a possible way to convert magnetic energy into thermal energy, but MHD models increasingly suggest this is not an efficient enough mechanism. Aims. We modelled heating by phase mixing transverse MHD waves in various configurations in order to investigate whether certain circumstances can enhance the heating sufficiently to sustain the million deg…
MHD modeling of coronal loops: the transition region throat
2014
The expansion of coronal loops in the transition region may considerably influence the diagnostics of the plasma emission measure. The cross sectional area of the loops is expected to depend on the temperature and pressure, and might be sensitive to the heating rate. The approach here is to study the area response to slow changes in the coronal heating rate, and check the current interpretation in terms of steady heating models. We study the area response with a time-dependent 2D MHD loop model, including the description of the expanding magnetic field, coronal heating and losses by thermal conduction and radiation from optically thin plasma. We run a simulation for a loop 50 Mm long and qu…
Black hole state counting in loop quantum gravity: a number-theoretical approach
2008
4 pages, 1 figure.-- PACS nrs.: 04.70.Dy, 04.60.Pp.-- ArXiv pre-print available at: http://arxiv.org/abs/0802.4077