arXiv subjects
Publications and source records attributed to Sergio Szpigel.
The Solar Submillimeter Telescope (SST) is an unique instrument that has been observing the Sun daily since 2001 bringing a wealth of information and raising new questions about the particle acceleration and transport, and emission mechanisms during flares. We are now designing its successor, the SSTng, that will expand the scientific goals of the instrument, including non-solar source observations.
We use the Monte Carlo particle physics code FLUKA (Fluktuierende Kaskade) to calculate $γ$-ray spectra expected from solar flare energetic ion distributions. The FLUKA code includes robust physics-based models for electromagnetic, hadronic and nuclear interactions, sufficiently detailed for it to be a useful tool for calculating nuclear de-excitation, positron annihilation and neutron capture line fluxes and shapes, as well as $\approx \; {\rm GeV}$ continuum radiation from pion decay products. We show nuclear de-excitation $γ$-ray line model spectra from a range of assumed primary accelerated ion distributions and find them to be in good agreement with those found using the code of Murphy et al. (2009). We also show full $γ$-ray model spectra which exhibit all the typical structures of $γ$-ray spectra observed in solar flares. From these model spectra we build templates which are incorporated into the software package Objective Spectral Executive (OSPEX) and used to fit the combined Fermi Gamma-ray Burst Monitor (GBM)/Large Area Telescope (LAT) spectrum of the 2010 June 12 solar flare, providing a statistically acceptable result. To the best of our knowledge, the fit carried out with the FLUKA templates for the full $γ$-ray spectrum can be regarded as the first attempt to use a single code to implement a self-consistent treatment of the several spectral components in the photon energy range from $\approx 100$s ${\rm keV}$ to $\approx 100$s ${\rm MeV}$.
Effective interactions have been used to compute the pairing gap for nuclear and neutron matter in several schemes. In this work we analyze the impact of phase-shift equivalent interactions within the BCS theory on the $^1S_0$-channel pairing gap for a translational invariant many-fermion system such as nuclear and neutron matter. We solve the BCS pairing gap equation on a finite momentum grid for a toy model separable Gaussian potential in the $^1S_0$-channel explicitly evolved through the Similarity Renormalization Group (SRG) transformation and show that in the on-shell and continuum limits the pairing gap vanishes. For finite size systems the momentum is quantized and the on-shell limit is realized for SRG cutoffs comparable to the momentum resolution. In this case the pairing gap can be computed directly from the scattering phase-shifts by an energy-shift formula. While the momentum grid is usually used as an auxiliary way of solving the BCS pairing gap equation, we show that it actually encodes some relevant physical information, suggesting that in fact finite grids may represent the finite size of the system.
We compute the Bertsch parameter for neutron matter by using nucleon-nucleon interactions that are fully diagonal in momentum space. We analyze the on-shell limit with the similarity renormalization group and compare the results for a simple separable toy model to realistic calculations with high precision $NN$ potentials.
We analyze quantitatively the interplay between explicit and implicit renormalization in Nuclear Physics. By explicit renormalization we mean to integrate out higher energy modes below a given cutoff scale using the similarity renormalization group (SRG) with a block-diagonal evolution generator, which separates the total Hilbert-space into a model space and its complementary. In the implicit renormalization we impose given conditions at low energies for a cutoff theory. In both cases we compare the outcoming effective interactions as functions of the cutoff scale. We carry out a comprehensive analysis of a toy-model which captures the main features of the nucleon-nucleon (NN) S-wave interaction at low energies. We find a wide energy region where both approaches overlap. This amounts to a great simplification in the determination of the effective interaction. Actually, the outcoming scales are within the expected ones relevant for the physics of light nuclei.
We apply five subtractions in the Lippman-Schwinger (LS) equation in order to perform a non-perturbative renormalization of chiral N3LO nucleon-nucleon interactions. Here we compute the phase shifts for the uncoupled peripheral waves at renormalization scales between $0.1~ \rm{fm}^{-1}$ and $1 ~ \rm{fm}^{-1}$. In this range, the results are scale invariant and provide an overall goof agreement with the Nijmegen partial wave analysis up to at least $E_{\rm{lab}} = 150 ~ \rm{MeV}$, with a cutoff at $Λ= 30~\rm{fm}^{-1}$.
We use an S-wave toy model for the two-nucleon system to show that the implicit renormalization of a contact theory matches the explicit renormalization through a flow equation which integrates out the high momentum components. By fitting the low-momentum interaction with a new contact theory, we show that the running of the contact strengths in both original and fitted contact theories match over a wide cutoff range.
We analyze the role played by Long Distance Symmetries within the context of the Similarity Renormalization Group (SRG) approach, which is based on phase-shift preserving continuous unitary transformations that evolve hamiltonians with a cutoff on energy differences. We find that there is a SRG cutoff for which almost perfect fulfillment of Wigner symmetry is found. We discuss the possible consequences of such finding.
Methods based on Wilson's renormalization group have been successfully applied in the context of nuclear physics to analyze the scale dependence of effective nucleon-nucleon ($NN$) potentials, as well as to consistently integrate out the high-momentum components of phenomenological high-precision $NN$ potentials in order to derive phase-shift equivalent softer forms, the so called $V_{low-k}$ potentials. An alternative renormalization group approach that has been applied in this context is the Similarity Renormalization Group (SRG), which is based on a series of continuous unitary transformations that evolve hamiltonians with a cutoff on energy differences. In this work we study the SRG evolution of a leading order (LO) chiral effective $NN$ potential in the $^1 S_0$ channel derived within the framework of the Subtracted Kernel Method (SKM), a renormalization scheme based on a subtracted scattering equation.
Quantum field theories require a cutoff to regulate divergences that result from local interactions, and yet physical results can not depend on the value of this cutoff. The renormalization group employs a transformation that changes the cutoff to isolate hamiltonians that produce cutoff-independent eigenvalues. The similarity renormalization group is based on similarity transformations that regulate off-diagonal matrix elements, forcing the hamiltonian towards a band-diagonal form as the cutoff is lowered. This avoids pathologies that plagued tradition transformations acting on hamiltonians, making it possible to produce a well-behaved perturbative approximation of renormalized hamiltonians in asymptotically free theories. We employ a simple two-dimensional delta function example to illustrate this new renormalization technique.
We use two renormalization techniques, Effective Field Theory and the Similarity Renormalization Group, to solve simple Schr{ö}dinger equations with delta-function potentials in one and two dimensions. The familiar one-dimensional delta-function does not require renormalization, but it provides the simplest example of a local interaction that can be replaced by a sequence of effective cutoff interactions that produce controllable power-law errors. The two-dimensional delta-function leads to logarithmic divergences, dimensional transmutation and asymptotic freedom, providing an example of some of the most important renormalization problems in gauge field theories. We concentrate on the power-law analysis of errors in low-energy observables. The power-law suppression of the effects of irrelevant operators is critical to the success of field theory, and understanding them turns renormalization group techniques into powerful predictive tools for complicated problems where exact solutions are not available.
The Schrodinger equation with a two-dimensional delta-function potential is a simple example of an asymptotically free theory that undergoes dimensional transmutation. Renormalization requires the introduction of a mass scale, which can be lowered perturbatively until an infrared cutoff produced by non-perturbative effects such as bound state formation is encountered. We outline the effective field theory and similarity renormalization group techniques for producing renormalized cutoff hamiltonians, and illustrate the control of logarithmic and inverse-power-law errors both techniques provide.
A constituent parton picture of hadrons with logarithmic confinement arises naturally in light-front QCD when the hamiltonian is computed using a perturbative renormalization group; however, rotational symmetry is not manifest as a simple kinematic symmetry. Relevant operators must typically be fine-tuned when a perturbative renormalization group is used, and in light-front field theory such operators contain functions of longitudinal momenta. We explore the possibility that a gluon mass operator with simple longitudinal momentum dependence may improve approximate kinematic rotational symmetry, as revealed by the potential between heavy quarks.