Search results for "Field theory"
showing 10 items of 1188 documents
Nucleon isovector charges and twist-2 matrix elements with Nf=2+1 dynamical Wilson quarks
2019
We present results from a lattice QCD study of nucleon matrix elements at vanishing momentum transfer for local and twist-2 isovector operator insertions. Computations are performed on gauge ensembles with nonperturbatively improved ${N}_{f}=2+1$ Wilson fermions, covering four values of the lattice spacing and pion masses down to ${M}_{\ensuremath{\pi}}\ensuremath{\approx}200\text{ }\text{ }\mathrm{MeV}$. Several source-sink separations (typically $\ensuremath{\sim}1.0$ to $\ensuremath{\sim}1.5\text{ }\text{ }\mathrm{fm}$) allow us to assess excited-state contamination. Results on individual ensembles are obtained from simultaneous two-state fits across all observables and all available sou…
Effective Field Theory for Jet Processes
2015
Processes involving narrow jets receive perturbative corrections enhanced by logarithms of the jet opening angle and the ratio of the energies inside and outside the jets. Analyzing cone-jet processes in effective field theory, we find that in addition to soft and collinear fields their description requires degrees of freedom which are simultaneously soft and collinear to the jets. These collinear-soft particles can resolve individual collinear partons, leading to a complicated multi-Wilson-line structure of the associated operators at higher orders. Our effective field theory provides, for the first time, a factorization formula for a cone-jet process, which fully separates the physics at …
Finite size effects at phase transitions
2008
For many models of statistical thermodynamics and of lattice gauge theory computer simulation methods have become a valuable tool for the study of critical phenomena, to locate phase transitions, distinguish whether they are of first or second order, and so on. Since simulations always deal with finite systems, analysis of finite size effects by suitable finite size scaling concepts is a key ingredient of such applications. The phenomenological theory of finite size scaling is reviewed with emphasis on the concept of probability distributions of order parameter and/or energy. Attention is also drawn to recent developments concerning anisotropic geometries and anisotropic critical behavior, …
Light-Induced Excited Spin State Trapping in Iron(II) Complexes
1987
In the course of our studies on the thermally induced high spin (HS) ↔ low spin (LS) transition in iron(II) complexes /1/, \({\!^5{\text{T}}_2}_{\text{g}}\) ↔ \({\!^1{\text{A}}_1}_{\text{g}}\) in the approximation of Oh symmetry, we have observed in 1984 a new photophysical effect /2/: If, at sufficiently low temperature, the solid spin crossover complex is irradiated with green light into the \({\!^1{\text{A}}_1}\)→ \({\!^1{\text{T}}_1}\) ligand field absorption band, the thermodynamically stable LS state can be converted to the metastable HS state and trapped with practically infinite lifetime. We have called this unusual phenomenon “Light-Induced Excited Spin State Trapping (LIESST)”.
In Silico Molecular Engineering of Dysprosocenium-Based Complexes to Decouple Spin Energy Levels from Molecular Vibrations
2019
Molecular nanomagnets hold great promise for spintronics and quantum technologies, provided that their spin memory can be preserved above liquid-nitrogen temperatures. In the past few years, the magnetic hysteresis records observed for two related dysprosocenium-type complexes have highlighted the potential of molecular engineering to decouple vibrational excitations from spin states and thereby enhance magnetic memory. Herein, we study the spin-vibrational coupling in [(CpiPr5)Dy(Cp*)]+ (CpiPr5 = pentaisopropylcyclopentadienyl, Cp* = pentamethylcyclopentadienyl), which currently holds the hysteresis record (80 K), by means of a computationally affordable methodology that combines first-pri…
Weak Interactions at Low Energy
1985
In this series of lectures, I would like to discuss the basic aspects of the standard electroweak theory, as well as some alternatives. The phenomenology associated with charged and neutral currents weak interaction will show the success of the standard theory in giving a quantitative account of the existing data. The framework of most of our considerations will be that of an effective theory at low momentum transfer, low with respect to the masses of the intermediate vector bosons, 80–90 GeV. As an extension of the standard theory for the leptonic sector, I will contemplate the possibility of massive neutrinos being described by Majorana fields, leading to exotic phenomena induced by lepto…
Carrier-induced ferromagnetism in two-dimensional magnetically doped semiconductor structures
2021
We show theoretically that the magnetic ions, randomly distributed in a two-dimensional (2D) semiconductor system, can generate a ferromagnetic long-range order via the RKKY interaction. The main physical reason is the discrete (rather than continuous) symmetry of the 2D Ising model of the spin-spin interaction mediated by the spin-orbit coupling of 2D free carriers, which precludes the validity of the Mermin-Wagner theorem. Further, the analysis clearly illustrates the crucial role of the molecular field fluctuations as opposed to the mean field. The developed theoretical model describes the desired magnetization and phase-transition temperature ${T}_{c}$ in terms of a single parameter, na…
Investigation of nuclear collectivity in the neutron mid-shell nucleusPb186
2007
For the first time, non-yrast structures of the neutron mid-shell nucleus $^{186}\mathrm{Pb}$ have been identified in an in-beam \ensuremath{\gamma}-ray spectroscopy measurement using the recoil-decay tagging technique. The yrast band has been tentatively extended up to ${I}^{\ensuremath{\pi}}=20{}^{+},$ revealing a similar backbend to that observed in the Pt and Hg isotones. Three new bands and several other transitions have been observed. Calculations carried out in the framework of the interacting boson model together with mean-field studies using the generator coordinate method provide arguments for the association of one of the new bands with an oblate shape. The present data also show…
Particle creation by wormholes: A 1 + 1 model
2020
The propagation of a free massless scalar field in a [Formula: see text]-dimensional Minkowski space modeling, a wormhole is considered. The wormhole model consists on two timelike trajectories, which represent the entrance and the exit of the wormhole, connected via some transfer function that specifies how incoming modes that reach the entrance are transferred to the exit. We find that particles and energy fluxes are generically produced except for transfer functions that represent global conformal transformations. We consider several examples involving exit trajectories which are asymptotically inertial, asymptotically null, and also involving a faster-than-light motion to illustrate th…
A one-loop study of matching conditions for static-light flavor currents
2012
Heavy Quark Effective Theory (HQET) computations of semi-leptonic decays, e.g. B -> pi l nu, require the knowledge of the parameters in the effective theory for all components of the heavy-light flavor currents. So far non-perturbative matching conditions have been employed only for the time component of the axial current. Here we perform a check of matching conditions for the time component of the vector current and the spatial component of the axial vector current up to one-loop order of perturbation theory and to lowest order of the 1/m-expansion. We find that the proposed observables have small higher order terms in the 1/m-series and are thus excellent candidates for a non-perturbat…