118845565
Spectral densities from the lattice
We discuss a method to extract the K\"all\'{e}n-Lehmann spectral density of a particle (be it elementary or bound state) propagator by means of 4d lattice data. We employ a linear regularization strategy, commonly known as the Tikhonov method with Morozov discrepancy principle. An important virtue over the popular maximum entropy method is the possibility to also probe unphysical spectral densities, as, for example, of a confined gluon. We apply our proposal to the SU(3) glue sector.
62820378
Glueball spectral densities from the lattice
The propagator of a physical degree of freedom ought to obey a Kallen-Lehmann spectral representation, with positive spectral density. The latter quan tity is directly related to a cross section based on the optical theorem. The spectral density is a crucial ingredient of a quantum field theory with elementary and bound states, with a direct experimental connection as the masses of the excitations reflect themselves into (continuum) δ -singularities. In usual lattice simulational approaches to the QCD spectrum the spectral density itself is not accessed. The (bound state) masses are extracted from the asymptotic exponential decay of the two-point function. Given the importance of the spectral density, each nonperturbative continuum approach to QCD should be able to adequately describe it or to take into proper account . In this work, we wish to present a first trial in extracting an estimate for the scalar glueball spectral density in SU(3) gluodynamics using lattice gauge theory.
119119666
Shadow Hamiltonians, Poisson Brackets, and Gauge Theories
Numerical lattice gauge theory computations to generate gauge field configurations including the effects of dynamical fermions are usually carried out using algorithms that require the molecular dynamics evolution of gauge fields using symplectic integrators. Sophisticated integrators are commonly used but hard to optimize, and force-gradient integrators show promise especially for large lattice volumes. We explain that symplectic integrators lead to very efficient Monte Carlo algorithms because they exactly conserve a shadow Hamiltonian. The shadow Hamiltonian may be expanded in terms of Poisson brackets and can be used to optimize the integrators. We show how this may be done for gauge theories by extending the formulation of Hamiltonian mechanics on Lie groups to include Poisson brackets and shadows and by giving a general method for the practical computation of forces, force gradients, and Poisson brackets for gauge theories.
7542535
Exploring the infrared Landau gauge propagators using large asymmetric lattices
We report on the infrared limit of the quenched lattice Landau gauge gluon and ghost propagators computed from large asymmetric lattices. In particular, the compatibility of the pure power law infrared solutions of the Dyson-Schwinger equations with the lattice data is investigated and the exponent $\kappa$ is measured. The gluon lattice data favour $\kappa \sim 0.52$, which would imply a vanishing zero momentum gluon propagator. For the subset of lattices where the ghost propagator was computed, the data is not compatible with a pure power law. Our data also show a decreasing running coupling in the infrared region. Furthermore, positivity violation for the gluon propagator is also verified.
117914077
Force Gradient Integrators
We present initial results of the use of Force Gradient integ rators for lattice field theories. These promise to give significant performance improvements, espe cially for light fermions and large lattices. Our results show that this is indeed the case, indi cating a speed-up of more than a factor of two, which is expected to increase as the integration step size becomes smaller for larger lattices and smaller fermion masses.
15178500
Gauge fixing methods and Gribov copies effects in lattice QCD
We compare two Landau gauge fixing methods, aiming to find the global maximum of the gauge fixing functional. Moreover, a systematic effect of Gribov copies in the gluon and ghost propagators computed in Landau gauge is presented and discussed.
118710451
A minimal 3-3-1 model with naturally sub-eV neutrinos
In the original version of the minimal SU(3) C × SU(3) L × U(1) N model the masses of all quarks are correctly obtained by introducing three scalar triplets into the model; meanwhile the lepton mass generation requires the introduction of at least one scalar sextet. In this work we show that this scalar sextet is unable to yield the correct neutrino masses and mixing. In order to solve this puzzle in the most economical way, we evoke an additional Z 3 discrete symmetry, without including this sextet in the scalar spectrum, and propose a truly minimal 3-3-1 model capable of generating the correct masses and mixing of all fermions. Moreover, we show that our proposal leads to naturally light neutrinos with masses in the eV range, obtained with three scalar triplets only. Finally, the so-called minimal 3-3-1 model is also in danger due to the presence of undesirable effective operators that lead to proton decay unless they are suppressed by extremely small couplings or, as we choose to employ in this work, are eliminated by some discrete symmetry.
73558306
Precision measurements in the top quark sector with the CMS experiment at the LHC
A review of the measurements of the t production cross section (inclusive and differential) performed by the CMS experiment at the LHC is given. The results are used to constrain the magnitude of most of the relevant systematics affecting precision measurements in the top quark sector. In addition from the determination of the top quark pair production cross section it is possible to determine the strong coupling constant as ?S(mZ) = 0.1178+0.0046?0.0040 in agreement with the current world average. After the combination of several channels and techniques explored to measure the top quark mass in data, a precise determination of this quantity is made at CMS: mt = 173.4?0.4stat ? 0.9syst GeV. The measurement of the difference ?mt = mt ? m is also presented.
1436056
Critical coupling for dynamical chiral-symmetry breaking with an infrared finite gluon propagator
Abstract We compute the critical coupling constant for the dynamical chiral-symmetry breaking in a model of quantum chromodynamics, solving numerically the quark self-energy using infrared finite gluon propagators found as solutions of the Schwinger-Dyson equation for the gluon, and one gluon propagator determined in numerical lattice simulations. The gluon mass scale screens the force responsible for the chiral breaking, and the transition occurs only for a larger critical coupling constant than the one obtained with the perturbative propagator. The critical coupling shows a great sensibility to the gluon mass scale variation, as well as to the functional form of the gluon propagator.
119424038
Chiral parameters in a QCD-motivated model with confined and massive gluons
Abstract A one parameter model of a confined-gluon propagator has been formulated by Frank and Roberts recently, which has a great success explaining π − and ρ −meson observables. We show, computing few chiral parameters, that a small variation of this model considering an infrared finite gluon propagator with a dynamically generated gluon mass, can also fit data related to the chiral symmetry breaking. This allows a direct interpretation for the unique parameter involved in the model as the gluon mass scale.
122818028
Fine Synchronization of the CMS Muon Drift-Tube Local Trigger using Cosmic Rays
The CMS experiment uses self-triggering arrays of drift tubes in the barrel muon trigger to perform the identification of the correct bunch crossing. The identification is unique only if the trigger chain is correctly synchronized. In this paper, the synchronization performed during an extended cosmic ray run is described and the results are reported. The random arrival time of cosmic ray muons allowed several synchronization aspects to be studied and a simple method for the fine synchronization of the Drift Tube Local Trigger at LHC to be developed.