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At least 721 records · Page 40Linked to original sources
arXiv2010

Phase Transitions in Disordered Systems

We face the problem of phase transitions in diluted systems both from theoretical and numerical sides. We study the effects of quenched site-dilution in classical models (Heisenberg, Ising and Potts) in 2, 3, and 4 dimensions both by using the Renormalization Group and numerical simulations in the canonical and microcanonical ensembles. We propose and check a new formulation of the Finite Size Scaling ansatz (FSS) inside the microcanonical ensemble. We use microcanonical simulations to obtain a clear fist-order behavior for the diluted Potts model in 3D, estimating the tricritical dilution. We perform large-scale simulations of the 3D diluted Heisenberg model, checking its self-averaging properties. Finally we study the 4D diluted Ising model obtaining from the FSS of the specific heat a clear differentiation between the existing conflicting theories. We also compiled a large number of appendix that we expect to be used as future reference.

cond-mat.dis-nn↗
arXiv2010

General linear matrix model, Minkowski spacetime and the Standard Model

The Hermitian matrix model with general linear symmetry is argued to decouple into a finite unitary matrix model that contains metastable multidimensional lattice configurations and a fermion determinant. The simplest metastable state is a Hermitian Weyl kinetic operator of either handedness on a 3+1 D lattice with general nonlocal interactions. The Hermiticity produces 16 effective Weyl fermions by species doubling, 8 left- and 8 right-handed. These are identified with a Standard Model generation. Only local non-anomalous gauge fields within the soup of general fluctuations can survive at long distances, and the degrees of freedom for gauge field subgroups of $U(8)_L X U(8)_R$ are present. Standard Model gauge symmetries associate with particular species symmetries, for example change of QCD color associates with permutation of doubling status amongst space directions. Vierbein gravity is probably also generated. While fundamental Higgs fields are not possible, low fermion current masses can arise from chiral symmetry breaking solutions of the fermion self-energy Schwinger-Dyson equations, generating W mass and composite Higgs states, similar to a scheme proposed by Gribov. Specific higher dimensional lattices with larger spinors are potentially stable but produce non-Riemannian spaces without conserved quadratic distances. However if the extra dimensions are compactified, the Minowski space persists at low energy accompanied by SM generations, potentially doubled further by duplicate zero modes in the compact directions and potentially containing dark matter. The model is conjectured to have an origin in infinite dimensional conformal invariance and the concept of Bare Particulars.

hep-th↗
arXiv2010

Spectrum of Spin 1 Dirac Operators on the Fuzzy 2-Sphere

We numerically find out the spectrum of the $3$ spin $1$ Dirac operators found in~\cite{ApbPP}. We give an analytic and numerical proof that they are unitarily inequivalent. Since these operators come paired with an anticommuting chirality operator, we find their spectrums to resemble those of fermions with positive and negative eigenvalues along with a number of zero modes. We give a method to count the number of zero modes which can be extended to higher spins on $S_F^2$. An universal relation between the energy eigenvalues of the spin 1 Dirac operator and their multiplicities is found. This helps us predict the energy eigenvalues for an arbitrarily large cut-off $L$, a problem which is computationally difficult to handle.

hep-th↗
arXiv2010

Kaon physics from lattice QCD

I review lattice calculations and results for hadronic parameters relevant for kaon physics, in particular the vector form factor f+(0) of semileptonic kaon decays, the ratio fK/fpi of leptonic decay constants and the kaon bag parameter BK. For each lattice calculation a colour code rating is assigned, by following a procedure which is being proposed by the Flavianet Lattice Averaging Group (FLAG), and the following final averages are obtained: f+(0)=0.962(3)(4), fK/fpi = 1.196(1)(10) and \hat BK = 0.731(7)(35). In the last part of the talk, the present status of lattice studies of non-leptonic K--> pi pi decays is also briefly summarized.

hep-lat↗
arXiv2010

Lattice searches for tetraquarks and mesonic molecules: light scalar mesons and XYZ states

Searches for tetraquarks and mesonic molecules in lattice QCD are briefly reviewed. In the light quark sector the most serious candidates are the lightest scalar resonances sigma, kappa, a0 and f0. In the hidden-charm sector I discuss lattice simulations of X(3872), Y(4260), Y(4140) and Z^+(4430). The most serious challenge in all these lattice studies is the presence of scattering states in addition to possible tetraquark/molecular states. The available methods for distinguishing both are reviewed and the main conclusions of the simulations are presented.

hep-lat↗
arXiv2010

Finite Temperature Schwinger Model

The temperature dependence of the order parameter of the Schwinger model is calculated in the euclidean functional integral approach. For that we solve the model on a finite torus and let the spatial extension tend to infinity at the end of the computations. The induced actions, fermionic zero-modes, relevant Green functions and Wilson loop correlators on the torus are derived. We find the analytic form of the chiral condensate for any temperature and in particular show that it behaves like \< \barΨΨ\> \sim -2 T\exp(-π\sqrtπT/e) for temperatures large compared to the induced photon mass.

hep-th↗
arXiv2010

mc4qcd: Online Analysis Tool for Lattice QCD

mc4qcd is a web based collaboration tool for analysis of Lattice QCD data. Lattice QCD computations consists of a large scale Markov Chain Monte Carlo. Multiple measurements are performed at each MC step. Our system acquires the data by uploading log files, parses them for results of measurements, filters the data, mines for required information by aggregating results, represents the results as plots and histograms, and it further allows refining and interaction by fitting the results. The system computes moving averages and autocorrelations, builds bootstrap samples and bootstrap errors, and allows modeling the data using Bayesian correlated constrained linear and non-linear fits. It can be scripted to allow real time visualization of results form an ongoing computation. The system is modular and it can be adapted to automating the analysis workflow of different types of MC computations.

hep-lat↗
arXiv2010

Two-Loop Fermionic Integrals in Perturbation Theory on a Lattice

A comprehensive number of one-loop integrals in a theory with Wilson fermions at $r=1$ is computed using the Burgio-Caracciolo-Pelissetto algorithm. With the use of these results, the fermionic propagator in the coordinate representation is evaluated, thus making it possible to extend the Luscher-Weisz procedure for two-loop integrals to the fermionic case. Computations are performed with FORM and REDUCE packages.

hep-lat↗
arXiv2011

A Modified Hard Thermal Loop Perturbation Theory

Based on the external perturbation that disturbs the system only slightly from its equilibrium position we make the Taylor expansion of the pressure of a quark gas. It turns out that the first term was used in the literature to construct a Hard Thermal Loop perturbation theory (HTLpt) within the variation principle of the lowest order of the thermal mass parameter. Various thermodynamic quantities within the 1-loop HTLpt encountered overcounting of the leading order (LO) contribution and also required a separation scale for soft and hard momenta. Using same variational principle we reconstruct the HTLpt at the first derivative level of the pressure that takes into account the effect of the variation of the external source through the conserved density fluctuation. This modification markedly improves those quantities in 1-loop HTLpt in a simple way instead of pushing the calculation to a considerably more complicated 2-loop HTLpt. Moreover, the results also agree with those obtained in the 2-loop approximately self-consistent Φ-derivable Hard Thermal Loop resummation. We also discuss how this formalism can be extended for the higher order contributions

hep-ph↗
arXiv2010

Polarised correlators at finite temperature

QCD undergoes a deconfining transition at high temperature to a "quark-gluon plasma" phase where hadrons may become unbound. In this work, meson correlation functions at non-zero momentum are studied both in the confined and plasma phases using the Maximum Entropy Method. In particular, both the longitudinal and transverse modes of the vector correlation functions are considered. Only in the case of light quarks in the plasma phase, we find that both longitudinal and transverse spectral functions have a non-zero intercept at zero energy.

hep-lat↗
arXiv2010

The spectrum of static-light baryons in twisted mass lattice QCD

We compute the static-light baryon spectrum with N_f = 2 flavors of sea quarks using Wilson twisted mass lattice QCD. As light valence quarks we consider quarks, which have the same mass as the sea quarks with corresponding pion masses in the range 340 MeV < m_PS < 525 MeV, as well as partially quenched quarks, which have the mass of the physical s quark. We extract masses of states with isospin I = 0, 1/2, 1, with strangeness S = 0, -1, -2, with angular momentum of the light degrees of freedom j = 0, 1 and with parity P = +, -. We present a preliminary extrapolation in the light u/d and an interpolation in the heavy b quark mass to the physical point and compare with available experimental results.

hep-lat↗
arXiv2010

Relevant momentum components of gluons for confinement and chiral symmetry breaking

We investigate which momentum components of gluons induce color confinement and spontaneous chiral symmetry breaking in lattice QCD. For this purpose, we formulate a lattice framework to introduce the momentum cutoff of the gluon field. Using this framework, we calculate the quark-antiquark potential, the color flux tube, the chiral condensate, and the Dirac spectrum. Our results suggest that confinement and chiral symmetry breaking are induced by somehow different momentum components of gluons.

hep-lat↗
arXiv2010

Nucleon Magnetic Moments and Electric Polarizabilities

Electromagnetic properties of the nucleon are explored with lattice QCD using a novel technique. Focusing on background electric fields, we show how the electric polarizability can be extracted from nucleon correlation functions. A crucial step concerns addressing contributions from the magnetic moment, which affects the relativistic propagation of nucleons in electric fields. By properly handing these contributions, we can determine both magnetic moments and electric polarizabilities. Lattice results from anisotropic clover lattices are presented. Our method is not limited to the neutron; we show results for the proton as well.

hep-lat↗
arXiv2010

Towards the N_f=2 deconfinement transition temperature with O(a) improved Wilson fermions

A lot of effort in lattice simulations over the last years has been devoted to studies of the QCD deconfinement transition. Most state-of-the-art simulations use rooted staggered fermions, while Wilson fermions are affected by large systematic uncertainties, such as coarse lattices or heavy sea quarks. Here we report on an ongoing study of the transition, using two degenerate flavours of nonperturbatively $\Ord(a)$ improved Wilson fermions. We start with $N_{t}=12$ and 16 lattices and pion masses of 600 to 450 MeV, aiming at chiral and continuum limits with light quarks.

hep-lat↗
arXiv2010

Egalitarian Improvement to Democracy: Quark renormalization constants in N_f=2 QCD

We present our results on the non-perturbative evaluation of the renormalization constant for the quark field, $Z_q$, in Landau gauge within RI-MOM scheme. Using three lattice spacing we are able to isolate lattice artefacts of various origin, both perturbative and non-perturbative. In particular, the existence of the dimension-two gluon-condensate is discussed, and confirmed.

hep-lat↗
arXiv2010

Poincaré Invariant Quantum Mechanics based on Euclidean Green functions

We investigate a formulation of Poincaré invariant quantum mechanics where the dynamical input is Euclidean invariant Green functions or their generating functional. We argue that within this framework it is possible to calculate scattering observables, binding energies, and perform finite Poincaré transformations without using any analytic continuation. We demonstrate, using a toy model, how matrix elements of $e^{-βH}$ in normalizable states can be used to compute transition matrix elements for energies up to 2 GeV. We discuss some open problems.

math-ph↗