arXiv subjects

Alberto Ramos

Publications and source records attributed to Alberto Ramos.

62 records · Page 4Linked to original sources

arXiv2014

SU(3) gauge theory with 12 flavours in a twisted box

We present preliminary result for the step-scaling study of the coupling constant with the Yang-Mills gradient flow, in the twelve-favour SU(3) gauge theory. In this work, the lattice simulation is performed using unimproved staggered fermions and the Wilson plaquette gauge action, from which the gradient flow is also implemented. Imposing twisted boundary condition a'la t'Hooft and Parisi, our calculation is performed at zero fermion mass. The renormalised coupling constant is extracted via the computation of the energy density. In order to examine the reliability of the continuum extrapolation, we investigate this coupling constant using two different lattice discretisations. Our result shows that in order to control the systematic effects in the continuum extrapolation, it is necessary to implement a large enough gradient-flow time. In the current calculation, the gauge-field averaging radius corresponding to the flow time has to be as large as 40% of the lattice size.

hep-lat↗

arXiv2013

Critical slowing down and the gradient flow coupling in the Schrödinger functional

We study the sensitivity of the gradient flow coupling to sectors of different topological charge and its implications in practical situations. Furthermore, we investigate an alternative definition of the running coupling that is expected to be less sensitive to the problems of the HMC algorithm to efficiently sample all topological sectors.

hep-lat↗

arXiv2013

The gradient flow coupling in the Schrödinger Functional

We study the perturbative behavior of the Yang-Mills gradient flow in the Schrödinger Functional, both in the continuum and on the lattice. The energy density of the flow field is used to define a running coupling at a scale given by the size of the finite volume box. From our perturbative computation we estimate the size of cutoff effects of this coupling to leading order in perturbation theory. On a set of Nf=2 gauge field ensembles in a physical volume of L ~ 0.4 fm we finally demonstrate the suitability of the coupling for a precise continuum limit due to modest cutoff effects and high statistical precision.

hep-lat↗

arXiv2012

Playing with the kinetic term in the HMC

The HMC algorithm, combining the advantages of molecular dynamics and Monte-Carlo methods, is the most efficient algorithm to simulate QCD including the effects of sea quarks. In the standard approach momentum fields are generated with a Gaussian probability density. In this work I will explore another possibility. By using a Lorentz distribution one can dynamically impose a cutoff in the rate of change of the coordinates that potentially could have a better behavior. I will present some results in pure SU(2) gauge theory.

hep-lat↗

arXiv2012

B $\to$ $π$ form factor with 2 flavours of $O(a)$ improved Wilson quarks

The determinations of $|V_{\rm ub}|$ from the exclusive branching ratios of $B\to τν$ and $B \to πl ν$ tend to show a tension at the level of $3σ$ \cite{Beringer:1900zz}. On the theoretical side they depend on the lattice computation of the hadronic matrix elements $f_{\rm B}$ and the $B\to π$ form factor $f_+(q^2)$. To understand the tension, improved precision and a careful analysis of the systematics involved are necessary. Working towards this goal, we present preliminary lattice results of the ALPHA collaboration for the $B\to π$ form factor $f_+(q^2)$ with $N_{\rm f}=2$ flavours of $O(a)$-improved Wilson fermions. Our computation uses HQET in the static limit, pion masses ranging down to $\sim250$ MeV, large volumes with $m_πL >4$, three lattice spacings, and non-perturbative renormalization. We describe the techniques adopted to reduce the statistical noise (stochastic all-to-all with full time dilution) and the contamination from excited states (smearing for the B and the pion). We estimate the size of the chiral and continuum extrapolations. We discuss the impact our result could have to clarify the above mentioned discrepancy in the determination of $|V_{\rm ub}|$.

hep-lat↗

arXiv2012

Systematic errors in partially-quenched QCD plus QED lattice simulations

At the precision reached in current lattice QCD calculations, electromagnetic effects are becoming numerically relevant. Here, electromagnetic effects are included by superimposing $\mathrm{U}(1)$ degrees of freedom on $N_f = 2+1$ QCD configurations from the Budapest-Marseille-Wuppertal Collaboration. We present preliminary results for the electromagnetic corrections to light pseudoscalars mesons masses and discuss some of the associated systematic errors.

hep-lat↗

arXiv2006

Dynamics of critical vortices on the torus and on the plane

We use the Bradlow parameter expansion to construct the metric tensor in the space of solutions of the Bogomolny equations for the Abelian Higgs model on a two-dimensional torus. Using this metric we study the dynamics and scattering of vortices on the torus within the geodesic approximation. For small torus volumes the metric is determined in terms of a small number of parameters. For large volumes the results provide a very precise approximation to the metric and dynamics on the plane.

hep-th↗

arXiv2004

Expansion for the solutions of the Bogomolny equations on the torus

We show that the solutions of the Bogomolny equations for the Abelian Higgs model on a two-dimensional torus, can be expanded in powers of a quantity epsilon measuring the departure of the area from the critical area. This allows a precise determination of the shape of the solutions for all magnetic fluxes and arbitrary position of the Higgs field zeroes. The expansion is carried out to 51 orders for a couple of representative cases, including the unit flux case. We analyse the behaviour of the expansion in the limit of large areas, in which case the solutions approach those on the plane. Our results suggest convergence all the way up to infinite area.

hep-th↗