arXiv subjects

K. Jansen

Publications and source records attributed to K. Jansen.

At least 55 records · Page 3Linked to original sources

arXiv2014

Progress in Simulations with Twisted Mass Fermions at the Physical Point

In this contribution, results from $N_f=2$ lattice QCD simulations at one lattice spacing using twisted mass fermions with a clover term at the physical pion mass are presented. The mass splitting between charged and neutral pions (including the disconnected contribution) is shown to be around $20(20)~\mathrm{MeV}$. Further, a first measurement using the clover twisted mass action of the average momentum fraction of the pion is given. Finally, an analysis of pseudoscalar meson masses and decay constants is presented involving linear interpolations in strange and charm quark masses. Matching to meson mass ratios allows the calculation of quark mass ratios: $μ_s/μ_l=27.63(13)$, $μ_c/μ_l=339.6(2.2)$ and $μ_c/μ_s=12.29(10)$. From this mass matching the quantities $f_K=153.9(7.5)~\mathrm{MeV}$, $f_D=219(11)~\mathrm{MeV}$, $f_{D_s}=255(12)~\mathrm{MeV}$ and $M_{D_s}=1894(93)~\mathrm{MeV}$ are determined without the application of finite volume or discretization artefact corrections and with errors dominated by a preliminary estimate of the lattice spacing.

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arXiv2014

Baryon spectrum with $N_f=2+1+1$ twisted mass fermions

The masses of the low lying baryons are evaluated using a total of ten ensembles of dynamical twisted mass fermion gauge configurations. The simulations are performed using two degenerate flavors of light quarks, and a strange and a charm quark fixed to approximately their physical values. The light sea quarks correspond to pseudo scalar masses in the range of about 210~MeV to 430~MeV. We use the Iwasaki improved gluonic action at three values of the coupling constant corresponding to lattice spacing $a=0.094$~fm, 0.082~fm and 0.065~fm determined from the nucleon mass. We check for both finite volume and cut-off effects on the baryon masses. We examine the issue of isospin symmetry breaking for the octet and decuplet baryons and its dependence on the lattice spacing. We show that in the continuum limit isospin breaking is consistent with zero, as expected. We performed a chiral extrapolation of the forty baryon masses using SU(2) $χ$PT. After taking the continuum limit and extrapolating to the physical pion mass our results are in good agreement with experiment. We provide predictions for the mass of the doubly charmed $Ξ_{cc}^*$, as well as of the doubly and triply charmed $Ω$s that have not yet been determined experimentally.

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arXiv2014

Sigma-terms and axial charges for hyperons and charmed baryons

We present results for the $σ$-terms and axial charges for various hyperons and charmed baryons using $N_f=2+1+1$ twisted mass fermions. For the computation of the three-point function we use the fixed current method. For one of the $N_f=2+1+1$ ensembles with pion mass of 373 MeV we compare the results of the fixed current method with those obtained with a stochastic method for computing the all-to-all propagator involved in the evaluation of the three point functions.

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arXiv2014

Evaluation of disconnected quark loops for hadron structure using GPUs

A number of stochastic methods developed for the calculation of fermion loops are investigated and compared, in particular with respect to their efficiency when implemented on Graphics Processing Units (GPUs). We assess the performance of the various methods by studying the convergence and statistical accuracy obtained for observables that require a large number of stochastic noise vectors, such as the isoscalar nucleon axial charge. The various methods are also examined for the evaluation of sigma-terms where noise reduction techniques specific to the twisted mass formulation can be utilized thus reducing the required number of stochastic noise vectors.

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arXiv2014

Disconnected quark loop contributions to nucleon observables in lattice QCD

We perform a high statistics calculation of disconnected fermion loops on Graphics Processing Units for a range of nucleon matrix elements extracted using lattice QCD. The isoscalar electromagnetic and axial vector form factors, the sigma-terms and the momentum fraction and helicity are among the quantities we evaluate. We compare the disconnected contributions to the connected ones and give the physical implications on nucleon observables that probe its structure.

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arXiv2013

Nucleon generalized form factors with twisted mass fermions

We present results on the nucleon form factors, momentum fraction and helicity moment for $N_f=2$ and $N_f=2+1+1$ twisted mass fermions for a number of lattice volumes and lattice spacings. First results for a new $N_f=2$ ensemble at the physical pion mass are also included. The implications of these results on the spin content of the nucleon are discussed taking into account the disconnected contributions at one pion mass.

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arXiv2013

The quark contents of the nucleon and their implication for dark matter search

We present results concerning the light and strange quark contents of the nucleon using $N_f=2+1+1$ flavours of maximally twisted mass fermions. The corresponding $σ$-terms are casting light on the origin of the nucleon mass and their values are important to interpret experimental data from direct dark matter searches. We discuss our strategy to estimate systematic uncertainties arising in our computations. Our preliminary results for the $σ-$terms read $σ_{πN} = 37(2.6)(24.7) \mev$ and $σ_s=28(8)(10) \mev$. We present our recent final analysis of the $y_N$ parameter and found $y_N=0.135(46)$ including systematics\cite{Alexandrou:2013nda}.

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arXiv2013

Experiences with OpenMP in tmLQCD

An overview is given of the lessons learned from the introduction of multi-threading using OpenMP in tmLQCD. In particular, programming style, performance measurements, cache misses, scaling, thread distribution for hybrid codes, race conditions, the overlapping of communication and computation and the measurement and reduction of certain overheads are discussed. Performance measurements and sampling profiles are given for different implementations of the hopping matrix computational kernel.

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arXiv2013

A first look at maximally twisted mass lattice QCD calculations at the physical point

In this contribution, a first look at simulations using maximally twisted mass Wilson fermions at the physical point is presented. A lattice action including clover and twisted mass terms is presented and the Monte Carlo histories of one run with two mass-degenerate flavours at a single lattice spacing are shown. Measurements from the light and heavy-light pseudoscalar sectors are compared to previous $N_f = 2$ results and their phenomenological values. Finally, the strategy for extending simulations to $N_f = 2 + 1 + 1$ is outlined.

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arXiv2013

The mass spectrum of the Schwinger model with Matrix Product States

We show the feasibility of tensor network solutions for lattice gauge theories in Hamiltonian formulation by applying matrix product states algorithms to the Schwinger model with zero and non-vanishing fermion mass. We introduce new techniques to compute excitations in a system with open boundary conditions, and to identify the states corresponding to low momentum and different quantum numbers in the continuum. For the ground state and both the vector and scalar mass gaps in the massive case, the MPS technique attains precisions comparable to the best results available from other techniques.

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arXiv2013

Quasi-Monte Carlo methods for lattice systems: a first look

We investigate the applicability of Quasi-Monte Carlo methods to Euclidean lattice systems for quantum mechanics in order to improve the asymptotic error behavior of observables for such theories. In most cases the error of an observable calculated by averaging over random observations generated from an ordinary Markov chain Monte Carlo simulation behaves like N^{-1/2}, where N is the number of observations. By means of Quasi-Monte Carlo methods it is possible to improve this behavior for certain problems to N^{-1}, or even further if the problems are regular enough. We adapted and applied this approach to simple systems like the quantum harmonic and anharmonic oscillator and verified an improved error scaling.

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arXiv2013

Nucleon form factors and moments of generalized parton distributions using $N_f=2+1+1$ twisted mass fermions

We present results on the axial and the electromagnetic form factors of the nucleon, as well as, on the first moments of the nucleon generalized parton distributions using maximally twisted mass fermions. We analyze two N_f=2+1+1 ensembles having pion masses of 210 MeV and 354 MeV at two values of the lattice spacing. The lattice scale is determined using the nucleon mass computed on a total of 18 N_f=2+1+1 ensembles generated at three values of the lattice spacing, $a$. The renormalization constants are evaluated non-perturbatively with a perturbative subtraction of ${\cal O}(a^2)$-terms. The moments of the generalized parton distributions are given in the $\bar{\rm MS}$ scheme at a scale of $ μ=2$ GeV. We compare with recent results obtained using different discretization schemes. The implications on the spin content of the nucleon are also discussed.

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arXiv2013

Nucleon Structure using lattice QCD

A review of recent nucleon structure calculations within lattice QCD is presented. The nucleon excited states, the axial charge, the isovector momentum fraction and helicity distribution are discussed, assessing the methods applied for their study, including approaches to evaluate the disconnected contributions. Results on the spin carried by the quarks in the nucleon are also presented.

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arXiv2012

Sigma terms and strangeness content of the nucleon with $Nf=2+1+1$ twisted mass fermions

We investigate excited state contaminations in a direct computation of the nucleon $σ$-terms. This is an important source of systematic effects that needs to be controlled besides the light quark mass dependence and lattice artefacts. We use maximally twisted mass fermions with dynamical light ($u$,$d$), strange and charm degrees of freedom. Employing an efficient stochastic evaluation of the disconnected contribution available for twisted mass fermions, we show that the effect of excited states is large in particular for the strange $σ$-terms, where it can be as big as $O(\gtrsim 40%$). This leads to the unfortunate conclusion that even with a source-sink separation of $\sim 1.5 \fm$ and a good statistical accuracy it is not clear, whether excited state effects are under control for this quantity.

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arXiv2012

A first look at quasi-Monte Carlo for lattice field theory problems

In this project we initiate an investigation of the applicability of Quasi-Monte Carlo methods to lattice field theories in order to improve the asymptotic error behavior of observables for such theories. In most cases the error of an observable calculated by averaging over random observations generated from an ordinary Monte Carlo simulation behaves like 1/sqrt(N), where N is the number of observations. By means of Quasi-Monte Carlo methods it is possible to improve this behavior for certain problems to up to 1/N. We adapted and applied this approach to simple systems like the quantum harmonic and anharmonic oscillator and verified an improved error scaling.

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arXiv2012

Evaluation of disconnected contributions using GPUs

We calculate on GPUs the disconnected diagrams associated with the nucleon form factors and moments of generalized parton distributions using Nf=2+1+1 twisted mass fermions. We employ the truncated solver method (TSM) for estimating the all-to-all propagators. Due to the fact that the TSM involves many low precision stochastic estimators, the usage of GPUs is essential to perform efficiently the contractions and the inversions.

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arXiv2012

A quenched study of the Schroedinger functional with chirally rotated boundary conditions: applications

In a previous paper [1], we have discussed the non-perturbative tuning of the chirally rotated Schroedinger functional (XSF). This tuning is required to eliminate bulk O(a) cutoff effects in physical correlation functions. Using our tuning results obtained in [1] we perform scaling and universality tests analyzing the residual O(a) cutoff effects of several step-scaling functions and we compute renormalization factors at the matching scale. As an example of possible application of the XSF we compute the renormalized strange quark mass using large volume data obtained from Wilson twisted mass fermions at maximal twist.

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arXiv2012

A quenched study of the Schroedinger functional with chirally rotated boundary conditions: non-perturbative tuning

The use of chirally rotated boundary conditions provides a formulation of the Schroedinger functional that is compatible with automatic O(a) improvement of Wilson fermions up to O(a) boundary contributions. The elimination of bulk O(a) effects requires the non-perturbative tuning of the critical mass and one additional boundary counterterm. We present the results of such a tuning in a quenched setup for several values of the renormalized gauge coupling, from perturbative to non-perturbative regimes, and for a range of lattice spacings. We also check that the correct boundary conditions and symmetries are restored in the continuum limit.

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