Publications (132)194.27 Total impact
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ABSTRACT: We present results on the nucleon scalar, axial and tensor charges as well as on the momentum fraction, and the helicity and transversity moments. The pion momentum fraction is also presented. The computation of these key observables is carried out using lattice QCD simulations at a physical value of the pion mass. The evaluation is based on gauge configurations generated with two degenerate sea quarks of twisted mass fermions with a clover term. We investigate excited states contributions with the nucleon quantum numbers by analyzing three sinksource time separations. We find that, for the scalar charge, excited states contribute significantly and to a less degree to the nucleon momentum fraction and helicity moment. Our analysis yields a value for the nucleon axial charge agrees with the experimental value and we predict a value of 1.027(62) in the $\overline{\text{MS}}$ scheme at 2 GeV for the isovector nucleon tensor charge directly at the physical point. The pion momentum fraction is found to be $\langle x\rangle_{ud}^{\pi^\pm}=0.214(15)(^{+12}_{9})$ in the $\overline{\rm MS}$ at 2 GeV. 
Article: Simulating QCD at the Physical Point with $N_f=2$ Wilson Twisted Mass Fermions at Maximal Twist
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ABSTRACT: We present simulations of QCD using Nf=2 dynamical Wilson twisted mass lattice QCD with physical value of the pion mass and at one value of the lattice spacing. Such simulations at ~0.09 fm became possible by adding the clover term to the action. While O(a) improvement is still guaranteed by Wilson twisted mass fermions at maximal twist, the introduction of the clover term reduces cutoff effects related to isospin symmetry breaking. We give results for a set of phenomenologically interesting observables like pseudoscalar masses and decay constants, quark masses and the anomalous magnetic moments of leptons. We mostly find remarkably good agreement with phenomenology, even though we cannot take the continuum and thermodynamic limits. 
Article: HadronHadron Interactions from $N_f=2+1+1$ Lattice QCD: isospin2 $\pi\pi$ scattering length
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ABSTRACT: We present results for the $I=2$ $\pi\pi$ scattering length using $N_f=2+1+1$ twisted mass lattice QCD for three values of the lattice spacing and a range of pion mass values. Due to the use of Laplacian Heaviside smearing our statistical errors are reduced compared to previous lattice studies. A detailed investigation of systematic effects such as discretisation effects, volume effects, and pollution of excited and thermal states is performed. After extrapolation to the physical point using chiral perturbation theory at NLO we obtain $M_\pi a_0=0.0442(2)_\mathrm{stat}(^{+4}_{0})_\mathrm{sys}$.  [Show abstract] [Hide abstract]
ABSTRACT: We study η and mesons and their mixing angle in a mixed action approach with socalled Osterwalder–Seiler valence quarks on a Wilson twisted mass sea. The gauge configurations have been generated by ETMC for dynamical quark flavours and for three values of the lattice spacing. The main results are that differences in between the mixed action and the unitary approach vanish towards the continuum limit with the expected rate of . The individual size of the lattice artifacts depends, however, strongly on the observable used to match unitary and valence actions. Moreover, we show that for the η mass valence and valence plus sea quark mass dependence differ significantly. Hence, in this case a retuning of the simulation parameters in the valence sector only is not sufficient to compensate for mismatches in the original quark masses.  [Show abstract] [Hide abstract]
ABSTRACT: We compute both sides of the WittenVeneziano formula using lattice techniques. For the one side we perform dedicated quenched simulations and use the spectral projector method to determine the topological susceptibility in the pure YangMills theory. The other side we determine in lattice QCD with $N_f=2+1+1$ dynamical Wilson twisted mass fermions including for the first time also the flavour singlet decay constant. After taking the continuum and the SU$(2)$ chiral limits we compare both sides and find good agreement within uncertainties.  [Show abstract] [Hide abstract]
ABSTRACT: We study eta and eta' mesons and their mixing angle in a mixed action approach with socalled OsterwalderSeiler valence quarks on a Wilson twisted mass sea. The gauge configurations have been generated by ETMC for 2+1+1 dynamical quark flavours and for three values of the lattice spacing. The main results are that differences in between the mixed action and the unitary approach vanish towards the continuum limit with the expected rate of O(a^2). The individual size of the lattice artifacts depends, however, strongly on the observable used to match unitary and valence actions. Moreover, we show that for the eta mass valence and valence plus sea quark mass dependence differ significantly. Hence, in this case a retuning of the simulation parameters in the valence sector only to account for small mismatches in the sea parameters is not enough.  [Show abstract] [Hide abstract]
ABSTRACT: PiPi scattering is investigated for the first time for Nf=2+1+1 dynamical quark flavours using Wilson twisted mass fermions. L\"uscher's finite size method is used to relate energy shifts in finite volume to scattering quantities like the scattering length in the I=2 channel. The computation is performed at several pion masses and lattice spacings utilising the stochastic LapH method.  [Show abstract] [Hide abstract]
ABSTRACT: 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: $\mu_s/\mu_l=27.63(13)$, $\mu_c/\mu_l=339.6(2.2)$ and $\mu_c/\mu_s=12.29(10)$. 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.  [Show abstract] [Hide abstract]
ABSTRACT: We present a comparison of different definitions of the topological charge on the lattice, using a smallvolume ensemble with 2 flavours of dynamical twisted mass fermions. The investigated definitions are: index of the overlap Dirac operator, spectral projectors, spectral flow of the Hermitian WilsonDirac operator and field theoretic with different kinds of smoothing of gauge fields (HYP and APE smearings, gradient flow, cooling). We also show some results on the topological susceptibility.  [Show abstract] [Hide abstract]
ABSTRACT: We present a lattice QCD calculation of the up, down, strange and charm quark masses performed using the gauge configurations produced by the European Twisted Mass Collaboration with Nf = 2 + 1 + 1 dynamical quarks, which include in the sea, besides two light mass degenerate quarks, also the strange and charm quarks with masses close to their physical values. The simulations are based on a unitary setup for the two light quarks and on a mixed action approach for the strange and charm quarks. The analysis uses data at three values of the lattice spacing and pion masses in the range 210  450 MeV, allowing for accurate continuum limit and controlled chiral extrapolation. The quark mass renormalization is carried out nonperturbatively using the RIMOM method. The results for the quark masses converted to the bar{MS} scheme are: mud(2 GeV) = 3.70(17) MeV, ms(2 GeV) = 99.6(4.1) MeV and mc(mc) = 1.348(42) GeV. We obtain also the quark mass ratios ms/mud = 26.66(32) and mc/ms = 11.62(16). By studying the mass splitting between the neutral and charged kaons and using available lattice results for the electromagnetic contributions, we evaluate mu/md = 0.470(56), leading to mu = 2.36(24) MeV and md = 5.03(26) MeV.Nuclear Physics B 03/2014; 887. DOI:10.1016/j.nuclphysb.2014.07.025 · 3.95 Impact Factor 
Article: Investigating a mixed action approach for {\eta} and {\eta}' mesons in Nf=2+1+1 lattice QCD
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ABSTRACT: We present results for a test of a mixed action approach with OsterwalderSeiler valence quarks on a Wilson twisted mass sea for the example {\eta}, {\eta}' mesons. Flavour singlet pseudoscalar mesons obtain significant contributions from disconnected diagrams and are, therefore, expected to be particularly sensitive to mixed regularisations. We employ different procedures for matching valence and sea quark actions and show that the results agree in the continuum limit.  [Show abstract] [Hide abstract]
ABSTRACT: We present first results using the stochastic LaplaceHeaviside (sLapH) method in the twisted mass formulation. The calculations are performed on gauge configurations provided by the European Twisted Mass (ETM) collaboration with 2+1+1 dynamical quark flavours at a single value of the lattice spacing. In particular, we compute disconnected contributions to flavour singlet pseudoscalar mesons and compare sLapH to standard volume noise methods.  [Show abstract] [Hide abstract]
ABSTRACT: We investigate the masses and decay constants of eta and eta' mesons using the Wilson twisted mass formulation with N_f=2+1+1 dynamical quark flavours based on gauge configurations of ETMC. We show how to efficiently subtract excited state contributions to the relevant correlation functions and estimate in particular the eta' mass with improved precision. After investigating the strange quark mass dependence and the continuum and chiral extrapolations, we present our results for masses and mixing angle(s) at the physical point. Using chiral perturbation theory we also extract the decay constants f_l and f_s and use them to estimate the decay widths of eta,\eta' to gamma gamma and the transition form factor in the limit of large momentum transfer.  [Show abstract] [Hide abstract]
ABSTRACT: We present an overview of recent developments in the tmLQCD software suite. We summarise the features of the code, including actions and operators implemented. In particular, we discuss the optimisation efforts for modern architectures using the Blue Gene/Q system as an example. 
Article: Experiences with OpenMP in tmLQCD
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ABSTRACT: An overview is given of the lessons learned from the introduction of multithreading 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.  [Show abstract] [Hide abstract]
ABSTRACT: 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 massdegenerate flavours at a single lattice spacing are shown. Measurements from the light and heavylight 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. 
Article: η and η^{′} Mixing from Lattice QCD
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ABSTRACT: We present a lattice QCD computation of η and η^{′} masses and mixing angles, for the first time controlling continuum and quark mass extrapolations. The results for M_{η}=551(8)_{stat}(6)_{syst} MeV and M_{η^{′}}=1006(54)_{stat}(38)_{syst}(+61)_{ex} MeV are in excellent agreement with experiment. Our data show that the mixing in the quark flavor basis can be described by a single mixing angle of ϕ=46(1)_{stat}(3)_{syst}° indicating that the η^{′} is mainly a flavor singlet state.Physical Review Letters 10/2013; 111(18). DOI:10.1103/PhysRevLett.111.181602 · 7.51 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We review the status of an ongoing longterm lattice investigation of the spectrum and structure of tetraquark candidates. We focus on the light scalar meson $a_0(980)$. First steps regarding the study of a possibly existing $c c \bar{c} \bar{c}$ tetraquark are also outlined.Journal of Physics Conference Series 10/2013; 503(1). DOI:10.1088/17426596/503/1/012031 
Article: Study of the a_0(980) on the lattice
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ABSTRACT: We present lattice results for the a_0(980) state using a variational approach with a variety of creation operators: quarkantiquark, mesonic molecule, diquarkantidiquark as well as twomeson type. Focus is put on recent technical advances, in particular the computation of singly disconnected diagrams.  [Show abstract] [Hide abstract]
ABSTRACT: By matching Wilson twisted mass lattice QCD determinations of pseudoscalar meson masses to Wilson Chiral Perturbation Theory we determine the lowenergy constants $ W_6^{\prime } $ , $ W_8^{\prime } $ and their linear combination c 2. We explore the dependence of these lowenergy constants on the choice of the lattice action and on the number of dynamical flavours.Journal of High Energy Physics 05/2013; 2013(5). DOI:10.1007/JHEP05(2013)038 · 6.22 Impact Factor
Publication Stats
2k  Citations  
194.27  Total Impact Points  
Top Journals
Institutions

2009–2013

University of Bonn
 Helmholtz Institute for Radiation and Nuclear Physics
Bonn, North RhineWestphalia, Germany


2009–2010

HumboldtUniversität zu Berlin
 Department of Physics
Berlín, Berlin, Germany


2004–2008

University of Liverpool
 Department of Mathematical Sciences
Liverpool, ENG, United Kingdom


2007

University of Rome Tor Vergata
 Dipartimento di Fisica
Roma, Latium, Italy 
University of Glasgow
 School of Physics and Astronomy
Glasgow, Scotland, United Kingdom


2004–2006

Freie Universität Berlin
 Institute of Theoretical Physics
Berlin, Land Berlin, Germany


2

Peking University
Peping, Beijing, China
