We present physics results from simulations of QCD using N f = 2 dynamical Wilson twisted mass fermions at the physical value of the pion mass. These simulations were enabled by the addition of the clover term to the twisted mass quark action. We show evidence that compared to previous simulations without this term, the pion mass splitting due to isospin breaking is almost completely eliminated. Using this new action, we compute the masses and decay constants of pseudoscalar mesons involving the dynamical up and down as well as valence strange and charm quarks at one value of the lattice spacing, a ≈ 0.09 fm. Further, we determine renormalized quark masses as well as their scale-independent ratios, in excellent agreement with other lattice determinations in the continuum limit. In the baryon sector, we show that the nucleon mass is compatible with its physical value and that the masses of the ∆ baryons do not show any sign of isospin breaking. Finally, we compute the electron, muon and tau lepton anomalous magnetic moments and show the results to be consistent with extrapolations of older ETMC data to the continuum and physical pion mass limits. We mostly find remarkably good agreement with phenomenology, even though we cannot take the continuum and thermodynamic limits.

We present results for the I = 2 ππ 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 π a 0 = −0.0442(2) stat ( +4 −0 ) sys .

We present results for the isospin-0 ππ s-wave scattering length calculated with Osterwalder-Seiler valence quarks on Wilson twisted mass gauge configurations. We use three N f = 2 ensembles with unitary (valence) pion mass at its physical value (250 MeV), at 240 MeV (320 MeV) and at 330 MeV (400 MeV), respectively. By using the stochastic Laplacian Heaviside quark smearing method, all quark propagation diagrams contributing to the isospin-0 ππ correlation function are computed with sufficient precision. The chiral extrapolation is performed to obtain the scattering length at the physical pion mass.Our result M π a I=0 0 = 0.198(9)(6) agrees reasonably well with various experimental measurements and theoretical predictions. Since we only use one lattice spacing, certain systematics uncertainties, especially those arising from unitary breaking, are not controlled in our result.

We present an investigation of the ρ-meson from N f = 2 + 1 + 1 flavour lattice QCD.The calculation is performed based on gauge configuration ensembles produced by the ETM collaboration with three lattice spacing values and pion masses ranging from 230 MeV to 500 MeV. Applying the Lüscher method phase shift curves are determined for all ensembles separately. Assuming a Breit-Wigner form, the ρ-meson mass and width are determined by a fit to these phase shift curves. Mass and width combined are then extrapolated to the chiral limit, while lattice artefacts are not detectable within our statistical uncertainties. For the ρmeson mass extrapolated to the physical point we find good agreement with experiment. The corresponding decay width differs by about two standard deviations from the experimental value.

We present results for the interaction of two kaons at maximal isospin. The calculation is based on N f = 2 + 1 + 1 flavour gauge configurations generated by the European Twisted Mass Collaboration with pion masses ranging from about 230 MeV to 450 MeV at three values of the lattice spacing. The elastic scattering length a I=1 0 is calculated at several values of the bare strange and light quark masses. We find M K a 0 = −0.385(16) stat ( +0 −12 ) ms ( +0 −5 ) Z P (4) r f as the result of a combined extrapolation to the continuum and to the physical point, where the first error is statistical, and the three following are systematical. This translates to a 0 = −0.154(6) stat ( +0 −5 ) ms ( +0 −2 ) Z P (2) r f fm.1 arXiv:1703.04737v2 [hep-lat]