72 research outputs found

### Flavor-singlet meson decay constants from $N_f=2+1+1$ twisted mass lattice QCD

We present an improved analysis of our lattice data for the $\eta$--$\eta'$
system, including a correction of the relevant correlation functions for
residual topological finite size effects and employing consistent chiral and
continuum fits. From this analysis we update our physical results for the
masses $M_\eta=557(11)_\mathrm{stat}(03)_{\chi\mathrm{PT}}\,\mathrm{MeV}$ and
$M_{\eta'}=911(64)_\mathrm{stat}(03)_{\chi\mathrm{PT}}\,\mathrm{MeV}$, as well
as the mixing angle in the quark flavor basis
$\phi=38.8(2.2)_\mathrm{stat}(2.4)_{\chi\mathrm{PT}}^\circ$ in excellent
agreement with other results from phenomenology. Similarly, we include an
analysis for the decay constant parameters, leading to
$f_l=125(5)_\mathrm{stat}(6)_{\chi\mathrm{PT}}\,\mathrm{MeV}$ and
$f_s=178(4)_\mathrm{stat}(1)_{\chi\mathrm{PT}}\,\mathrm{MeV}$. The second error
reflects the uncertainty related to the chiral extrapolation. The data used for
this study has been generated on gauge ensembles provided by the European
Twisted Mass Collaboration with $N_f=2+1+1$ dynamical flavors of Wilson twisted
mass fermions. These ensembles cover a range of pion masses from
$220\,\mathrm{MeV}$ to $500\,\mathrm{MeV}$ and three values of the lattice
spacing. Combining our data with a prediction from chiral perturbation theory,
we give an estimate for the physical $\eta,\eta' \rightarrow \gamma\gamma$
decay widths and the singly-virtual $\eta,\eta'\rightarrow\gamma\gamma^*$
transition form factors in the limit of large momentum transfer.Comment: 22 pages, 8 figures, 12 tables; matching version accepted for
publicatio

### Properties of pseudoscalar flavor singlet mesons from lattice QCD

The central topic of this work are masses and mixing parameters of the Î·-Î·' system, which are investigated within the framework of Wilson twisted mass lattice QCD, using gauge configurations provided by the European Twisted Mass Collaboration. We present the first calculation with Nf=2+1+1 dynamical quark flavors performed at three different values of the lattice spacing and multiple values of the light quark mass, corresponding to charged pion masses ranging from âˆ¼ 230 MeV to âˆ¼ 500 MeV. Moreover, we use selected ensembles which differ only by the value of the strange quark mass while all other parameters are kept fixed in order to obtain information on the strange quark mass dependence of our observables. This allows us to carry out chiral and continuum extrapolations with well-controlled systematics for the mass of the Î· meson. Using the standard method, the statistical error for the Î·' turns out significantly larger due to the large contributions of quark disconnected diagrams and autocorrelation effects. However, employing an improved analysis method based on an excited state subtraction in the connected pieces of the correlation function matrix, it becomes feasible to obtain a result for the Î·' mass with controlled systematics as well. The values for both masses MÎ·=551(8)stat(6)sys MeV and MÎ·' = 1006(54)stat(38)sys(+64)ex MeV turn out to be in excellent agreement with experiment. Considering matrix elements in the quark-flavor basis, one expects the mixing in the Î·-Î·' system to be described reasonably well by a single mixing angle Ï† and two decay constants fl, fs. The required accuracy of the matrix elements is again guaranteed by the aforementioned, improved analysis method, yielding a value of Ï†=46.0(0.9)stat(2.7)sysÂ° for the mixing angle extrapolated to the physical point. In addition we obtain results for the ratios flfPS = 0.859(07)stat(64)sys and fs/fK = 1.166(11)stat(31)sys. We find that our data is indeed described well by a single mixing angle, indicating that the Î·' is mostly a flavor singlet state. Moreover, our results confirm that the charm quark does not contribute to any of the two states within errors. Apart from the flavor singlet sector, we also perform calculations of masses for the remaining light pseudoscalar octet mesons. Matching these masses to two-flavor Wilson chiral perturbation theory allows for a determination of the low energy constants W'6, W'8 and their linear combination c2 which controls the O(a2) mass splitting between charged and neutral pion. We study the dependence of these low energy constants on the number of dynamical quark flavors and for different choices of the lattice action

### A Mixed Action Analysis of $\eta$ and $\eta'$ Mesons

We study $\eta$ and $\eta'$ mesons and their mixing angle in a mixed action
approach with so-called Osterwalder-Seiler valence quarks on a Wilson twisted
mass sea. The gauge configurations have been generated by ETMC for $N_f=2+1+1$
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
$\mathcal{O}(a^2)$. 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 re-tuning of the
simulation parameters in the valence sector only is not sufficient to
compensate for mismatches in the original quark masses.Comment: 30 pages, 8 figures, version accepted for publicatio

### A new method for suppressing excited-state contaminations on the nucleon form factors

One of the most challenging tasks in lattice calculations of baryon form
factors is the analysis and control of excited-state contaminations. Taking the
isovector axial form factors of the nucleon as an example, both a dispersive
representation and a calculation in chiral effective field theory show that the
excited-state contributions become dominant at fixed source-sink separation
when the axial current is spatially distant from the nucleon source location.
We address this effect with a new method in which the axial current is
localized by a Gaussian wave-packet and apply it on a CLS ensemble with
$N_f=2+1$ flavors of O($a$) improved Wilson fermions with a pion mass of
$m_\pi=200\,$MeV.Comment: 7 pages, 6 figures, 1 table, Proceedings for the 36th Annual
International Symposium on Lattice Field Theory, 22-28 July 2018, Michigan
State University, East Lansing, Michigan, US

### Testing the Witten-Veneziano Formula on the Lattice

In this proceeding contribution we report on a test of the famous
Witten-Veneziano formula using lattice techniques. We perform dedicated
quenched simulations and apply the spectral projector method to determine the
topological susceptibility in pure Yang-Mills theory. In order to compute the
relevant meson masses and the flavor singlet decay constant we employ lattice
QCD with $N_f=2+1+1$ dynamical Wilson twisted mass fermions. Taking the
continuum and the $SU(2)$ chiral limits we find good agreement within
uncertainties for both sides of the formula.Comment: 7 pages, 4 figures, presented at the 33rd International Symposium on
Lattice Field Theory, 14 -18 July 2015, Kobe International Conference Center,
Kobe, Japa

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