Calculation of the pion electromagnetic form factor from lattice QCD

We present a lattice calculation of the vector form factor of the pion for two flavours of non-perturbatively O(a) improved Wilson fermions. For the measurements we utilise the CLS ensembles which include various lattice spacings and pion masses down to about 250 MeV. To obtain a fine momentum resolution near zero momentum transfer (q^2) partially twisted boundary conditions are employed using several twist angles. Due to the fine resolution around q^2=0 we are able to determine the slope of the form factor and, in turn, extract the charge radius of the pion without any model dependence. The results for the form factor and the charge radius are then compared to chiral perturbation theory and phenomenological models which are used to extrapolate the results to the physical point.Comment: 4 pages, 4 figures, talk presented at Hadron 2011: 14th International Conference on Hadron Spectroscopy, Munich, German

Twisted-mass reweighting for O(a) improved Wilson fermions

We test the reweighting of the quark determinant of O(a) improved Wilson fermions in the domain-decomposed hybrid Monte-Carlo algorithm. Specifically, we implement a reweighting in a twisted-mass parameter proposed by Palombi and L\"uscher in $N_{\rm f}=2$ QCD. We find that at equal acceptance rate, the algorithm is significantly more stable on a $32\times64^3$ lattice upon switching on the reweighting parameter. At the same time, the reweighting factor does not fluctuate strongly and hence is under control. At equal statistics, the uncertainty on the pion correlator is comparable to the case of the standard, unreweighted algorithm.Comment: 7 pages, 5 figures, XXIX International Symposium On Lattice Field Theor

QCD thermodynamics with O(a) improved Wilson fermions at Nf=2

We present an update of our study of the phase diagram of two-flavour QCD at zero baryon density with dynamical $O(a)$ improved Wilson quarks. All simulations are done on lattices with a temporal extent of $N_t=16$ and spatial extent $L=32,48$ and 64, ensuring that discretisation effects are small and finite size effects can be controlled. In the approach to the chiral limit we currently have three scans with pion masses between 540 and 200 MeV. In this proceedings article the focus is on the new scan at $m_\pi=200$ MeV and the measurement of screening masses. We also present first results concerning a test of scaling in the approach to the chiral limit and the chiral extrapolation of the difference of screening masses in scalar and pseudoscalar channels, which provides a measure for the strength of the anomalous breaking of the $U_A(1)$ symmetry.Comment: 7 pages, 5 figures, talk presented at the 31st International Symposium on Lattice Field Theory (Lattice 2013), 29 July - 3 August 2013, Mainz, German

Towards the N_f=2 deconfinement transition temperature with O(a) improved Wilson fermions: An update

We give an update on our current project to determine the transition temperature and the order of the deconfinement transition in the chiral limit of two flavour QCD. We use nonperturbatively O(a) improved Wilson fermions of the Sheikholeslami-Wohlert type, employing the efficient deflation accelerated DDHMC algorithm. We start at lattices with N_t>=12 and pion masses below 600 MeV, aiming at chiral and continuum limits with light quarks.Comment: 3 pages, 4 figures, talk given at "Quark Confinement and the Hadron Spectrum IX", Madrid, Spain, Aug. 30-Sep. 3, 2010, submitted to AIP Conference Proceeding

Towards the Nf = 2 deconfinement transition temperature with O(a) improved Wilson fermions

A lot of effort in lattice simulations over the last years has been devoted to studies of the QCD deconfinement transition. Most state-of-the-art simulations use rooted staggered fermions, while Wilson fermions are affected by large systematic uncertainties, such as coarse lattices or heavy sea quarks. Here we report on an ongoing study of the transition, using two degenerate flavours of nonperturbatively O(a) improved Wilson fermions. We start with Nt = 12 and 16 lattices and pion masses of 600 to 450 MeV, aiming at chiral and continuum limits with light quarks

A new representation of the Adler function for lattice QCD

We address several aspects of lattice QCD calculations of the hadronic vacuum polarization and the associated Adler function. We implement a representation derived previously which allows one to access these phenomenologically important functions for a continuous set of virtualities, irrespective of the flavor structure of the current. Secondly we present a theoretical analysis of the finite-size effects on our particular representation of the Adler function, based on the operator product expansion at large momenta and on the spectral representation of the Euclidean correlator at small momenta. Finally, an analysis of the flavor structure of the electromagnetic current correlator is performed, where a recent theoretical estimate of the Wick-disconnected diagram contributions is rederived independently and confirmed.Comment: 9 pages, 5 figure

Testing the strength of the UA(1)\text{U}_A(1) anomaly at the chiral phase transition in two-flavour QCD

We study the thermal transition of QCD with two degenerate light flavours by lattice simulations using $\mathcal{O}(a)$-improved Wilson quarks. Particular emphasis lies on the pattern of chiral symmetry restoration, which we probe via the static screening correlators. On $32^3$ volumes we observe that the screening masses in transverse iso-vector vector and axial-vector channels become degenerate at the transition temperature. The splitting between the screening masses in iso-vector scalar and pseudoscalar channels is strongly reduced compared to the splitting at zero temperature and is actually consistent with zero within uncertainties. In this proceedings article we extend our studies to matrix elements and iso-singlet correlation functions. Furthermore, we present results on larger volumes, including first results at the physical pion mass.Comment: 10 pages, 9 figures, invited contribution to the 9th International Workshop on Chiral Dynamics, Sept. 17-21, 2018, Duke University, Durham, NC, US