Improving the lattice axial vector current

For Wilson and clover fermions traditional formulations of the axial vector current do not respect the continuum Ward identity which relates the divergence of that current to the pseudoscalar density. Here we propose to use a point-split or one-link axial vector current whose divergence exactly satisfies a lattice Ward identity, involving the pseudoscalar density and a number of irrelevant operators. We check in one-loop lattice perturbation theory with SLiNC fermion and gauge plaquette action that this is indeed the case including order $O(a)$ effects. Including these operators the axial Ward identity remains renormalisation invariant. First preliminary results of a nonperturbative check of the Ward identity are also presented.Comment: 7 pages, 3 figures, Proceedings of the 33rd International Symposium on Lattice Field Theory, 14-18 July 2015, Kobe, Japa

Particle-number conservation in static-path approximation for thermal superfluid systems

By applying particle-number projection to the static-path approximation (SPA), the heat capacity and the breakdown of pairing correlations are investigated in the thermally excited, superfluid systems 172Yb, 94Mo, and 56Fe. For the heavy nucleus 172Yb, the heat capacities in both the SPA and the number-projected SPA (NPSPA) exhibit an S shape; the difference between the SPA and NPSPA heat-capacity curves is not very large and the particle-number projection thereby enhances the S shape already seen in the SPA. The temperature at which the S-shape of heat capacity curve occurs parallels the temperature of the breakdown of pairing correlations as indicated by the effective pairing gap. However, for the comparatively lighter nuclei 94Mo and 56Fe, the SPA does not produce an S-shaped heat capacity on its own; only after particle-number projection the S shape appears in the heat-capacity curve. For 94Mo, we compare the NPSPA result with thermal odd-even mass differences, which are regarded as a direct measure of the pairing gap.Comment: 7 pages, 5 figures, accepted for publication in Phys. Rev.

The electric dipole moment of the neutron from 2+1 flavor lattice QCD

We compute the electric dipole moment d_n of the neutron from a fully dynamical simulation of lattice QCD with 2+1 flavors of clover fermions and nonvanishing theta term. The latter is rotated into the pseudoscalar density in the fermionic action using the axial anomaly. To make the action real, the vacuum angle theta is taken to be purely imaginary. The physical value of d_n is obtained by analytic continuation. We find d_n = -3.8(2)(9) x 10^{-16} [theta e cm], which, when combined with the experimental limit on d_n, leads to the upper bound theta < 7.6 x 10^{-11}.Comment: 12 pages, 8 figures, matches PRL published versio

Demonstration of a Transportable 1 Hz-Linewidth Laser

We present the setup and test of a transportable clock laser at 698 nm for a strontium lattice clock. A master-slave diode laser system is stabilized to a rigidly mounted optical reference cavity. The setup was transported by truck over 400 km from Braunschweig to D\"usseldorf, where the cavity-stabilized laser was compared to a stationary clock laser for the interrogation of ytterbium (578 nm). Only minor realignments were necessary after the transport. The lasers were compared by a Ti:Sapphire frequency comb used as a transfer oscillator. The thus generated virtual beat showed a combined linewidth below 1 Hz (at 1156 nm). The transport back to Braunschweig did not degrade the laser performance, as was shown by interrogating the strontium clock transition.Comment: 3 pages, 4 figure

Renormalisation of composite operators in lattice QCD: perturbative versus nonperturbative

The perturbative and nonperturbative renormalisation of quark-antiquark operators in lattice QCD with two flavours of clover fermions is investigated within the research programme of the QCDSF collaboration. Operators with up to three derivatives are considered. The nonperturbative results based on the RI-MOM scheme are compared with estimates from one- and two-loop lattice perturbation theory.Comment: 7 pages, 4 figures, talk given at the XXVIII International Symposium on Lattice Field Theory (Lattice 2010), June 14-19 2010, Villasimius, Ital

Isospin splittings of meson and baryon masses from three-flavor lattice QCD + QED

Lattice QCD simulations are now reaching a precision where isospin breaking effects become important. Previously, we have developed a program to systematically investigate the pattern of flavor symmetry beaking within QCD and successfully applied it to meson and baryon masses involving up, down and strange quarks. In this Letter we extend the calculations to QCD + QED and present our first results on isospin splittings in the pseudoscalar meson and baryon octets. In particular, we obtain the nucleon mass difference of M_n-M_p=1.35(18)(8)\,\mbox{MeV} and the electromagnetic contribution to the pion splitting M_{\pi^+}-M_{\pi^0}=4.60(20)\,\mbox{MeV}. Further we report first determination of the separation between strong and electromagnetic contributions in the $\bar{MS}$ scheme.Comment: 14 pages, 10 figures, text and figures added, Journal versio

Determining the scale in Lattice QCD

We discuss scale setting in the context of 2+1 dynamical fermion simulations where we approach the physical point in the quark mass plane keeping the average quark mass constant. We have simulations at four beta values, and after determining the paths and lattice spacings, we give an estimation of the phenomenological values of various Wilson flow scales.Comment: 7 pages; talk presented at Lattice 2015, the 33rd International Symposium on Lattice Field Theory 4 -18 July 2015 at Kobe International Conference Center, Kobe, Japan; PoS(LATTICE2015)26

Wilson flow and scale setting from lattice QCD

We give a determination of the phenomenological value of the Wilson (or gradient) flow scales t0 and w0 for 2+1 flavours of dynamical quarks. The simulations are performed keeping the average quark mass constant, which allows the approach to the physical point to be made in a controlled manner. O(a) improved clover fermions are used and together with four lattice spacings this allows the continuum extrapolation to be taken.Comment: 24 pages, 11 figure