Bastian B. Brandt

Thermal correlators in the ρ channel of two-flavor QCD

A bstractWe present a lattice QCD calculation with two dynamical flavors of the isovector vector correlator in the high-temperature phase. We analyze the correlator in terms of the associated spectral function, for which we review the theoretical expectations. In our main analysis, we perform a fit for the difference of the thermal and vacuum spectral functions, and we use an exact sum rule that constrains this difference. We also perform a direct fit for the thermal spectral function, and obtain good agreement between the two analyses for frequencies below the two-pion threshold. Under the assumption that the spectral function is smooth in that region, we give an estimate of the electrical conductivity.

Probing boundary-corrections to Nambu-Goto open string energy levels in 3d SU(2) gauge theory

We measure the energy levels of the excitations of the flux tube between static quark and antiquark in three-dimensional SU(2) gauge theory. Combining exponential error reduction techniques and a variational method we are able to reduce the errors for the excited states significantly and to extract excited states in distinct parity and charge conjugation channels. It is conjectured that the infrared behavior (at large

Spectrum of the QCD flux tube in 3d SU(2) lattice gauge theory

q\overline q

The pion vector form factor from lattice QCD and NNLO chiral perturbation theory

separation R) of the flux tube is governed by an effective string theory. Indeed previous simulations show good agreement between lattice data and predictions from Nambu-Goto string theory. Recently, new results on the effective string theory obtained corrections to the Nambu-Goto predictions and showed that for the open string in three dimensions first corrections should appears at order 1/R4. They correspond to boundary terms in the worldsheet field theory. These corrections are presumably small for the ground state, but significantly larger for the excited states and lift the degeneracies of the free theory. Assuming this functional form of the correction, we obtain for the coefficient b2 = −0.5(2)(2).

Charge transport and vector meson dissociation across the thermal phase transition in lattice QCD with two light quark flavors

Abstract Evidence from the lattice suggests that formation of a flux tube between a q q ¯ pair in the QCD vacuum leads to quark confinement. For large separations between the quarks, it is conjectured that the flux tube has a behavior similar to an oscillating bosonic string, supported by lattice data for the groundstate q q ¯ potential. We measure the excited states of the flux tube in 3d SU ( 2 ) gauge theory with three different couplings inside the scaling region. We compare our results to predictions of effective string theories.

On the strength of the U A (1) anomaly at the chiral phase transition in N f = 2 QCD

A bstractWe present a comprehensive study of the electromagnetic form factor, the decay constant and the mass of the pion computed in lattice QCD with two degenerate O(a)-improved Wilson quarks at three different lattice spacings in the range 0.05 – 0.08 fm and pion masses between 280 and 630 MeV at mπL ≥ 4. Using partially twisted boundary conditions and stochastic estimators, we obtain a dense set of precise data points for the form factor at very small momentum transfers, allowing for a model-independent extraction of the charge radius. Chiral Perturbation Theory (ChPT) augmented by terms which model lattice artefacts is then compared to the data. At next-to-leading order the effective theory fails to produce a consistent description of the full set of pion observables but describes the data well when only the decay constant and mass are considered. By contrast, using the next-to-next-to-leading order expressions to perform global fits result in a consistent description of all data. We obtain

Form factors in lattice QCD

\left\langle {r_{\pi}^2} \right\rangle =0.481\left( {33} \right)\left( {13} \right)

A relation between screening masses and real-time rates

fm2 as our final result for the charge radius at the physical point. Our calculation also yields estimates for the pion decay constant in the chiral limit, Fπ/F = 1.080(16)(6), the quark condensate,

QCD thermodynamics with O(a) improved Wilson fermions at

\sum {\frac{{{1 \left/ {3} \right.}}}{\mathrm{MS}}\left( {2\;\mathrm{GeV}} \right)} =261\left( {13} \right)(1)

N_f=2

MeV and several low-energy constants of SU(2) ChPT.