arXiv subjects

Bastian B. Brandt

Publications and source records attributed to Bastian B. Brandt.

66 records · Page 4Linked to original sources

arXiv2013

Spectrum of the open QCD flux tube in d=2+1 and its effective string description

Simulations in lattice gauge theory suggest that the formation of a flux tube between quark and antiquark leads to quark confinement. It is conjectured that the infrared behaviour of the flux tube is governed by an effective string theory and simulations show good agreement between lattice data and its predictions. To next-to leading order ($R^{-3}$) in the inverse $q\bar{q}$ separation $R$ the effective string theory is equivalent to Nambu-Goto string theory. For the open flux tube in three dimensions corrections appear at order $R^{-4}$. We compare these predictions to high-accuracy measurements of the groundstate energy of the flux tube in 3d SU(2) and SU(3) gauge theory and extract the coefficient of the leading order boundary term in the effective action.

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arXiv2013

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

We 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 0.05-0.08fm and pion masses between 280 and 630MeV at m_pi L >~ 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 =0.481(33)(13)fm^2 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, F_pi/F=1.080(16)(6), the quark condensate, Sigma^{1/3}_MSbar(2GeV)=261(13)(1)MeV and several low-energy constants of SU(2) ChPT.

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arXiv2013

Two-flavour lattice QCD correlation functions in the deconfinement transition region

We report on 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 by performing a fit for the difference of the thermal and vacuum spectral functions, using also an exact sum rule that constrains this difference. Additonally we carry out 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.

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arXiv2013

The electromagnetic form factor of the pion in two-flavour lattice QCD

We present the current status of our lattice calculation of the electromagnetic form factor of the pion with two flavours of non-perturbatively O(a)-improved Wilson fermions. Using twisted boundary conditions and stochastic sources we obtain accurate results with a fine momentum resolution near $q^2=0$. This enables the computation of the charge radius without model dependence. The ensembles cover various lattice spacings and pion masses, ranging down to 250 MeV. This allows to compare the data to continuum chiral perturbation theory to NNLO including corrections of finite lattice spacing to perform a simultaneous chiral and continuum extrapolation. An estimate for the systematic error resulting from the extrapolation can be obtained by looking at the spread of results obtained from other functional forms such as polynomials.

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arXiv2012

Thermal Correlators in the ρ channel of two-flavor QCD

We 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.

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arXiv2012

QCD thermodynamics with two flavours of Wilson fermions on large lattices

We explore the phase diagram of two flavour QCD at vanishing chemical potential using dynamical O(a)-improved Wilson quarks. In the approach to the chiral limit we use lattices with a temporal extent of N_t=16 and spatial extent L=32,48 and 64 to enable the extrapolation to the thermodynamic limit with small discretisation effects. In addition to an update on the scans at constant κ, reported earlier, we present first results from scans along lines of constant physics at a pion mass of 290 MeV. We probe the transition using the Polyakov loop and the chiral condensate, as well as spectroscopic observables such as screening masses.

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arXiv2011

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.

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arXiv2011

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 q\bar{q} 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/R^4. 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 b_2=-0.5(2)(2).

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arXiv2010

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.

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arXiv2010

Towards the N_f=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 $\Ord(a)$ improved Wilson fermions. We start with $N_{t}=12$ and 16 lattices and pion masses of 600 to 450 MeV, aiming at chiral and continuum limits with light quarks.

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arXiv2009

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

Evidence from the lattice suggests that formation of a flux tube between a $q\bar{q}$ pair in the QCD vacuum leads to quark confinement. For large separations between the quarks, it is conjectured that the flux tube has a behaviour similar to an oscillating bosonic string, supported by lattice data for the groundstate $q\bar{q}$ 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.

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arXiv2007

Luescher-Weisz algorithm for excited states of the QCD flux-tube

We present a version of the Luescher-Weisz multilevel algorithm ideally suited for studying excited states of the QCD flux tube. While the original version achieved error reduction only in the temporal direction, the new algorithm reduces fluctuations in the sources as well. We report on the implementation of this algorithm as well as improvement over the older method. We also present first results, where we see a good agreement with theoretical predictions from bosonic string models.

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