arXiv subjects
Publications and source records attributed to Andreas Schäfer.
We reexamine the scale dependence of the ETQS (Efremov-Teryaev-Qiu-Sterman) twist-3 matrix element which has been studied already by the four different groups with conflicting results Ref.[1-4]. We find that we can in fact reproduce the results of Ref.[4] with the method Ref.[4] when we treat some subtleties with greater care, thus easing the mentioned conflict.
We study the longitudinal-transverse double spin asymmetry $A_{LT}$ for direct photon production in nucleon-nucleon scattering by using the collinear twist-3 approach. This asymmetry, which, for instance, could be measured at RHIC, contains a complete set of collinear twist-3 correlation functions in a transversely polarized nucleon.
We present a calculation of generalized baryon form factors in the framework of three-flavor covariant baryon chiral perturbation theory at leading one-loop order. This is needed for lattice calculations of the first moments of generalized parton distribution functions. The formulae we derive can be used to guide the chiral extrapolation of such lattice results.
We calculate the transverse correlation of fluctuations of the deposited energy density in nuclear collisions in the framework of the Gaussian color glass condensate model.
We investigate how the topological charge density in lattice QCD simulations is affected by violations of chiral symmetry caused by the fermion action. To this end we compare lattice configurations generated with a number of different actions including first configurations generated with exact dynamical overlap quarks. We visualize the topological profiles after mild smearing. In the topological charge correlator we measure the size of the positive core, which is known to shrink to zero extension in the continuum limit. To leading order we find the core size to scale linearly with the lattice spacing with the same coefficient for all actions, even including quenched simulations. In the subleading term the different actions vary over a range of about 10 %. Our findings suggest that non-chiral lattice actions at current lattice spacings do not differ much for observables related to topology, both among themselves and compared to overlap fermions.
We compute corrections to both the isovector anomalous magnetic moment and the isovector electromagnetic current of the nucleon to $O(p^3)$ in the framework of covariant two-flavor Baryon Chiral Perturbation Theory. We then apply these corrections to lattice data for the anomalous magnetic moment from the LHPC, RBC & UKQCD and QCDSF collaborations.
We investigate how the topological charge density in lattice QCD simulations is affected by violations of chiral symmetry in different fermion actions. To this end we compare lattice configurations generated with a number of different actions including first configurations generated with exact overlap quarks. We visualize the topological profiles after mild smearing. In the topological charge correlator we measure the size of the positive core, which is known to vanish in the continuum limit. To leading order we find the core size to scale linearly with the lattice spacing with the same coefficient for all actions, even including quenched simulations. In the subleading term the different actions vary over a range of about 10 %. Our findings suggest that non-chiral lattice actions at current lattice spacings do not differ much for this specific observable related to topology, both among themselves and compared to overlap fermions.
The energy loss parameter $\hat q$ is one of the fundamental transport parameters of hadronic matter. Using the twist-4 collinear approach, we show that the $\cos ϕ$ azimuthal asymmetry in unpolarized semi-inclusive deeply inelastic scattering (SIDIS) off a large nucleus at intermediate transverse momentum is a sensitive observable for its determination. The effect is due to the suppression of the azimuthal asymmetry by final-state multiple scattering.
We review current ideas on entropy production during the different stages of a relativistic nuclear collision. This includes recent results on decoherence entropy and the entropy produced during the hydrodynamic phase by viscous effects. We start by a discussion of decoherence caused by gluon bremsstrahlung in the very first interactions of gluons from the colliding nuclei. We then present a general framework, based on the Husimi distribution function, for the calculation of entropy growth in quantum field theories, which is applicable to the early ("glasma") phase of the collision during which most of the entropy is generated. The entropy calculated from the Husimi distribution exhibits linear growth when the quantum field contains unstable modes and the growth rateis asymptotically equal to the Kolmogorov-Sinaï (KS) entropy. We outline how the approach can be used to investigate the problem of entropy production in a relativistic heavy-ion reaction from first principles. Finally we discuss some recent results on entropy production in the strong coupling limit, as obtained from AdS/CFT duality.
Within the framework of two-flavor covariant baryon chiral perturbation theory we have expressed the Chernyak-Zhitnitsky, Ioffe and Dosch currents in terms of chiral fields to provide leading one-loop extrapolation formulae for the leading and next-to-leading twist normalization constants $f_N$, $λ_1$ and $λ_2$. Finite volume effects due to pion loops have been taken into account. The occurring low energy constants are fitted to data obtained from recent lattice QCD simulations in order to extract the values at the physical point.
We present a study of the Drell-Yan process in pion-proton collisions including next-to-leading-logarithmic threshold-resummed contributions. We analyze rapidity-integrated as well as rapidity-differential cross sections in the kinematic regime relevant for the COMPASS fixed target experiment. We find that resummation leads to a significant enhancement of the cross section compared to fixed-order calculations in this regime. Particularly large corrections arise at large forward and backward rapidities of the lepton pair. We also study the scale dependence of the cross section and find it to be substantially reduced by threshold resummation.
Using the AdS/CFT correspondence, we probe the scale-dependence of thermalization in strongly coupled field theories following a quench, via calculations of two-point functions, Wilson loops and entanglement entropy in d=2,3,4. In the saddlepoint approximation these probes are computed in AdS space in terms of invariant geometric objects - geodesics, minimal surfaces and minimal volumes. Our calculations for two-dimensional field theories are analytical. In our strongly coupled setting, all probes in all dimensions share certain universal features in their thermalization: (1) a slight delay in the onset of thermalization, (2) an apparent non-analyticity at the endpoint of thermalization, (3) top-down thermalization where the UV thermalizes first. For homogeneous initial conditions the entanglement entropy thermalizes slowest, and sets a timescale for equilibration that saturates a causality bound over the range of scales studied. The growth rate of entanglement entropy density is nearly volume-independent for small volumes, but slows for larger volumes.
Results of hadron spectroscopy with two dynamical mass-degenerate chirally improved quarks are presented. Three ensembles with pion masses of 322(5), 470(4) and 525(7) MeV, lattices of size 16^3 \times 32, and lattice spacings close to 0.15 fm are investigated. We discuss the possible appearance of scattering states, considering masses and eigenvectors. Partially quenched results in the scalar channel suggest the presence of a 2-particle state, however, in most channels we cannot identify them. Where available, we compare to results from quenched simulations using the same action.
We determine the valence parton distribution function of the pion by performing a new analysis of data for the Drell-Yan process $π^- N\to μ^+μ^-X$. Compared to previous analyses, we include next-to-leading-logarithmic threshold resummation effects in the calculation of the Drell-Yan cross section. As a result of these, we find a considerably softer valence distribution at high momentum fractions $x$ than obtained in previous next-to-leading order analyses, in line with expectations based on perturbative-QCD counting rules or Dyson-Schwinger equations.
In QCD chiral symmetry is explicitly broken by quark masses, the effect of which can be described reliably by chiral perturbation theory. Effects of explicit chiral symmetry breaking by the lattice regularisation of the Dirac operator, typically parametrised by the residual mass, should be negligible for almost all observables if the residual mass of the Dirac operator is much smaller than the quark mass. However, maintaining a small residual mass becomes increasingly expensive as the quark mass decreases towards the physical value and the continuum limit is approached. We investigate the feasibility of using a new approximately chiral Dirac operator with a small residual mass as an alternative to overlap and domain wall fermions for lattice simulations. Our Dirac operator is constructed from a Zolotarev rational approximation for the matrix sign function that is optimal for bulk modes of the Hermitian kernel Dirac operator but not for the low-lying parts of its spectrum. We test our operator on various 32^3\times64 lattices, comparing the residual mass and the performance of the Hybrid Monte Carlo algorithm at a similar lattice spacing and pion mass with a hyperbolic tangent operator as used by domain wall fermions. We find that our approximations have a significantly smaller residual mass than domain wall fermions at a similar computational cost, and still admit topological charge change.
We derive a nonlocal effective Lagrangian for the chiral magnetic effect. An electric field is generated by winding number fluctuations of the nonabelian gauge field in the presence of a strong magnetic field. We estimate the magnitude of charge asymmetry fluctuations with respect to the reaction plane induced by the chiral magnetic effect in relativistic heavy ion collisions to be less than $10^{-6}$, several orders of magnitude smaller than the signal observed in the STAR experiment.
In high energy heavy ion collisions as well as in astrophysical objects like magnetars extreme magnetic field strengths are reached. Thus, there exists a need to calculate divers QED processes to all orders in the magnetic field. We calculate the vacuum polarization graph in second order of the electric field and all orders of the magnetic field resulting in a generalization of the Euler-Heisenberg Lagrangian. We perform the calculation in the effective Lagrangian approach of J. Schwinger as well as using modified Feynman rules. We find that both approaches give the same results provided that the different finite renormalization terms are taken into account. Our results imply that any quantitative explanation of the recently proposed Chiral Magnetic Effect has to take 'Strong QED' effects into account, because these corrections are huge.
We present results of meson and baryon spectroscopy using the Chirally Improved Dirac operator on lattices of size 16**3 x 32 with two mass-degenerate light sea quarks. Three ensembles with pion masses of 322(5), 470(4) and 525(7) MeV and lattice spacings close to 0.15 fm are investigated. Results on ground and excited states for several channels are given, including spin two mesons and hadrons with strange valence quarks. The analysis of the states is done with the variational method, including two kinds of Gaussian sources and derivative sources. We obtain several ground states fairly precisely and find radial excitations in various channels. Excited baryon results seem to suffer from finite size effects, in particular at small pion masses. We discuss the possible appearance of scattering states in various channels, considering masses and eigenvectors. Partially quenched results in the scalar channel suggest the presence of a 2-particle state, however, in most channels we cannot identify them. Where available, we compare our results to results of quenched simulations using the same action.