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Schäfer, A.

Publications and source records attributed to Schäfer, A..

13 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Azimuthal single- and double-spin asymmetries in semi-inclusive deep-inelastic lepton scattering by transversely polarized protons

A comprehensive set of azimuthal single-spin and double-spin asymmetries in semi-inclusive leptoproduction of pions, charged kaons, protons, and antiprotons from transversely polarized protons is presented. These asymmetries include the previously published HERMES results on Collins and Sivers asymmetries, the analysis of which has been extended to include protons and antiprotons and also to an extraction in a three-dimensional kinematic binning and enlarged phase space. They are complemented by corresponding results for the remaining four single-spin and four double-spin asymmetries allowed in the one-photon-exchange approximation of the semi-inclusive deep-inelastic scattering process for target-polarization orientation perpendicular to the direction of the incoming lepton beam. Among those results, significant non-vanishing cos ( Φ–Φ S ) modulations provide evidence for a sizable worm-gear (II) distribution, ${g}_{1\mathrm{T}}^q\left(x,{\mathrm{p}}_T^2\right)$. Most of the other modulations are found to be consistent with zero with the notable exception of large sin ( Φ S ) modulations for charged pions and K + .

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2017

Ratios of helicity amplitudes for exclusive ρ 0 electroproduction on transversely polarized protons

Exclusive ρ 0 -meson electroproduction is studied by the HERMES experiment, using the 27.6 GeV longitudinally polarized electron/positron beam of HERA and a transversely polarized hydrogen target, in the kinematic region 1.0 GeV 2 < Q 2 < 7.0 GeV 2 , 3.0 GeV < W < 6.3 GeV, and –t' < 0.4 GeV 2 . Using an unbinned maximum-likelihood method, 25 parameters are extracted. These determine the real and imaginary parts of the ratios of several helicity amplitudes describing ρ 0 -meson production by a virtual photon. The denominator of those ratios is the dominant amplitude, the nucleon-helicity-non-flip amplitude F 01/201/2 , which describes the production of a longitudinal ρ 0 -meson by a longitudinal virtual photon. The ratios of nucleon-helicity-non-flip amplitudes are found to be in good agreement with those from the previous HERMES analysis. The transverse target polarization allows for the first time the extraction of ratios of a number of nucleon-helicity-flip amplitudes to F 01/201/2 . Results obtained in a handbag approach based on generalized parton distributions taking into account the contribution from pion exchange are found to be in good agreement with these ratios. Within the model, the data favor a positive sign for the π - ρ transition form factor. By also exploiting the longitudinal beam polarization, a total of 71 ρ 0 spin-density matrix elements is determined from the extracted 25 parameters, in contrast to only 53 elements as directly determined in earlier analyses.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2016

Lattice QCD study of the Boer-Mulders effect in a pion

The three-dimensional momenta of quarks inside a hadron are encoded in transverse momentumdependent parton distribution functions (TMDs). Our work presents an exploratory lattice QCD study of a TMD observable in the pion describing the Boer-Mulders effect, which is related to polarized quark transverse momentum in an unpolarized hadron. The primary goal is to gain insight into the behavior of TMDs as a function of a Collins-Soper evolution parameter, $\hat{ζ}$, which quantifies the rapidity difference between the hadron momentum and a vector describing the trajectory of the struck quark, e.g., in a semiinclusive deep-inelastic scattering (SIDIS) process. The lattice calculation, performed at the pion mass m π = 518 MeV, utilizes a definition of TMDs via hadronic matrix elements of a quark bilocal operator with a staple-shaped gauge connection; in this context, the evolution parameter is related to the staple direction. By parametrizing the aforementioned matrix elements in terms of invariant amplitudes, the problem can be cast in a Lorentz frame suited for the lattice calculation. Aided by the lower mass of the pion, compared to the nucleon studied previously, the present investigation of pion TMD observables constitutes an important step towards the quantitative study of the physically important regime of large relative rapidity where the dependence on $\hat{ζ}$ appears to approach a limit. Even though matching to perturbative evolution equations in $\hat{ζ}$ is not yet available, extrapolations based on Ansätze containing inverse powers of $\hat{ζ}$ yield stable results with an uncertainty as low as 20%, and both upper and lower bounds for the asymptotics are obtained. In passing, the similarity between the Boer-Mulders effects extracted in the pion and the nucleon is indicated.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2014

Exploring the use of dynamic linear panel data models for evaluating energy/economy/environment models — an application for the transportation sector

This paper evaluates the RoSE transportation sector scenarios of the GCAM and REMIND models for the U.S. region by comparing these models’ projected future final energy use levels (1) to historical trends and (2) across each other. To estimate and compare the intrinsic model elasticities, we explore the use of dynamic linear panel data models. On the basis of 26 scenarios (panels) from each of the two energy system models, our analysis suggests that most intrinsic model elasticities with respect to final energy use are roughly comparable to each other and to those observed historically. The key difference is the income elasticity of final energy use, which is significantly smaller for the two energy models than that derived from the historical development. This difference and other minor inconsistencies are interpreted through key assumptions underlying both energy system models.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2011

Thermalization of Strongly Coupled Field Theories

Using the holographic mapping to a gravity dual, we calculate 2-point functions, Wilson loops, and entanglement entropy in strongly coupled field theories in d=2, 3, and 4 to probe the scale dependence of thermalization following a sudden injection of energy. For homogeneous initial conditions, the entanglement entropy thermalizes slowest and sets a time scale for equilibration that saturates a causality bound. The growth rate of entanglement entropy density is nearly volume-independent for small volumes but slows for larger volumes. In this setting, the UV thermalizes first.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗