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Andreas Schäfer

Publications and source records attributed to Andreas Schäfer.

At least 109 records · Page 6Linked to original sources
arXiv2016

Novel quark smearing for hadrons with high momenta in lattice QCD

Hadrons in lattice QCD are usually created employing smeared interpolators. We introduce a new quark smearing that allows us to maintain small statistical errors and good overlaps of hadronic wavefunctions with the respective ground states, also at high spatial momenta. The method is successfully tested for the pion and the nucleon at a pion mass $m_π\approx 295$ MeV and momenta as high as 2.8 GeV. We compare the results obtained to dispersion relations and suggest further optimizations.

hep-lat↗
arXiv2016

Direct determinations of the nucleon and pion $σ$ terms at nearly physical quark masses

We present a high statistics study of the pion and nucleon light and strange quark sigma terms using $N_f=2$ dynamical non-perturbatively improved clover fermions with a range of pion masses down to $m_π\sim 150$ MeV and several volumes, $Lm_π=3.4$ up to $6.7$, and lattice spacings, $a=0.06-0.08$ fm, enabling a study of finite volume and discretisation effects for $m_π\gtrsim 260$ MeV. Systematics are found to be reasonably under control. For the nucleon we obtain $σ_{πN}=35(6)$ MeV and $σ_s=35(12)$ MeV, or equivalently in terms of the quark fractions, $f_{T_u}=0.021(4)$, $f_{T_d}=0.016(4)$ and $f_{T_s}=0.037(13)$, where the errors include estimates of both the systematic and statistical uncertainties. These values, together with perturbative matching in the heavy quark limit, lead to $f_{T_c}=0.075(4)$, $f_{T_b}=0.072(2)$ and $f_{T_t}=0.070(1)$. In addition, through the use of the (inverse) Feynman-Hellmann theorem our results for $σ_{πN}$ are shown to be consistent with the nucleon masses determined in the analysis. For the pion we implement a method which greatly reduces excited state contamination to the scalar matrix elements from states travelling across the temporal boundary. This enables us to demonstrate the Gell-Mann-Oakes-Renner expectation $σ_π=m_π/2$ over our range of pion masses.

hep-lat↗
arXiv2016

The decays $Λ_{b,c}\to N^*\, l\,ν$ in QCD

We present an exploratory study of the $Λ_{c,b}\to N^*$-form factors and the semileptonic decay width within the framework of light-cone sum rules. We use two different methods and two different interpolating currents for the $Λ_{c,b}$. As interpolating currents we choose an axial-vector like and a pseudoscalar like current. Our results show that the procedure of eliminating negative parity partners is not well suited for the case at hand and that the second approach using structures with highest powers of $p_+$ with an axial-vector like interpolating current gives the most reliable results. Our predictions are based on the models obtained in \cite{Anikin:2015ita,Braun:2014wpa}. The largest uncertainty comes from the uncertainty of the twist 4 parameters $η_{10},\,η_{11}$ and we take the spread between the two models in \cite{Anikin:2015ita} as a measure for this. We get \begin{eqnarray} Γ(Λ_b\to N^*(1535) l ν)&=&\left(0.0058^{+0.0010}_{-0.0009}\right)\cdot\left(\dfrac{V_{ub}}{3.5\cdot10^{-3}}\right)^2,\quad\mbox{LCSR(1)} \nonumber Γ(Λ_b\to N^*(1535) l ν)&=&\left(0.00070^{+0.00012}_{-0.00011}\right)\cdot\left(\dfrac{V_{ub}}{3.5\cdot10^{-3}}\right)^2,\quad\mbox{LCSR(2)} \nonumber Γ(Λ_c\to N^*(1535) l ν)&=&\left(0.0064^{+0.0012}_{-0.0011}\right)\cdot \left(\dfrac{V_{cd}}{0.225}\right)^2,\quad\mbox{LCSR(1)}\nonumber Γ(Λ_c\to N^*(1535) l ν)&=&\left(0.00077^{+0.00016}_{-0.00014}\right)\cdot \left(\dfrac{V_{cd}}{0.225}\right)^2,\quad\mbox{LCSR(2)}\nonumber \end{eqnarray} as predictions for the respective decay widths, where LCSR(1) and LCSR(2) refer to the two different models of the distribution amplitudes of the $N^*$. It is seen that even a rough measurement of these decays will greatly help to discriminate different models.

hep-ph↗
arXiv2016

$ρ$ and $K^*$ resonances on the lattice at nearly physical quark masses and $N_f=2$

Working with a pion mass $m_π\approx 150$ MeV, we study $ππ$ and $Kπ$ scattering using two flavours of non-perturbatively improved Wilson fermions at a lattice spacing $a\approx 0.071$ fm. Employing two lattice volumes with linear spatial extents of $N_s=48$ and $N_s=64$ points and moving frames, we extract the phase shifts for p-wave $ππ$ and $Kπ$ scattering near the $ρ$ and $K^*$ resonances.Comparing our results to those of previous lattice studies, that used pion masses ranging from about 200 MeV up to 470 MeV, we find that the coupling $g_{ρππ}$ appears to be remarkably constant as a function of $m_π$.

hep-lat↗
arXiv2016

Light-cone distribution amplitudes of the baryon octet

We present results of the first ab initio lattice QCD calculation of the normalization constants and first moments of the leading twist distribution amplitudes of the full baryon octet, corresponding to the small transverse distance limit of the associated S-wave light-cone wave functions. The P-wave (higher twist) normalization constants are evaluated as well. The calculation is done using $N_f=2+1$ flavors of dynamical (clover) fermions on lattices of different volumes and pion masses down to 222 MeV. Significant SU(3) flavor symmetry violation effects in the shape of the distribution amplitudes are observed.

hep-lat↗
arXiv2016

Nucleon generalized form factors from lattice QCD with nearly physical quark masses

We determine generalized form factors of the nucleon from lattice simulations with $N_f = 2$ mass-degenerate non-perturbatively improved Wilson-Sheikholeslami-Wohlert fermions down to a pion mass of 150 MeV. We also present the resulting isovector quark angular momentum. Possible excited-state contaminations are investigated with correlated simultaneous fits.

hep-lat↗
arXiv2015

Spin Decomposition of Electron in QED

We perform a systematic study on the spin decomposition of an electron in QED at one-loop order. It is found that the electron orbital angular momentum defined in Jaffe-Manohar and Ji spin sum rules agrees with each other, and the so-called potential angular momentum vanishes at this order. The calculations are performed in both dimensional regularization and Pauli-Villars regularization for the ultraviolet divergences, and they lead to consistent results. We further investigate the calculations in terms of light-front wave functions, and find a missing contribution from the instantaneous interaction in light-front quantization. This clarifies the confusing issues raised recently in the literature on the spin decomposition of an electron, and will help to consolidate the spin physics program for nucleons in QCD.

hep-ph↗
arXiv2015

Model-independent calculation of $SU(3)_f$ violation in baryon octet light-cone distribution amplitudes

In this work we present a minimal parametrization of the light-cone distribution amplitudes of the baryon octet including higher twist contributions. Simultaneously we obtain the quark mass dependence of the amplitudes at leading one-loop accuracy by the use of three-flavor baryon chiral perturbation theory (BChPT), which automatically yields model-independent results for the leading $SU(3)$ flavor breaking effects. For that purpose we have constructed the nonlocal light-cone three-quark operators in terms of baryon octet and meson fields and have carried out a next-to-leading order BChPT calculation. We were able to find a minimal set of distribution amplitudes (DAs) that do not mix under chiral extrapolation towards the physical point and naturally embed the $Λ$ baryon. Additionally they are chosen in such a way that all DAs of a certain symmetry class have a similar quark mass dependence (independent of the twist of the corresponding amplitude), which allows for a compact presentation. The results are well-suited for the extrapolation of lattice data and for model building.

hep-ph↗
arXiv2015

Threshold Resummation for Polarized High-$p_T$ Hadron Production at COMPASS

We study the cross section for the photoproduction process $γN\rightarrow h X$ where the incident photon and nucleon are longitudinally polarized and a hadron $h$ is observed at high transverse momentum. Specifically, we address the "direct" part of the cross section, for which the photon interacts in a pointlike way. For this contribution we perform an all-order resummation of logarithmic threshold corrections generated by soft or collinear gluon emission to next-to-leading logarithmic accuracy. We present phenomenological results relevant for the COMPASS experiment and compare to recent COMPASS data.

hep-ph↗
arXiv2015

One-Loop Matching for Generalized Parton Distributions

We present the one-loop matching condition for the unpolarized and polarized generalized quark distributions in the non-singlet case. The matching condition links the quasi distributions defined in terms of spacelike correlators at finite nucleon momentum to the light-cone distributions, and is useful for extracting the latter from the former in a lattice QCD calculation. Our results show that at one-loop and leading power accuracy the matching for the light-cone generalized quark distribution $H$ ($\tilde H$) is non-trivial, whereas no matching is required for $E$ ($\tilde E$). Therefore, $E$ ($\tilde E$) can be smoothly approached by its quasi counterpart in the large momentum limit. We also present the matching for the distribution amplitude of the pion.

hep-ph↗
arXiv2015

Center clusters in full QCD at finite temperature and background magnetic field

We study the center structure of full dynamical QCD at finite temperatures and nonzero values of the background magnetic field using continuum extrapolated lattice data. We concentrate on two particular observables characterizing center clusters: their fractality and the probability for percolation. For temperatures below and around the transition region, the fractal dimension is found to be significantly smaller than three, leading to a vanishing mean free path inside the cluster structure. This finding might be relevant for center symmetry-based models of heavy-ion collisions. In addition, the percolation probability is employed to define the transition temperature and to map out the QCD phase diagram in the magnetic field-temperature plane.

hep-lat↗
arXiv2015

Nucleon isovector couplings from $N_f=2$ lattice QCD

We compute the axial, scalar, tensor and pseudoscalar isovector couplings of the nucleon as well as the induced tensor and pseudoscalar charges in lattice simulations with $N_f=2$ mass-degenerate non-perturbatively improved Wilson-Sheikholeslami-Wohlert fermions. The simulations are carried out down to a pion mass of 150 MeV and linear spatial lattice extents of up to 4.6 fm at three different lattice spacings ranging from approximately 0.08 fm to 0.06 fm. Possible excited state contamination is carefully investigated and finite volume effects are studied. The couplings, determined at these lattice spacings, are extrapolated to the physical pion mass. In this limit we find agreement with experimental results, where these exist, with the exception of the magnetic moment. A proper continuum limit could not be performed, due to our limited range of lattice constants, but no significant lattice spacing dependence is detected. Upper limits on discretization effects are estimated and these dominate the error budget.

hep-lat↗
arXiv2015

A note on the scale evolution of tri-gluon correlations

We reexamine the scale dependence of the gluonic pole tri-gluon correlations relevant for the transverse single spin asymmetries. Our results differ from those of all previous similar calculations. The corresponding asymptotic behavior of tri-gluon correlations at small x is also briefly discussed.

hep-ph↗
arXiv2015

Transverse single spin asymmetry in direct photon production in polarized pA collisions

We study the transverse single spin asymmetry in direct photon production in pA collisions with incoming protons being transversely polarized. To facilitate the calculation, we formulate a hybrid approach in which the nucleus is treated in the Color Glass Condensate (CGC) framework while the collinear twist-3 formalism is applied on the proton side. It has been found that an additional term which arises from color entanglement shows up in the spin dependent differential cross section. The fact that this additional term is perturbatively calculable allows us to quantitatively study color entanglement effects.

hep-ph↗
arXiv2014

First moments of nucleon generalized parton distributions in chiral perturbation theory at full one-loop order

In this paper we present chiral extrapolation formulas for the generalized form factors connected to the first moments of chiral-even generalized parton distributions. For the description of QCD at low energies we employ the framework of two-flavor covariant baryon chiral perturbation theory at full one-loop accuracy. Our results are well suited for the extrapolation of lattice QCD data to physical quark masses.

hep-ph↗
arXiv2014

Finite Volume corrections to $< x>_{u\pm d}$

In this paper we calculate the full one-loop finite volume corrections to the quantities $< x >_{u\pm d}$ within the framework of two-flavor baryon chiral perturbation theory and show estimates of these effects for the leading one-loop corrections based on fits carried out in previously published works.

hep-lat↗
arXiv2014

Spectral functions of charmonium with 2+1 flavours of dynamical quarks

Finite temperature charmonium spectral functions in the pseudoscalar(PS) and vector(V) channels are studied in lattice QCD with 2+1 flavours of dynamical Wilson quarks, on fine isotropic lattices (with a lattice spacing of 0.057fm), with a non-physical pion mass of 545MeV. The highest temperature studied is approximately 1.4Tc. Up to this temperature no significant variation of the spectral function is seen in the PS channel. The V channel shows some temperature dependence, which seems to be consistent with a temperature dependent low frequency peak related to heavy quark transport, plus a temperature independent term at omega > 0. These results are in accord with previous calculations using the quenched approximation.

hep-lat↗