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U. -G. Meißner

Publications and source records attributed to U. -G. Meißner.

At least 55 records · Page 3Linked to original sources
arXiv2016

The electromagnetic form factors of the $Λ$ in the timelike region

The reaction $e^+e^- \to \bar ΛΛ$ is investigated for energies close to the threshold. Specific emphasis is put on the role played by the interaction in the final $\bar ΛΛ$ system which is taken into account rigorously. For that interaction a variety of $\bar ΛΛ$ potential models is employed that have been constructed for the analysis of the reaction $\bar p p \to \bar ΛΛ$ in the past. The enhancement of the effective form factor for energies close to the $\bar ΛΛ$ threshold, seen in pertinent experiments, is reproduced. Predictions for the $Λ$ electromagnetic form factors $G_M$ and $G_E$ in the timelike region and for spin-dependent observables such as spin-correlation parameters are presented.

nucl-th↗
arXiv2016

Role of the total isospin 3/2 component in three-nucleon reactions

We discuss the role of the three-nucleon isospin T=3/2 amplitude in elastic neutron-deuteron scattering and in the deuteron breakup reaction. The contribution of this amplitude originates from charge-independence breaking of the nucleon-nucleon potential and is driven by the difference between neutron-neutron (proton-proton) and neutron-proton forces. We study the magnitude of that contribution to the elastic scattering and breakup observables, taking the locally regularized chiral N4LO nucleon-nucleon potential supplemented by the chiral N2LO three-nucleon force. For comparison we employ also the Av18 nucleon-nucleon potential combined with the Urbana IX three-nucleon force. We find that the isospin T=3/2 component is important for the breakup reaction and the proper treatment of charge-independence breaking in this case requires the inclusion of the 1S0 state with isospin T=3/2. For neutron-deuteron elastic scattering the T=3/2 contributions are insignificant and charge-independence breaking can be accounted for by using the effective t-matrix generated with the so-called "2/3-1/3" rule.

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arXiv2015

Jacobi no-core shell model for p-shell nuclei

We introduce an algorithm to obtain coefficients of fractional parentage for light $p$-shell nuclei. The coefficients enable to use Jacobi coordinates in no-core shell model calculations separating off the center-of-mass motion. Fully antisymmetrized basis states are given together with recoupling coefficients that allow one to apply two- and three-nucleon operators. As an example, we study the dependence on the harmonic oscillator frequency of $^3$H, $^4$He, $^6$He, $^6$Li and $^7$Li and extract their binding and excitation energies. The coefficients will be made openly accessible as HDF5 data files.

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arXiv2015

$J/ψ(η_c) N$ and $Υ(η_b) N$ cross sections

Inspired by the recent findings of the two $P_c^+$ states in the $J/ψp$ mass spectrum at LHCb, we investigate the elastic and inelastic cross sections of the $J/ψN$, $η_c N$, $ΥN$ and $η_b N$ channels within the constraints from heavy quark spin and flavour symmetry. The $\bar{D}^{(*)} Σ_c^{(*)}$ ($B^{(*)} Σ_b^{(*)}$) bound states predicted in earlier works should be accessible in elastic and/or inelastic processes of the $J/ψN$ and/or $η_c N$ ($ΥN$ and/or $η_b N$) interactions.

hep-ph↗
arXiv2015

Eta photoproduction in a combined analysis of pion- and photon-induced reactions

The $ηN$ final state is isospin-selective and thus provides access to the spectrum of excited nucleons without being affected by excited $Δ$ states. To this end, the world database on eta photoproduction off the proton up to a center-of-mass energy of $E\sim 2.3$ GeV is analyzed, including data on differential cross sections, and single and double polarization observables. The resonance spectrum and its properties are determined in a combined analysis of eta and pion photoproduction off the proton together with the reactions $πN\to πN$, $ηN$, $KΛ$ and $KΣ$. For the analysis, the so-called Jülich coupled-channel framework is used, incorporating unitarity, analyticity, and effective three-body channels. Parameters tied to photoproduction and hadronic interactions are varied simultaneously. The influence of recent MAMI $T$ and $F$ asymmetry data on the eta photoproduction amplitude is discussed in detail.

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arXiv2015

New structures in the proton-antiproton system

In the most recent measurements of the reaction $e^+e^- \rightarrow p\bar{p}$ by the BABAR collaboration, new structures have been found with unknown origin. We examine a possible relation of the most distinct peak to the recently observed $Φ(2170)$. Alternatively, we analyse possible explanations due to the nucleon$\,\barΔ$ and $Δ\barΔ$ thresholds. The latter could explain a periodicity found in the data.

hep-ph↗
arXiv2015

Few-nucleon systems with state-of-the-art chiral nucleon-nucleon forces

We apply improved nucleon-nucleon potentials up to fifth order in chiral effective field theory, along with a new analysis of the theoretical truncation errors, to study nucleon-deuteron (Nd) scattering and selected low-energy observables in 3H, 4He, and 6Li. Calculations beyond second order differ from experiment well outside the range of quantified uncertainties, providing truly unambiguous evidence for missing three-nucleon forces within the employed framework. The sizes of the required three-nucleon force contributions agree well with expectations based on Weinberg's power counting. We identify the energy range in elastic Nd scattering best suited to study three-nucleon force effects and estimate the achievable accuracy of theoretical predictions for various observables.

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arXiv2015

Photocouplings at the Pole from Pion Photoproduction

The reactions $γp\toπ^0 p$ and $γp\toπ^+ n$ are analyzed in a semi-phenomenological approach up to $E\sim2.3$ GeV. Fits to differential cross section and single and double polarization observables are performed. A good overall reproduction of the available photoproduction data is achieved. The Jülich2012 dynamical coupled-channel model -which describes elastic $πN$ scattering and the world data base of the reactions $πN\toηN$, $KΛ$, and $KΣ$ at the same time - is employed as the hadronic interaction in the final state. The framework guarantees analyticity and, thus, allows for a reliable extraction of resonance parameters in terms of poles and residues. In particular, the photocouplings at the pole can be extracted and are presented.

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arXiv2014

Precision nucleon-nucleon potential at fifth order in the chiral expansion

We present a nucleon-nucleon potential at fifth order in chiral effective field theory. We find a substantial improvement in the description of nucleon-nucleon phase shifts as compared to the fourth-order results of Ref. [E. Epelbaum, H. Krebs, U.-G. Meißner, arXiv:1412.0142 [nucl-th]]. This provides clear evidence of the corresponding two-pion exchange contributions with all low-energy constants being determined from pion-nucleon scattering. The fifth-order corrections to nucleon-nucleon observables appear to be of a natural size which confirms the good convergence of the chiral expansion for nuclear forces. Furthermore, the obtained results provide strong support for the novel way of quantifying the theoretical uncertainty due to the truncation of the chiral expansion proposed in Ref. [E. Epelbaum, H. Krebs, U.-G. Meißner, arXiv:1412.0142 [nucl-th]]. Our work opens up new perspectives for precision ab initio calculations in few- and many-nucleon systems and is especially relevant for ongoing efforts towards a quantitative understanding the structure of the three-nucleon force in the framework of chiral effective field theory.

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arXiv2014

Improved chiral nucleon-nucleon potential up to next-to-next-to-next-to-leading order

We present improved nucleon-nucleon potentials derived in chiral effective field theory up to next-to-next-to-next-to-leading order. We argue that the nonlocal momentum-space regulator employed in the two-nucleon potentials of Refs. [E. Epelbaum, W. Gloeckle, U.-G. Meißner, Nucl. Phys. A747 (2005) 362], [D.R. Entem, R. Machleidt, Phys. Rev. C68 (2003) 041001] is not the most efficient choice, in particular since it affects the long-range part of the interaction. We are able to significantly reduce finite-cutoff artefacts by using an appropriate regularization in coordinate space which maintains the analytic structure of the amplitude. The new potentials do not require the additional spectral function regularization employed in Ref. [E. Epelbaum, W. Gloeckle, U.-G. Meißner, Nucl. Phys. A747 (2005) 362] to cut off the short-range components of the two-pion exchange and make use of the low-energy constants c_i and d_i determined from pion-nucleon scattering without any fine tuning. We discuss in detail the construction of the new potentials and convergence of the chiral expansion for two-nucleon observables. We also introduce a new procedure for estimating the theoretical uncertainty from the truncation of the chiral expansion that replaces previous reliance on cutoff variation.

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arXiv2014

Properties of effective massive Yang-Mills theory in the limit of vanishing vector boson mass

Two-loop corrections to the pole mass of the vector boson and the pole masses and the magnetic moments of fermions are calculated in the framework of an effective field theory of massive Yang-Mills fields interacting with fermions. It is shown that the limit of vanishing vector boson mass is finite for all these quantities. Implications of the obtained results are discussed.

hep-ph↗
arXiv2014

Reduction of the proton radius discrepancy by 3 sigma

We show that in previous analyses of electron-proton scattering, the uncertainties in the statistical procedure to extract the proton charge radius are underestimated. Using a fit function based on a conformal mapping, we can describe the scattering data with high precision and extract a radius value in agreement with the one obtained from muonic hydrogen.

hep-ph↗
arXiv2014

The electromagnetic form factors of the proton in the timelike region

The reactions ppbar -> e+e- and e+e- -> ppbar are analyzed in the near-threshold region. Specific emphasis is put on the role played by the interaction in the initial- or final antinucleon-nucleon state which is taken into account rigorously. For that purpose a recently published NNbar potential derived within chiral effective field theory and fitted to results of a new partial-wave analysis of ppbar scattering data is employed. Our results provide strong support for the conjecture that the pronounced energy dependence of the e+e- <-> ppbar cross section, seen in pertinent experiments, is primarily due to the ppbar interaction. Predictions for the proton electromagnetic form factors G_E and G_M in the timelike region, close to the NNbar threshold, and for spin-dependent observables are presented. The steep rise of the effective form factor for energies close to the ppbar threshold is explained solely in terms of the ppbar interaction. The corresponding experimental information is quantitatively described by our calculation.

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arXiv2013

Dispersive analysis for $η\to γγ^*$

A dispersion integral is derived that connects data on $η\to π^+π^-γ$ to the $η\to γγ^*$ transition form factor. A detailed analysis of the uncertainties is provided. We find for the slope of the $η$ transition form factor at the origin $b_η= (2.05 {}^{+0.22}_{-0.10})\, {\rm GeV}^{-2}$. Using an additional, plausible assumption, one finds for the corresponding slope of the $η'$ transition form factor, $b_{η'} = (1.58 {}^{+0.15}_{-0.08})\, {\rm GeV}^{-2}$. Both values are consistent with all recent data, but differ from some previous theoretical analyses.

hep-ph↗
arXiv2013

On the renormalization of the one-pion exchange potential and the consistency of Weinberg's power counting

Nogga, Timmermans and van Kolck recently argued that Weinberg's power counting in the few-nucleon sector is inconsistent and requires modifications. Their argument is based on the observed cutoff dependence of the nucleon-nucleon scattering amplitude calculated by solving the Lippmann-Schwinger equation with the regularized one-pion exchange potential and the cutoff Lambda varied in the range Lambda = 2...20 fm^(-1). In this paper we discuss the role the cutoff plays in the application of chiral effective field theory to the two-nucleon system and study carefully the cutoff-dependence of phase shifts and observables based on the one-pion exchange potential. We show that (i) there is no need to use the momentum-space cutoff larger than Lambda ~ 3 fm^(-1); (ii) the neutron-proton low-energy data show no evidence for an inconsistency of Weinberg's power counting if one uses Lambda ~ 3 fm^{-1}.

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arXiv2013

The magnetic moment of the ρ-meson

The magnetic moment of the ρ-meson is calculated in the framework of a low-energy effective field theory of the strong interactions. We find that the complex-valued strong interaction corrections to the gyromagnetic ratio are small leading to a value close to the real leading tree level result, g_ρ= 2. This is in a reasonably good agreement with the available lattice QCD calculations for this quantity.

hep-ph↗