arXiv subjects

Ulf-G. Meißner

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

At least 343 records · Page 19Linked to original sources

arXiv2015

Regularization Methods for Nuclear Lattice Effective Field Theory

We investigate Nuclear Lattice Effective Field Theory for the two-body system for several lattice spacings at lowest order in the pionless as well as in the pionful theory. We discuss issues of regularizations and predictions for the effective range expansion. In the pionless case, a simple Gaussian smearing allows to demonstrate lattice spacing independence over a wide range of lattice spacings. We show that regularization methods known from the continuum formulation are necessary as well as feasible for the pionful approach.

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arXiv2015

Parity violation in neutron capture on the proton: determining the weak pion-nucleon coupling

We investigate the parity-violating analyzing power in neutron capture on the proton at thermal energies in the framework of chiral effective field theory. By combining this analysis with a previous analysis of parity violation in proton-proton scattering, we are able to extract the size of the weak pion-nucleon coupling constant. The uncertainty is significant and dominated by the experimental error which is expected to be reduced soon.

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arXiv2015

Ab initio alpha-alpha scattering

Processes involving alpha particles and alpha-like nuclei comprise a major part of stellar nucleosynthesis and hypothesized mechanisms for thermonuclear supernovae. In an effort towards understanding alpha processes from first principles, we describe in this letter the first ab initio calculation of alpha-alpha scattering. We use lattice effective field theory to describe the low-energy interactions of nucleons and apply a technique called the adiabatic projection method to reduce the eight-body system to an effective two-cluster system. We find good agreement between lattice results and experimental phase shifts for S-wave and D-wave scattering. The computational scaling with particle number suggests that alpha processes involving heavier nuclei are also within reach in the near future.

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arXiv2015

A new tool in nuclear physics: Nuclear lattice simulations

In the last years, chiral effective field theory has been successfully developed for and applied to systems with few nucleons. Here, I present a new approach for ab initio calculations of nuclei that combines these precise and systematic forces with Monte Carlo simulation techniques that allow for exact solutions of the nuclear A-body problem. A short introduction of this method is given and a few assorted results concerning the spectrum and structure of 12C and 16O are presented. The framework further allows one to study the properties of nuclei in worlds that have fundamental parameters different from the ones in Nature. This allows for a physics test of the anthropic principle by addressing the question how strongly the generation of the life-relevant elements depends on the light quark masses and the electromagnetic fine structure constant.

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arXiv2015

Nuclear Electric Dipole Moments in Chiral Effective Field Theory

We provide a consistent and complete calculation of the electric dipole moments of the deuteron, helion, and triton in the framework of chiral effective field theory. The CP-conserving and CP-violating interactions are treated on equal footing and we consider CP-violating one-, two-, and three-nucleon operators up to next-to-leading-order in the chiral power counting. In particular, we calculate for the first time EDM contributions induced by the CP-violating three-pion operator. We find that effects of CP-violating nucleon-nucleon contact interactions are larger than those found in previous studies based on phenomenological models for the CP-conserving nucleon-nucleon interactions. Our results which apply to any model of CP violation in the hadronic sector can be used to test various scenarios of CP violation. As examples, we study the implications of our results on the QCD $θ$-term and the minimal left-right symmetric model.

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arXiv2015

P- and T-Violating Lagrangians in Chiral Effective Field Theory and Nuclear Electric Dipole Moments

A scheme to derive hadronic interactions induced by effective multi-quark terms is presented within the framework of chiral effective field theory. It is employed to work out the list of parity- and time-reversal-symmetry-violating hadronic interactions that are relevant for the computation of nuclear contributions to the electric dipole moments of the hydrogen-2, helium-3 and hydrogen-3 nuclei. We also derive the scattering and Faddeev equations required to compute electromagnetic form factors in general and electric dipole moments in particular.

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arXiv2015

Breaking and restoration of rotational symmetry for irreducible tensor operators on the lattice

We study the breaking of rotational symmetry on the lattice for irreducible tensor operators and practical methods for suppressing this breaking. We illustrate the features of the general problem using an $α$ cluster model for $^{8}$Be. We focus on the lowest states with non-zero angular momentum and examine the matrix elements of multipole moment operators. We show that the physical reduced matrix element is well reproduced by averaging over all possible orientations of the quantum state, and this is expressed as a sum of matrix elements weighted by the corresponding Clebsch-Gordan coefficients. For our $α$ cluster model we find that the effects of rotational symmetry breaking can be largely eliminated for lattice spacings of $a\leq 1.7$ fm, and we expect similar improvement for actual lattice Monte Carlo calculations.

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arXiv2015

Predictions for the $Λ_b \to J/ψ~ Λ(1405)$ decay

We calculate the shape of the $πΣ$ and $\bar K N$ invariant mass distributions in the $Λ_b \to J/ψ\, πΣ$ and $Λ_b \to J/ψ\,\bar K N$ decays that are dominated by the $Λ(1405)$ resonance. The weak interaction part is the same for both processes and the hadronization into the different meson-baryon channels in the final state is related by SU(3) symmetry. The most important feature is the implementation of the meson-baryon final-state interaction using two chiral unitary models from different theoretical groups. Both approaches give a good description of antikaon-nucleon scattering data, the complex energy shift in kaonic hydrogen and the line shapes of $πΣK$ in photoproduction, based on the two-pole scenario for the $Λ(1405)$. We find that this reaction reflects more the higher mass pole and we make predictions of the line shapes and relative strength of the meson-baryon distributions in the final state.

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arXiv2015

Constraints on the chiral unitary $\bar KN$ amplitude from $πΣK^+$ photoproduction data

A chiral unitary approach for antikaon-nucleon scattering in on-shell factorization is studied. We find multiple sets of parameters for which the model describes all existing hadronic data similarly well. We confirm the two-pole structure of the $Λ(1405)$. The narrow $Λ(1405)$ pole appears at comparable positions in the complex energy plane, whereas the location of the broad pole suffers from a large uncertainty. In the second step, we use a simple model for photoproduction of $K^+πΣ$ off the proton and confront it with the experimental data from the CLAS collaboration. It is found that only a few of the hadronic solutions allow for a consistent description of the CLAS data within the assumed reaction mechanism.

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arXiv2015

On the near-threshold $\bar pp$ invariant mass spectrum measured in $J/ψ$ and $ψ'$ decays

A systematic analysis of the near-threshold enhancement in the $\bar pp$ invariant mass spectrum seen in the decay reactions $J/ψ\to x \bar pp$ and $ψ(3686) \to x \bar pp$ $(x = γ,\, ω,\, ρ,\, π,\, η)$ is presented. The enhancement is assumed to be due to the $\bar NN$ final-state interaction (FSI) and the pertinent FSI effects are evaluated in an approach that is based on the distorted-wave Born approximation. For the $\bar NN$ interaction a recent potential derived within chiral effective field theory and fitted to results of a partial-wave analysis of $\bar pp$ scattering data is considered and, in addition, an older phenomenological model constructed by the Jülich group. It is shown that the near-threshold spectrum observed in various decay reactions can be reproduced simultaneously and consistently by our treatment of the $\bar pp$ FSI. It turns out that the interaction in the isospin-1 $^1S_0$ channel required for the description of the $J/ψ\to γ\bar pp$ decay predicts a $\bar NN$ bound state.

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arXiv2015

A study of hyperons in nuclear matter based on chiral effective field theory

The in-medium properties of a hyperon-nucleon potential, derived within chiral effective field theory and fitted to Lambda-N and Sigma-N scattering data, are investigated. Results for the single-particle potentials of the Lambda and Sigma hyperons in nuclear matter are reported, based on a conventional first-order Brueckner calculation. The Sigma-nucleus potential is found to be repulsive, in agreement with phenomenological information. A weak Lambda-nucleus spin-orbit interaction can be achieved by an appropriate adjustment of a low-energy constant corresponding to an antisymmetric Lambda-N -- Sigma-N spin-orbit interaction that arises at next-to-leading order in the effective field theory approach.

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arXiv2015

Theoretical Constraints and Systematic Effects in the Determination of the Proton Form Factors

We calculate the two-photon exchange corrections to electron-proton scattering with nucleon and $Δ$ intermediate states. The results show a dependence on the elastic nucleon and nucleon-$Δ$-transition form factors used as input which leads to significant changes compared to previous calculations. We discuss the relevance of these corrections and apply them to the most recent and precise data set and world data from electron-proton scattering. Using this, we show how the form factor extraction from these data is influenced by the subsequent inclusion of physical constraints. The determination of the proton charge radius from scattering data is shown to be dominated by the enforcement of a realistic spectral function. Additionally, the third Zemach moment from the resulting form factors is calculated. The obtained radius and Zemach moment are shown to be consistent with Lamb shift measurements in muonic hydrogen.

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arXiv2015

Remarks on the study of the X(3872) from Effective Field Theory with Pion-Exchange Interaction

In a recent paper Phys.Rev.Lett. 111, 042002 (2013) (arXiv:1304.0846), the charmonium state X(3872) is studied in the framework of an effective field theory. In that work it is claimed that (i) the one-pion exchange (OPE) alone provides sufficient binding to produce the X as a shallow bound state at the $D^0\bar{D}^{*0}$ threshold, (ii) short-range dynamics (described by a contact interaction) provides only moderate corrections to the OPE, and (iii) the X-pole disappears as the pion mass is increased slightly and therefore the X should not be seen on the lattice, away from the pion physical mass point, if it were a molecular state. In this paper we demonstrate that the results of Phys.Rev.Lett. 111, 042002 (2013) (arXiv:1304.0846) suffer from technical as well as conceptual problems and therefore do not support the conclusions drawn by the authors.

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arXiv2014

Do $ΞΞ$ bound states exist?

The existence of baryon-baryon bound states in the strangeness sector is examined in the framework of SU(3) chiral effective field theory. Specifically, the role of SU(3) symmetry breaking contact terms that arise at next-to-leading order in the employed Weinberg power counting scheme is explored. We focus on the 1S0 partial wave and on baryon-baryon channels with maximal isospin since in this case there are only two independent SU(3) symmetry breaking contact terms. At the same time, those are the channels where most of the bound states have been predicted in the past. Utilizing $pp$ phase shifts and $Σ^+ p$ cross section data allows us to pin down one of the SU(3) symmetry breaking contact terms and a clear indication for the decrease of attraction when going from the NN system to strangeness S=-2 is found, which rules out a bound state for $ΣΣ$ with isospin I=2. Assuming that the trend observed for S=0 to S=-2 is not reversed when going to $ΞΣ$ and $ΞΞ$ makes also bound states in those systems rather unlikely.

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arXiv2014

Production of the spin partner of the X(3872) in e^+e^- collisions

We study the production of the spin partner of the X(3872), which is a D^{*}\bar D^{*} bound state with quantum numbers J^{PC}=2^{++} and named X_2(4012) here, with the associated emission of a photon in electron--positron collisions. The results show that the ideal energy region to observe the X_2(4012) in e^+e^- annihilations is from 4.4~GeV to 4.5~GeV, due to the presence of the S-wave \bar D^{*} D_1(2420) and \bar D^{*} D_2(2460) thresholds, respectively. We also point out that it will be difficult to observe the γX(4012) at the e^+e^- center-of-mass energy around 4.26~GeV.

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arXiv2014

Anthropic considerations in nuclear physics

In this short review, I discuss the sensitivity of the generation of the light and the life-relevant elements like carbon and oxygen under changes of the parameters of the Standard Model pertinent to nuclear physics. Chiral effective field theory allows for a systematic and precise description of the forces between two, three, and four nucleons. In this framework, variations under the light quark masses and the electromagnetic fine structure constant can also be consistently calculated. Combining chiral nuclear effective field theory with Monte Carlo simulations allows to further calculate the properties of nuclei, in particular of the Hoyle state in carbon, that plays a crucial role in the generation of the life-relevant elements in hot, old stars. The dependence of the triple-alpha process on the fundamental constants of Nature is calculated and some implications for our anthropic view of the Universe are discussed.

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arXiv2014

Low-energy neutron-deuteron reactions with N3LO chiral forces

We solve three-nucleon Faddeev equations with nucleon-nucleon and three-nucleon forces derived consistently in the framework of chiral perturbation theory at next-to-next-to-next-to-leading order in the chiral expansion. In this first investigation we include only matrix elements of the three-nucleon force for partial waves with the total two-nucleon (three-nucleon) angular momenta up to 3 (5/2). Low-energy neutron-deuteron elastic scattering and deuteron breakup reaction are studied. Emphasis is put on Ay puzzle in elastic scattering and cross sections in symmetric-space-star and neutron-neutron quasi-free-scattering breakup configurations, for which large discrepancies between data and theory have been reported.

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arXiv2014

Uncertainties of Euclidean Time Extrapolation in Lattice Effective Field Theory

Extrapolations in Euclidean time form a central part of Nuclear Lattice Effective Field Theory (NLEFT) calculations using the Projection Monte Carlo method, as the sign problem in many cases prevents simulations at large Euclidean time. We review the next-to-next-to-leading order NLEFT results for the alpha nuclei up to $^{28}$Si, with emphasis on the Euclidean time extrapolations, their expected accuracy and potential pitfalls. We also discuss possible avenues for improving the reliability of Euclidean time extrapolations in NLEFT.

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