arXiv subjects

U. -G. Meißner

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

At least 73 records · Page 4Linked to original sources

arXiv2013

Electromagnetic Structure of the Z_c(3900)

The observation of the exotic quarkonium state Z_c(3900) by the BESIII and Belle collaborations supports the concept of hadronic molecules. Charmonium states interpreted as such molecules would be bound states of heavy particles with small binding energies. This motivates their description using an effective theory with contact interactions. In particular, we focus on the electromagnetic structure of the charged state Z_c(3900). Using first experimental results concerning spin and parity, we interpret it as an S-wave molecule and calculate the form factors as well as charge and magnetic radii up to next-to-leading order. We also present first numerical estimations of some of these observables at leading order.

hep-ph↗

arXiv2013

Quark mass dependence of the X(3872) binding energy

We explore the quark-mass dependence of the pole position of the X(3872) state within the molecular picture. The calculations are performed within the framework of a nonrelativistic Faddeev-type three-body equation for the $D\bar{D}π$ system in the $J^{PC}=1^{++}$ channel. The $πD$ interaction is parametrised via a $D^*$ pole, and a three-body force is included to render the equations well defined. Its strength is adjusted such that the X(3872) appears as a $D\bar{D}^*$ bound state 0.5 MeV below the neutral threshold. We find that the trajectory of the X(3872) depends strongly on the assumed quark-mass dependence of the short-range interactions which can be determined in future lattice QCD calculations. At the same time we are able to provide nontrivial information on the chiral extrapolation in the $X$ channel.

hep-ph↗

arXiv2013

Search for a dark photon in the $π^0 \to e^+e^-γ$ decay

The presently world largest data sample of pi0 --> gamma e+e- decays containing nearly 5E5 events was collected using the WASA detector at COSY. A search for a dark photon U produced in the pi0 --> gamma U --> gamma e+e- decay from the pp-->ppπ^0 reaction was carried out. An upper limit on the square of the U-gamma mixing strength parameter epsilon^2 of 5e-6 at 90% CL was obtained for the mass range 20 MeV <M_U< 100 MeV. This result together with other recent experimental limits significantly reduces the M_U vs. ε^2 parameter space preferred by the measured value of the muon anomalous magnetic moment.

hep-ex↗

arXiv2013

Varying the light quark mass: impact on the nuclear force and Big Bang nucleosynthesis

The quark mass dependences of light element binding energies and nuclear scattering lengths are derived using chiral perturbation theory in combination with non-perturbative methods. In particular, we present new, improved values for the quark mass dependence of meson resonances that enter the nuclear force. A detailed analysis of the theoretical uncertainties arising in this determination is presented. As an application we derive from a comparison of observed and calculated primordial deuterium and helium abundances a stringent limit on the variation of the light quark mass, $δm_q/m_q = 0.02 \pm 0.04$. Inclusion of the neutron lifetime modification under the assumption of a variation of the Higgs vacuum expectation value that translates into changing quark, electron, and weak gauge boson masses, leads to a stronger limit, $|δm_q/m_q| < 0.009$.

nucl-th↗

arXiv2013

Coupled-channel dynamics in the reactions piN --> piN, etaN, KLambda, KSigma

Elastic piN scattering and the world data of the family of reactions pi^- p --> eta n, K^0 Lambda$, K^0 Sigma^0, K^+ Sigma^-, and pi^+ p --> K^+ Sigma^+ are described simultaneously in an analytic, unitary, coupled-channel approach. SU(3) flavor symmetry is used to relate both the t- and the u- channel exchanges that drive the meson-baryon interaction in the different channels. Angular distributions, polarizations, and spin-rotation parameters are compared with available experimental data. Partial-wave amplitudes are determined and the resonance content is extracted from the analytic continuation, including resonance positions and branching ratios, and possible sources of uncertainties are discussed. The results provide the final-state interactions for the ongoing analysis of photo- and electroproduction data.

nucl-th↗

arXiv2013

Hyperon-nucleon interaction at next-to-leading order in chiral effective field theory

Results for the Lambda N and Sigma N interactions obtained at next-to-leading order in chiral effective field theory are reported. At the order considered there are contributions from one- and two-pseudoscalar-meson exchange diagrams and from four-baryon contact terms without and with two derivatives. SU(3) flavor symmetry is imposed for constructing the hyperon-nucleon interaction while the explicit SU(3) symmetry breaking by the physical masses of the pseudoscalar mesons (pi, K, eta) is taken into account. An excellent description of the hyperon-nucleon system can be achieved at next-to-leading order. It is on the same level of quality as the one obtained by the most advanced phenomenological hyperon-nucleon interaction models.

nucl-th↗

arXiv2012

Improving the Hadron Physics of Non-Standard-Model Decays: Example Bounds on R-parity Violation

Using the example of selected decays driven by R-parity-violating supersymmetric operators, we demonstrate how strong final-state interactions can be controlled quantitatively with high precision, thus allowing for a more accurate extraction of effective parameters from data. In our examples we focus on the lepton-flavor-violating decays tau --> mu pi+ pi-. In R-parity violation these can arise due to the product of two couplings. We find bounds that are an order of magnitude stronger than previous ones.

hep-ph↗

arXiv2012

The electric dipole moment of the deuteron from the QCD $θ$-term

The two-nucleon contributions to the electric dipole moment (EDM) of the deuteron, induced by the QCD $θ$-term, are calculated in the framework of effective field theory up-to-and-including next-to-next-to-leading order. In particular we find for the difference of the deuteron EDM and the sum of proton and neutron EDM induced by the QCD $θ$-term a value of $(-5.4\pm 3.9)\,\barθ\times 10^{-4}\, e$ fm. The by far dominant uncertainty comes from the CP- and isospin-violating $πNN$ coupling constant.

hep-ph↗

arXiv2012

New insights into the spin structure of the nucleon

We analyze the low-energy spin structure of the nucleon in a covariant effective field theory with explicit spin-3/2 degrees of freedom to third order in the small scale expansion. Using the available data on the strong and electromagnetic width of the Delta-resonance, we give parameter-free predictions for various spin-polarizabilities and moments of spin structure functions. We find an improved description of the nucleon spin structure at finite photon virtualities for some observables and point out the necessity of a fourth order calculation.

hep-ph↗

arXiv2012

Finite volume effects in pion-kaon scattering and reconstruction of the kappa(800) resonance

Simulating the kappa(800) on the lattice is a challenging task that starts to become feasible due to the rapid progress in recent-years lattice QCD calculations. As the resonance is broad, special attention to finite-volume effects has to be paid, because no sharp resonance signal as from avoided level crossing can be expected. In the present article, we investigate the finite volume effects in the framework of unitarized chiral perturbation theory using next-to-leading order terms. After a fit to meson-meson partial wave data, lattice levels for piK scattering are predicted. In addition, levels are shown for the quantum numbers in which the sigma(600), f_0(980), a_0(980), phi(1020), K*(892), and rho(770) appear, as well as the repulsive channels. Methods to extract the kappa(800) signal from the lattice spectrum are presented. Using pseudo-data, we estimate the precision that lattice data should have to allow for a clear-cut extraction of this resonance. To put the results into context, in particular the required high precision on the lattice data, the sigma(600), the P-wave resonances K*(892) and rho(770), and the repulsive piK, pipi phases are analyzed as well.

hep-lat↗

arXiv2012

Scalar mesons moving in a finite volume and the role of partial wave mixing

Phase shifts and resonance parameters can be obtained from finite-volume lattice spectra for interacting pairs of particles, moving with nonzero total momentum. We present a simple derivation of the method that is subsequently applied to obtain the pi pi and pi K phase shifts in the sectors with total isospin I=0 and I=1/2, respectively. Considering different total momenta, one obtains extra data points for a given volume that allow for a very efficient extraction of the resonance parameters in the infinite-volume limit. Corrections due to the mixing of partial waves are provided. We expect that our results will help to optimize the strategies in lattice simulations, which aim at an accurate determination of the scattering and resonance properties.

hep-lat↗

arXiv2012

Roy-Steiner equations for pion-nucleon scattering

Starting from hyperbolic dispersion relations, we derive a closed system of Roy-Steiner equations for pion-nucleon scattering that respects analyticity, unitarity, and crossing symmetry. We work out analytically all kernel functions and unitarity relations required for the lowest partial waves. In order to suppress the dependence on the high-energy regime we also consider once- and twice-subtracted versions of the equations, where we identify the subtraction constants with subthreshold parameters. Assuming Mandelstam analyticity we determine the maximal range of validity of these equations. As a first step towards the solution of the full system we cast the equations for the $ππ\to\bar NN$ partial waves into the form of a Muskhelishvili-Omnès problem with finite matching point, which we solve numerically in the single-channel approximation. We investigate in detail the role of individual contributions to our solutions and discuss some consequences for the spectral functions of the nucleon electromagnetic form factors.

hep-ph↗

arXiv2012

Dispersive analysis of the scalar form factor of the nucleon

Based on the recently proposed Roy-Steiner equations for pion-nucleon scattering, we derive a system of coupled integral equations for the pi pi --> N-bar N and K-bar K --> N-bar N S-waves. These equations take the form of a two-channel Muskhelishvili-Omnes problem, whose solution in the presence of a finite matching point is discussed. We use these results to update the dispersive analysis of the scalar form factor of the nucleon fully including K-bar K intermediate states. In particular, we determine the correction Delta_sigma=sigma(2M_pi^2)-sigma_{pi N}, which is needed for the extraction of the pion-nucleon sigma term from pi N scattering, as a function of pion-nucleon subthreshold parameters and the pi N coupling constant.

hep-ph↗

arXiv2012

Matrix elements of unstable states

Using the language of non-relativistic effective Lagrangians, we formulate a systematic framework for the calculation of resonance matrix elements in lattice QCD. The generalization of the Lüscher-Lellouch formula for these matrix elements is derived. We further discuss in detail the procedure of the analytic continuation of the resonance matrix elements into the complex energy plane and investigate the infinite-volume limit.

hep-lat↗

arXiv2011

Model-independent approach to eta -> pi+ pi- gamma and eta' -> pi+ pi- gamma

We present a new, model-independent method to analyze radiative decays of mesons to a vector, isovector pair of pions of invariant mass square below the first significant pion-pion threshold in the vector channel. It is based on a combination of chiral perturbation theory and dispersion theory. This allows for a controlled inclusion of resonance physics without the necessity to involve vector meson dominance explicitly. As an example, the method is applied to an analysis of the reactions eta -> pi+ pi- gamma and eta'->pi+ pi- gamma.

nucl-th↗

arXiv2011

Dynamical coupled-channel approaches on a momentum lattice

Dynamical coupled-channel approaches are a widely used tool in hadronic physics that allow to analyze different reactions and partial waves in a consistent way. In such approaches the basic interactions are derived within an effective Lagrangian framework and the resulting pseudo-potentials are then unitarized in a coupled-channel scattering equation. We propose a scheme that allows for a solution of the arising integral equation in discretized momentum space for periodic as well as twisted boundary conditions. This permits to study finite size effects as they appear in lattice QCD simulations. The new formalism, at this stage with a restriction to S-waves, is applied to coupled-channel models for the sigma(600), f0(980), and a0(980) mesons, and also for the Lambda(1405) baryon. Lattice spectra are predicted.

hep-lat↗

arXiv2011

Exotic bound states of two baryons in light of chiral effective field theory

Baryon-baryon bound states in the strangeness S=-2, S=-3, and S=-4 sectors are considered. In particular, the dependence of the corresponding binding energies on the quark mass (or equivalently the pion mass) is explored in the framework of chiral effective field theory, in order to connect with current lattice QCD calculations. For a bound state in the Xi-Xi 1S0 channel, predicted by our leading-order effective field theory interaction, binding energies are inferred that are roughly in line with a recent lattice QCD result at meson and baryon masses that correspond to those in the lattice simulation. With regard to the so-called H-dibaryon it is shown that the SU(3) breaking effects induced by the differences of the pertinent two-baryon thresholds (Lambda-Lambda, Xi-N, Sigma-Sigma) have a very pronounced impact on its binding energy. A bound H-dibaryon as found in two lattice calculations could be shifted above the Lambda-Lambda or even above the Xi-N threshold for physical masses of the involved baryons.

nucl-th↗

arXiv2011

Exclusive Measurement of the eta --> pi+ pi- gamma Decay

An exclusive measurement of the decay eta --> pi+ pi- gamma has been performed at the WASA facility at COSY. The eta mesons were produced in the fusion reaction pd --> 3He X at a proton beam momentum of 1.7 GeV/c. Efficiency corrected differential distributions have been extracted based on 13340\pm140 events after background subtraction. The measured pion angular distribution is consistent with a relative p-wave of the two-pion system, whereas the measured photon energy spectrum was found at variance with the simplest gauge invariant matrix element of eta --> pi+ pi- gamma. A parameterization of the data can be achieved by the additional inclusion of the empirical pion vector form factor multiplied by a first-order polynomial in the squared invariant mass of the pi+ pi- system.

nucl-ex↗