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Ulf-G. Meißner

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

At least 289 records · Page 16Linked to original sources
arXiv2017

Universal dimer-dimer scattering in lattice effective field theory

We consider two-component fermions with short-range interactions and large scattering length. This system has universal properties that are realized in several different fields of physics. In the limit of large fermion-fermion scattering length $a_\mathrm{ff}$ and zero-range interaction, all properties of the system scale proportionally with $a_\mathrm{ff}$. For the case with shallow bound dimers, we calculate the dimer-dimer scattering phase shifts using lattice effective field theory. We extract the universal dimer-dimer scattering length $a_\mathrm{dd}/a_\mathrm{ff}=0.618(30)$ and effective range $r_\mathrm{dd}/a_\mathrm{ff}=-0.431(48)$. This result for the effective range is the first calculation with quantified and controlled systematic errors. We also benchmark our methods by computing the fermion-dimer scattering parameters and testing some predictions of conformal scaling of irrelevant operators near the unitarity limit.

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arXiv2017

Chiral study of the $a_0(980)$ resonance and $πη$ scattering phase shifts in light of a recent lattice simulation

We investigate the $a_0(980)$ resonance within chiral effective field theory through a three-coupled-channel analysis, namely $πη$, $K\bar{K}$ and $πη'$. A global fit to recent lattice finite-volume energy levels from $πη$ scattering and relevant experimental data on a $πη$ event distribution and the $γγ\toπη$ cross section is performed. Both the leading and next-to-leading-order analyses lead to similar and successful descriptions of the finite-volume energy levels and the experimental data. However, these two different analyses yield different $πη$ scattering phase shifts for the physical masses for the $π, K, η$ and $η'$ mesons. The inelasticities, the pole positions in the complex energy plane and their residues are calculated both for unphysical and physical meson masses.

hep-ph↗
arXiv2017

Effects of $Z_b$ states and bottom meson loops on $Υ(4S) \to Υ(1S,2S) π^+π^-$ transitions

We study the dipion transitions $Υ(4S) \rightarrow Υ(nS) π^+π^-$ $(n=1,2)$. In particular, we consider the effects of the two intermediate bottomoniumlike exotic states $Z_b(10610)$ and $Z_b(10650)$ as well as bottom meson loops. The strong pion-pion final-state interactions, especially including channel coupling to $K\bar{K}$ in the $S$-wave, are taken into account model-independently by using dispersion theory. Based on a nonrelativistic effective field theory we find that the contribution from the bottom meson loops is comparable to those from the chiral contact terms and the $Z_b$-exchange terms. For the $Υ(4S) \rightarrow Υ(2S) π^+π^-$ decay, the result shows that including the effects of the $Z_b$-exchange and the bottom meson loops can naturally reproduce the two-hump behavior of the $ππ$ mass spectra. Future angular distribution data are decisive for the identification of different production mechanisms. For the $Υ(4S) \rightarrow Υ(1S) π^+π^-$ decay, we show that there is a narrow dip around 1 GeV in the $ππ$ invariant mass distribution, caused by the final-state interactions. The distribution is clearly different from that in similar transitions from lower $Υ$ states, and needs to be verified by future data with high statistics. Also we predict the decay width and the dikaon mass distribution of the $Υ(4S) \rightarrow Υ(1S) K^+ K^-$ process.

hep-ph↗
arXiv2017

Neutron-proton scattering at next-to-next-to-leading order in Nuclear Lattice Effective Field Theory

We present a systematic study of neutron-proton scattering in Nuclear Lattice Effective Field Theory (NLEFT), in terms of the computationally efficient radial Hamiltonian method. Our leading-order (LO) interaction consists of smeared, local contact terms and static one-pion exchange. We show results for a fully non-perturbative analysis up to next-to-next-to-leading order (NNLO), followed by a perturbative treatment of contributions beyond LO. The latter analysis anticipates practical Monte Carlo simulations of heavier nuclei. We explore how our results depend on the lattice spacing a, and estimate sources of uncertainty in the determination of the low-energy constants of the next-to-leading-order (NLO) two-nucleon force. We give results for lattice spacings ranging from a = 1.97 fm down to a = 0.98 fm, and discuss the effects of lattice artifacts on the scattering observables. At a = 0.98 fm, lattice artifacts appear small, and our NNLO results agree well with the Nijmegen partial-wave analysis for S-wave and P-wave channels. We expect the peripheral partial waves to be equally well described once the lattice momenta in the pion-nucleon coupling are taken to coincide with the continuum dispersion relation, and higher-order (N3LO) contributions are included. We stress that for center-of-mass momenta below 100 MeV, the physics of the two-nucleon system is independent of the lattice spacing.

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arXiv2017

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

Results for the antinucleon-nucleon ($\bar NN$) interaction obtained at next-to-next-to-next-to-leading order in chiral effective field theory (EFT) are reported. A new local regularization scheme is used for the pion-exchange contributions that has been recently suggested and applied in a pertinent study of the $NN$ force within chiral EFT. Furthermore, an alternative strategy for estimating the uncertainty is utilized that no longer depends on a variation of the cutoffs. The low-energy constants associated with the arising contact terms are fixed by a fit to the phase shifts and inelasticities provided by a phase-shift analysis of $\bar pp$ scattering data. An excellent description of the $\bar NN$ amplitudes is achieved at the highest order considered. Moreover, because of the quantitative reproduction of partial waves up to $J=3$, there is also a nice agreement on the level of $\bar pp$ observables. Specifically, total and integrated elastic and charge-exchange cross sections agree well with the results from the partial-wave analysis up to laboratory energies of $300$ MeV, while differential cross sections and analyzing powers are described quantitatively up to $200$-$250$ MeV. The low-energy structure of the $\bar NN$ amplitudes is also considered and compared to data from antiprotonic hydrogen.

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arXiv2017

Where does the X(5568) structure come from?

We study the semi-exclusive production of $π^\pm B_s^0$ pairs in hadron colliders which is associated with the $X(5568)$ structure observed by the D0 Collaboration in 2016, but that was not confirmed by LHCb and CMS later. The reason of its appearance in the D0 and absence in LHCb and CMS is discussed in this letter. In a semi-exclusive process, one might miss the third particle which is produced together with the $π^\pm B_s^0$ simultaneously. In the three-body Dalitz plot, once the remaining region is narrow enough after the kinematic cuts, its reflection to another invariant mass distribution will accumulate a large number of events within a specific energy region. If there is an enhancement in the remaining region, it will make the reflection structure more pronounced. The precise line shape of the reflection will depend on the specific interaction form. A combined study of different cone cuts and the low-energy dynamics, e.g. the Landau singularity, demonstrates that the $X(5568)$ structure could come from this kinematic reflection. This conclusion can be checked by both searching for the enhancement in another invariant mass distribution, such as $B_s^0\bar{B}^0$, and the cone cut dependence of the $X(5568)$ mass. Such a combined study can be used to distinguish the effects of the triangle singularity from a genuine state. We also propose how to avoid this kinematic reflection in future experimental analysis.

hep-ph↗
arXiv2016

Nuclear matter properties with nucleon-nucleon forces up to fifth order in the chiral expansion

The properties of nuclear matter are studied using state-of-the-art nucleon-nucleon forces up to fifth order in chiral effective field theory. The equations of state of symmetric nuclear matter and pure neutron matter are calculated in the framework of the Brueckner-Hartree-Fock theory. We discuss in detail the convergence pattern of the chiral expansion and the regulator dependence of the calculated equations of state and provide an estimation of the truncation uncertainty. For all employed values of the regulator, the fifth-order chiral two-nucleon potential is found to generate nuclear saturation properties similar to the available phenomenological high precision potentials. We also extract the symmetry energy of nuclear matter, which is shown to be quite robust with respect to the chiral order and the value of the regulator.

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arXiv2016

Renormalization of the three-boson system with short-range interactions revisited

We consider renormalization of the three-body scattering problem in low-energy effective field theory of self-interacting scalar particles by applying time-ordered perturbation theory to the manifestly Lorentz-invariant formulation. The obtained leading-order equation is perturbatively renormalizable and non-perturbatively finite and does not require a three-body counter term in contrast to its non-relativistic approximation.

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arXiv2016

$P$-wave coupled channel effects in electron-positron annihilation

$P$-wave coupled channel effects arising from the $D\bar{D}$, $D\bar{D}^*+c.c.$, and $D^*\bar{D}^*$ thresholds in $e^+e^-$ annihilations are systematically studied. We provide an exploratory study by solving the Lippmann-Schwinger equation with short-ranged contact potentials obtained in the heavy quark limit. These contact potentials can be extracted from the $P$-wave interactions in the $e^+e^-$ annihilations, and then be employed to investigate possible isosinglet $P$-wave hadronic molecules. In particular, such an investigation may provide information about exotic candidates with quantum numbers $J^{PC}=1^{-+}$. In the mass region of the $D\bar{D}$, $D\bar{D}^*+c.c.$, and $D^*\bar{D}^*$ thresholds, there are two quark model bare states, i.e. the $ψ(3770)$ and $ψ(4040)$, which are assigned as $(1^3D_1)$ and $(3^1S_1)$ states, respectively. By an overall fit of the cross sections of $e^+e^-\to D\bar{D}$, $D\bar{D}^*+c.c.$, $D^*\bar{D}^*$, we determine the physical coupling constants to each channel and extract the pole positions of the $ψ(3770)$ and $ψ(4040)$. The deviation of the ratios from that in the heavy quark spin symmetry (HQSS) limit reflects the HQSS breaking effect due to the mass splitting between the $D$ and the $D^*$. Besides the two poles, we also find a pole a few MeV above the $D\bar{D}^*+c.c.$ threshold which can be related to the so-called $G(3900)$ observed earlier by BABAR and Belle. This scenario can be further scrutinized by measuring the angular distribution in the $D^*\bar{D}^*$ channel with high luminosity experiments.

hep-ph↗
arXiv2016

Heavy-quark spin symmetry partners of the $X(3872)$ revisited

We revisit the consequences of the heavy-quark spin symmetry for the possible spin partners of the $X(3872)$. We confirm that, if the $X(3872)$ were a $D\bar{D}^*$ molecular state with the quantum numbers $J^{PC}=1^{++}$, then in the strict heavy-quark limit there should exist three more hadronic molecules degenerate with the $X(3872)$, with the quantum numbers $0^{++}$, $1^{+-}$, and $2^{++}$ in line with previous results reported in the literature. We demonstrate that this result is robust with respect to the inclusion of the one-pion exchange interaction between the $D$ mesons. However, this is true only if all relevant partial waves as well as particle channels which are coupled via the pion-exchange potential are taken into account. Otherwise, the heavy-quark symmetry is destroyed even in the heavy-quark limit. Finally, we solve the coupled-channel problem in the $2^{++}$ channel with nonperturbative pions beyond the heavy-quark limit and, contrary to the findings of previous calculations with perturbative pions, find for the spin-2 partner of the $X(3872)$ a significant shift of the mass as well as a width of the order of 50 MeV.

hep-ph↗
arXiv2016

Subtraction of power counting breaking terms in chiral perturbation theory: spinless matter fields

When matter fields are included in chiral perturbation theory, the nonvanishing mass in the chiral limit introduces a new energy scale so that the loop diagrams including such matter field propagators spoil the usual power counting. However, the power counting breaking terms can be absorbed into counterterms in the chiral Lagrangian. In this paper, we systematically derive these terms to leading one-loop order (next-to-next-to leading order in the chiral expansion) at once by calculating the generating functional using the path integral. They are then absorbed by counterterms in the next-to-leading order Lagrangian. The method can be extended to calculating power counting breaking terms for other matter fields.

hep-ph↗
arXiv2016

Aspects of the low-energy constants in the chiral Lagrangian for charmed mesons

We investigate the numerical values of the low-energy constants in the chiral effective Lagrangian for the interactions between the charmed mesons and the lightest pseudoscalar mesons, the Goldstone bosons of the spontaneous breaking of chiral symmetry for QCD. This problem is tackled from two sides: estimates using the resonance exchange model, and positivity constraints from the general properties of the $S$-matrix including analyticity, crossing symmetry and unitarity. These estimates and constraints are compared with the values determined from fits to lattice data of the scattering lengths. Tensions are found, and possible reasons are discussed. We conclude that more data from lattice calculations and experiments are necessary to fix these constants better. As a by-product, we also estimate the coupling constant $g_{DDa_2}$, with $a_2$ the light tensor meson, via the QCD sum rule approach.

hep-ph↗
arXiv2016

Remarks on the $P_c$ structures and triangle singularities

It was proposed that the narrow $P_c(4450)$ structure observed by the LHCb Collaboration in the reaction $Λ_b\to J/ψp K$ might be due to a triangle singularity around the $χ_{c1}$--proton threshold at 4.45~GeV. We discuss the occurrence of a similar triangle singularity in the $J/ψp$ invariant mass distribution for the decay $Λ_b\to J/ψp π$, which could explain the bump around 4.45~GeV in the data. More precise measurements of this process would provide valuable information towards an understanding of the $P_c$ structures.

hep-ph↗
arXiv2016

Spectrum of three-body bound states in a finite volume

The spectrum of a bound state of three identical particles with a mass $m$ in a finite cubic box is studied. It is shown that in the unitary limit, the energy shift of a shallow bound state is given by $ΔE=c (κ^2/m)\,(κL)^{-3/2}|A|^2\exp(-2κL/\sqrt{3})$, where $κ$ is the bound-state momentum, $L$ is the box size, $|A|^2$ denotes the three-body analog of the asymptotic normalization coefficient of the bound state wave function and $c$ is a numerical constant. The formula is valid for $κL\gg 1$.

hep-lat↗
arXiv2016

Nuclear binding near a quantum phase transition

How do protons and neutrons bind to form nuclei? This is the central question of ab initio nuclear structure theory. While the answer may seem as simple as the fact that nuclear forces are attractive, the full story is more complex and interesting. In this work we present numerical evidence from ab initio lattice simulations showing that nature is near a quantum phase transition, a zero-temperature transition driven by quantum fluctuations. Using lattice effective field theory, we perform Monte Carlo simulations for systems with up to twenty nucleons. For even and equal numbers of protons and neutrons, we discover a first-order transition at zero temperature from a Bose-condensed gas of alpha particles (4He nuclei) to a nuclear liquid. Whether one has an alpha-particle gas or nuclear liquid is determined by the strength of the alpha-alpha interactions, and we show that the alpha-alpha interactions depend on the strength and locality of the nucleon-nucleon interactions. This insight should be useful in improving calculations of nuclear structure and important astrophysical reactions involving alpha capture on nuclei. Our findings also provide a tool to probe the structure of alpha cluster states such as the Hoyle state responsible for the production of carbon in red giant stars and point to a connection between nuclear states and the universal physics of bosons at large scattering length.

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arXiv2016

Precise determination of lattice phase shifts and mixing angles

We introduce a general and accurate method for determining lattice phase shifts and mixing angles, which is applicable to arbitrary, non-cubic lattices. Our method combines angular momentum projection, spherical wall boundaries and an adjustable auxiliary potential. This allows us to construct radial lattice wave functions and to determine phase shifts at arbitrary energies. For coupled partial waves, we use a complex-valued auxiliary potential that breaks time-reversal invariance. We benchmark our method using a system of two spin-1/2 particles interacting through a finite-range potential with a strong tensor component. We are able to extract phase shifts and mixing angles for all angular momenta and energies, with precision greater than that of extant methods. We discuss a wide range of applications from nuclear lattice simulations to optical lattice experiments.

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arXiv2016

Density-dependent effective baryon-baryon interaction from chiral three-baryon forces

A density-dependent effective potential for the baryon-baryon interaction in the presence of the (hyper)nuclear medium is constructed, based on the leading (irreducible) three-baryon forces derived within SU(3) chiral effective field theory. We evaluate the contributions from three classes: contact terms, one-pion exchange and two-pion exchange. In the strangeness-zero sector we recover the known result for the in-medium nucleon-nucleon interaction. Explicit expressions for the Lambda-nucleon in-medium potential in (asymmetric) nuclear matter are presented. Our results are suitable for implementation into calculations of (hyper)nuclear matter. In order to estimate the low-energy constants of the leading three-baryon forces we introduce the decuplet baryons as explicit degrees of freedom and construct the relevant terms in the minimal non-relativistic Lagrangian. With these, the constants are estimated through decuplet saturation. Utilizing this approximation we provide numerical results for the effect of the three-body force in symmetric nuclear matter and pure neutron matter on the Lambda-nucleon interaction. A moderate repulsion that increases with density is found in comparison to the free Lambda-nucleon interaction.

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