arXiv subjects

R. Machleidt

Publications and source records attributed to R. Machleidt.

At least 73 records · Page 4Linked to original sources

arXiv2003

Modelling nucleon-nucleon scattering above 1 GeV

Motivated by the recent measurement of proton-proton spin-correlation parameters up to 2.5 GeV laboratory energy, we investigate models for nucleon-nucleon (NN) scattering above 1 GeV. Signatures for a gradual failure of the traditional meson model with increasing energy can be clearly identified. Since spin effects are large up to tens of GeV, perturbative QCD cannot be invoked to fix the problems. We discuss various theoretical scenarios and come to the conclusion that we do not have a clear phenomenological understanding of the spin-dependence of the NN interaction above 1 GeV.

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arXiv2003

Towards a Model-Independent Low Momentum Nucleon-Nucleon Interaction

We provide evidence for a high precision model-independent low momentum nucleon-nucleon interaction. Performing a momentum-space renormalization group decimation, we find that the effective interactions constructed from various high precision nucleon-nucleon interaction models, such as the Paris, Bonn, Nijmegen, Argonne, CD Bonn and Idaho potentials, are identical. This model-independent low momentum interaction, called V_{low k}, reproduces the same phase shifts and deuteron pole as the input potential models, without ambiguous assumptions on the high momentum components, which are not constrained by low energy data and lead to model-dependent results in many-body applications. V_{low k} is energy-independent and does not necessitate the calculation of the Brueckner G matrix.

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arXiv2003

The nuclear force problem: Are we seeing the end of the tunnel?

Embedded in the historical context, we review recent progress in the development of nucleon-nucleon (NN) potentials based upon chiral effective field theory. A major breakthrough is the construction of the first NN potential at next-to-next-to-next-to-leading order (fourth order) of chiral perturbation theory (ChPT). The accuracy of this potential concerning the reproduction of the NN data below 290 MeV lab. energy is comparable to the one of phenomenological high-precision potentials. Since NN potentials of order three or less of ChPT are known to be deficient in quantitative terms, the recent advances show that the fourth order of ChPT is necessary and sufficient for a reliable NN potential derived from chiral effective Lagrangians. This recent substantial progress raises hopes that we might be getting closer to a solution of the nuclear force problem at low energies that has plagued the community for more than half a century.

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arXiv2003

Comment on "Determination of the chiral coupling constants c_3 and c_4 in new pp and np partial-wave analyses"

In a recent study [M.C.M. Rentmeester et al., Phys. Rev. C 67, 044001 (2003)], the Nijmegen group reports on the determination of the chiral low-energy constants (LEC), c_3 and c_4, involved in the 2-pi-exchange part of the NN amplitude at next-to-next-to-leading order (NNLO) of chiral perturbation theory. This analysis does not apply the uniquely-determined and model-independent NN amplitudes at NNLO and uses, instead, amplitudes that are up 90% smaller. We point out that this flaw produces a large systematic error, rendering the Nijmegen method unsuitable for a reliable determination of the LEC.

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arXiv2003

Accurate Charge-Dependent Nucleon-Nucleon Potential at Fourth Order of Chiral Perturbation Theory

We present the first nucleon-nucleon potential at next-to-next-to-next-to-leading order (fourth order) of chiral perturbation theory. Charge-dependence is included up to next-to-leading order of the isospin-violation scheme. The accuracy for the reproduction of the NN data below 290 MeV lab. energy is comparable to the one of phenomenological high-precision potentials. Since NN potentials of order three and less are known to be deficient in quantitative terms, the present work shows that the fourth order is necessary and sufficient for a reliable NN potential derived from chiral effective Lagrangians. The new potential provides a promising starting point for exact few-body calculations and microscopic nuclear structure theory (including chiral many-body forces derived on the same footing).

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arXiv2002

Microscopic Nuclear Structure Based upon a Chiral NN Potential

We report on shell-model calculations employing effective interactions derived from a new realistic nucleon-nucleon (NN) potential based on chiral effective field theory. We present results for 18O, 134Te, and 210Po. Our results are in excellent agreement with experiment indicating a remarkable predictive power of the chiral NN potential for low-energy microscopic nuclear structure.

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arXiv2002

Chiral 2pi exchange at order four and peripheral NN scattering

We calculate the impact of the complete set of two-pion exchange contributions at chiral order four (also known as next-to-next-to-next-to-leading order, N3LO) on peripheral partial waves of nucleon-nucleon scattering. Our calculations are based upon the analytical studies by Kaiser. It turns out that the contribution of order four is substantially smaller than the one of order three, indicating convergence of the chiral expansion. We compare the prediction from chiral pion-exchange with the corresponding one from conventional meson-theory as represented by the Bonn Full Model and find, in general, good agreement. Our calculations provide a sound basis for investigating the issue whether the low-energy constants determined from pi-N lead to reasonable predictions for NN.

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arXiv2001

Chiral NN model and Ay puzzle

We analyze the results by chiral NN models for the two-nucleon system and calculate the predictions for the nucleon vector analyzing power of elastic nucleon-deuteron (Nd) scattering, Ay, by these models. Our conclusion is that a QUANTITATIVE chiral two-nucleon potential does not resolve the Nd Ay puzzle (when only two-body forces are included).

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arXiv2001

$Δ(1232)$ Isobar Excitations and the Ground State of Nuclei

The influence of $Δ$ isobar components on the ground state properties of nuclear systems is investigated for nuclear matter as well as finite nuclei. Many-body wave functions, including isobar configurations, and binding energies are evaluated employing the framework of the coupled-cluster theory. It is demonstrated that the effect of isobar configurations depends in a rather sensitive way on the model used for the baryon-baryon interaction. As examples for realistic baryon-baryon interactions with explicit inclusion of isobar channels we use the local ($V28$) and non-local meson exchange potentials (Bonn$_{2000}$) but also a model recently developed by the Salamanca group, which is based on a quark picture. The differences obtained for the nuclear observables are related to the treatment of the interaction, the $π$-exchange contributions in particular, at high momentum transfers.

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arXiv2001

Accurate Nucleon-Nucleon Potential Based upon Chiral Perturbation Theory

We present an accurate nucleon-nucleon ($NN$) potential based upon chiral effective Lagrangians. The model includes one- and two-pion exchange contributions up to chiral order three. We show that a quantitative fit of the $NN$ $D$-wave phase shifts requires contact terms (which represent the short range force) of order four. Within this framework, the $NN$ phase shifts below 300 MeV lab. energy and the properties of the deuteron are reproduced with high-precision. This chiral $NN$ potential represents a reliable starting point for testing the chiral effective field theory approach in exact few-nucleon and microscopic nuclear many-body calculations. An important implication of the present work is that the chiral $2π$ exchange at order four is of crucial interest for future chiral $NN$ potential development.

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arXiv2001

Chiral Symmetry and the Nucleon-Nucleon Interaction: Developing an Accurate NN Potential Based upon Chiral Effective Field Theory

We present an accurate nucleon-nucleon (NN) potential based upon chiral effective Lagrangians. The model includes one- and two-pion exchange contributions up to chiral order three and contact terms (which represent the short range force) up to order four. Within this framework, the NN phase shifts below 300 MeV lab. energy and the properties of the deuteron are reproduced with high-precision. This chiral NN potential may serve as a reliable starting point for testing the chiral effective field theory approach in exact few-nucleon and microscopic nuclear many-body calculations.

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arXiv2001

The nucleon-nucleon interaction

We review the major progress of the past decade concerning our understanding of the nucleon-nucleon interaction. The focus is on the low-energy region (below pion production threshold), but a brief outlook towards higher energies is also given. The items discussed include charge-dependence, the precise value of the $πNN$ coupling constant, phase shift analysis and high-precision NN data and potentials. We also address the issue of a proper theory of nuclear forces. Finally, we summarize the essential open questions that future research should be devoted to.

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arXiv2000

Charge symmetry breaking of the nucleon-nucleon interaction: $ρ$-$ω$ mixing VERSUS nucleon mass splitting

We investigate three models for the charge symmetry breaking (CSB) of the nucleon-nucleon ($NN$) interaction (based upon $ρ$-$ω$ mixing, nucleon mass splitting, and phenomenology) that all reproduce the empirical value for the CSB of the $^1S_0$ scattering length ($Δa_{CSB}$) accurately. We reveal that these models make very different predictions for CSB in $^3P_J$ waves and examine the impact of this on some observable quantities of $A\geq 3$ nuclear systems. It turns out that the $^3$H-$^3$He binding energy difference is essentially ruled by $Δa_{CSB}$ and not very sensitive to CSB from $P$ waves. However, the Coulomb displacement energies (which are the subject of the Nolen-Schiffer anomaly) receive about 50% of their CSB contribution from $NN$ partial waves beyond $^1S_0$. Consequently, the predictions by the various CSB models differ here substantially (10-20%). Unfortunately, the evaluation of the leading Coulomb contributions carry a large uncertainty such that no discrimination between the competing CSB models can presently be made. To decide the issue we suggest to look into nuclear few-body reactions that are sensitive to CSB of the nuclear force.

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arXiv2000

The nuclear force in the third millennium

I will review recent progress in our understanding of the nuclear force. In the course of the 1990's, so-called high-precision, charge-dependent nucleon-nucleon potentials have been constructed which are, essentially, phenomenological models. These potentials are now commonly used as input for exact few-body calculations and microscopic nuclear many-body theory. I will critically analyses those models and point out their strenghts and weaknesses. Particular emphasis will be on charge dependence. Other recent research was conducted on a more basic level: understanding the nuclear force in terms of the fundamental theory of strong interactions, QCD. Predictions from this sector are typically qualitative in nature. Thus, the main problem of the current status in the field is that quantitative models for the nuclear force have a poor theoretical background, while theory based models yield poor results. The chief challenge for the new millenium is to overcome this discrepancy. Chiral effective field theory may be a suitable tool to solve the problem.

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arXiv2000

The high-precision, charge-dependent Bonn nucleon-nucleon potential (CD-Bonn)

We present a charge-dependent nucleon-nucleon (NN) potential that fits the world proton-proton data below 350 MeV available in the year of 2000 with a chi^2 per datum of 1.01 for 2932 data and the corresponding neutron-proton data with chi^2/datum = 1.02 for 3058 data. This reproduction of the NN data is more accurate than by any phase-shift analysis and any other NN potential. The charge-dependence of the present potential (that has been dubbed `CD-Bonn') is based upon the predictions by the Bonn Full Model for charge-symmetry and charge-independence breaking in all partial waves with J <= 4. The potential is represented in terms of the covariant Feynman amplitudes for one-boson exchange which are nonlocal. Therefore, the off-shell behavior of the CD-Bonn potential differs in a characteristic and well-founded way from commonly used local potentials and leads to larger binding energies in nuclear few- and many-body systems, where underbinding is a persistent problem.

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arXiv1999

Brueckner Theory of Nuclear Matter with Nonnucleonic Degrees of Freedom and Relativity

For the past 40 years, Brueckner theory has proven to be a most powerful tool to investigate systematically models for nuclear matter. I will give an overview of the work done on nuclear matter theory, starting with the simplest model and proceeding step by step to more sophisticated models by extending the degrees of freedom and including relativity. The final results of a comprehensive hadronic theory of nuclear matter are compared to the predictions by currently fashionable two-nucleon force models. It turns out that a two-nucleon force can, indeed, reproduce those results if the potential is nonlocal, since nonlocality is an inherent quality of the more fundamental fieldtheoretic approach. This nonlocality is crucial for creating sufficient nuclear binding.

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arXiv1999

How sensitive are various NN observables to changes in the $πNN$ coupling constant?

The deuteron, NN analyzing powers A_y, and the singlet scattering length show great sensitivity to the $πNN$ coupling constant $g_π$. While the pp A_y data favor $g^2_π/4π\leq 13.6$, the np A_y data and the deuteron quadrupole moment imply $g^2_π/4π\geq 14.0$. The two diverging values could be reconciled by the assumption of (substantial) charge-splitting of $g_π$. However, the established theoretical explanation of the charge-dependence of the $^1S_0$ scattering length (based upon pion mass splitting) is very sensitive to a difference between $g_{π^0}$ and $g_{π^\pm}$ and rules out any substantial charge-splitting of $g_π$. Thus, there are real and large discrepancies between the values for $g_π$ extracted from different NN observables. Future work that could resolve the problems is suggested.

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arXiv1999

Charge-dependence of the $πNN$ coupling constant and charge-dependence of the NN interaction

The recent determination of the charged $πNN$ coupling constant, $g_{π^\pm}$, by the Uppsala Neutron Research Group implies that there may be considerable charge-splitting of the pion coupling constant. We investigate the consequences of this for the charge-independence breaking (CIB) of the $^1S_0$ scattering length, $Δa_{CIB}$. We find that $Δa_{CIB}$ depends sensitively on the difference between $g_{π^\pm}$ and the neutral $πNN$ coupling constant, $g_{π^0}$. Moreover, if $g^2_{π^\pm}$ is only about 3% larger than $g^2_{π^0}$, then the established theoretical explanation of $Δa_{CIB}$ (in terms of pion mass splitting) is completely wiped out.

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