Two-pion Exchange Research Articles

Alpha-clustered hypernuclei and chiral SU(3) dynamics

Light hypernuclei with an $\alpha$ cluster substructure of the core nucleus are studied using an accurate cluster approach (the Hyper-THSR wave function) in combination with a density-dependent $\Lambda$ hyperon-nuclear interaction derived from chiral SU(3) effective field theory. This interaction includes important two-pion exchange processes involving $\Sigma N$ intermediate states and associated three-body mechanisms as well as effective mass and surface terms arising in a derivative expansion of the in-medium $\Lambda$ self-energy. Applications and calculated results are presented and discussed for $_\Lambda^9$Be and $^{13}_\Lambda$C. Furthermore, the result of the lightest $\alpha$ clustered hypernucleus, $^5_{\Lambda}$He using realistic $ab initio$ four nucleon density is shown.

Error Analysis of Nuclear Matrix Elements

We estimate the expected errors of nuclear matrix elements coming from the uncertainty on the NN interaction. We use a coarse grained (GR) interaction fitted to NN scattering data, with several prescriptions for the long-part of the interaction, including one pion exchange and chiral two-pion exchange interactions.

Revisiting the spectrum of baryonium in heavy baryon chiral perturbation theory

In the framework of the heavy baryon perturbation theory, in which the two-pion exchange is considered, the physical properties of heavy-baryon-anti-heavy-baryon systems are revisited. The potentials between heavy-baryon and anti-heavy-baryon are extracted in a holonomic form. Based on the extracted potentials, the s-wave scattering phase shifts and scattering lengths of $\Lambda_c$-$\bar{\Lambda}_c$ and $\Sigma_c$-$\bar{\Sigma}_c$ are calculated. From these scattering features, it is found that the $\Lambda_c$-$\bar{\Lambda}_c$ system can be bound only when the value of the coupling constant $g_2$ is larger than that from the decay data of the $\Sigma_c(\Sigma_c^*) \to \Lambda_c \pi$ process. The binding condition for the $\Sigma_c$-$\bar{\Sigma}_c$ system is also examined. The binding possibilities of these systems deduced from the scattering calculations are also checked by the bound state calculation and the binding energies are obtained if the system can be really bound. The binding possibility of the $\Lambda_b$-$\bar{\Lambda}_b$ system is investigated as well.

Improved convergence of chiral effective field theory for1S0ofNNscattering

It is argued that the fine-tuning due to the unnaturally large, generalized effective range in the 1S0 channel of NN scattering must be incorporated in order for one to obtain satisfactory convergence for chiral effective field theory. Without the proposition of perturbative one-pion exchange, an effective field theory with the spin-0, isospin-1 dibaryon is developed to account for this fine-tuning, and is demonstrated up to O(Q) where the leading irreducible two-pion exchange arises. The approach shown in the paper results in rapid convergence of the 1S0 partial-wave amplitude.

Polarization observables in low-energy antiproton-proton scattering

We investigate the polarization parameters in low-energy antiproton-proton elastic ($\bar{p}p\rightarrow\bar{p}p$) and charge-exchange ($\bar{p}p\rightarrow\bar{n}n$) scattering. The predictions for unmeasured observables are based on our new energy-dependent partial-wave analysis of all antiproton-proton scattering data below 925 MeV/$c$ antiproton laboratory momentum, which gives an optimal description of the existing database. Sizable and remarkable spin effects are observed, in particular for charge-exchange scattering. These result from the spin dependence of the long- and medium-range one- and two-pion exchange antinucleon-nucleon potential and the state dependence of the parametrized short-range interaction. We study the possibility of polarizing a circulating antiproton beam with a polarized proton target by exploiting the spin dependence of the $\bar{p}p$ total cross section. It appears feasible to achieve a significant transverse polarization of an antiproton beam within a reasonable time.

Two-pseudoscalar exchanges between heavy hadrons

We calculate the two-pion and two-kaon exchange potentials of and by using the Gaussian form factor to regularize loop integrals. Although the magnitude of the two-pseudo-goldstone exchange is small as expected, it is not negligible in dynamics because it modifies the shape of the potential. We find the bound state of starts to appear at the cut-off Λ = 1360 MeV after consideration of the two-pseudo-goldstone exchange, significantly reducing the discrepancy between chiral perturbation theory and other phenomenological approaches.

Possible bound state of the double heavy meson-baryon system

We calculate the two-pion exchange potential between a heavy meson and a heavy baryon. We find this potential is as strong as the one-pion exchange potential between two heavy mesons and is enough to bind ${\ensuremath{\Lambda}}_{b}\ensuremath{-}\overline{B}$. Though our result is sensitive to the cutoff, the value of the cutoff is in the reasonable region.

Parity-nonconserving cold neutron-parahydrogen interactions

Three pion dominated observables of the parity nonconserving interactions between the cold neutrons and parahydrogen are calculated. The transversely polarized neutron spin rotation, unpolarized neutron longitudinal polarization, and photon-asymmetry of the radiative polarized neutron capture are considered. For the numerical evaluation of the observables, the strong interactions are taken into account by the Reid93 potential and the parity nonconserving interactions by the DDH model along with the two-pion exchange.

Energy-dependent partial-wave analysis of all antiproton-proton scattering data below 925 MeV/c

We present a new energy-dependent partial-wave analysis of all antiproton-proton elastic and charge-exchange scattering data below 925 MeV/c antiproton laboratory momentum. The long-range parts of the chiral one- and two-pion exchange interactions are included exactly. The short-range interactions, including the coupling to the mesonic annihilation channels, are parametrized by a complex boundary condition at a radius of r=1.2 fm. The updated database, which includes significantly more high-quality charge-exchange data, contains 3749 scattering data. The fit results in chi^2_min/N_df=1.048, where N_df=3578 is the number of degrees of freedom. We discuss the description of the experimental data and we present the antiproton-proton phase-shift parameters.

Parity nonconserving proton-proton elastic scattering

The parity nonconserving longitudinal analyzing power A_L is calculated in elastic pp scattering at the energies below the approximate inelastic region T_lab = 350 MeV. The short-ranged heavy meson rho and omega exchanges as well as the longer-ranged two pion exchanges are considered as the mediators of the parity nonconserving interactions. The DDH "best" coupling values are used as the parity nonconserving meson-NN couplings. Also three different parity nonconserving two-pion exchange potentials by various authors are compared.

Short-range nuclear forces in singlet channels

Continuing our effort to build a consistent power counting for chiral nuclear effective field theory (EFT), we discuss the subleading contact interactions, or counterterms, in the singlet channels of nucleon-nucleon scattering, with renormalization group invariance as the constraint. We argue that the rather large cutoff error of the leading amplitude requires $\mathcal{O}(Q)$ of the EFT expansion to be nonvanishing, contrary to Weinberg's original power counting. This, together with the ultraviolet divergences of two-pion exchanges in the distorted-wave expansion, leads to enhancement of the ${}^{1}{S}_{0}$ counterterms and results in a pionless-theory-like power counting for the singlet channels.

Energy of partial A-body problem for 9ΛBe and 10ΛΛBe

The energy of the degenerate doublet (3+/2, 5+/2) of 9ΛBe, treating it as a partial nine-body system in the Λαα cluster model, has been calculated in the variational Monte Carlo framework. A simplified treatment, with the central two-body Urbana type ΛN and the three-body dispersive and two-pion exchange ΛNN forces along with the central two- and three-body correlations, is found to be adequate in explaining the energy of observed γ-ray transition from the excited degenerate doublet to the ground state. This partial nine-body model of 9ΛBe has been extended for the case of partial ten-body problem 10ΛΛBe in the ΛΛ + αα model where nucleonic degrees of freedom of as are taken into consideration ignoring antisymmetrization between two αs. The central two-body ΛN and ΛΛ and the three-body dispersive and two-pion exchange ΛNN forces, constrained by the Λp scattering data and the observed ground state energies of 5ΛHe and 6ΛΛ, are employed. The product type trial wavefunction predicts binding energy for the ground state considerably less than for the event reported by Danysz et al but however, consistent with the value deduced assuming a γ-ray of 3.04 MeV must have escaped undetected in the decay of the product 9ΛBe* → 9ΛBe + γ of the emulsion event 10ΛΛBe → π− + p + 9ΛBe* and for the excited 2+ state closer to the value measured in the Demachi-Yanagi event. The results of the present work are consistent with the earlier three- and four-body cluster model approaches where as are assumed to be structureless entities.

Nuclear thermodynamics and the in-medium chiral condensate

The temperature dependence of the chiral condensate in isospin-symmetric nuclear matter at varying baryon density is investigated using thermal in-medium chiral effective field theory. This framework provides a realistic approach to the thermodynamics of the correlated nuclear many-body system and permits calculating systematically the pion-mass dependence of the free energy per particle. One- and two-pion exchange processes, Δ(1232)-isobar excitations, Pauli-blocking corrections and three-body correlations are treated up to and including three loops in the expansion of the free energy density. It is found that nuclear matter remains in the Nambu–Goldstone phase with spontaneously broken chiral symmetry in the temperature range T≲100 MeV and at baryon densities at least up to about twice the density of normal nuclear matter, 2ρ0≃0.3 fm−3. Effects of the nuclear liquid–gas phase transition on the chiral condensate at low temperatures are also discussed.

Short-range nuclear effects on axion emissivities by nucleon–nucleon bremsstrahlung

The rates of axion emission by nucleon–nucleon (NN) bremsstrahlung are reconsidered by taking into account the NN short-range correlations. The analytical formulas for the neutron–neutron, proton–proton and neutron–proton processes with the inclusion of the full momentum dependence of one- and two-pion exchange nuclear potentials, in the non-degenerate limit, are explicitly given. We find that the two-pion exchange (short-range) effects can give a significant contribution to the emission rates, and are temperature dependent. Also, other short-range nuclear effects such as effective nucleon mass, polarization effects and use of correlated wave functions are discussed. The trend of all these nuclear effects is to diminish the corresponding axion emission rates. Furthermore, we estimate that the values of the emission rates calculated with the inclusion of all these effects can differ from the corresponding ones derived with constant nuclear matrix elements by a factor of ∼24. This leads to an uncertainty factor of ∼4.9 when extracting bounds of the axion parameters.

Chiral three-nucleon force at N4LO: Longest-range contributions

We derive the sub-subleading two-pion exchange contributions to the three-nucleon force which appear at next-to-next-to-next-to-next-to-leading order in chiral effective field theory. In order to determine the low-energy constants, a complete analysis of pion-nucleon scattering at the subleading-loop order in the heavy-baryon expansion is carried out utilizing the power counting scheme employed in the derivation of the nuclear forces. We discuss the convergence of the chiral expansion for this particular three-nucleon force topology and give the values of the low-energy constants which provide the most realistic description of the three-nucleon force when the chiral expansion is truncated at next-to-next-to-leading order.

Chiral thermodynamics of nuclear matter

The free energy and the equation of state of isospin-asymmetric nuclear matter are calculated at finite temperature up to three loop order in the framework of in-medium chiral perturbation theory, systematically incorporating one- and two-pion exchange dynamics to this order. Effects from the two-pion exchange with explicit {\Delta}-isobar excitation are included, as well as three-body forces. We construct the phase diagram of nuclear matter for different proton fractions x_p and investigate the dependence of nuclear matter properties on the isospin-asymmetry. A detailed study of the liquid-gas phase transition is performed. For isospin-symmetric nuclear matter we find a critical temperature of 15.1 MeV; as the isospin-asymmetry is increased, the liquid-gas coexistence region decreases until it disappears at x_p \approx 0.05, while nuclear matter becomes unbound at x_p \approx 0.12. The quadratic expansion of the free energy in the asymmetry parameter {\delta} is a good approximation at low temperature even for large {\delta}. An estimate of chiral four-body correlations in nuclear and neutron matter is also performed and the corrections from such four-body interactions are found to be small.

Study of parity violating observables in few-nucleon systems

Parity violation in few-nucleon systems is studied using a nucleon-nucleon parity-violating (PV) potential derived within an effective field theory framework at next-to-next-to-leading order. The potential includes one- and two-pion exchanges, contact interactions and relativistic corrections and depends on six low-energy constants: the pion-nucleon coupling constant h1π and five parameters multiplying the independent contact interaction terms (with one four-gradient). This potential is used to study the p⃗-p longitudinal asymmetry, the neutron spin rotation in n⃗-d scattering, and the longitudinal asymmetry in the 3He(n⃗,p)3H reaction.

Holographic equations of state and astrophysical compact objects

We solve the Tolman-Oppenheimer-Volkoff equation using an equation of state (EoS) calculated in holographic QCD. The aim is to use compact astrophysical objects like neutron stars as an indicator to test holographic equations of state. We first try an EoS from a dense D4/D8/D8 model. In this case, however, we could not find a stable compact star, a star satisfying pressure-zero condition with a radius R, p(R) = 0, within a reasonable value of the radius. This means that the EoS from the D4/D8/D8 model may not support any stable compact stars or may support one whose radius is very large. This might be due to a deficit of attractive force from a scalar field or two-pion exchange in the D4/D8/D8 model. Then, we consider D4/D6 type models with different number of quark flavors, N f = 1, 2, 3. Though the mass and radius of a holographic star is larger than those of normal neutron stars, the D4/D6 type EoS renders a stable compact star.

Nuclear energy density functional from chiral two-nucleon aaand three-nucleon interactions

An improved density-matrix expansion is used to calculate the nuclear energy density functional from chiral two- and three-nucleon interactions. The two-body interaction comprises long-range one- and two-pion exchange contributions and a set of contact terms contributing up to fourth power in momenta. In addition we employ the leading-order chiral three-nucleon interaction with its parameters c E , c D and c 1,3,4 fixed in calculations of nuclear few-body systems. With this input the nuclear energy density functional is derived to first order in the two- and three-nucleon interaction. We find that the strength functions F ∇(ρ) and F so (ρ) of the surface and spin-orbit terms compare in the relevant density range reasonably with the results of phenomenological Skyrme forces. However, an improved description requires (at least) the treatment of the two-body interaction to second order. This observation is in line with the deficiencies in the nuclear matter equations of state $$\bar E(\rho )$$ that remain in the Hartree-Fock approximation with low-momentum two- and three-nucleon interactions.

Signatures of the chiral two-pion exchange electromagnetic currents in theH2andHe3photodisintegration reactions

The recently derived long-range two-pion exchange (TPE) contributions to the nuclear current operator which appear at next-to-leading order (NLO) of the chiral expansion are used to describe electromagnetic processes. We study their role in the photodisintegration of 2H and 3He and compare our predictions with experimental data. The bound and scattering states are calculated using five different parametrizations of the chiral next-to-next-to-leading order (N2LO) nucleon-nucleon (NN) potential which allows us to estimate the theoretical uncertainty at a given order in the chiral expansion. For some observables the results are very close to the predictions based on the AV18 NN potential and the current operator (partly) consistent with this force. In the most cases, the addition of long-range TPE currents improved the description of the experimental data.