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Renato Higa

Publications and source records attributed to Renato Higa.

18 recordsLinked to original sources
arXiv2025

Precision calculation of $^3$He$(α,γ)^7$Be for solar physics

We calculate the cross section for radiative capture $^3$He$(α,γ)^7$Be at next-to-next-to-leading order (NNLO). At this order of perturbation, momentum dependent two-body currents make their first appearance. We provide a model-independent construction of these currents from gauge and Galilean invariance, where the general framework for constructing higher-order two-body currents in low-energy effective field theories becomes evident. The $^3$He$(α,γ)^7$Be astrophysical S-factor $S_{34}(0)= 0.564^{+0.17}_{-0.015}$ keV b is obtained from a Bayesian analysis at NNLO, with an additional nominal theoretical uncertainty $\pm0.017$ keV b of 3%.

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arXiv2024

Coupled-channels treatment of $^7\mathrm{Be}(p,γ)^8\mathrm{B}$ in effective field theory

The E1 and M1 contributions to $^7\mathrm{Be}(p,γ)^8\mathrm{B}$ at low energies are calculated in halo effective field theory. The excited $^7\mathrm{Be}^\star$ core is included as an explicit degree of freedom in a coupled-channels calculation. The E1 transition is calculated up to next-to-next-to-leading order. The leading contribution from M1 transition that gives significant contribution in a narrow energy region around the $1^+$ resonance state of $^8$B is included. We compare our results with previous halo effective field theory calculations that also included the $^7\mathrm{Be}^\star$ as an explicit degree of freedom. We disagree with these previous calculations in both the formal expressions and also in the analysis. Bayesian inference of the data gives $S_{17}(0)=21.0(7)$ eV b when combined with the expected theory error.

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arXiv2024

Coupled-channel treatment of $^7\mathrm{Li}(n,γ)^8\mathrm{Li}$ in effective field theory

The E1 contribution to the capture reaction $^7\mathrm{Li}(n,γ)^8\mathrm{Li}$ is calculated at low energies. We employ a coupled-channel formalism to account for the $^7\mathrm{Li}^\star$ excited core contribution. We develop a halo effective field theory power counting where capture in the spin $S=2$ channel is enhanced over the $S=1$ channel. A next-to-leading order calculation is presented where the excited core contribution is shown to affect only the overall normalization of the cross section. The momentum dependence of the capture cross section, as a consequence, is the same in a theory with or without the excited $^7\mathrm{Li}^\star$ degree of freedom at this order of the calculation. The kinematical signature of the $^7\mathrm{Li}^\star$ core is negligible at momenta below 1 MeV and significant only beyond the $3^+$ resonance energy, though still compatible with a next-to-next-to-leading order correction. We compare our formalism with a previous halo effective field theory calculation [Zhang, Nollett, and Phillips, Phys. Rev. C 89, 024613 (2014)] that also treated the $^7\mathrm{Li}^\star$ core as an explicit degree of freedom. Our formal expressions and analysis disagree with this earlier work in several aspects.

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arXiv2018

Dipole-dipole interactions between neutrons

In this work we present results of the dipole-dipole interactions between two neutrons, a neutron and a conducting wall, and a neutron between two walls. As input, we use dynamical electromagnetic dipole polarizabilities fitted to chiral EFT results up to the pion production threshold and at the onset of the Delta resonance. Our work can be relevant to the physics of confined ultracold neutrons inside bottles.

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arXiv2018

Fate of the neutron-deuteron virtual state as an Efimov level

The emergence of Efimov levels in a three-body system is investigated near the unitarity limit characterized by resonating two-body interaction. No direct evidence of Efimov levels is seen in the three-nucleon system since the triton is the only physical bound state. We provide a model-independent analysis of nucleon-deuteron scattering at low energy by formulating a consistent effective field theory. We show that virtual states evolve into shallow bound states, which emerge as excited triton levels as we drive the system towards unitarity. Even though we consider this specific system, our results for the emergence of the Efimov levels are universal.

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arXiv2018

Radiative 3He-alpha reaction in Halo Effective Field Theory

In this work we study the radiative capture of ${\rm {}^3He}$ on ${\rm {}^4He}$ within the halo effective field theory (EFT) framework. At leading order the capture amplitude comprises the initial state $s$-wave strong and Coulomb interactions summed to all orders. At the same order in the expansion, leading two-body currents contribute as well. We find delicate cancelations between the various contributions, and the two-body current contributions can be replaced by appropriately enhancing the asymptotic normalizations of the $^7$Be ground and first excited state wave functions. The next-to-leading order corrections come from the $s$-wave shape parameter and the pure Coulomb $d$-wave initial state interactions. We fit the EFT parameters to available scattering data and most recent capture data. Our zero-energy astrophysical $S$-factor estimate, $S_{34}\sim 0.55$ keV b, is consistent within error bars with the average in the literature.

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arXiv2017

Dipole-dipole dispersion interactions between neutrons

We investigate the long-range interactions between two neutrons utilizing recent data on the neutron static and dynamic electric and magnetic dipole polarizabilities. The resulting long-range potentials are used to make quantitative comparisons between the collisions of a neutron with a neutron and a neutron with a proton. We also assess the importance of the first pion production threshold and first excited state of the nucleon, the $Δ$-resonance ($J^π$ = + 3/2, I = 3/2). We found both dynamical effects to be quite relevant for distances r between ~ 50 fm up to ~$10^3$ fm in the nn system, the neutron-wall system and in the wall-neutron-wall system, reaching the expected asymptotic limit beyond that. Relevance of our findings to the confinement of ultra cold neutrons inside bottles is discussed.

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arXiv2012

Electric Dipole Moments of Light Nuclei from Chiral EFT

Recent calculations of EDMs of light nuclei in the framework of chiral effective field theory are presented. We argue that they can be written in terms of the leading six low-energy constants encoding CP-violating physics. EDMs of the deuteron, triton, and helion are explicitly given in order to corroborate our claim. An eventual non-zero measurement of these EDMs can be used to disentangle the different sources and strengths of CP-violation.

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arXiv2011

Resonance Contribution to Radiative Neutron Capture on Lithium-7

Using halo effective field theory, we provide a model-independent calculation of the radiative neutron capture on lithium-7 over an energy range where the contribution from the 3+ resonance becomes important. One finds that a satisfactory description of the capture reaction, in the present single-particle approximation, suggests the use of a resonance width about three times larger than the experimental value. We also present power counting arguments that establish a hierarchy for electromagnetic one- and two-body currents.

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arXiv2010

Radiative Neutron Capture on Lithium-7

The radiative neutron capture on lithium-7 is calculated model independently using a low energy halo effective field theory. The cross section is expressed in terms of scattering parameters directly related to the S-matrix element. The cross section depends on the poorly known p-wave effective range parameter r. This constitutes the leading order uncertainty in traditional model calculations. It is explicitly demonstrated by comparing with potential model calculations. A single parameter fit describes the low energy data extremely well and yields r = -1.47 fm^{-1}.

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arXiv2010

Electromagnetic interactions in Halo Effective Field Theory

After a brief discussion of effective field theory applied to nuclear clusters, I concentrate on the inclusion of two particular aspects, namely, narrow resonances and electromagnetic interactions. As examples of applications, I present the details of our studies on alpha-alpha and proton-alpha scattering.

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arXiv2009

NN potentials from IR chiral EFT

Chiral perturbation theory is nowadays a well-established approach to incorporate the chiral constraints from QCD. Nevertheless, for systems involving one baryon, the power counting which dictates the chiral order of observables is not as simple and consensual as in the purely mesonic case. The heavy baryon approach, which relies on a non-relativistic expansion around the limit of infinitely heavy baryon, recovers the usual power counting but destroys some analytic properties of the scattering amplitude. Some years ago, Becher and Leutwyler proposed a Lorentz-invariant formulation of chiral perturbation theory that maintains the required analytic properties, but at the expense of a less intuitive power counting. Aware of the shortcomings of the heavy baryon formalism, the São Paulo group derived the two-pion exchange component of the nucleon-nucleon potential in line with the works of Becher and Leutwyler. A striking result was that the long distance properties of the potential is determined by the specific low energy region of the pion-nucleon scattering amplitude where the heavy baryon expansion fails. In this talk I will discuss the origin of such failure and how it reflects in the asymptotics of the nucleon-nucleon interaction. Some results for phase shifts and deuteron properties will be shown, followed by a comparison with the heavy baryon predictions.

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arXiv2008

Nuclear clusters with Halo Effective Field Theory

After a brief discussion of effective field theory applied to nuclear clusters, I present the aspect of Coulomb interactions, with applications to low-energy alpha-alpha and nucleon-alpha scattering.

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arXiv2004

Low-energy constants and relativity in peripheral nucleon-nucleon scattering

In our recent works we derived a chiral $O(q^4)$ two-pion exchange nucleon-nucleon potential (TPEP) formulated in a relativistic baryon (RB) framework, expressed in terms of the so called low energy constants (LECs) and functions representing covariant loop integrations. We showed that the expansion of these functions in powers of the inverse of the nucleon mass reproduces most of the terms of the TPEP derived from the heavy baryon (HB) formalism, but such an expansion is ill defined and does not converge at large distances. In the present work we perform a study of the phase shifts in nucleon-nucleon ($NN$) scattering for peripheric waves ($L\geq 3$), which are sensitive to the tail of the potential. We assess quantitatively the differences between the RB and HB results, as well as variations due to different values of the LECs. By demanding consistency between the LECs used in $πN$ and $NN$ scattering we show how $NN$ peripheral phase shifts could constrain these values. We demonstrate that this idea, first proposed by the Nijmegen group, favors a smaller value for the LEC $c_3$ than the existing ones, when considering the TPEP up to order $q^4$ in the chiral expansion.

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arXiv2003

Relativistic $O(q^4)$ two-pion exchange nucleon-nucleon potential: configuration space

We have recently performed a relativistic $O(q^4)$ chiral expansion of the two-pion exchange $NN$ potential, and here we explore its configuration space content. Interactions are determined by three families of diagrams, two of which involve just $g_A$ and $f_π$, whereas the third one depends on empirical coefficients fixed by subthreshold $πN$ data. In this sense, the calculation has no adjusted parameters and gives rise to predictions, which are tested against phenomenological potentials. The dynamical structure of the eight leading non-relativistic components of the interaction is investigated and, in most cases, found to be clearly dominated by a well defined class of diagrams. In particular, the central isovector and spin-orbit, spin-spin, and tensor isoscalar terms are almost completely fixed by just $g_A$ and $f_π$. The convergence of the chiral series in powers of the ratio (pion mass/nucleon mass) is studied as a function of the internucleon distance and, for $r>$ 1 fm, found to be adequate for most components of the potential. An important exception is the dominant central isoscalar term, where the convergence is evident only for $r>$ 2.5 fm. Finally, we compare the spatial behavior of the functions that enter the relativistic and heavy baryon formulations of the interaction and find that, in the region of physical interest, they differ by about 5%.

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