arXiv subjects

Tae-Sun Park

Publications and source records attributed to Tae-Sun Park.

At least 37 records · Page 2Linked to original sources

arXiv2008

Effective field theory approach for the M1 properties of A=2 and 3 nuclei

The magnetic moments of ${}^2{H}$, ${}^3{He}$ and ${}^3{H}$ as well as the thermal neutron capture rate on the proton are calculated using heavy baryon chiral perturbation theory {\it à la} Weinberg. The M1 operators have been derived up to {N$^3$LO}. The nuclear matrix elements are evaluated with the use of wave functions obtained by carrying out variational Monte Carlo calculations for a realistic nuclear Hamiltonian involving high-precision phenomenological potentials like Argonne Av18 and Urbana IX tri-nucleon interactions. We discuss the potential- and cutoff-dependence of the results.

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arXiv2004

The Solar $Hep$ Process

The $Hep$ process is a weak-interaction reaction, $He3 + p \to He4 + e^+ + ν_e$, which occurs in the sun. There is renewed interest in $Hep$ owing to current experimental efforts to extract from the observed solar neutrino spectrum information on non-standard physics in the neutrino sector. $Hep$ produces highest-energy solar neutrinos, although their flux is quite modest. This implies that the $Hep$ neutrios can at some level influence the solar neutrino spectrum near its upper end. Therefore, a precise interpretation of the observed solar neutrino spectrum requires an accurate estimate of the $Hep$ rate. This is an interesting but challenging task. We describe the difficulties involved and how the recent theoretical developments in nuclear physics have enabled us to largely overcome these difficulties. A historical survey of $Hep$ calculations is followed by an overview of the latest developments. We compare the results obtained in the conventional nuclear physics approach and those obtained in a newly developed effective field theory approach. We also discuss the current status of the experiments relevant to $Hep$.

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arXiv2004

An Effective Field Theory at Finite Density

An effective theory to treat the dense nuclear medium by the perturbative expansion method is proposed as a natural extension of the Heavy Baryon Chiral Perturbation Theory (HBChPT). Treating the Fermi momentum scale as a separate scale of the system, we get an improved convergence and the conceptually clear interpretation. We compute the pion decay constant and the pion velocity in the nuclear medium, and find their characters different from what the usual HBChPT predicts. We also obtain the Debye screening scale at the normal nuclear matter density, and the damping scale of the pion wave. Those results indicate that the present theory, albeit its improvement over the HBChPT, has the limitation yet to go over to the medium of about 1.3 times of normal matter density due to the absence of the intrinsic density dependence of the coupling constants. We discuss how we overcome this limitation in terms of the renormalization method.

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arXiv2003

Hen Process in Effective Field Theory

An effective field theory technique that combines the standard nuclear physics approach and chiral perturbation theory is applied to the $hen$ process, ${}^{3}{He}+n\to {}^{4}{He}+ γ$. For the initial and final nuclear states, high-precision wave functions are generated via the variational Monte Carlo method using the Argonne $v_{14}$ potential and Urbana VIII trinucleon interactions, while the relevant transition operators are calculated up to ${\cal O}(Q^4)$ in HB$χ$PT. The imposition of the renormalization condition that the magnetic moments of ${}^{3}{He}$ and ${}^{3}{H}$ be reproduced allows us to carry out a parameter-free calculation of the $hen$ cross section. The result, $σ=(60\pm 3\pm 1) μb$, is in reasonable agreement with the experimental values, $(54\pm 6) μb$ and $(55\pm 3) μb$. This agreement demonstrates the validity of the calculational method previously used for estimating the reaction rate of the solar $hep$ process.

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arXiv2003

EFT for electroweak processes of light nuclei

Recently we succeeded to make a reliable EFT prediction in a totally parameter-free manner for the $S$ factors for the solar $pp$ and $hep$ processes, $p+p\to d + e^+ +ν_e$ and $\He3+p \to \He4 + e^+ + ν_e$. The strategy used in there is to embed a highly sophisticated standard nuclear physics approach (SNPA) exploiting realistic potentials into an EFT framework, that we refer to as EFT*. Up to next-next-to-leading order in chiral expansion, it turnes out that there is effectively only one counter-term relevant to this process, the coefficient of which -- ($\hat d^R$) -- has been renormalized to reproduce the experimental value of the tritium-beta decay. Our study has also led to {\em very} accurate EFT calculations on two-body weak processes that also receive contributions from the $\hat d^R$ term, $μ-d$ capture rate, and $ν-d$ scattering cross section.

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arXiv2001

The Solar $hep$ Processes in Effective Field Theory

By combining effective field theory with the standard nuclear physics approach (SNPA) we obtain a high-precision estimate of the $S$ factor for the solar $hep$ process. The accurate wave functions available in SNPA are used to evaluate the nuclear matrix elements for the transition operators that result from chiral perturbation theory (ChPT). All the contributions up to \nlo3 in ChPT are included. The resulting parameter-free, error-controlled prediction is: $S(hep)=(8.6 \pm 1.3)\times 10^{-20}$ keV-b.

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arXiv2001

Asymmetry in $\vec{n} + p -> d + γ$

Heavy-baryon chiral perturbation theory (HBChPT) is applied to the asymmetry $A_γ$ in $\vec{n} + p \to d + γ$ at threshold, which arises due to the weak parity non-conserving interactions. Instead of appealing to Siegert's theorem, transition operators up to next-to-leading chiral order are derived and the corresponding amplitudes are evaluated with the Argonne $v_{18}$ wavefunctions. In addition to the impulse contribution, both parity-conserving and parity-non-conserving two-body one-pion-exchange diagrams appear up to this order. Our prediction for the asymmetry is $A_γ= - 0.10 \hpiNN$, which is close to the Siegert's theorem based result, $A_γ\simeq -0.11 \hpiNN$. This illustrates that HBChPT is effective in the parity-non-conserving physics.

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arXiv2001

In-Medium Effective Pion Mass from Heavy-Baryon Chiral Perturbation Theory

Using heavy-baryon chiral perturbation theory, we have calculated all the diagrams up to two-loop order which contribute to the S-wave pion self-energy in symmetric nuclear matter. Some subtleties related to the definition of pion fields are discussed. The in-medium pion mass is turned out to be increased by only (5 - 6) per cents in normal nuclear matter density, without any off-shell ambiguity.

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arXiv2001

Threshold $pp\to ppπ^0$ up to one-loop accuracy

The $pp\to ppπ^0$ cross section near threshold is computed up to one-loop order including the initial and final state interactions using the hybrid heavy baryon chiral perturbation theory and the counting rule a la Weinberg. With the counter terms whose coefficients are fixed by the resonance-saturation assumption, we find that the one-loop contributions are as important as the tree-order contribution and bring the present theoretical estimation of the total cross section close to the experimental data. The short-ranged contributions are controlled by means of a cutoff, and a mild cutoff dependence is observed when all diagrams of the given chiral order are summed. To the order treated, however, the expansion is found to converge rather slowly, calling for further studies of the process.

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arXiv2000

Effective Field Theory For Nuclei: Confronting Fundamental Questions in Astrophysics

Fundamental issues involving nuclei in the celebrated solar neutrino problem are discussed in terms of an effective field theory adapted to nuclear few-body systems, with a focus on the proton fusion process and the hep process. Our strategy in addressing these questions is to combine chiral perturbation theory -- an effective field theory of QCD -- with an accurate nuclear physics approach to arrive at a more effective effective field theory that reveals and exploits a subtle role of the chiral-symmetry scale in short-distance effects encoded in short-range nuclear correlations. Our key argument is drawn from the close analogy of the principal weak matrix element figuring in the hep process to the suppressed matrix elements in the polarized neutron-proton capture at threshold currently being measured in the laboratories.

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arXiv1999

On Making Predictions With Effective Field Theories In Nuclear Physics

Based on the effective field theory previously formulated by us to accurately postdict all low-energy two-nucleon properties as well as predict certain electroweak transitions in heavy nuclei, we make parameter-free predictions for the polarized np capture process \vec{n}+\vec{p} --> d +γpresently being measured at the Institut Laue-Langevin in Grenoble. Other participants of this meeting are invited to make their own predictions using their preferred approaches and join the bet for the best prediction to confront the forthcoming experiment.

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arXiv1998

The Power Of Effective Field Theories In Nuclei: The Deuteron, NN Scattering and Electroweak Processes

We show how effectively effective quantum field theories work in nuclear physics. Using the physically transparent cut-off regularization, we study the simplest nuclear systems of two nucleons for both bound and scattering states at a momentum scale much less than the pion mass. We consider all the static properties of the deuteron, the two-nucleon scattering phase-shifts, the n + p --> d + γprocess at thermal energy and the solar proton fusion process p + p --> d + e(+) + ν(e), and we demonstrate that these are all described with great accuracy in the expansion to the next-to-leading order. We explore how a "new" degree of freedom enters in an effective theory by turning on and off the role of the pion in the Lagrangian.

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arXiv1998

Effective Field Theory for Low-Energy Two-Nucleon Systems

We illustrate how effective field theories work in nuclear physics by using an effective Lagrangian in which all other degrees of freedom than the nucleonic one have been integrated out to calculate the low-energy properties of two-nucleon systems, viz, the deuteron properties, the np 1S0 scattering amplitude and the M1 transition amplitude entering into the radiative np capture process. Exploiting a finite cut-off regularization procedure, we find all the two-nucleon low-energy properties to be accurately described with little cut-off dependence, in consistency with the general philosophy of effective field theories.

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arXiv1998

The Solar Proton Burning Process Revisited In Chiral Perturbation Theory

The proton burning process p + p -> d + e(+) + ν(e), important for the stellar evolution of main-sequence stars of mass equal to or less than that of the Sun, is computed in effective field theory using chiral perturbation expansion to the next-to-next-to leading chiral order. This represents a model-independent calculation consistent with low-energy effective theory of QCD comparable in accuracy to the radiative np capture at thermal energy previously calculated by first using very accurate two-nucleon wavefunctions backed up by an effective field theory technique with a finite cut-off. The result obtained thereby is found to support within theoretical uncertainties the previous calculation of the same process by Bahcall and his co-workers.

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arXiv1998

Effective Field Theory for Low-Energy np Systems

The properties of low-energy neutron-proton systems are studied in an effective field theory where only nucleons figure as relevant degrees of freedom. With a finite momentum cut-off regularization scheme, we show that the large scattering lengths of the np systems do not spoil the convergence of the effective field theory, which turns out to be extremely successful in reproducing, with little cut-off dependence, the deuteron properties, the np 1S0 scattering amplitude and most significantly, the M1 transition amplitude entering into the radiative np capture process.

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arXiv1997

In-Medium Effective Axial-Vector Coupling Constant

The axial-vector coupling constant in nuclear medium is systematically studied using chiral perturbation theory. For normal nuclear matter, the first non-trivial corrections are estimated based on the independent particle approximation taking into account nuclear correlations in nuclear medium. The one-pion-exchange (OPE) two-body interactions turn out to enhance the coupling constant by $δg_A/g_A\simeq (0.09 \sim 0.15)$. The role of the four-nucleon contact interaction, whose low-energy constants are yet to be determined, is analyzed in connection with the empirical quenching factor of $δg_A/g_A\simeq -0.2$ in the presence of the nuclear short-range correlation. Conventional approaches are discussed in the light of our result.

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arXiv1995

Chiral Lagrangian Approach to Exchange Vector Currents in Nuclei

Exchange vector currents are calculated up to one-loop order (corresponding to next-to-next-to-leading order) in chiral perturbation theory. As an illustration of the power of the approach, we apply the formalism to the classic nuclear process $n+p\rightarrow d +γ$ at thermal energy. The exchange current correction comes out to be $(4.5 \pm 0.3)$ \% in amplitude giving a predicted cross section $σ= (334\pm 3)\ {\mbox mb}$ in excellent agreement with the experimental value $(334.2\pm 0.5)\ {\mbox mb}$. Together with the axial charge transitions computed previously, this result provides a strong support for the power of chiral Lagrangians in nuclear physics. As a by-product of our results, we suggest an open problem in the application of chiral Lagrangian approach to nuclear processes that has to do with giving a physical meaning to the short-range correlations that play an important role in nuclei.

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arXiv1994

Radiative Neutron-Proton Capture in Effective Chiral Lagrangians

We calculate the cross-section for the thermal $n+p\rightarrow d+γ$ process in chiral perturbation theory to next-to-next-to-leading order using heavy-fermion formalism. The exchange current correction is found to be $(4.5\pm 0.3)~\%$ in amplitude and the chiral perturbation at one-loop order gives the cross section $σ_{th}^{np}=(334\pm 2)\ {\mbox mb}$ which is in agreement with the experimental value $(334.2\pm 0.5)\ {\mbox mb}$. Together with the axial charge transitions, this provides a strong support for the power of chiral Lagrangians for nuclear physics.

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