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K. Kubodera

Publications and source records attributed to K. Kubodera.

At least 55 records · Page 3Linked to original sources
arXiv2001

The $μ^-d$ capture rate in effective field theory

Muon capture on the deuteron is studied in heavy baryon chiral perturbation theory (HBChPT). It is found that by far the dominant contribution to $μd$ capture comes from a region of the final three-body phase-space in which the energy of the two neutrons is sufficiently small for HBChPT to be applicable. The single unknown low-energy constant having been fixed from the tritium beta decay rate, our calculation contains no free parameter. Our estimate of the $μd$ capture rate is consistent with the existing data. The relation between $μd$ capture and the $νd$ reactions, which are important for the SNO experiments, is briefly discussed.

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arXiv2001

The Solar pp and hep Processes in Effective Field Theory

The strategy of modern effective field theory is exploited to pin down accurately the flux $S$ factors for the $pp$ and $hep$ processes in the Sun. The technique used is to combine the high accuracy established in few-nucleon systems of the "standard nuclear physics approach" (SNPA) and the systematic power counting of chiral perturbation theory (ChPT) into a consistent effective field theory framework. Using highly accurate wave functions obtained in the SNPA and working to \nlo3 in the chiral counting for the current, we make totally parameter-free and error-controlled predictions for the $pp$ and $hep$ processes in the Sun.

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arXiv2001

The Solar hep Process in Effective Field Theory

Using effective field theory, we calculate the S-factor for the hep process in a totally parameter-free formulation. The transition operators are organized according to chiral counting, and their matrix elements are evaluated using the realistic nuclear wave functions obtained in the correlated-hyperspherical-harmonics method. Terms of up to next-to-next-to-next-to-leading order in heavy-baryon chiral perturbation theory are considered. Fixing the only parameter in the theory by fitting the tritium β-decay rate, we predict the hep S-factor with accuracy better than \sim 20 %.

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arXiv2001

Parameter-Free Calculation of the Solar Proton Fusion Rate in Effective Field Theory

Spurred by the recent complete determination of the weak currents in two-nucleon systems up to ${\cal O}(Q^3)$ in heavy-baryon chiral perturbation theory, we carry out a parameter-free calculation of the solar proton fusion rate in an effective field theory that combines the merits of the standard nuclear physics method and systematic chiral expansion. Using the tritium beta-decay rate as an input to fix the only unknown parameter in the effective Lagrangian, we can evaluate with drastically improved precision the ratio of the two-body contribution to the well established one-body contribution; the ratio is determined to be (0.86\pm 0.05) %. This result is essentially independent of the cutoff parameter for a wide range of its variation (500 MeV \le Λ\le 800 MeV), a feature that substantiates the consistency of the calculation.

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arXiv2001

Electroweak Probes and Nuclear Effective Field Theory

There is growing interest in nuclear physics applications of effective field theory. I give a brief account of some of the latest developments in this area. I also describe interplay between this new approach and the traditional nuclear physics approach.

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arXiv2000

Neutrino Reactions on Deuteron

The cross sections for the ν-d and \barν-d reactions are calculated for the incident energy up to E_ν= 170 MeV, with the use of a phenomenological Lagrangian approach. We assess and improve the reliability of the employed calculational method by examining the dependence of the results on various input and approximations that go into the calculation. The main points of improvements over the existing work are: (1) use of the "modern" NN potentials; (2) use of the more accurate nucleon weak-interaction form factors; (3) monitoring the strength of a vertex that governs the exchange-current contribution, with the use of data on the related process, n+p\to d+γ. In addition to the total cross sections, we present various differential cross sections that are expected to be useful for the SNO and other experiments. In the low energy regime relevant to the solar neutrinos, the newly calculated total cross sections agree with the existing literature values. The origins of slight differences found for higher energies are discussed. The ratio between the neutral-current and charged-current reaction cross sections is found to be extremely stable against any variations in the input of our calculation.

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arXiv1999

A next-to-next-to-leading order pp -> ppπ^0 transition operator in chiral perturbation theory

We discuss the results of a systematic calculation of the next-to-next-to-leading order amplitude for the pp -> ppπ^0 S-wave production at the threshold in heavy-baryon chiral perturbation theory. We find six new diagrams, two of which can be viewed as vertex corrections, of 15 - 20 %, to the pion-exchange graph, whereas the rest are much larger, one exceeding 900 %. We discuss the reasons for this enhancement, as well as the steps necessary to be taken before the final comparison with experiment.

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arXiv1999

A next-to-next-to-leading-order $pp \to ppπ^0$ transition operator in chiral perturbation theory

We present a systematic analysis of next-to-next-to-leading-order diagrams that contribute to the $pp \to ppπ^0$ production at threshold. Analytic expressions are given for the effective transition operators, and the relative importance of various types of diagrams is discussed. The vertex-correction-type graphs are found to give only small corrections to lower order graphs in conformity with expectations. By contrast, we find very large contributions from the two-pion graphs which can be interpreted as a part of effective $σ$-meson exchange diagrams. The recoil correction to the pion rescattering diagram also turns out to be large.

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arXiv1999

Effective Field Theory Approach To \vec{n} + \vec{p} -> d + γAt Threshold

Previously, in an effective field theory formulated by us, we have carried out parameter-free calculations of a large number of low-energy two-nucleon properties. An experiment at the Institut Laue-Langevin is currently measuring spin-dependent effects in the polarized np capture process \vec{n}+\vec{p} -> d +γat threshold. Noting that spin-dependent observables for this reaction are sensitive to terms of chiral orders higher than hitherto studied, we extend our effective theory approach to this process and make parameter-free predictions on the spin-dependent observables.

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arXiv1998

Solar Fusion Cross Sections

We review and analyze the available information for nuclear fusion cross sections that are most important for solar energy generation and solar neutrino production. We provide best values for the low-energy cross-section factors and, wherever possible, estimates of the uncertainties. We also describe the most important experiments and calculations that are required in order to improve our knowledge of solar fusion rates.

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arXiv1998

Current Issues in the Nuclear Chiral Perturbation Approach

After outlining the basic ideas of nuclear chiral perturbation theory, I discuss its application, presenting three examples which I believe are of great current interest: (1) Exchange currents in those cases where the leading chiral-order terms dominate; (2) The near-threshold $p+p\ra p+p+π^0$ reaction, for which the leading order terms are suppressed and consequently higher-order corrections play a prominent role; (3) Radiative $μ$-capture on the proton.

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arXiv1997

Radiative Muon Capture by a Proton in Chiral Perturbation Theory

The first measurement of the radiative muon capture (RMC) rate on a proton was recently carried out at TRIUMF. The TRIUMF group analyzed the RMC rate in terms of the theoretical formula of Beder and Fearing, and found the surprising result that the pseudoscalar coupling g_P is 1.5 times the value expected from PCAC. To assess the reliability of the theoretical framework used by the TRIUMF group to relate the RMC rate to the pseudoscalar form factor, we calculate the RMC rate in chiral perturbation theory, which provides a systematic framework to describe all the vertices involved in RMC, fulfilling gauge-invariance and chiral-symmetry requirements in a transparent manner. As a first step we present a chiral perturbation calculation at tree level which includes sub-leading order terms.

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arXiv1997

Possible solution to basic problems regarding the coupling constants $G_V$ and $G_A$

The existing experimental evidence on the vector and axial-vector coupling constants, $G_V$ and $G_A$, for neutron $β$-decay exhibits two prominent problems: (i) the unitarity for the first row of the CKM matrix seems violated; (ii) one obtains different values of $G_A/G_V$ according to whether one uses as input the neutron lifetime or neutron-decay correlation observables. We show that the in-medium modification of $G_A$ can influence both the inner and outer radiative corrections used in deducing $G_V$ from the super-allowed Fermi transitions and that this effect may resolve the long-standing problems (i) and (ii) simultaneously.

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arXiv1997

Chiral Perturbation Theory and the pp -> pp pi0 Reaction Near Threshold

A chiral-perturbative consideration of the near-threshold pp -> pp pi0 reaction indicates that the pion-rescattering term has a substantial energy and momentum dependence. The existing calculations that incorporate this dependence give pion rescattering contributions significantly larger than those of the conventional treatment, and this enhanced rescattering term interferes destructively with the one-body impulse term, leading to theoretical cross sections that are much smaller than the observed values. However, since the existing calculations are based on coordinate-space representation, they involve a number of simplifying assumptions about the energy-momentum flow in the rescattering diagram, even though the delicate interplay between the one-body and two-body terms makes it desirable to avoid these kinematical assumptions. We carry out here a momentum-space calculation that retains the energy-momentum dependence of the vertices as predicted by chiral perturbation theory. Our improved treatment increases the rescattering amplitude by a factor of 3 over the value obtained in the r-space calculations. The pp -> pp pi0 transition amplitude, which is now dominated by the rescattering term, leads to the cross section much larger than what was reported in the approximate r-space calculations. Thus, the extremely small cross sections obtained in the previous chiral perturbative treatments of this reaction should be considered as an accidental consequence of the approximations employed rather than a general feature.

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arXiv1996

Chiral Perturbation Approach to the pp -> pp pi0 Reaction Near Threshold

The usual theoretical treatments of the near-threshold $pp \rightarrow ppπ^0$ reaction are based on various phenomenological Lagrangians. In this work we examine the relationship between these approaches and a systematic chiral perturbation method. Our chiral perturbation calculation indicates that the pion rescattering term should be significantly enhanced as compared with the traditional phenomenological treatment, and that this term should have substantial energy and momentum dependence. An important consequence of this energy-momentum dependence is that, for a representative threshold kinematics and within the framework of our semiquantitative calculation, the rescattering term interferes destructively with the Born-term in sharp contrast to the constructive interference obtained in the conventional treatment. This destructive interference makes theoretical cross sections for $pp \rightarrow ppπ^0$ much smaller than the experimental values, a feature that suggests the importance of the heavy-meson exchange contributions to explain the experimental data.

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arXiv1994

Meson Condensation in Dense Matter Revisited

The results for meson condensation in the literature vary markedly depending on whether one uses chiral perturbation theory or the current-algebra-plus-PCAC approach. To elucidate the origin of this discrepancy, we re-examine the role of the sigma-term in meson condensation. We find that the resolution of the existing discrepancy requires a knowledge of terms in the Lagrangian that are higher order in density than hitherto considered.

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