Nuclear polarizability of helium isotopes in atomic transitions

We estimate the nuclear polarizability correction to atomic transition frequencies in various helium isotopes. This effect is non-negligible for high precision tests of quantum electrodynamics or accurate determination of the nuclear charge radius from spectroscopic measurements in helium atoms and ions. In particular, it amounts to $28(3)$ kHz for 1S-2S transition in 4He+.Comment: 11 pages, 4 figures, to be published in Phys. Rev. A. Revised version: misprints corrected, new references adde

Photodisintegration of light nuclei for testing a correlated realistic interaction in the continuum

An exact calculation of the photodisintegration cross section of 3H, 3He and 4He is performed using as interaction the correlated Argonne V18 potential, constructed within the Unitary Correlation Operator Method (VUCOM). Calculations are carried out using the Lorentz Integral Transform method in conjunction with an hyperspherical harmonics basis expansion. A comparison with other realistic potentials and with available experimental data is discussed. The VUCOM potential leads to a very similar description of the cross section as the Argonne V18 interaction with the inclusion of the Urbana IX three-body force for photon energies 45< w < 120 MeV, while larger differences are found close to threshold.Comment: 9 pages, 6 figure

Two-body Photodisintegration of 4^{4}He with Full Final State Interaction

The cross sections of the processes $^4$He($\gamma,p$)$^3$H and $^4$He($\gamma,n$)$^3$He are calculated taking into account the full final state interaction via the Lorentz integral transform (LIT) method. This is the first consistent microscopic calculation beyond the three--body breakup threshold. The results are obtained with a semirealistic central NN potential including also the Coulomb force. The cross sections show a pronounced dipole peak at 27 MeV which lies within the rather broad experimental band. At higher energies, where experimental uncertainties are considerably smaller, one finds a good agreement between theory and experiment. The calculated sum of three-- and four--body photodisintegration cross sections is also listed and is in fair agreement with the data.Comment: 18 pages, 6 figure

Influence of Gamma-Ray Emission on the Isotopic Composition of Clouds in the Interstellar Medium

We investigate one mechanism of the change in the isotopic composition of cosmologically distant clouds of interstellar gas whose matter was subjected only slightly to star formation processes. According to the standard cosmological model, the isotopic composition of the gas in such clouds was formed at the epoch of Big Bang nucleosynthesis and is determined only by the baryon density in the Universe. The dispersion in the available cloud composition observations exceeds the errors of individual measurements. This may indicate that there are mechanisms of the change in the composition of matter in the Universe after the completion of Big Bang nucleosynthesis. We have calculated the destruction and production rates of light isotopes (D, 3He, 4He) under the influence of photonuclear reactions triggered by the gamma-ray emission from active galactic nuclei (AGNs). We investigate the destruction and production of light elements depending on the spectral characteristics of the gamma-ray emission. We show that in comparison with previous works, taking into account the influence of spectral hardness on the photonuclear reaction rates can increase the characteristic radii of influence of the gamma-ray emission from AGNs by a factor of 2-8. The high gamma-ray luminosities of AGNs observed in recent years increase the previous estimates of the characteristic radii by two orders of magnitude. This may suggest that the influence of the emission from AGNs on the change in the composition of the medium in the immediate neighborhood (the host galaxy) has been underestimated.Comment: 13 pages, 13 figures, 3 table

Electromagnetic Polarizabilities of Nucleons bound in 40^{40}Ca, 16^{16}O and 4^4He

Differential cross sections for elastic scattering of photons have been measured for $^{40}$Ca at energies of 58 and 74 MeV and for $^{16}$O and $^4$He at 61 MeV, in the angular range from 45$^o$ to 150$^o$. Evidence is obtained that there are no significant in-medium modifications of the electromagnetic polarizabilities except for those originating from meson exchange currents.Comment: 20 pages including 5 Figure

alpha-particle photoabsorption with a realistic nuclear force

The 4He total photoabsorption cross section is calculated with the realistic nucleon-nucleon potential Argonne V18 and the three-nucleon force (3NF) Urbana IX. Final state interaction is included rigorously via the Lorentz Integral Transform method. A rather pronounced giant resonance with peak cross sections of 3 (3.2) mb is obtained with (without) 3NF. Above 50 MeV strong 3NF effects, up to 35%, are present. Good agreement with experiment is found close to threshold. A comparison in the giant resonance region is inconclusive, since present data do not show a unique picture.Comment: 13 pages, 4 figures; corrected experimental data in Fig.4b and slightly modified discussion of Fig.4

The Lorentz Integral Transform (LIT) method and its applications to perturbation induced reactions

The LIT method has allowed ab initio calculations of electroweak cross sections in light nuclear systems. This review presents a description of the method from both a general and a more technical point of view, as well as a summary of the results obtained by its application. The remarkable features of the LIT approach, which make it particularly efficient in dealing with a general reaction involving continuum states, are underlined. Emphasis is given on the results obtained for electroweak cross sections of few--nucleon systems. Their implications for the present understanding of microscopic nuclear dynamics are discussed.Comment: 83 pages, 31 figures. Topical review. Corrected typo

Electromagnetic Cascades and Cascade Nucleosynthesis in the Early Universe

We describe a calculation of electromagnetic cascading in radiation and matter in the early universe initiated by the decay of massive particles or by some other process. We have used a combination of Monte Carlo and numerical techniques which enables us to use exact cross sections, where known, for all the relevant processes. In cascades initiated after the epoch of big bang nucleosynthesis $\gamma$-rays in the cascades will photodisintegrate $^4$He, producing $^3$He and deuterium. Using the observed $^3$He and deuterium abundances we are able to place constraints on the cascade energy deposition as a function of cosmic time. In the case of the decay of massive primordial particles, we place limits on the density of massive primordial particles as a function of their mean decay time, and on the expected intensity of decay neutrinos.Comment: compressed and uuencoded postscript. We now include a comparison with previous work of the photon spectrum in the cascade and the limits we calculate for the density of massive particles. The method of calculation of photon spectra at low energies has been improved. Most figures are revised. Our conclusions are substantially unchange

Big-Bang Nucleosynthesis and Hadronic Decay of Long-Lived Massive Particles

We study the big-bang nucleosynthesis (BBN) with the long-lived exotic particle, called X. If the lifetime of X is longer than \sim 0.1 sec, its decay may cause non-thermal nuclear reactions during or after the BBN, altering the predictions of the standard BBN scenario. We pay particular attention to its hadronic decay modes and calculate the primordial abundances of the light elements. Using the result, we derive constraints on the primordial abundance of X. Compared to the previous studies, we have improved the following points in our analysis: The JETSET 7.4 Monte Carlo event generator is used to calculate the spectrum of hadrons produced by the decay of X; The evolution of the hadronic shower is studied taking account of the details of the energy-loss processes of the nuclei in the thermal bath; We have used the most recent observational constraints on the primordial abundances of the light elements; In order to estimate the uncertainties, we have performed the Monte Carlo simulation which includes the experimental errors of the cross sections and transfered energies. We will see that the non-thermal productions of D, He3, He4 and Li6 provide stringent upper bounds on the primordial abundance of late-decaying particle, in particular when the hadronic branching ratio of X is sizable. We apply our results to the gravitino problem, and obtain upper bound on the reheating temperature after inflation.Comment: 94 pages, 49 figures, to appear in Phys. Rev. D. This is a full length paper of the preprint astro-ph/040249

Is there a Pronounced Giant Dipole Resonance in ^4He?

A four-nucleon calculation of the total ^4He photodisintegration cross section is performed. The full final-state interaction is taken into account for the first time. This is achieved via the method of the Lorentz integral transform. Semi-realistic NN interactions are employed. Different from the known partial two-body ^4He(\gamma,n)^3He and ^4He(\gamma,p)^3H cross sections our total cross section exhibits a pronounced giant resonance. Thus, in contrast to older $(\gamma,np)$ data, we predict quite a strong contribution of the $(\gamma,np)$ channel at the giant resonance peak energy.Comment: 10 pages, Latex (REVTEX), 4 Postscript figures, to appear in Phys. Rev. Let