14347496
The BBN manifesto
Abstract In this manifesto I review the current status of standard big bang nucleosynthesis in light of recent observational data and discuss the importance of near-future observations as direct tests of the standard BBN.
18626728
Testing Two Nuclear Physics Approximations Used in the Standard Leaky-Box Model for the Spallogenic Production of LiBeB
The spallative production rates of lithium, beryllium, and boron (LiBeB) are a necessary component in any calculation of the evolution of these nuclei in the Galaxy. Previous calculations of these rates relied on two assumptions relating to the nuclear physics aspects: the straight-ahead approximation that describes the distribution of fragment energies and the assumption that the major contributor to the production rate arises from single-step reactions between primary cosmic-ray projectiles and interstellar medium targets. We examine both assumptions by using a semiempirical description for the spall's energy distribution and by including the reactions that proceed via intermediary fragments. After relaxing the straight-ahead approximation, we find that the changes in the production rates and emerging fluxes are small and do not warrant rejection of this approximation. In contrast, we discover that two-step reactions can alter the production rate considerably, leading to noticeable increases in the efficiency of producing the LiBeB nuclei. Motivated by this result, we introduce a cascade technique to compute the production rates exactly and find that the results differ only slightly from those of our two-step calculations. We thus conclude that terminating the reaction network at the two-step order is sufficiently accurate for current studies of spallation.
8649559
Interactions of ultrahigh-energy neutrinos.
Cross sections for the charged-current interactions of ultrahigh-energy neutrinos with nucleons are evaluated in light of recent improvements in our knowledge of nucleon structure functions. For 10/sup 19/-eV neutrinos, the cross section is an order of magnitude larger than the values traditionally used in astrophysical calculations. Some consequences for event rates from generic astrophysical neutrino sources are noted.
45996526
Bounds on halo-particle interactions from interstellar calorimetry.
The light from distant stars makes cartography of the luminous material within galaxies possible, the simplest observation being the distinctive shape of the galaxy. In spiral galaxies, for example, most of the light emanates from a small central nucleus and an extended diskshaped region. In addition to the obvious density information this light provides, by examining the optical and 21-cm spectral lines we obtain "rotation curves," maps of the orbital velocities of the components of a galaxy as a function of their distance from the center. From this we deduce that the total mass distribution in spiral galaxies is approximately that of an isothermal sphere: The mass density falls as r~ 2 , where r is the distance from the center of the galaxy. l The gravitational potential and the mass inferred from these rotation curves is in conflict with the distribution of luminous matter observed—gravitating matter outweighs luminous matter by a factor of 3 to 10. This discrepancy is observed in elliptical galaxies as well. The resolution of this paradox lies in "dark matter," nonluminous, gravitating, material in spherical halos surrounding galaxies. Many suggestions have been made as to the form this matter might take, from "condensed matter" such as substellar size macroscopic objects made of baryons, to a diffuse collection of some species of "elementary" particles such as axions or massive neutrinos. It has generally been assumed that if the galactic halo is composed of elementary particles, then these halo particles must be weakly interacting; otherwise, it is argued, they would be dragged into the disk of the galaxy as ordinary matter collapses. Recently, De Riijula, Glashow, and Sarid have pointed out that this reasoning could be false: If halo particles are very massive, they may not lose energy fast enough to collapse into the disk with baryons during the formation of the galaxy. Moreover, if halo particles interact relatively strongly with ordinary matter, they will not show up in traditional searches for dark matter which rely on the particles interacting weakly enough to penetrate the Earth. In particular, the authors of Ref. 2 propose that the halo of our Galaxy could be composed of CHAMPs, charged particles with a mass of between a few tens to 10 TeV. 4 In this Letter we show that the existence of neutral interstellar clouds constrains the interaction of any particulate dark-matter candidate with atomic hydrogen to be quite small. We show that, even for a halo particle of mass 1 PeV (10 GeV), the cross section with hydrogen must be smaller than the typical atomic cross section that we expect for a positively charged particle bound to an electron. This argument eliminates such positively charged particles as a dark-matter possibility, as well as any other object that has a similar cross section. In outline, the argument is simple: If the clouds are in equilibrium, the rate at which energy is deposited by collisions with dark-matter particles must be smaller than the rate at which the cloud can cool. In the remainder of this paper we use this argument to constrain the interaction cross section of dark matter with hydrogen, and conclude with some remarks on the general viability of charged dark matter. If the dark-matter particles which form the galactic halo interact with hydrogen, neutral or ionized, their interactions will be largely elastic as the relative velocity is of order of the average velocity of a halo particle (here we assume a spherical halo with a Maxwellian velocity distribution having an rms velocity Vh of approximately 9x 10~c) and is less than aemc, the typical "velocity" of an electron bound to a proton. The recoiling hydrogen atom or proton will gain a momentum less than or of order rripVh and will quickly share most of its energy through elastic collisions with other hydrogen atoms and protons. Therefore, most of the energy deposited by the interactions of a halo particle will go into heating the interstellar medium and will not be quickly lost in radiation. The heating rate per nucleon due to halo-particle interactions may be computed in terms of the transport cross section crtr for hydrogen-halo-particle or protonhalo-particle scattering,
115618593
The nineteen neutrinos of SN 1987A: mass or no mass?
119396716
Big Bang nucleosynthesis constraints on brane cosmologies
Abstract We examine constraints from Big Bang nucleosynthesis on type II Randall–Sundrum brane cosmologies with both a dark radiation component and a quadratic term that depends on the 5-dimensional Planck mass, M 5 . Using limits on the abundances of deuterium and helium-4, we calculate the allowed region in the M 5 –dark radiation plane and derive the precise BBN bound on M 5 alone with no dark radiation: M 5 >13 TeV.
55911739
Spin exchange collisions and their consequences for spin polarized gas targets of hydrogen and deuterium
Abstract The effects of spin-exchange collisions on the polarization of dense spin-polarized samples of hydrogen and deuterium are analyzed. It is shown that even in large magnetic fields spin-exchange collisions transfer angular momentum between the electrons and the nuclei. This effect has important implications for the operation of densed spin-polarized targets and sources of hydrogen and deuterium. For tensor polarized targets care will be required to obtain a high tensor polarization, especially at low fields. For the specific case of sources that are spin-polarized by spin-exchange collisions with optically pumped alkali atoms, spin-exchange not only polarizes the hydrogen and deuterium electron spins, but polarizes the nuclear spins as well. For high-density vector-polarized targets this may eliminate the need for rf transitions to polarize the nuclei.
8930049
Spin-exchange optical pumping of noble-gas nuclei
Spin-exchange optical pumping of mixtures of alkali-metal vapors and noble gases can be used to efficiently polarize the nuclei of the noble-gas atoms. Liters of noble gases at standard temperature and pressure and with nuclear spin polarizations of several tens of percent are now used in many applications. The authors describe the basic phenomena that govern the spin-exchange process and review the physics of angular momentum transfer and loss in optical pumping and spin-exchange collisions. {copyright} {ital 1997} {ital The American Physical Society}
350710
Magnetocardiography with a modular spin-exchange relaxation-free atomic magnetometer array.
We present a portable four-channel atomic magnetometer array operating in the spin-exchange relaxation-free regime. The magnetometer array has several design features intended to maximize its suitability for biomagnetic measurement, specifically foetal magnetocardiography, such as a compact modular design and fibre-coupled lasers. The modular design allows the independent positioning and orientation of each magnetometer. Using this array in a magnetically shielded room, we acquire adult magnetocadiograms. These measurements were taken with a 6-11 fT Hz(-1/2) single-channel baseline sensitivity that is consistent with the independently measured noise level of the magnetically shielded room.
32501332
Hybrid spin-exchange optical pumping of 3He.
We demonstrate spin-exchange optical pumping of 3He using a "hybrid" K-Rb vapor mixture. The Rb atoms absorb light from a standard laser at 795 nm, then collisionally polarize the potassium atoms. Spin-exchange collisions of K and 3He atoms then transfer the angular momentum to the 3He with much greater efficiency than Rb-3He. For a K-rich vapor, the efficiency of the hybrid spin-exchange collisions approaches 1/4, an order of magnitude greater than achieved by pure Rb pumping. We present the first measurements of actual photon efficiencies (polarized nuclei produced per absorbed photon), and show that a new parasitic absorption process limits the total efficiencies for both hybrid and pure Rb pumping.