arXiv subjects

David Thomas

Publications and source records attributed to David Thomas.

24 records · Page 2Linked to original sources

arXiv1998

Generalized Limits to the Number of Light Particle Degrees of Freedom from Big Bang Nucleosynthesis

We compute the big bang nucleosynthesis limit on the number of light neutrino degrees of freedom in a model-independent likelihood analysis based on the abundances of He4 and Li7. We use the two-dimensional likelihood functions to simultaneously constrain the baryon-to-photon ratio and the number of light neutrinos for a range of He4 abundances $Y_p$ = 0.225 -- 0.250, as well as a range in primordial Li7 abundances from (1.6 to 4.1) $ \times 10^{-10}$. For (Li7/H)$_p = 1.6 \times 10^{-10}$, as can be inferred from the Li7 data from Population II halo stars, the upper limit to $N_ν$ based on the current best estimate of the primordial He4 abundance of $Y_p = 0.238$, is $N_ν< 4.3$ and varies from $N_ν< 3.3$ (at 95% C.L.) when $Y_p=0.225$ to $N_ν< 5.3$ when $Y_p=0.250$. If Li7 is depleted in these stars the upper limit to $N_ν$ is relaxed. Taking (Li7/H)$_p = 4.1 \times 10^{-10}$, the limit varies from $N_ν< 3.9$ when $Y_p = 0.225$ to $N_ν\la 6$ when $Y_p = 0.250$. We also consider the consequences on the upper limit to $N_ν$ if recent observations of deuterium in high-redshift quasar absorption-line systems are confirmed.

hep-ph↗

arXiv1996

A Big Bang Nucleosynthesis Likelihood Analysis of the Baryon-to-Photon Ratio and the Number of Light Particle Degrees of Freedom

We extend the model-independent likelihood analysis of big bang nucleosynthesis (BBN) based on He4 and Li7 to allow for numbers of degrees of freedom which differ from the standard model value characterized by $N_ν= 3$. We use the two-dimensional likelihood functions to simultaneously constrain the baryon-to-photon ratio and the number of light neutrinos. The upper limit thus obtained is $N_ν< 4.0$ (at 95% C.L.). We also consider the consequences if recent observations of deuterium in high-redshift QSO absorption-line systems (QSOALS) are confirmed.

hep-ph↗

arXiv1996

Model Independent Predictions of Big Bang Nucleosynthesis from \he4 and \li7: Consistency and Implications

We examine in detail how BBN theory is constrained, and what predictions it can make, when using only the most model-independent observational constraints. We avoid the uncertainties and model-dependencies that necessarily arise when solar neighborhood D and \he3 abundances are used to infer primordial D and \he3 via chemical and stellar evolution models. Instead, we use \he4 and \li7, thoroughly examining the effects of possible systematic errors in each. Via a likelihood analysis, we find near perfect agreement between BBN theory and the most model-independent data. Given this agreement, we then {\it assume} the correctness of BBN to set limits on the single parameter of standard BBN, the baryon-to-photon ratio, and to predict the primordial D and \he3 abundances. We also repeat our analysis including recent measurements of D/H from quasar absorption systems and find that the near perfect agreement between theory and observation of the three isotopes, D, \he4 and \li7 is maintained. These results have strong implications for the chemical and stellar evolution of the light elements, in particular for \he3. In addition, our results (especially if the D/H measurements are confirmed) have implications for the stellar depletion of \li7. Finally, we set limits on the number \nnu\ of neutrino flavors, using an analysis which carefully and systematically includes all available experimental constraints. The value \nnu = 3.0 fits best with BBN and a 95\% CL upper limit of \nnu $\la 4$ is established.

astro-ph↗

arXiv1994

Monte-Carlo Analysis of Big Bang Production of Beryllium and Boron

There is continued interest in the possibility that big bang nucleosynthesis may produce significant quantities of Be and B. In this paper we reevaluate the primordial abundances taking into account uncertainties in reactions rates. We discuss the implications for primordial nucleosynthesis, and for galactic cosmic ray spallation.

astro-ph↗

arXiv1993

Production of Li, Be \& B from Baryon Inhomogeneous Primordial Nucleosynthesis

We investigate the possibility that inhomogeneous nucleosynthesis may eventually be used to explain the abundances of \li6, \be9 and B in population II stars. The present work differs from previous studies in that we have used a more extensive reaction network. It is demonstrated that in the simplest scenario the abundances of the light elements with $A\le7$ constrain the separation of inhomogeneities to sufficiently small scales that the model is indistinguishable from homogeneous nucleosynthesis and that the abundances of \li6, \be9\ and B are then below observations by several orders of magnitude. This conclusion does not depend on the \li7 constraint. We also examine alternative scenarios which involve a post-nucleosynthesis reprocessing of the light elements to reproduce the observed abundances of Li and B, while allowing for a somewhat higher baryon density (still well below the cosmological critical density). Future B/H measurements may be able to exclude even this exotic scenario and further restrict primordial nucleosynthesis to approach the homogeneous model conclusions.

astro-ph↗

arXiv1992

Primordial Nucleosynthesis and the Abundances of Beryllium and Boron

The ability to now make measurements of Be and B as well as put constraints on \lisix\ abundances in metal-poor stars has led to a detailed reexamination of Big Bang Nucleosynthesis in the $A\groughly6$ regime. The nuclear reaction network has been significantly expanded with many new rates added. It is demonstrated that although a number of $A>7$ reaction rates are poorly determined, even with extreme values chosen, the standard homogeneous model is unable to produce significant yields (Be/H and B/H $<10^{-17}$ when $A\le7$ abundances fit) above $A=7$ and the \liseven/\lisix\ ratio always exceeds 500. We also preliminarily explore inhomogeneous models, such as those inspired by a first order quark-hadron phase transition, where regions with high neutron/proton ratios can allow some leakage up to $A>7$. However models that fit the $A\le7$ abundances still seem to have difficulty in obtaining significant $A>7$ yields.

astro-ph↗