Top-Quark Mass and Bottom-Quark Decay

The possibility of a long B-meson lifetime is explored, in which case the weak mixing angles θ_2 and θ_3 are quite small. This allows the derivation of a lower bound on the top-quark mass as a function of the B-meson lifetime, by comparison of the short-distance prediction for the CP-nonconservation parameter ε with its experimental value. The bound is significant for τ_B>4×10^(-13) s

Late Pop III Star Formation During the Epoch of Reionization: Results from the Renaissance Simulations

We present results on the formation of Pop III stars at redshift 7.6 from the Renaissance Simulations, a suite of extremely high-resolution and physics-rich radiation transport hydrodynamics cosmological adaptive-mesh refinement simulations of high redshift galaxy formation performed on the Blue Waters supercomputer. In a survey volume of about 220 comoving Mpc$^3$, we found 14 Pop III galaxies with recent star formation. The surprisingly late formation of Pop III stars is possible due to two factors: (i) the metal enrichment process is local and slow, leaving plenty of pristine gas to exist in the vast volume; and (ii) strong Lyman-Werner radiation from vigorous metal-enriched star formation in early galaxies suppresses Pop III formation in ("not so") small primordial halos with mass less than $\sim$ 3 $\times$ 10$^7$ M$_\odot$. We quantify the properties of these Pop III galaxies and their Pop III star formation environments. We look for analogues to the recently discovered luminous Ly $\alpha$ emitter CR7 (Sobral et al. 2015), which has been interpreted as a Pop III star cluster within or near a metal-enriched star forming galaxy. We find and discuss a system similar to this in some respects, however the Pop III star cluster is far less massive and luminous than CR7 is inferred to be.Comment: 8 pages, 4 figures, 3 tables. Accepted by Ap

Why are Retirement Rates So High at Age 65?

In most data sets of labor force participation of the elderly, an empirical regularity that emerges is that retirement rates are particularly high at age 65. While there are numerous economic reasons why individuals may choose to retire at 65, empirical models that have attempted to explain the age-65 spike have met with limited success. Interpreted another way, while many models would predict a jump in the hazard rate at age 65, the magnitude of the spike indicates excessive response given the economic considerations that retirees typically face. This paper considers the puzzle of why retirement rates are so high at age 65 and explores a variety of explanations.

Three Models of Retirement: Computational Complexity Versus Predictive Validity

Empirical analysis often raises questions of approximation to underlying individual behavior. Closer approximation may require more complex statistical specifications, On the other hand, more complex specifications may presume computational facility that is beyond the grasp of most real people and therefore less consistent with the actual rules that govern their behavior, even though economic theory may push analysts to increasingly more complex specifications. Thus the issue is not only whether more complex models are worth the effort, but also whether they are better. We compare the in-sample and out-of-sample predictive performance of three models of retirement -- "option value," dynamic programming, and probit -- to determine which of the retirement rules most closely matches retirement behavior in a large firm. The primary measure of predictive validity is the correspondence between the model predictions and actual retirement under the firm's temporary early retirement window plan. The "option value" and dynamic programming models are considerably more successful than the less complex probit model in approximating the rules individuals use to make retirement decisions, but the more complex dynamic programming rule approximates behavior no better than the simpler option value rule.

Probing The Ultraviolet Luminosity Function of the Earliest Galaxies with the Renaissance Simulations

In this paper, we present the first results from the Renaissance Simulations, a suite of extremely high-resolution and physics-rich AMR calculations of high redshift galaxy formation performed on the Blue Waters supercomputer. These simulations contain hundreds of well-resolved galaxies at $z \sim 25-8$, and make several novel, testable predictions. Most critically, we show that the ultraviolet luminosity function of our simulated galaxies is consistent with observations of high-z galaxy populations at the bright end of the luminosity function (M$_{1600} \leq -17$), but at lower luminosities is essentially flat rather than rising steeply, as has been inferred by Schechter function fits to high-z observations, and has a clearly-defined lower limit in UV luminosity. This behavior of the luminosity function is due to two factors: (i) the strong dependence of the star formation rate on halo virial mass in our simulated galaxy population, with lower-mass halos having systematically lower star formation rates and thus lower UV luminosities; and (ii) the fact that halos with virial masses below $\simeq 2 \times 10^8$ M$_\odot$ do not universally contain stars, with the fraction of halos containing stars dropping to zero at $\simeq 7 \times 10^6$ M$_\odot$. Finally, we show that the brightest of our simulated galaxies may be visible to current and future ultra-deep space-based surveys, particularly if lensed regions are chosen for observation.Comment: 7 pages, 4 figures, accepted by The Astrophysical Journal Letter

Scaling Relations for Galaxies Prior to Reionization

The first galaxies in the Universe are the building blocks of all observed galaxies. We present scaling relations for galaxies forming at redshifts $z \ge 15$ when reionization is just beginning. We utilize the ``Rarepeak' cosmological radiation hydrodynamics simulation that captures the complete star formation history in over 3,300 galaxies, starting with massive Population III stars that form in dark matter halos as small as ~$10^6 M_\odot$. We make various correlations between the bulk halo quantities, such as virial, gas, and stellar masses and metallicities and their respective accretion rates, quantifying a variety of properties of the first galaxies up to halo masses of $10^9 M_\odot$. Galaxy formation is not solely relegated to atomic cooling halos with virial temperatures greater than $10^4$ K, where we find a dichotomy in galaxy properties between halos above and below this critical mass scale. Halos below the atomic cooling limit have a stellar mass -- halo mass relationship $\log M_\star \simeq 3.5 + 1.3\log(M_{\rm vir} / 10^7 M_\odot)$. We find a non-monotonic relationship between metallicity and halo mass for the smallest galaxies. Their initial star formation events enrich the interstellar medium and subsequent star formation to a median of $10^{-2} Z_\odot$ and $10^{-1.5} Z_\odot$, respectively, in halos of total mass $10^7 M_\odot$ that is then diluted by metal-poor inflows, well beyond Population III pre-enrichment levels of $10^{-3.5} Z_\odot$. The scaling relations presented here can be employed in models of reionization, galaxy formation and chemical evolution in order to consider these galaxies forming prior to reionization.Comment: 10 pages, 10 figures. Accepted to Ap