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| RESEARCH
SUMMARY |
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I continue to test the theory of big bang nucleosynthesis by comparing the output of the theoretical calculations
of the abundances of the light elements with their observational determinations. Of the light elements under consideration,
perhaps the most important is helium. Next to hydrogen, helium is the most abundant element in the Universe. I
continue to work on accurately determining the primordial abundance of this element. I recently studied the chemical
evolution of the 4He abundance with Fields (a former postdoc at Minnesota).
Recent observations of Li show that its intrinsic dispersion in metal-poor stars is essentially zero, and the random
error in the mean Li abundance is negligible. However, a decreasing trend in the Li abundance towards lower metallicity,
plus 6Li detections, indicate that its primordial abundance can be inferred only after allowing for nucleosynthesis
processes in the Galaxy. Ryan, Beers, Fields, Norris and I showed that the Li vs Fe trend provides a tough discriminant
between alternative models for the evolution of light-elements. A critical assessment of current systematic uncertainties
was made, and the primordial Li abundance within new, much tighter limits was determined.
Deuterium is also a primordial isotope and in recent work with M. Casse, E. Vangioni-Flam, and J. Audouze, I considered
the effects of different star formation histories on overall luminosity density as a function of redshift. Recent
observations by the Canada-France redshift survey have shown that the rate of star formation was about a factor
of 10 higher than the present at a redshift of about z ~1. Since deuterium is very sensitive to the star formation
history, we compared the predictions of various galactic models to these observations with an emphasis on the total
amount of deuterium destruction. We found that if our galaxy is rather typical of the light producing objects in
the Universe, then deuterium has been significantly destroyed in our galaxy. On the other hand, if the deuterium
abundance in our galaxy is only a factor of 2-3 down from its primordial value, then our galaxy must have had a
very different star formation history than the typical light-producing object in the Universe.
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 4He and 7Li was computed recently by Olive and Thomas. Two-dimensional likelihood
functions to simultaneously constrain the baryon-to-photon ratio and the number of light neutrinos for a range
of 4He abundances Yp = 0.225 - 0.250, as well as a range in primordial 7Li abundances from (1.6 to 4.1) ¥ 10-10
were used.
Li, Be, and B measurements in metal poor stars have had a major impact on our understanding of the origin of the
light elements in the universe. Due to the roughly linear correlation between Be/H and Fe/H in low metallicity
halo stars, it has been argued that a "primary" component in the nucleosynthesis of Be must be present
in addition to the "secondary" component from standard Galactic cosmic ray nucleosynthesis. Fields and
Olive have examined the evidence for the primary versus secondary character of Li, Be, and B evolution, analyzing
both the observations and Galactic chemical evolution models. While it appears that [Be/H] versus [Fe/H] has a
logarithmic slope near 1, it is rather the Be-O trend that directly arises from the physics of spallation production.
Using new abundances for oxygen in halo stars based on UV OH lines, it was found that in Pop II stars for which
O has been measured, the Be-O slope has a large uncertainty due to systematic effects. Namely, the Be-O logarithmic
slope lies in the range 1.3 - 1.8, rendering it difficult to distinguish from the data between the secondary slope
of 2 and the primary slope of 1. Based on these results, it is possible that a good fit to the LiBeB evolution
requires only traditional the Galactic cosmic ray spallation, and the (primary) neutrino-process contribution to
11B. We also compared the Galactic chemical evolution of 6Li with recent observational determinations of the lithium
isotopic ratio in the same type of models.
In a series of papers, I have examined in detail the possibilities for supersymmetric dark matter. Despite its
simplicity, the minimal supersymmetric standard model (MSSM) has a large number of unmeasured parameters. Many
of these determine not only the mass and identity of the lightest supersymmetric particle (the LSP) but also its
relic abundance. The most common assumption is that the LSP is some linear combination of the neutral gauginos
and Higgsinos, collectively referred to as neutralinos.
Ellis, Falk, Schmitt and I have recently taken a detailed look at accelerator constraints on the mass of lightest
neutralino in connection with cosmology. In particular, they made use of the most recent results from current accelerator
searches for neutralinos and charginos at LEP 2. These limits were improved significantly by combining the results
from s particle searches at LEP 2 at center-of-mass energies up to 183 GeV with cosmological considerations. The
constraints on supersymmetric parameter space are further strengthened if LEP constraints on supersymmetric Higgs
bosons are taken into account. If universality at the GUT scale is assumed for soft supersymmetry-breaking scalar
masses, including those of the Higges, the Higgs boson searches play a dramatic role, and we found that mc >
40 GeV.
Also concerning the limits on the mass of the neutralino, we considered the effects of neutralino-stau coannihilations
on the cosmological relic density of the lightest supersymmetric particle in the minimal supersymmetric extension
of the Standard Model (MSSM), particularly in the constrained MSSM in which universal supergravity inputs at the
GUT scale are assumed. For much of the parameter space in these models, c is approximately a U(1) gaugino , and
constraints on the cosmological relic density W h2 yield an upper bound on m . It was shown that in regions of
parameter space for which the cosmological bound is nearly saturated, coannihilations of the , with the , the next
lightest particle, are important and may reduce significantly the relic density. Including also coannihilations
with the and, it was found that the upper limit on mc is increased from about 200~GeV to about 600~GeV in the constrained
MSSM, with a similar new upper limit expected in the MSSM.
The role of CP-violating phases in the Constrained Minimal Supersymmetric Standard Model were also considered recently.
It was found that in principle, large CP violating phases are compatible with the bounds on the electric dipole
moments of the neutron and electron, as well as remaining compatible with the cosmological upper bound on the relic
density of neutralinos. The contribution of CP-violating phases to T-violating nuclear forces were calculated.
These forces induce a Schiff moment in the 199Hg nucleus, which is strongly limited by experiments aimed at the
detection of the electric dipole moment of the mercury atom. The result for dHg is found to be very sensitive to
the CP-violating phases of the MSSM and the calculation carries far fewer QCD uncertainties than the corresponding
calculation of the neutron EDM.
The phases also affect the scattering of neutralinos on matter and therefore affect the possible dark matter detection
rates. The four-Fermi neutralino-quark interaction Lagrangian including contributions from the CP violating phases
in the MSSM was computed. It was found that neutralino-nucleus scattering cross-sections relevant for direct detection
experiments show a strong dependence on the value of the CP-violating phase.
The extent to which the cosmological fine-tuning problem--why the relic density of neutralino cold dark matter
particles c is similar to that of baryons--is related to the fine-tuning aspect of the gauge hierarchy problem--how
one arranges that MW « MP without unnatural choices of MSSM parameters was studied. Working in the minimal
supergravity framework with universal soft supersymmetry-breaking parameters as inputs, it was found that the hierarchical
fine-tuning is minimized for Wch2 ~0.1. Conversely, imposing Wch2 < 1 does not require small hierarchical fine
tuning, but the exceptions to this rule are rather special, with parameters chosen such that mc ~ MZ/2 or Mh/2,
or else mc ³mt.
I have also worked on inflation-related problems. Related to the Prebig Bang scenario, Kaloper, Kogan and I considered
solutions to the cosmological equations of motion in 11 dimensions with and without 4-form charges. These types
of solutions are motivated by the recent interest in M-theory. They showed explicitly the correspondence between
some of these solutions and known solutions in 10 dimensional string gravity. New solutions involving combinations
of 4-form charges were explored.
The possibility that higher-curvature corrections could drive inflation after the compactification to four dimensions
was also considered. Assuming that the low energy limit of the fundamental theory is eleven-dimensional supergravity
to the lowest order, including curvature corrections which do not directly contradict the current lore of M-theory,
and taking the descent from eleven dimensions to four via an intermediate five-dimensional theory, as favored by
recent considerations of unification at some scale around ~1016 GeV, one may obtain a simple model of inflation
in four dimensions.
The conditions necessary for a successful implementation of so-called assisted inflation was considered by Kanti
and Olive. The applicability of assisted inflation was generalized beyond exponential potentials as originally
proposed to include standard chaotic (lf4 or m2 f2)) models as well. It was also demonstrated that in a purely
4-dimensional theory, unless the assisted sector is in fact decoupled, the additional fields of the assisted sector
actually impede inflation. As a specific example of an assisted sector, a 5-dimensional KK model for which the
extra dimension may be somewhat or much larger than the inverse Planck scale was considered. In this case, the
assisted sector (coming from a KK compactification) eliminates the need for a fine-tuned quartic coupling to drive
chaotic inflation. This is a general result of models with one or more "large" extra dimensions. |
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| Updated: 01/06/02 |
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