INHOMOGENEOUS BIG BANG NUCLEOSYNTHESIS WITH A NEW NEUTRON LIFETIME

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INHOMOGENEOUS BIG BANG NUCLEOSYNTHESIS WITH A NEW
NEUTRON LIFETIME

• Dr. Juan F. Lara
• Department of Physics and Astronomy
• Clemson University
• Professor Toshitaka Kajino
• National Astronomical Observatory of Japan
• Department of Astronomy, University of Tokyo

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INTRODUCTION TO BBN

At high temperatures free neutrons and protons
are kept in thermal equilibrium with each other
by the following weak reactions.
n ?e ? p e- n e ? p ?e n ? p e-
?e
?
?
These weak reactions become ineffective at
temperature T 13 GK.
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At T 0.9 GK nuclear reactions become
significant.
n p ? d ? n d ? t ? p t ? 4He ? etc.
Nearly all free neutrons are incorporated into
4He nuclei
X4He 2(n/p) / (n/p) 1
The ratio (n/p) at the time when T 0.9 GK
depends on the value of the neutron lifetime tn.
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World Average tn 885.7 0.8 sec.

• From results collected by Particle Data Group in
  S. Eidelman et al., Phys. Lett. B 592 1 ( 2004 )

NEW RESULT tn 878.5 0.7 0.3 sec.

• A. Serebrov et al. in Proceedings of the
  Conference on Precision Measurements with Slow
  Neutrons, in press, nucl-ex/0408009
• An experiment using Ultra Cold Neutrons in a
  gravitational trap.

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THE GRAVITATIONAL TRAP
(-h2/2m) d2/dz2?(z) mgz ?(z) E ?(z)

• UCNs have such low energy that quantum effects
  from the gravitational potential can be observed.

Figure (1) One kind of UCN gravitational
trap. Image stolen from Nature, Volume 415, Page
267
N No exp(-t/tst) tst UCN storage time
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1/tst 1/tn 1/tloss

• 1/tloss is the rate of the loss of neutrons in
  the trap due to reasons other than decay.
• 1/ tloss can be factored into ?(E), the effective
  frequency of collisions, and ?(T), the loss
  factor of the neutron in the collision.

1/tst 1/tn ?(T)?(E)

• Measurements of 1/tst can be extrapolated to
  zero ?(E).
• Two storage times t1 and t2 of the same average
  UCN energy E can be used to get rid of ?(T)

1/tn 1/t1 ( 1/t2 1/t1 )/ ?2(E)/?1(E)
1
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Final Result tn 878.5 0.7stat 0.3syst.
sec.
The storage times were closer to tn than in
previous experiments.
The Cabibbo-Kobayashi-Maskawa Matrix Test

• The CKM matrix relates the quark weak
  eigenstates to the quark mass eigenstates.

Vud2 Vus2 Vub2 1 - ?

• The standard model predicts that ? 0.
• Plugging their value for tn into Vud, Serebrov
  et al. get good agreement with standard model
  predictions.

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INHOMOGENEOUS BBN
Figure (2) Initial distributions for
cylindrical shell model

• At the start the free neutrons and protons have
  an inhomogeneous baryon distribution.
• The neutrons diffuse from the high density region
  to the low density region, the time depending on
  the size of the model.

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Observational Constraints

• X4He 0.242 0.002, measured in metal poor HII
  regions by Y. Izotov T. Thuan, ApJ, 602, 200
  (2004).
• Deuterium abundance ratio Y(d)/Y(p)
  2.78-0.380.44 10-5, measured in Quasi Stellar
  Objects by D. Kirkman et al., ApJS, 149, 1
  (2003).
• Y(7Li)/Y(p) 1.23-0.320.68 10-10, measured in
  metal poor halo stars by S. Ryan et al., ApJ,
  530, L57 (2000).

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Figure (3) 4He and deuterium constraints on
IBBN model. ? baryon to photon ratio, ri
distance scale at T 100 GK

• Shorter lifetime fewer neutrons at the time of
  nucleosynthesis.
• The new tn shifts the maximum X4He to higher ? by
  1 10-10 but does not effect deuterium results.
• Ranges of 5.6, 6.7 10-10 for ? and ri lt 6.4
  105 cm are possible.

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Figure (4) Final 4He distribution
Figure (5) Final deuterium distribution

• For ri 3.2 105 cm neutron back diffusion is
  effective only around boundary between high, low
  density regions.
• For ri 2 106 cm neutrons cant homogenize
  before nucleosynthesis, leading to large 4He
  production in outer edge and around boundary.
  Large deuterium production occurs in trough
  between those regions.

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Figure (6) 7Li constraints on IBBN model

• Increased production of 7Be occurs for models
  with ri 25000 cm.
• A depletion factor from 2.8 to 9.3 could bring
  7Li constraints in concordance with 4He and
  deuterium constraints.

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FUTURE RESEARCH

• Compare our depletion factors to upcoming
  measurements of 7Li in extra-solar interstellar
  media.
• Resolve debate over the correct way to calculate
  rates for the neutron-proton interconversion
  reactions.
• See if we can match a specific baryon
  inhomogeneity with predictions from baryogenesis
  models.