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Theory Center Seminar<br>
Mon., Feb. 11, 2013<br>
1:00 p.m. (coffee at 12:45 p.m.)<br>
CEBAF Center, Room F113<br>
<br>
<br>
Andre Walker-Loud<br>
Lawrence Berkeley National Laboratory<br>
<br>
<b>M_n - M_p </b><br>
<br>
Our understanding of the formation of light nuclei in the early
universe, known as Big Bang Nucleosynthesis, is now a precision
science. For example, given the baryon to photon ratio determined
from the Cosmic Microwave Background, the mass fraction of He(4) in
the early universe is predicted with 0.1% precision. The predicted
abundances of the primordial nuclei match well with astrophysical
observations in all cases except Li(7). The resulting picture is
highly constraining on possible physics beyond the Standard Model,
one such example being the time-variation of fundamental constants.<br>
<br>
Big Bang Nucleosynthesis is most sensitive to the neutron-proton
mass splitting as this quantity controls the initial ratio of
neutrons to protons and also impacts the neutron lifetime. There are
two sources of isospin violation in the Standard Model which give
rise to the neutron-proton mass spitting: the masses of the up and
down quarks and their electromagnetic couplings. The electromagnetic
correction can be determined via contour integrals and the
experimentally measured elastic and inelastic structure functions of
the nucleons. The contribution from the down-quark up-quark mass
splitting can only be determined with lattice QCD. I will describe
in detail our understanding of these two contributions and summarize
with implied constraints from primordial nucleosynthesis.
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