arXiv:hep-ph/0612357v3 25 Mar 2007
Unified TeV Scale Picture of Baryogenesis and Dark Matter
K. S. Babua ‡, R. N. Mohapatrab †, and Salah Nasric §
aDepartment of Physics, Oklahoma State University, Stillwater, OK 74078, USA
bDepartment of Physics, University of Maryland,College Park, MD 20742,USA and
cDepartment of Physics, University of Florida, Gainesville, Florida 32611, USA
We present a simple extension of MSSM which provides a unified picture of cosmological baryon
asymmetry and dark matter. Our model introduces a gauge singlet field N and a color triplet field
X which couple to the right–handed quark fields. The out–of equilibrium decay of the Majorana
fermion N mediated by the exchange of the scalar field X generates adequate baryon asymmetry
for MN∼ 100 GeV and MX∼ TeV. The scalar partner of N (denoted˜ N1) is naturally the lightest
SUSY particle as it has no gauge interactions and plays the role of dark matter.
into quarks efficiently in the early universe via the exchange of the fermionic˜ X field. The model
is experimentally testable in (i) neutron–antineutron oscillations with a transition time estimated
to be around 1010sec, (ii) discovery of colored particles X at LHC with mass of order TeV, and
(iii) direct dark matter detection with a predicted cross section in the observable range.
. The origin of matter–anti-matter asymmetry of the Universe and that of dark matter
are two of the major cosmological puzzles that rely heavily on particle physics beyond the
standard model for their resolution. It is a common practice to address these two puzzles
separately by invoking unrelated new physics. For instance, a widely held belief is that either
the lightest supersymmetric particle (LSP) or the near massless invisible axion constitutes
the dark matter, while baryogenesis occurs through an unrelated mechanism involving either
the decay of a heavy right–handed neutrino (leptogenesis), or new weak scale physics which
makes use of the electroweak sphalerons. A closer examination of the minimal versions
of SUSY would suggest that to generate the required amount of dark matter density one
needs some tuning of parameters. The LSP should either have the right amount of Higgsino
component, or another particle, usually the right–handed stau, should be nearly degenerate
with the Bino LSP to facilitate dark matter co-annihilation. Similarly, the leptogenesis
mechanisms requires the heavy right–handed neutrino to have its mass in the right range to
generate the adequate amount of matter. Despite these possible problems, these ideas are
attractive since they arise in connection with physics scenarios which are strongly motivated
by other puzzles of the standard model, e.g., resolving the gauge hierarchy problem (in the
case of LSP dark matter), or generating small neutrino masses (in the case of leptogenesis).
In the absence of any experimental confirmation of these ideas, it is quite appropriate to
entertain alternate explanations which could be motivated on other grounds. Our motivation
here is to seek a unified picture of both these cosmological puzzles within the context of
weak scale supersymmetry without fine–tuning of parameters. We propose a class of models
where a very minimal extension of the MSSM resolves these puzzles in a natural manner
with testable consequences for the near future.
Our extension of MSSM involves the addition of two new particles: a SM singlet su-
perparticle denoted by N with mass in the 100 GeV range and an iso-singlet color triplet
particle X with mass in the TeV range. These particles, consistent with the usual R–parity
assignment, couple only to the right–handed quark fields. We discuss two models, one in
which the electric charge of X is 2/3 and another where it is −1/3. We show that in these
models, baryon asymmetry arises by the mechanism of post–sphaleron baryogenesis sug-
gested by us in a recent paper  involving the decay of the Majorana fermion N. The
dation Grant No. Phy-0354401 and S. Nasri by DOE Grant No. DE-FG02-97ER41029.
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