Physics Letters B 752 (2016) 252–257
Contents lists available at ScienceDirect
Physics Letters B
www.elsevier.com/locate/physletb
Predictions of SU(9) orbifold family unification
Yuhei Goto
∗
, Yoshiharu Kawamura
Department of Physics, Shinshu University, Matsumoto 390-8621, Japan
a r t i c l e i n f o a b s t r a c t
Article history:
Received
7 September 2015
Received
in revised form 11 November 2015
Accepted
12 November 2015
Available
online 17 November 2015
Editor:
J. Hisano
We study predictions of orbifold family unification models with SU(9) gauge group on a six-dimensional
space–time including the orbifold T
2
/Z
2
, and obtain relations among sfermion masses in the supersym-
metric
extension of models. The models have an excellent feature that just three families of the standard
model fermions exist in a pair of Weyl fermions in the 84 representation as four-dimensional zero modes,
without accompanying any mirror particles.
© 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license
(http://creativecommons.org/licenses/by/4.0/). Funded by SCOAP
3
.
1. Introduction
Gauge theories on a higher-dimensional space–time including
an orbifold as an extra space possess suitable properties to realize
a family unification [1–14]. Three families of the standard model
(SM) fermions are embedded into a few multiplets of a large gauge
group. Extra fermions including mirror particles can be eliminated
and the SM fermions can survive as zero modes, through the orb-
ifold
breaking mechanism.
In
our previous work, we have found a lot of possibilities that
three families of the SM fermions appear as zero modes from a
pair of Weyl fermions in SU(N) gauge theories (N = 9∼13) on the
six-dimensional (6D) space–time M
4
× T
2
/Z
M
(M = 2, 3, 4, 6) [10].
The subjects left behind are to construct realistic models and to
find out model-dependent predictions.
In
this paper, we focus on the orbifold family unification in the
minimal setup, because models tend to be more complex and less
realistic by extending the structure of space–time and/or the in-
gredients
of models such as gauge symmetries and representations
of matters. We take orbifold family unification models based on
SU(9) gauge symmetry on M
4
× T
2
/Z
2
as a starting point, exam-
ine
the reality of models, and find out some predictions. For the
reality, we use the appearance of Yukawa interactions from inter-
actions
in the 6D bulk as a selection rule. For the predictions, we
search specific relations among sfermion masses in the supersym-
metric
(SUSY) extension of models.
The
contents of this paper are as follows. In Sec. 2, we explain
our setup and its properties. In Sec. 3, we carry out the examina-
*
Corresponding author.
E-mail
addresses: 14st302a@shinshu-u.ac.jp (Y. Goto),
haru@azusa.shinshu-u.ac.jp (Y. Kawamura).
tion for the reality of models and the search for predictions. Sec. 4
is
devoted to conclusions and discussions.
2. SU(9 ) orbifold family unification
2.1. Setup
Our space–time is assumed to be the product of four-dimen-
sional
(4D) Minkowski space–time M
4
and two-dimensional (2D)
orbifold T
2
/Z
2
. The T
2
/Z
2
is obtained by dividing 2D lattice T
2
by
the Z
2
transformation: z →−z, where z is a complex coordinate.
Then, z is identified with (−1)
l
z + me
1
+ ne
2
, where l, m and n are
integers, and e
1
and e
2
are basis vectors of T
2
.
We
impose the following boundary conditions (BCs) on a 6D
field (x, z),
(x, −z) = T
[P
0
](x, z), (1)
(x, e
1
− z) = T
[P
1
](x, z), (2)
(x, e
2
− z) = T
[P
2
](x, z), (3)
where T
[P
0
], T
[P
1
] and T
[P
2
] represent the representation
matrices, and P
0
, P
1
and P
2
stand for the representation matrices
of Z
2
transformations z →−z, z → e
1
− z and z → e
2
− z for fields
with the fundamental representation.
The
eigenvalues of T
[P
0
], T
[P
1
] and T
[P
2
] are interpreted
as the Z
2
parities on T
2
/Z
2
. The fields with even Z
2
parities
have zero modes. Here, zero modes mean 4D massless fields sur-
viving
after compactification. Massive modes are called “Kaluza–
Klein
modes”, and they do not appear in the low-energy world
because they have heavy masses of O (1/L) where L is the size
of extra space. Fields including an odd Z
2
parity do not have
zero modes. Hence, the reduction of symmetry occurs upon com-
pactification,
unless all components of multiplet have common Z
2
http://dx.doi.org/10.1016/j.physletb.2015.11.034
0370-2693/
© 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Funded by
SCOAP
3
.