Physics Letters B 780 (2018) 538–542
Contents lists available at ScienceDirect
Physics Letters B
www.elsevier.com/locate/physletb
Enhanced axion–photon coupling in GUT with hidden photon
Ryuji Daido
a,∗
, Fuminobu Takahashi
a,b
, Norimi Yokozaki
a
a
Department of Physics, Tohoku University, Sendai, Miyagi 980-8578, Japan
b
Kavli Institute for the Physics and Mathematics of the Universe (WPI), University of Tokyo, Kashiwa 277-8583, Japan
a r t i c l e i n f o a b s t r a c t
Article history:
Received
9 February 2018
Received
in revised form 13 March 2018
Accepted
15 March 2018
Available
online 16 March 2018
Editor:
A. Ringwald
We show that the axion coupling to photons can be enhanced in simple models with a single Peccei–
Quinn
field, if the gauge coupling unification is realized by a large kinetic mixing χ = O(0.1) between
hypercharge and unbroken hidden U(1)
H
. The key observation is that the U(1)
H
gauge coupling should
be rather strong to induce such large kinetic mixing, leading to enhanced contributions of hidden matter
fields to the electromagnetic anomaly. We find that the axion–photon coupling is enhanced by about a
factor of 10–100 with respect to the GUT-axion models with E/N = 8/3.
© 2018 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
The axion, a, is a pseudo-Nambu–Goldstone boson associated
with the spontaneous breakdown of a global U(1)
PQ
symmetry in
the Peccei–Quinn (PQ) mechanism [1–4]. Not only does the axion
solve the strong CP problem, but it can also explain dark matter
[5–7], which makes it one of the well-motivated physics beyond
the standard model.
The
axion or axion-like particles have been searched for by nu-
merous
experiments (see e.g. Refs. [8,9]for recent reviews). Many
of the on-going and planned experiments exploit the axion cou-
pling
to photons,
L =
g
aγγ
4
aF
μν
˜
F
μν
, (1)
where F
μν
is the photon field strength, and
˜
F
μν
denotes its dual.
Therefore, the expected region of the axion–photon coupling g
aγγ
for a given decay constant f
a
or axion mass m
a
is a very important
input for such experiments.
The
axion–photon coupling is induced by one-loop diagrams
where those particles running in the loop have both electric and
U(1)
PQ
charges. In fact, the coupling depends on the detailed struc-
ture
of the PQ sector [10]. As an extreme example, one can con-
sider
a clockwork (or aligned) QCD axion model [11–13], which
include multiple PQ fields with a clockwork structure [14–18]. In
*
Corresponding author.
E-mail
address: daido @tuhep .phys .tohoku .ac .jp (R. Daido).
this case, the axion can have a coupling to photons [19]or hid-
den
photons [12], which is exponentially larger than gluons. See
also Ref. [20]for related topics. However, in simple models with a
single PQ field, there is a preferred range of g
aγγ
for a given f
a
[21–23](see black dash-dotted lines in Fig. 2). In this paper we
limit ourselves to such simple models with a single PQ scalar
whose expectation value is equal to the decay constant (up to the
domain wall number, N
DW
).
Another
important motivation for physics beyond the standard
model is grand unified theories (GUTs). In a non-supersymmetric
GUT, however, the unification scale tends to be too low to satisfy
the proton decay constraint, and moreover, the gauge couplings
seem to fail unify at a single scale. One of the remedies for the
gauge coupling unification is to add a massless hidden photon with
a large kinetic mixing with hypercharge, U(1)
Y
[24]. According to
the recent analysis using the two-loop renormalization group (RG)
equations [25], the unification scale is shown to be at 10
16.5
GeV
and the required kinetic mixing is χ (m
Z
) ≈ 0.37. Interestingly, the
unification with a hidden photon is rather robust against adding
visible or hidden matters [25,28]. This finding enables us to incor-
porate
the axion into the framework in a consistent manner.
In
this paper, we study the axion coupling to photons in a GUT
scenario where a massless hidden photon has a large kinetic mix-
ing
with hypercharge.
1
Since the kinetic mixing between U(1)
Y
and U(1)
H
is induced by one-loop diagrams with bi-charged par-
ticles
running in the loop, it requires either many such particles
1
See Refs. [26,27]for axion models in other GUT scenarios.
https://doi.org/10.1016/j.physletb.2018.03.039
0370-2693/
© 2018 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
.