Physics Letters B 753 (2016) 389–394
Contents lists available at ScienceDirect
Physics Letters B
www.elsevier.com/locate/physletb
Cosmological problems of the string axion alleviated by high scale
SUSY of m
3/2
10–100 TeV
Masahiro Kawasaki
a,b
, Tsutomu T. Yanagida
b
, Norimi Yokozaki
c,∗
a
ICRR, University of Tokyo, Kashiwa, Chiba 277-8582, Japan
b
Kavli IPMU (WPI), UTIAS, University of Tokyo, Kashiwa, Chiba 277-8583, Japan
c
Istituto Nazionale di Fisica Nucleare, Sezione di Roma, Piazzale Aldo Moro 2, I-00185 Rome, Italy
a r t i c l e i n f o a b s t r a c t
Article history:
Received
18 October 2015
Received
in revised form 8 December 2015
Accepted
15 December 2015
Available
online 17 December 2015
Editor:
J. Hisano
The string axion may provide the most attractive solution to the strong CP problem in QCD. However,
the axion energy density easily exceeds the dark matter density in the present universe due to a large
decay constant around 10
16
GeV, unless the initial value of the axion field is finely tuned. We show
that this problem is alleviated if and only if the SUSY particle mass scale is 10–100 TeV, since the
decay of the saxion can produce a large enough amount of entropy after the QCD phase transition,
not disturbing the BBN prediction. The saxion decay also produces a large number of the lightest SUSY
particles (LSPs). As a consequence, R-parity needs to be violated to avoid the overproduction of the LSPs.
The saxion field can be stabilized with relatively simple Kähler potentials, not inducing a too large axion
dark radiation. Despite the large entropy production, the observed baryon number is explained by the
Affleck–Dine mechanism. Furthermore, the constraint from isocurvature perturbations is relaxed, and the
Hubble constant during inflation can be as large as several ×10
10
GeV.
© 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
The appearance of an axion in string theories [1–3] is extremely
attractive, since it may provide the most plausible solution to the
strong CP problem [4–6]. The axion decay constant f
a
is predicted
as f
a
∼ 10
16
GeV in many string axion models, which is, however,
several orders of magnitude higher than the standard cosmological
bounds f
a
≤ 10
12
GeV [7–9]. This upper-bound is obtained for the
cosmological axion energy density not to exceed the observed dark
matter density in the present universe. Therefore, one has to finely
tune the initial value of the axion field in order to suppress the
axion energy density sufficiently.
However,
it was pointed out a long time ago in Ref. [10] that
the above problem can be alleviated if we have a certain amount
of entropy production after the QCD phase transition. As a result,
the axion energy density can be consistent with the observed dark
matter density without a fine-tuning. We may identify the axion as
a dominant component of the cold dark matter in the present uni-
verse.
In addition, the upper bound of the Hubble constant during
*
Corresponding author.
E-mail
address: norimi.yokozaki@roma1.infn.it (N. Yokozaki).
inflation from the isocurvature perturbations is somewhat relaxed
as H
inf
several × 10
10
GeV, which is one order of magnitude
larger than the bound without the entropy production [11].
In
this letter, we show that the required and consistent entropy
production in the late time of the early universe is indeed provided
by the saxion decay if and only if the SUSY particle mass scale is
around 100 TeV, without spoiling the successful prediction of the
Big-Bang Nucleosynthesis (BBN). The axion dark radiation accom-
panied
by the saxion decay, which may be problematic [12], can
be suppressed with relatively simple Kähler potentials, consistent
with stabilization of the saxion field.
Since
a large number of the lightest SUSY particles (LSPs) are
produced from the saxion decay, we need R-parity violation so
that the LSPs never exceed the dark matter density in the present
universe. In this case, there is no astrophysical constraint on the
LSP such as wino. The high scale supersymmetry (SUSY) may be
realized by the pure gravity mediation [13], predicting the wino
mass in the range of 0.1–1 TeV. Thus, wino searches for the mass
≤ 1TeVat the LHC is highly motivated. Note that the required size
of the R-parity violation to avoid cosmological constraints is tiny,
and hence, the R-parity violation is negligible at the collider time
scale in most cases.
http://dx.doi.org/10.1016/j.physletb.2015.12.043
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
.
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