Physics Letters B 757 (2016) 493–500
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Physics Letters B
www.elsevier.com/locate/physletb
Diphoton excess and running couplings
Kyu Jung Bae
a,∗
, Motoi Endo
a,b
, Koichi Hamaguchi
a,b
, Takeo Moroi
a,b
a
Department of Physics, University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan
b
Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU), 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
22 February 2016
Accepted
14 April 2016
Available
online 18 April 2016
Editor: J.
Hisano
The recently observed diphoton excess at the LHC may suggest the existence of a singlet (pseudo-)scalar
particle
with a mass of 750 GeV which couples to gluons and photons. Assuming that the couplings
to gluons and photons originate from loops of fermions and/or scalars charged under the Standard
Model gauge groups, we show that there is a model-independent upper bound on the cross section
σ (pp → S → γγ) as a function of the cutoff scale and masses of the fermions and scalars in the
loop. Such a bound comes from the fact that the contribution of each particle to the diphoton event
amplitude is proportional to its contribution to the one-loop β functions of the gauge couplings. We also
investigate the perturbativity of running Yukawa couplings in models with fermion loops, and show the
upper bounds on σ(pp → S → γγ) for explicit models.
© 2016 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
Recently, the ATLAS and CMS Collaborations reported an excess
of diphoton events implying a resonance with a mass of around
750 GeV [1,2]. The ATLAS Collaboration has 3.2 fb
−1
of data, and
the largest excess is found at around the diphoton invariant mass
of m
γγ
750 GeV with the local (global) significance of 3.6σ
(2.0σ ) for a narrow width case. When a large width for the sig-
nal
component is assumed, the local (global) significance increases
to 3.9σ (2.3σ ) at the width of about 45 GeV. The CMS Collab-
oration,
with 2.6 fb
−1
of data, also reported an excess at around
m
γγ
750 GeV with the local (global) significance of 2.6σ (1.2σ )
for a narrow width case, while the significance does not increase
with a larger width. Possible explanations and implications of this
excess have been extensively discussed [3–7].
One
of the plausible explanations of the excess is that a scalar
or pseudoscalar particle S with a mass of 750 GeV is produced
through gluon fusion and decays into a pair of photons, gg → S →
γγ, via diagrams with new fermions and/or bosons charged under
the Standard Model (SM) gauge groups running in the loops [3–6].
In order to explain the excess with perturbative couplings, how-
ever,
the new particles in the loop should have large quantum
numbers and/or large multiplicity, which implies that the pertur-
bativity
of the SM gauge groups may break down at some high
*
Corresponding author.
E-mail
address: bae@hep-th.phys.s.u-tokyo.ac.jp (K.J. Bae).
scale below the Planck scale. In this letter, we address this issue
and investigate the perturbativity of such models.
Our
main conclusions are as follows:
1. We
point out that the contribution of each particle in the loop
to the diphoton event amplitude is proportional to its contri-
bution
to the one-loop β functions of the gauge couplings at
the leading order, independently of the representations of the
particles in the loop. Consequently, there is a generic upper
bound on the cross section σ (pp → S → γγ) as a function of
the cutoff scale and masses of the fermions and scalars in
the loop. We also numerically evaluate such a bound, taking
into account the following constraints:
(i) the
constraints from Landau pole, requiring that the gauge
couplings remain perturbative up to the scale , and
(ii) the
constraint from the scale dependence of the strong
coupling constant based on the LHC [8].
2. We
also investigate the running of the Yukawa coupling in
models with fermion loops. The upper bound on σ (pp →
S → γγ) is presented as a function of the fermion mass and
the cutoff scale for some explicit models with vector-like
quarks.
The
generic analysis in the first part, which can be applied to
models with fermions and scalars in the loop in arbitrary repre-
sentations,
was not considered in the previous works. The analysis
of the second part is close to those of Ref. [3], where the authors
http://dx.doi.org/10.1016/j.physletb.2016.04.031
0370-2693/
© 2016 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
.