Physics Letters B 755 (2016) 343–347
Contents lists available at ScienceDirect
Physics Letters B
www.elsevier.com/locate/physletb
750 GeV composite axion as the LHC diphoton resonance
Neil D. Barrie
∗
, Archil Kobakhidze
∗
, Matthew Talia
∗
, Lei Wu
∗
ARC Centre of Excellence for Particle Physics at the Terascale, School of Physics, The University of Sydney, NSW 2006, Australia
a r t i c l e i n f o a b s t r a c t
Article history:
Received
4 February 2016
Accepted
5 February 2016
Available
online 15 February 2016
Editor: J.
Hisano
We propose that the 750 GeV resonance, presumably observed in the early LHC Run 2 data, could
be a heavy composite axion that results from condensation of a hypothetical quark in a high-colour
representation of conventional QCD. The model, motivated by a recently proposed solution to the strong
CP problem, is very economical and is essentially defined by the properties of the additional quark –
its colour charge, hypercharge and mass. The axion mass and its coupling to two photons (via axial
anomaly) can be computed in terms of these parameters. The axion is predominantly produced via
photon fusion (γγ → A) which is followed by Z vector boson fusion and associated production at
the LHC. We find that the total diphoton cross section of the axion can be fitted with the observed
excess. Combining the requirement on the cross-section, such that it reproduces the diphoton excess
events, with the bounds on the total width (
tot
45 GeV), we obtain the effective coupling in the range
1.6 ×10
−4
GeV
−1
C
A
6.5 ×10
−5
GeV
−1
. Within this window of allowed couplings the model favours
a narrow width resonance and y
2
Q
∼ O(10). In addition, we observe that the associated production
q
¯
q → Aγ → γγγ can potentially produce a sizeable number of three photon events at future LHC.
However, the rare decay Z → A
∗
γ → γγγ is found to be too small to be probed at the LHC and e
+
e
−
colliders.
© 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
There is preliminary evidence for a new diphoton resonance at
∼750 GeV in the early LHC Run 2 data. ATLAS reports an excess
of 15 events (with detector efficiency 40%) in four bins around
750 GeV in their 3.2fb
−1
13 TeV data [1]. This presumably im-
plies
a relatively large width for the putative resonance, fitted to
= 45 GeV in one model template. The local (global) statistical
significance of the observed excess is 3.9σ (2.3σ ). CMS also sees
an excess, which consists of ∼10 events peaked at 760 GeV [2].
The CMS excess is consistent with the narrow width resonance at
2.6σ (1.2σ ) local (global) significance.
The
anomalous events are not accompanied by any significant
missing energy, neither leptons or jets are seen. No resonances at
invariant mass 750 GeV are seen in the Run 2 data in ZZ, W
+
W
−
,
t
¯
t or jj events and the diphoton resonance has not been observed
by ATLAS in 8 TeV Run 1 data. The pp → R → γγ cross section
needed to explain the excess is estimated as [3]:
*
Corresponding author.
E-mail
addresses: neil.barrie@sydney.edu.au (N.D. Barrie),
archil.kobakhidze@sydney.edu.au (A. Kobakhidze), matthew.talia@sydney.edu.au
(M. Talia),
lei.wu1@sydney.edu.au (L. Wu).
σ
ATLAS
(pp → R →γγ) ≈ (10 ±3) fb (1)
σ
CMS
(pp → R →γγ) ≈ (6 ±3) fb (2)
The 8 TeV Run 1 data is compatible with the 13 TeV Run 2 data
at the level of 2σ , providing the signal cross section grows by at
least a factor of ∼4–5. Together with the 8 TeV data, the diphoton
excess contributing to the combined production rate is given by [4]
σ (pp → R →γγ) = (4.4 ±1.1) fb . (3)
From the available data it is premature to draw any definite
conclusion about the properties of the diphoton resonance, includ-
ing
its very existence. Nevertheless, assuming the observed excess
is not a statistical fluke, the suggested large branching ratio of
R → γγ is strongly reminiscent of the composite neutral pion.
The vast majority of composite models discussed so far in rela-
tion
to the LHC diphoton resonance (see, presumably, an incom-
plete
list of Refs. [5–19]), assumed production of the resonance via
gluon fusion. It has been pointed, however, that the excess can be
accommodated in a more minimal scenario via photo-production
[20–24]. Drell–Yan production pp → RR → 4γ has also been dis-
cussed
in [16,17] within the effective field theory approach. Spe-
cific
models relying on the photo-production mechanism can be
found in [25–27].
http://dx.doi.org/10.1016/j.physletb.2016.02.010
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
.
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