Model-independent combination of diphoton constraints at 750 GeV
Eur. Phys. J. C
Model-independent combination of diphoton constraints at 750 GeV
Jong Soo Kim 2
Krzysztof Rolbiecki 1
Roberto Ruiz de Austri 0
0 Instituto de Física Corpuscular, IFIC-UV/CSIC , Valencia , Spain
1 Institute of Theoretical Physics, University of Warsaw , 02093 Warsaw , Poland
2 Instituto de Física Teórica, IFT-UAM/CSIC, C/ Nicolás Cabrera , 13-15, Cantoblanco, 28049 Madrid , Spain
Motivated by the recent diphoton excess reported by both the ATLAS and CMS collaborations, we provide a model-independent combination of diphoton results obtained at √s = 8 and 13 TeV at the LHC. We consider resonant schannel production of a spin-0 and spin-2 particle with a mass of 750 GeV that subsequently decays to two photons. The size of the excess reported by ATLAS appears to be in a slight tension with other measurements under the spin-2 particle hypothesis.
1 Introduction
The ATLAS and CMS collaboration have found an excess
in the search for a diphoton final state after the first 13 TeV
data have been analyzed [
1,2
]. The excess points to a
resonance with an invariant mass of about 750 GeV with a local
significance of 3.6 σ (ATLAS) and 2.6 σ (CMS).
The simplest explanation of the excess is through
resonant production of a spin-0 or spin-2 particle with a mass
of around 750 GeV that decays to photons. A spin-1
resonance is excluded by the Yang–Landau theorem [
3,4
]. There
have been many attempts to explain the excess both via direct
production of the 750 GeV resonance or through a heavier
particle that decays on-shell to a pair of 750 GeV scalars
finally decaying to photons [
5–50
]; see Ref. [
51
] for a recent
review.
In this letter we investigate whether the interpretation of
the diphoton excess via resonant s-channel production is
compatible with the full set of Run-I data [
52–54
] for both
the spin-0 and the spin-2 particle hypotheses. We work in
a model-independent framework in which we parametrize
the diphoton rate by the cross section and branching ratio
to photons and perform a simple statistical test to assess the
compatibility between different measurements.
This work is structured as follows. In Sect. 2 we explain the
methodology that we have employed, in Sect. 3 we present
the results and finally we give our conclusions in the last
section.
2 Methodology
pp → X → γ γ ,
We assume that the diphoton signal is resonantly produced,
where X denotes either a spin-0 or spin-2 particle. Here, we
consider the case where the resonance is only produced via
gluon fusion [
55
],
σ ( pp → X ) = (2 J + 1) (X → gg)
π 2 1 dx
× 8m3X τ τ x
g x, m2X g τ, m2X ,
where we have introduced the dimensionless variable τ =
ms2X . J and g(x , m2X ) denotes the spin of the resonance and
the gluon distribution function of the proton, respectively.
Note that the gluon luminosity ratio between 13 and 8 TeV
is 4.7 for m X = 750 GeV [
56
]. The branching ratio into the
diphoton final state is given by
BR(X → γ γ ) =
(X → γ γ )
(X → γ γ ) + (X → gg) + (X → Y Y )
where Y denotes all other particles which can couple to
the resonance X . Due to the much lower increase in the u
and d quarks luminosity between √s = 8 and 13 TeV of
(1)
(2)
(3)
ATLAS
pT (γ ) ≥25 GeV
|ηγ | ≤ 2.37
ETγ1 /mγ γ ≥ 0.4, ETγ2 /mγ γ ≥ 0.3
2.5–2.7 [
13
], the production in quark–antiquark annihilation
would lead to significant tensions between 8 and 13 TeV
results as we will see later. For this reason we will ignore
this possibility in the following. This is different for heavy
quark initial states: the cross section increase for
producing a 750 GeV resonance is 5.1–5.4 for charm and bottom
initial states, hence numerically close to the enhancement
in gluon–gluon production. Therefore, our results would be
qualitatively valid also in this case, albeit with a reduced
tension, for a detailed analysis see Ref. [
57
].1
A sample of signal events for the spin-0 case was
generated with POWHEG [
59–61
] at the parton level and
interfaced with Pythia 6.4 [
62
] for the parton shower and
hadronization with the CTEQ6L1 parton distribution
function [
63
]. A sample for the spin-2 case was generated with
Herwig++ 2.7.1 [
64
] using the MSTW parton
distribution functions [
65
]. For both hypotheses we assume a decay
width of 45 GeV. We have implemented the 8 TeV [
52–
54,66
] and 13 TeV [
1,2
] diphoton searches from ATLAS and
CMS into the CheckMATE 1.2.2 framework [
67
] with its
AnalysisManager [
68
]. CheckMATE 1.2.2 is based
on the fast detector simulation Delphes 3.10 [
69
] with
heavily modified detector tunes and it determines the
number of expected signal events passing the selection cuts of the
particular analysis. The cuts of the ATLAS and CMS
analyses are shown in Table 1. We do not follow the approach
of both experiments where the expected signal plus
background distribution is fitted to the measured mγ γ
distribution. Instead, we just perform a simple cut-and-count study.
Our simplified implementation of the a (...truncated)