Characterising the 750 GeV diphoton excess
Received: March
Characterising the 750 GeV diphoton excess
Jeremy Bernon 0 1 2 4
Andreas Goudelis 0 1 2 3
Sabine Kraml 0 1 2 4
Kentarou Mawatari 0 1 2 4
Dipan Sengupta 0 1 2 4
0 International Solvay Institutes , Pleinlaan 2, Brussels, B-1050 Belgium
1 Nikolsdorfergasse 18 , Vienna, 1050 Austria
2 53 Avenue des Martyrs , Grenoble, F-38026 France
3 Institute of High Energy Physics, Austrian Academy of Sciences
4 Laboratoire de Physique Subatomique et de Cosmologie, Universite Grenoble-Alpes , CNRS/IN2P3
We study kinematic distributions that may help characterise the recently observed excess in diphoton events at 750 GeV at the LHC Run 2. Several scenarios are considered, including spin-0 and spin-2 750 GeV resonances that decay directly into photon pairs as well as heavier parent resonances that undergo three-body or cascade decays. We nd that combinations of the distributions of the diphoton system and the leading photon can distinguish the topology and mass spectra of the di erent scenarios, while patterns of QCD radiation can help di erentiate the production mechanisms. Moreover, missing energy is a powerful discriminator for the heavy parent scenarios if they involve (e ectively) invisible particles. While our study concentrates on the current excess at 750 GeV, the analysis is general and can also be useful for characterising other potential diphoton signals in the future.
Phenomenological Models; Phenomenology of Field Theories in Higher Di-
1 Introduction
Scenarios for the 750 GeV diphoton excess
2
3
4
5
2.1
2.2
4.1
4.2
Event simulations
Results
Conclusions
750 GeV resonance
Heavier parent resonance
750 GeV resonance
Heavier parent resonance
A Sequential resonance with S1 = S2
B
Mass and width e ects for the antler topology
dijets, monojets and other search channels both at p
s = 8 and 13 TeV. The rst theory
s = 8 to 13 TeV and to evade the often stringent bounds from null results in
papers discussing various ways to reproduce the observed diphoton rate as well as possibly
a large width while avoiding existing constraints from Run 1 appeared on the arXiv already
on the day after the announcement of the excess [4{11]. More than 200 papers followed
to date.
Whatever one may think of this \ambulance chasing" [12], an interesting question that
arises is how to experimentally di erentiate between this variety of possible interpretations.
Needless to say this question will be of imminent importance should the observed excess
turn into a discovery with the accumulation of more data.
One approach consists of
observing the new state in di erent decay modes, as the predictions for the (ratios of) rates
1The ATLAS excess consists of 14 events in 3.2 fb 1 of data; it has a local (global) signi cance of 3:6
(2:0 ) and seems to favour a large width of about 45 GeV (see however [3]). The CMS excess consists of
10 events in 2.6 fb 1 of data; it has a local (global) signi cance of 2:6
(1:2
) and is consistent with a
narrow width.
{ 1 {
of speci c nal states vary between di erent concrete models. Another, complementary
approach is to rely on the diphoton signal itself and attempt its detailed characterisation in
terms of kinematic distributions. As a preparatory step in the latter direction, in this paper
we study the expectations for di erential distributions from various signal hypotheses and
discuss ways to discriminate between them. We note in passing that both approaches |
inclusive measurements in di erent nal states and kinematic distributions | have been
pursued successfully to scrutinise the 125 GeV Higgs signal in Run 1 [13{25].
Irrespective of the underlying model, the interpretations put forward generically fall
in just a few classes. First, if we are dealing with a new particle with mass of 750 GeV
which undergoes a two-body decay into two photons, the classi cation is by spin and
or pseudoscalar) particle produced in gluon fusion and decaying to photons e.g. via loops
of new vector-like quarks. Bottom-quark (bb) initiated production could also provide the
necessary increase in cross section from p
s = 8 to 13 TeV of about a factor
ve [10].
If it has electroweak couplings, a scalar resonance can also be produced in vector boson
fusion and vector-boson associated production. Photon-initiated production has also been
discussed [26{29].
Another option is a spin-2 resonance, like the Kaluza-Klein (KK) graviton in
RandallSundrum (RS)-type models [30], which might be produced from gg or qq initial states.
A spin-1 particle would not decay into photons,2 and higher spins are not considered
because they are disfavoured theoretically. In order to explain a large width, as seemingly
favoured by ATLAS, the resonance should couple not only to gluons and photons (and
perhaps quarks) but also to non-standard states such as dark matter or light hidden-valley
particles. Invisible decays are, however, fairly constrained (although not excluded) by the
8 TeV mono-X searches as discussed e.g. in [33]. (...truncated)