Characterising the 750 GeV diphoton excess

Journal of High Energy Physics, May 2016

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 find that combinations of the distributions of the diphoton system and the leading photon can distinguish the topology and mass spectra of the different scenarios, while patterns of QCD radiation can help differentiate the production mechanisms. Moreover, missing energy is a powerful discriminator for the heavy parent scenarios if they involve (effectively) 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.

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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)


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Jérémy Bernon, Andreas Goudelis, Sabine Kraml, Kentarou Mawatari, Dipan Sengupta. Characterising the 750 GeV diphoton excess, Journal of High Energy Physics, 2016, pp. 128, Volume 2016, Issue 5, DOI: 10.1007/JHEP05(2016)128