The 750 GeV Diphoton Excess: what can it be?
M. Dalchenko, B. Dutta, Y. Gao, T. Ghosh, T. Kamon, J.W.Walker ATLAS results (L. Hauswald, K.Black)
2 CMS results (S.W. Lee’s talk)
3 Overtake an ambulance! G. Landsberg:
2HDM??
Composite sector?? Sgoldstino scalar??
Graviton?? Walking technopion???
4 What do we really know?
● γγ peak around 750 GeV over flatland
σ(pp → γγ) CMS ATLAS 8 TeV (0.5 ± 0.6)fb (0.4 ± 0.8)fb 13 TeV (6±3)fb (10±3)fb
● ATLAS prefers large width Γ/M ∼ 0.06 ● CMS prefers narrow width. ● γγ not accompanied by hard extras ● Spin-1 excluded due to Landau-Yang theorem ● No excess observed in other final states
5 A very generic model
Production modes ● Gluon fusion with NP in the loop ○ SM disfavored due to huge tt branching p ○ Favored by 8→13 TeV xs scaling X ● Vector boson fusion ○ Predicts extra jets ● Photon fusion p ○ Slightly disfavored by xs scaling
Final states: γγ + N jets
6 Effective lagrangian and couplings
In terms of physical gauge boson states
The relative size between these couplings can help to determine the origin of the new physics 7 Generate, simulate, analyze! Generate with MadGraph5: ● different coupling selection allows to enhance/supress photon- or W- VBF production
Hadronize with Pythia8 ● apply jet matching in case of gluon fusion
cut here Simulate with Delphes
● default ATLAS card used ● no pile-up (pile-up rejection seems to be very efficient) ● Why ATLAS → because they provide kinematic distributions 8 Photon fusion ● Photon-fusion can obtain domination with ● e.g. when the mediators inside the loop are non-colored singlets like heavy right-handed charged leptons ● Has unique kinematic feature: photon’s collinear enhancement, which results in low-pT and/or high-eta jets in case of VBF ● Pure γγ→γγ has the dominant cross section and results in jetless final state ● Elastic photons from the protons make a subleading contribution and populate 0-jet final state
9 WW fusion ● Present if heavy mediators are charged under the ○ E.g. vector-like lepton doublets ● Always has two associated jets, which are not forward-enhanced ● Typical jet transverse momentum is quite flat with a plateau around and above ● Since the is not zero, the WW fusion coexist (and interfere) with photon fusion ○ In order to suppress photon fusion w.r.t. WW fusion we select a special case
10 gg fusion
● Can be leading production channel if heavy mediators are colored ● Also can be implemented along with composite X made of colored fields ● The jet multiplicity follows a power-law shape which is typical for the QCD radiation ○ It peaks at zero, however with a much less pronounced weight in the zero-jet final state w.r.t. photon fusion ● Jet transverse momentum spectrum is also power-law and is significantly harder than the one in case of photon fusion
11 Distinctive features
Detector-level distributions from photon (red), WW (blue-dashed) and gg (black-dotted) fusions
● We follow ATLAS spin-0 analysis 12 Jet multiplicities
● Signal plotted on top of Sherpa bkg ● Event selection corresponds to the one in ATLAS ● Data points extracted from ATLAS plots 13 Compatibility tests
● Photon fusion ● WW fusion ● gg fusion ● Favors the S+B over only ● Disfavors the signal ● The fit is not as good as in B hypothesis hypothesis a lot photon case ● Best fit for S/B~30/70 ● B-only hypothesis is within 1 standard deviation
14 Summary
● The excess around 750 GeV is reported by both CMS and ATLAS experiments ● Despite minor differences, the results of both experiments are well compatible (yet not enough statistics to claim discovery) ● Huge amount of theory papers with various models were published ● We propose a method to distinguish between different production modes ● The kinematic distributions favors the photon fusion production mechanism ● Photon fusion production also fits well along the other experimental constraints (monojet, dijet, diboson etc.)
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