Model-independent combination of diphoton constraints at 750 GeV

The European Physical Journal C, May 2016

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 \(\sqrt{s}=8\) and 13 TeV at the LHC. We consider resonant s-channel 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.

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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)


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Jong Soo Kim, Krzysztof Rolbiecki, Roberto Ruiz de Austri. Model-independent combination of diphoton constraints at 750 GeV, The European Physical Journal C, 2016, pp. 251, Volume 76, Issue 5, DOI: 10.1140/epjc/s10052-016-4102-0