DOI: 10.17182/hepdata.85728.v1/t9
title: "Table 9" of "Search for pairs of highly collimated photon-jets in $pp$ collisions at $\sqrt{s}$ = 13 TeV with the ATLAS detector"

description: Selection efficiency for reconstructed photons originating from the decay $a\rightarrow 2\gamma$ with $m_a$ = 0.5 GeV.
description: CERN-LHC. Search for the pair production of photon-jets---collimated groupings of photons---with the ATLAS detector. Highly collimated photon-jets can arise from the decay of new, highly boosted particles that can decay to multiple photons collimated enought to be identified in the electromagnetic calorimeter as a single, photon-like energy cluster. Data from proton–proton collisions at a centre-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 36.7 fb$^{-1}$, were collected in 2015 and 2016. Upper limits are placed on $\sigma\times \mathcal{B}(X\rightarrow aa)\times \mathcal{B}(a\rightarrow\gamma\gamma)^2$ and $\sigma\times \mathcal{B}(X\rightarrow aa)\times \mathcal{B}(a\rightarrow 3\pi^0)^2$ for 200 GeV &lt; $m_X$ &lt; 2TeV and $m_a$ &lt; 10 GeV.

Tables 8 to 35 are provided to allow the recasting of the cross-section upper limits to different signal models predicting final states with photon-jets. These tables present the selection efficiency (before categorisation) $\varepsilon_{\gamma_R}(E_\mathrm{T},\eta)$ for reconstructed photons originating from a photon-jet, and the fraction $f_{\gamma_R}(E_\mathrm{T},\eta)$ of reconstructed photons with a value of the shower shape variable $\Delta E$ lower than the threshold.

The fiducial region is defined as:
- $E_\mathrm{T,1}&gt;0.4\times m_X$
- $E_\mathrm{T,2}&gt;0.3\times m_X$
- $|\eta_i| &lt; 2.37 (i=1,2)$ (excluding $1.37 &lt; |\eta_i| &lt;1.52$)
where $E_\mathrm{T,1}, \eta_1$ ($E_\mathrm{T,2}, \eta_2$) are the transverse energy and the pseudorapidity of the $a$ particle with the higher (the lower) transverse energy, respectively.

For a resonance particle $X$ decaying into a pair of photon-jets via $X\rightarrow aa$, the total selection efficiency, $\varepsilon$, and the fraction of events in the low-$\Delta E$ category, $f$, can be computed by integrating over the p.d.f. of $(E_\mathrm{T,1},\eta_1,E_\mathrm{T,2},\eta_2)$ with the following procedure:
- apply the fiducial cuts to the two $a$ particles
- compute $\varepsilon$ from the integration of $\varepsilon_{\gamma_R}(E_\mathrm{T,1},\eta_1) \cdot \varepsilon_{\gamma_R}(E_\mathrm{T,2},\eta_2)$ 
- compute $f$ from the integration of $\varepsilon_{\gamma_R}(E_\mathrm{T,1},\eta_1) \cdot \varepsilon_{\gamma_R}(E_\mathrm{T,2},\eta_2) \cdot f_{\gamma_R}(E_\mathrm{T,1},\eta_1) \cdot f_{\gamma_R}(E_\mathrm{T,2},\eta_2)$ divided by $\varepsilon$

With the resulting value of $f$ for a given value of $m_X$, the 95% CL observed upper limit on the visible cross-section (i.e. $\sigma\times \mathcal{B}\times\varepsilon$) can be taken from Table 7, which is considered to be model-independent. The corresponding upper limit on the cross-section times branching ratios, $\sigma \times \mathcal{B}$, can be computed by dividing the obtained visible cross-section by $\varepsilon$.

The estimation procedure described above is validated by comparing the results for the benchmark signal scenario decaying via $X\rightarrow aa\rightarrow 4\gamma$ with the results presented in the paper (i.e. Table 3). It is found that the two results agree within 20%, and the result with the estimation procedure described above gives lower values. The main difference is found for large values of the mass ratio, $0.005