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A search for pairs of highly collimated photon-jets in $pp$ collisions at $\sqrt{s}$ = 13 TeV with the ATLAS detector

The collaboration
Phys.Rev., 2018

Abstract (data abstract)
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 < $m_X$ < 2TeV and $m_a$ < 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: - $E_\mathrm{T,1}>0.4\times m_X$ - $E_\mathrm{T,2}>0.3\times m_X$ - $|\eta_i| < 2.37 (i=1,2)$ (excluding $1.37 < |\eta_i| <1.52$) 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, $0.005<m_a/m_X$. This is because, for larger values of $m_a/m_X$, the width of the distribution of the reconstructed diphoton mass $m_{\gamma_{R}\gamma_{R}}$ increases. This is caused by the wider angular separation between the photons inside a photon-jet for larger $m_a/m_X$, leading to a greater part of the energy of the shower leaking out of the window defined in the cells of the electromagnetic calorimeter to collect energy for the photon reconstruction.

• #### Table 1

Data from Figure 6a

10.17182/hepdata.85728.v1/t1

Distribution of the reconstructed diphoton mass for data events passing the analysis selection, in the low-$\Delta E$ category. There are...

• #### Table 2

Data from Figure 6b

10.17182/hepdata.85728.v1/t2

Distribution of the reconstructed diphoton mass for data events passing the analysis selection, in the high-$\Delta E$ category. There are...

• #### Table 3

Data from Figure 8

10.17182/hepdata.85728.v1/t3

The observed upper limits on the production cross-section times the product of branching ratios for the benchmark signal scenario involving...

• #### Table 4

Data from Figure 8

10.17182/hepdata.85728.v1/t4

The expected upper limits on the production cross-section times the product of branching ratios for the benchmark signal scenario involving...

• #### Table 5

Data from Figure 10

10.17182/hepdata.85728.v1/t5

The observed upper limits on the production cross-section times the product of branching ratios for the benchmark signal scenario involving...

• #### Table 6

Data from Figure 10

10.17182/hepdata.85728.v1/t6

The expected upper limits on the production cross-section times the product of branching ratios for the benchmark signal scenario involving...

• #### Table 7

Data from Figure 1 of auxiliary material

10.17182/hepdata.85728.v1/t7

Observed 95% CL upper limits on the visible cross section as a function of $m_X$ and the fraction of events...

• #### Table 8

Data from Figure 8a of auxiliary material

10.17182/hepdata.85728.v1/t8

Selection efficiency for reconstructed photons originating from the decay $a\rightarrow 2\gamma$ with $m_a$ = 0.1 GeV.

• #### Table 9

Data from Figure 8b of auxiliary material

10.17182/hepdata.85728.v1/t9

Selection efficiency for reconstructed photons originating from the decay $a\rightarrow 2\gamma$ with $m_a$ = 0.5 GeV.

• #### Table 10

Data from Figure 8c of auxiliary material

10.17182/hepdata.85728.v1/t10

Selection efficiency for reconstructed photons originating from the decay $a\rightarrow 2\gamma$ with $m_a$ = 0.7 GeV.

• #### Table 11

Data from Figure 8d of auxiliary material

10.17182/hepdata.85728.v1/t11

Selection efficiency for reconstructed photons originating from the decay $a\rightarrow 2\gamma$ with $m_a$ = 1 GeV.

• #### Table 12

Data from Figure 8e of auxiliary material

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Selection efficiency for reconstructed photons originating from the decay $a\rightarrow 2\gamma$ with $m_a$ = 2 GeV.

• #### Table 13

Data from Figure 8f of auxiliary material

10.17182/hepdata.85728.v1/t13

Selection efficiency for reconstructed photons originating from the decay $a\rightarrow 2\gamma$ with $m_a$ = 5 GeV.

• #### Table 14

Data from Figure 8g of auxiliary material

10.17182/hepdata.85728.v1/t14

Selection efficiency for reconstructed photons originating from the decay $a\rightarrow 2\gamma$ with $m_a$ = 10 GeV.

• #### Table 15

Data from Figure 9a of auxiliary material

10.17182/hepdata.85728.v1/t15

Selection efficiency for reconstructed photons originating from the decay $a\rightarrow 3\pi^0\rightarrow 6\gamma$ with $m_a$ = 0.5 GeV.

• #### Table 16

Data from Figure 9b of auxiliary material

10.17182/hepdata.85728.v1/t16

Selection efficiency for reconstructed photons originating from the decay $a\rightarrow 3\pi^0\rightarrow 6\gamma$ with $m_a$ = 0.7 GeV.

• #### Table 17

Data from Figure 9c of auxiliary material

10.17182/hepdata.85728.v1/t17

Selection efficiency for reconstructed photons originating from the decay $a\rightarrow 3\pi^0\rightarrow 6\gamma$ with $m_a$ = 1 GeV.

• #### Table 18

Data from Figure 9d of auxiliary material

10.17182/hepdata.85728.v1/t18

Selection efficiency for reconstructed photons originating from the decay $a\rightarrow 3\pi^0\rightarrow 6\gamma$ with $m_a$ = 2 GeV.

• #### Table 19

Data from Figure 9e of auxiliary material

10.17182/hepdata.85728.v1/t19

Selection efficiency for reconstructed photons originating from the decay $a\rightarrow 3\pi^0\rightarrow 6\gamma$ with $m_a$ = 5 GeV.

• #### Table 20

Data from Figure 9f of auxiliary material

10.17182/hepdata.85728.v1/t20

Selection efficiency for reconstructed photons originating from the decay $a\rightarrow 3\pi^0\rightarrow 6\gamma$ with $m_a$ = 10 GeV.

• #### Table 21

Data from Figure 10a of auxiliary material

10.17182/hepdata.85728.v1/t21

Fraction of reconstructed photons with a value of shower shape variable $\Delta E$ lower than the threshold, for reconstructed photons...

• #### Table 22

Data from Figure 10b of auxiliary material

10.17182/hepdata.85728.v1/t22

Fraction of reconstructed photons with a value of shower shape variable $\Delta E$ lower than the threshold, for reconstructed photons...

• #### Table 23

Data from Figure 10c of auxiliary material

10.17182/hepdata.85728.v1/t23

Fraction of reconstructed photons with a value of shower shape variable $\Delta E$ lower than the threshold, for reconstructed photons...

• #### Table 24

Data from Figure 10d of auxiliary material

10.17182/hepdata.85728.v1/t24

Fraction of reconstructed photons with a value of shower shape variable $\Delta E$ lower than the threshold, for reconstructed photons...

• #### Table 25

Data from Figure 10e of auxiliary material

10.17182/hepdata.85728.v1/t25

Fraction of reconstructed photons with a value of shower shape variable $\Delta E$ lower than the threshold, for reconstructed photons...

• #### Table 26

Data from Figure 10f of auxiliary material

10.17182/hepdata.85728.v1/t26

Fraction of reconstructed photons with a value of shower shape variable $\Delta E$ lower than the threshold, for reconstructed photons...

• #### Table 27

Data from Figure 10g of auxiliary material

10.17182/hepdata.85728.v1/t27

Fraction of reconstructed photons with a value of shower shape variable $\Delta E$ lower than the threshold, for reconstructed photons...

• #### Table 28

Data from Figure 11a of auxiliary material

10.17182/hepdata.85728.v1/t28

Fraction of reconstructed photons with a value of shower shape variable $\Delta E$ lower than the threshold, for reconstructed photons...

• #### Table 29

Data from Figure 11b of auxiliary material

10.17182/hepdata.85728.v1/t29

Fraction of reconstructed photons with a value of shower shape variable $\Delta E$ lower than the threshold, for reconstructed photons...

• #### Table 30

Data from Figure 11c of auxiliary material

10.17182/hepdata.85728.v1/t30

Fraction of reconstructed photons with a value of shower shape variable $\Delta E$ lower than the threshold, for reconstructed photons...

• #### Table 31

Data from Figure 11d of auxiliary material

10.17182/hepdata.85728.v1/t31

Fraction of reconstructed photons with a value of shower shape variable $\Delta E$ lower than the threshold, for reconstructed photons...

• #### Table 32

Data from Figure 11e of auxiliary material

10.17182/hepdata.85728.v1/t32

Fraction of reconstructed photons with a value of shower shape variable $\Delta E$ lower than the threshold, for reconstructed photons...

• #### Table 33

Data from Figure 11f of auxiliary material

10.17182/hepdata.85728.v1/t33

Fraction of reconstructed photons with a value of shower shape variable $\Delta E$ lower than the threshold, for reconstructed photons...

• #### Table 34

Data from Figure 12a of auxiliary material

10.17182/hepdata.85728.v1/t34

Selection efficiency for photons originating from the BSM process $X\rightarrow\gamma\gamma$, where the $X$ particle is a high-mass narrow-width scalar particle...

• #### Table 35

Data from Figure 12b of auxiliary material

10.17182/hepdata.85728.v1/t35

Fraction of photons with a value of shower shape variable $\Delta E$ lower than the threshold, for photons originating from...