Photons and photon identification - Physics object definition

3.5 Physics object definition

3.5.4 Photons and photon identification

Photons (γ) are reconstructed with the same method as done for electrons. Energy deposits in the ECAL are clustered using the sliding-window algorithm. Photons are required to have a minimum transverse energy of E_T(γ) > 20 GeV. All photon candidates are required to have |η_cl| < 2.37, excluding the calorimeter crack-region⁵. For both electrons and photons, corrections to the energy scale in data and to the energy resolution in MC (energy smearing) are applied.

The photon identification is based on a set of rectangular cuts on the shower-shape of calorime-ter variables. The selection cuts do not depend on the photon transverse energy but vary as a function of η to account for variations associated with the total thickness of material in front of

5In addition a so-called “photon cleaning” is performed. Liquid Argon (LAr) cells with noise bursts and dead-regions are discarded. A timing cut is imposed, in order to reject out-of-time pile-up candidates.

the EM calorimeter. Two sets of cuts, loose and tight, are defined [110]. In addition to tighter cuts on theloose shower-shape variables, the tightmenu adds additional discriminating variables and it is optimised for unconverted and converted photon candidates separately. The different discriminant variables used in the photon identification are detailed below, while the cut values used in thetightmenu are given in table 3.4.

Range in pseudorapidity

Variable Cut [0,0.6[ [0.6,0.8[ [0.8,1.15[ [1.15,1.37[ [1.52,1.81[ [1.81,2.01[ [2.01,2.37]

Unconverted photon candidates

R_had max 0.009 0.007 0.006 0.008 0.019 0.015 0.014

Rη min 0.951 0.940 0.942 0.946 0.932 0.928 0.924

R_ϕ min 0.954 0.95 0.59 0.82 0.93 0.947 0.935

wη₂ max 0.011 0.011 0.011 0.011 0.011 0.011 0.013

w_s_tot max 2.95 4.4 3.26 3.4 3.8 2.4 1.64

ws₃ max 0.66 0.69 0.697 0.81 0.73 0.651 0.610

F_side max 0.284 0.36 0.36 0.514 0.67 0.211 0.181

∆E max 92 92 99 111 92 110 148

Eratio max 0.63 0.84 0.823 0.887 0.88 0.71 0.78

Converted photon candidates

Rhad max 0.008 0.007 0.005 0.008 0.015 0.016 0.011

R_η min 0.941 0.927 0.930 0.931 0.918 0.924 0.913

Rϕ min 0.4 0.426 0.493 0.437 0.535 0.479 0.692

w_η₂ max 0.012 0.011 0.013 0.013 0.014 0.012 0.013

ws_tot max 2.8 2.95 2.89 3.14 3.7 2.0 1.48

w_s₃ max 0.697 0.709 0.749 0.78 0.773 0.672 0.644

Fside max 0.32 0.428 0.483 0.51 0.508 0.252 0.215

∆E max 200 200 122 86 123 80 132

Eratio max 0.908 0.911 0.808 0.803 0.67 0.915 0.962

Table 3.4: Tightidentification cuts for unconverted and converted photon candidates [110]. Upper and lower cuts are referred as “max” and “min” respectively. ∆E is given in MeV. See text for a description of the different variables.

Hadronic leakage variable

The hadronic leakage (R_had) is the total transverse energy deposited in the hadronic calorime-ter normalised to the total transverse energy of the photon candidate. In the pseudorapidity range 0.8<|η(γ)|<1.37 the energy deposited in the whole hadronic calorimeter is used, while for |η(γ)|<0.8 and|η(γ)|>1.37 only the leakage in the first layer of the hadronic calorimeter is used.

EM second (middle) layer variables

• Rη(γ) (middle η energy ratio): ratio in η(γ)of cell energies in a3×7rectangle in η×ϕ (measured in cell units) versus the sum of energies in a7×7rectangle, centred around the cluster seed, see Fig. 3.7.

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Figure 3.7: The R_η(γ) distribution comparisons for tight photons (continuous line) with respect to loose photons (dotted line). The distributions are obtained from data and are normalised to their area. The left (right) plot shows distribution for the electron channel (muon) channel.

• R_ϕ(γ) (middle ϕ energy ratio): ratio in ϕ of cell energies in a 3×3 rectangle in η×ϕ (measured in cell units) versus the sum of energies in a3×7rectangle, centred around the cluster seed, see Fig. 3.8.

Rφ

Figure 3.8: TheR_ϕ(γ)distribution comparisons fortightphotons (continuous line) with respect to loosephotons (dotted line).The distributions are obtained from data and are normalised to their area. The left (right) plot shows distribution for the electron channel (muon) channel.

• wη2(γ) (middle lateral width): lateral width of the shower in the second layer of the EM calorimeter, using cells in a windowη×ϕ= 3×5 (measured in cell units), see Fig. 3.9.

η,2

0 0.005 0.01 0.015 0.02 0.025

)/0.02γ(N

0 0.005 0.01 0.015 0.02 0.025

)/0.02γ(N

Figure 3.9: Thewη2(γ) distribution comparisons for tight photons (continuous line) with respect toloosephotons (dotted line).The distributions are obtained from data and are normalised to their area. The left (right) plot shows distribution for the electron channel (muon) channel.

EM first (strip) layer variables

• w_s_tot(γ) (total lateral width): shower width inη in the first layer of the EM calorimeter using cells in a window ∆η×∆ϕ = 0.0625×0.2 (corresponding approximately to 20×2 strip cells inη×ϕ), see Fig. 3.10.

Figure 3.10: Thewstot(γ)distribution comparisons fortightphotons (continuous line) with respect toloosephotons (dotted line).The distributions are obtained from data and are normalised to their area. The left (right) plot shows distribution for the electron channel (muon) channel.

• w_s₃(γ) (front lateral width): shower width in η in the first layer of the EM calorimeter using three strip cells around the maximal energy deposit, see Fig. 3.11.

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Figure 3.11: Thew_s₃ distribution comparisons for tightphotons (continuous line) with respect to loosephotons (dotted line).The distributions are obtained from data and are normalised to their area. The left (right) plot shows distribution for the electron channel (muon) channel.

• F_side(γ) (front side energy ratio): lateral containment of the shower alongη. It is mea-sured as the fraction of energy outside a core of three central strips but within seven strips, see Fig. 3.12.

Figure 3.12: TheFside distribution comparisons fortightphotons (continuous line) with respect to loosephotons (dotted line).The distributions are obtained from data and are normalised to their area. The left (right) plot shows distribution for the electron channel (muon) channel.

• ∆E(γ) (front second maximum difference): difference between the energy associated with the second maximum in the strip layer, and the energy reconstructed in the strip with the minimal value found between the first and second maxima (∆E = 1 when there is no second maximum).

• Eratio(γ)(front maxima relative ratio): ratio of the energy difference associated with the largest and second largest strip cell energy deposits over the sum of these energies (E_ratio= 1 when there is no second maximum).

Appendix A contains plots illustrating the discrimination power of the shower shapes on data for converted and unconverted photons separately.

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Figure 11: Comparison of the weighted mean of the data-driven measurements of converted"_ID to the nominal and corrected MC predictions in the region 15 GeV<ET <300 GeV. The"_IDcurves are shown in four di↵erent⌘regions. The green uncertainty band corresponds to the addition in quadrature of the statistical and systematic uncertainties estimated for the combination of the data-driven methods. Only the statistical uncertainties are shown for the MC predictions. The bottom figures show the di↵erence between the data-driven curve and nominal and corrected MC predictions.

Figure 12: Comparison of weighted mean of the data-driven measurements of unconverted "_ID to the nominal and corrected MC predictions in the region 15 GeV< E_T <300 GeV. The"_IDcurves are shown in four di↵erent⌘regions. The green uncertainty band corresponds to the addition in quadrature of the statistical and systematic uncertainties estimated for the combination of the data-driven methods. Only the statistical uncertainties are shown for the MC predictions. The bottom figures show the di↵erence between the data-driven curve and nominal and corrected MC predictions.

Figure 12: Comparison of weighted mean of the data-driven measurements of unconverted"_ID to the nominal and corrected MC predictions in the region 15 GeV<ET<300 GeV. The"_IDcurves are shown in four di↵erent⌘regions. The green uncertainty band corresponds to the addition in quadrature of the statistical and systematic uncertainties estimated for the combination of the data-driven methods. Only the statistical uncertainties are shown for the MC predictions. The bottom figures show the di↵erence between the data-driven curve and nominal and corrected MC predictions.

Figure 3.13: Photon identification efficiencies for 20 GeV < E_T(γ) < 300 GeV for two differ-ent η regions (|η| < 0.6 on the left and 0.6 < |η|1.37 on the right).The plots on top (bottom) show the efficiency for converted (unconverted) photon candidates.The band corresponds to the stat⊕sys uncertainty. For each figure the bottom plot shows the differences between data and simulations [111].

The minimum photonE_T(γ)value of 20 GeV is motivated by the validity of the so-called Fudge-Factors (FF), shifting factors used to correct for the discrepancies observed between the 2011 data and simulations in the photon discriminating variables. These shifting factors are obtained from the comparison of the means of the distributions of the shower shapes for simulations and data respectively. This method has been cross-checked with the minimisation of a χ² between data and simulations. These corrections do not account for shape differences in the shower-shapes distributions between simulations and data, see Fig. 3.13. Moreover, the are assumed to be the same for photons and hadrons. However, the data-to-simulation comparisons shows an increased agreement after the correction [111, 112].

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Dans le document Enlightened Top Quark: measurements of the ttγ cross section and of its spectrum in transverse energy of the photon in the single lepton channel at √s = 7 TeV in 4.59 fb−1 of pp collision data collected with the ATLAS detector (Page 60-66)