Abstract
Reconstructed photons in ATLAS are seeded by clusters of energy in the EM calorimeter. They are distinguished from electrons, which are seeded by the same clusters, by the absence of a track whose trajectory is consistent with the candidate cluster. An example of such a cluster, without an associated track, is shown in Fig. 4.1a. A special case occurs when the photon pair-converts into an electron–positron system before reaching the EM calorimeter. In such cases, a separate reconstruction chain attempts to recover those photons from reconstructed electrons. An example of such a photon candidate is shown in Fig. 4.1b. After reconstruction, a series of selection criteria are used to separate single photons from backgrounds (primarily from light mesons decaying to multiple photons). This chapter will review the reconstruction of both converted and unconverted photons, and the identification criteria applied at the analysis level.
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Notes
- 1.
The current upper limit on the photon mass is \(1 \times 10^{-18}\) eV [2].
- 2.
In almost all cases, the loose cuts offline are identical to the loose trigger cuts. However, the photon energy used in the computation of (for example) \(R_\text{had}\) is not necessarily the same after full reconstruction as it is for the trigger.
- 3.
There do exist isolation variables calculated after noise-suppression; such variables have narrower widths, but show little improvement in signal/background discrimination over non-noise-suppressed variables.
- 4.
An updated set of corrections, implemented after the inclusive analyses presented in this document, make several improvements on the results described here. The primary changes are that separate corrections are defined for converted and unconverted photons, and that the single-particle simulation data used to derive the corrections are updated with an improved geometrical detector description. The differences between the new and old corrections are small (at the percent level) and have little effect for objects below \(E_{\text{T}}\approx 100\) GeV.
- 5.
The definition of the noise width is primarily driven by noise in the electronics. At high luminosities, however, the nominal noise width can be increased to take in-time pileup into account, effectively raising the threshold for TopoCluster creation.
- 6.
This definition is motivated by the limitations of the JETPHOX program, which does not model the underlying event.
- 7.
This is certainly true in fixed-order Monte Carlo programs like PYTHIA and HERWIG.
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Hance, M. (2013). Reconstruction and Identification of Prompt Photons . In: Photon Physics at the LHC. Springer Theses. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-642-33062-9_4
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