Wing Sheung Chan

64 Object reconstruction and identification in the order: jets, τ had - vis , photon s † , electrons and muons. The soft term is the negative vector sum of the transverse momenta of all unused ID tracks that are associated to the primary vertex but not to any reconstructed physics objects. This definition of the soft term is called the track-based soft term (TST). By including soft charged particles, the TST reduces the bias and improves the resolution of the reconstructed E miss T . A more inclusive version of the soft term can be defined by also taking into account the unused clusters in the calorimeters. The advantage of such a definition is that the momenta of soft neutral particles can also be considered. But on the flip side, the inclusion of energy deposits without a clear origin significantly increases the dependence on pile-up, which is generally undesirable. Depending on the selection criteria of jets in the forward region, four working points for E miss T reconstruction are defined: Loose , Tight , Tighter and Tenacious . In principle, a tighter selection provides a better resolution, while a looser selection offers possibly smaller bias but is less robust against pile-up. The resolution of E miss T can be measured using processes with no expected E miss T , such as Z → ee . The measured resolution, as the root-mean-square (RMS) deviation and as a function of pile-up activity, is shown in Figure 3.8. 0 5 10 15 20 25 30 35 40 PV Number of primary vertices N 10 15 20 25 30 RMS Resolution [GeV] y miss ,E x miss E Preliminary ATLAS -1 = 13 TeV 36 fb s miss T Tight E ee → Z EMTopo MC EMTopo MC Syst Err EMTopo Data PFlow Data Figure 3.8.: The measured resolution (RMS deviation) of E miss T as a function of pile-up activity measured by the number of primary vertices ( N PV ) [89] . Only the measurement labelled “EMTopo” is relevant to the work presented here. 3.6. Analysis-specific definitions and overlap removal Object selection criteria that are specific to the search for LFV Z → `τ decays are listed in Table 3.2. These object definitions ensure good reconstruction qualities, while maintaining a high acceptance and efficiency for the target Z → `τ signal events. † Photons are reconstructed in a way similar to electrons, but without an associated track. Since they are nowhere used in the work presented in this thesis, details of their reconstruction will be omitted.