Figure 5.16: OS−SS distribution as in Fig-ure 5.14, but here including event selec-tion cuts and randomly selecting events corresponding to an integrated luminos-ity of 5 fb−1, i.e. uncertainties and fluc-tuations correspond to those expected for 5 fb−1at√
s= 7 TeV. The star marks the
value of the chosen observable. ml l τ [GeV]
0 50 100 150 200 250
-1events/5fb
-5 0 5 10 15
The statistical uncertainties in Figure 5.15b were estimated in the following way.
Out of 10 000 simulated signal events, events have been randomly chosen to get a sub-sample corresponding to an integrated luminosity of 5 fb−1. This procedure was repeated 100 times for each point to obtain different samples. The observables of these sub-samples follow a Gaussian distribution, where its width corresponds to the statistical uncertainties in Figure 5.15b. Figure 5.14 shows an example of one sub-sample for the BC 1 scenario.
From the above investigations one can conclude that rough estimates of the stau LSP mass in BC 1-like scenarios should be possible with an integrated luminosity of 5 fb−1 at√
s = 7 TeV. An interpretation of candidate events within the assumed model will be feasible with a mass resolution of about 20 GeV depending on the actual eτ mass. More detailed studies of the systematic effects on the mass determination have to be done once a BSM signal has been established and their specific event topologies are known.
Not only the benchmark scenario itself shows a high discovery potential at the LHC, but also the mSUGRA parameter space around this specific point can be covered. With 1 fb−1 a 5σ discovery should be possible in the mass range up to about M1/2 .500 GeV over the theoretically allowed range of 5 . tan(β) . 30. The determination of the eτ1 mass, once a signal has been established, is not straight forward due to the many neutrinos in the final states for the assumed R/p coupling and due to the combinatorial background within the signal events. Anyhow it was shown that estimates of theτe1mass should be possible withR
Ldt≈5 fb−1, where different methods for the mass estimation were compared.
Conclusions and Outlook
The guiding topic of this thesis was the search for supersymmetric extensions of the Standard Model including R-parity violation with the ATLAS experiment. SUSY is very well studied in many models in theory and in experiment, because it is theoretically appealing and may lead to a grand unified theory of particle physics. However,R-parity is usually taken to be conserved giving signatures with a stable, undetectable lightest supersymmetric particle in collision experiments. Even thoughR-parity violating (R/p) models are equally well motivated, they were mostly ignored in previous studies by the ATLAS collaboration.
The present study concentrated on SUSY models in the mSUGRA framework and more precisely on models where the scalar partner of the tau lepton, the stau τe, is the lightest supersymmetric particle (LSP). Such models are empirically excluded in R-parity conserving models, but arise naturally when assuming lepton number violating
/
Rp. It was shown that multi-lepton signatures as they arise for many R/p couplings can be discovered with the ATLAS experiment already in the first years of operation. A full Monte Carlo investigation of one benchmark scenario (BC 1) was used to derive a set of cuts to separate the R/p signal events from the Standard Model background. The integrated luminosity of R
Ldt = 500 pb−1, which is available by summer 2011, would lead to a nearly background free sample of about 30 BC 1 events, corresponding to a significance of aboutZ0 ≈12, taking systematic uncertainties of the background estimate into account. A scan of the parameter space around the benchmark scenario allowed to derive estimates for the expected luminosity which is needed for a 5σ discovery.
Assuming 1 fb−1 a discovery should be possible in the parameter range up to about M1/2 .500 GeV.
After the discovery of a potential R/pSUSY signal one wants to measure the properties of the new phenomenon. The mass of the LSP will be one of the first parameters to be determined. It was shown that an estimate of theeτ1 mass should be possible with a few fb−1. However, the mass determination suffers from a large combinatorial background and the many neutrinos in the final state for the assumed class of models. A method was presented to derive the eτ1 mass from its visible decay products including a scheme to select the best matching combination of reconstructed particles.
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With this study the awareness for R-parity violating SUSY within the ATLAS col-laboration grew. The grid of parameter points prepared in Section 5.2 will be used as an official reference grid to be studied with the data, which will be collected in 2011 by the ATLAS experiment.
Important for the R/p SUSY studies, but also of more general interest, is the recon-struction of tau leptons and the fast simulation of detector effects. Both topics were addressed in this thesis as well. The fast track simulation FATRAS provides detailed Monte Carlo estimates of measurements in the ATLAS tracking system, while being about a factor of 50 faster than the fully fledged simulation of the Inner Detector. Still a good agreement with first collision data of the ATLAS experiment was achieved. FATRAS is of increasing importance for the ATLAS collaboration and as part of ATLFAST-IIF applied for many studies. It also played an important role in studies of upgrades for the Inner Detector. Generic detector simulations were compared to the simulation of the ATLAS detector and it was shown that the Delphes program yields reasonably good results for electrons, muons and jets in the SUSY signal events. Tau leptons, however, are badly modeled in this simple approach.
The reliable identification of (hadronically decayed) tau leptons relies strongly on the reconstruction of tracks from charged pions. Track selection criteria were developed to obtain the best results in the classification of tau leptons according to their decay mode. The studies showed that inefficiencies in the track reconstruction are mostly due to hadronic interactions in the Inner Detector. Systematic uncertainties due to Inner Detector misalignments on the other hand can be neglected.
Finally a new approach to the reconstruction of tau leptons in ATLAS was intro-duced, fully based on results of the energy flow algorithm eflowRec. This algorithm, named PanTau, has a high potential in the identification of individual tau decay modes, which is an important ingredient for certain SUSY studies. Relying on energy flow re-sults has conceptional advantages in the tau reconstruction. Furthermore the individual treatment of the different tau decay modes improves the identification performance. The dependency of the identification variables on the tau energy was studied in detail and methods were applied to reduce it. However, no general recommendation can be given, whether the reduced dependency improves the overall performance as this strongly de-pends on the application of the tau ID and the energy spectrum of the tau candidates under investigation.
The performance of eflowRecandPanTauon QCD jets in the first data of the ATLAS experiment at √
s= 7 TeV was found to be well modeled in the Monte Carlo prediction.
Effects from pile-up were found in some ID variables. The tuning of PanTauparameters and the selection of variables will be slightly modified to minimise pile-up effects. At the time of writing efforts are ongoing by the ATLAS tau performance group to incorporate the PanTau approach into the common tau reconstruction of the ATLAS experiment, thus making it available for all analyses. PanTau will be an important contribution to the aim of including more information about the sub-structure of tau jets and the identification of individual charged and neutral pions in the jets.
Monte Carlo samples and software versions
The Monte Carlo samples used in this thesis in general follow theMC09simulation of the ATLAS experiment. Base line release of the ATLAS software for event reconstruction is athena 15.6.9.8. All Monte Carlo samples have been simulated with a centre-of-mass energy of √
s = 7 TeV, despite the comparisons to first collision data at √
s = 900 GeV (Section 3.1.2) and the studies of misalignment (Section 4.2.2). In addition to theMC09 tunes [130] of the event generators the Pythia DW tune [131] was used for some of the tau studies.