Proton-Proton Physics with ATLAS Detector
The LHC will generate proton-proton collisions with unprecedented energies of 14 TeV. The high energy and luminosity of the LHC offers a large range of physics opportunities, from the precise measurement of the properties of known objects to the exploration of the high energy frontier. The desire to probe the origin of the electroweak scale leads to a major focus on the Higgs boson; ATLAS will be sensitive to it over the full range of allowed masses. Other important goals are searches for phenomena possibly related to the supersymmetry theories, as well as new gauge bosons and evidence for composite quarks and leptons. The investigation of CP violation in B decays and the precision measurements of W and top-quark masses and triple gauge boson couplings are also be important components of the ATLAS physics programme.
Efficient identification of all lepton species is crucial in ATLAS. The tau leptons are the most difficult to identify but they can provide unique and important information as they are present in the large spectrum of possible experimental signatures. Within the frame of the Standard Model, the discovery in the so called intermediate mass range of the Higgs boson, presently the most promising decay model Higgs->tau tau produced in the vector boson fusion, will be rather challenging. In the MSSM, almost full coverage of the model parameters can be achieved with neutral and charged Higgs bosons decaying to tau's. Hadronically decaying tau leptons are also very important discovery signatures for several models of the Supersymmetry.
Already in the first year of the physics data taking the key element of physics
program of the ATLAS experiment will be studies of the Standard Model background channels:
Z boson -> tau tau and W boson -> tau and neutrino as a crucial step towards monitoring and calibrating
performance of the detector itself (calorimetry, tracking) and determining observability efficiencies
for those extremely important signatures.
Sensitivity to all those channels depends strongly on the quality of the tau reconstruction and identification, since backgrounds from QCD jets are potentially very large.
The ATLAS group Krakow group is strongly involved in the following activities:
Reconstruction of hadronic tau decays
The reconstruction of tau leptons is usually understood as a reconstruction of the hadronic decay modes, since it would be difficult to distinguish leptonic modes from primary electrons and muons. Hadronically decaying tau leptons are distinguished from QCD jets on the basis of the low track multiplicity in the narrow cone, characteristics of the track system and shapes of the calorimetric showers. Isolation from the rest of the event is required both in the Inner Detector and the Calorimeter. From this information, a set of identification variables is built.
The algorithms for hadronic tau reconstruction are considered a higher level reconstruction as they use components reconstructed already by algorithms specific to different subdetectors, like track reconstruction in the Inner Detector or topological clustering of the energy deposits in the calorimeter. At present, two, different seeded, algorithms have been implemented for the official ATLAS offline reconstruction software and are developed and maintained by the ATLAS Krakow group:
- The calorimetry-seed based, which starts from the cluster reconstructed in the hadronic and electromagnetic calorimeters and then builds the identification variables based on information from the tracker and calorimeter.
- The track-seed based which starts from selecting good quality tracks which provide a seed and requiring low multiplicity of such tracks in the core region of the reconstructed object. The energy of the candidate is calculated with an energy-flow algorithm.
Identification of tau leptons
Reconstructed jets originating from hadronically decaying tau leptons have to be distinguished from the background (mainly QCD multi jet events, but also electrons). The QCD multi jet events dominating the backgrounds have much larger cross section, therefore the efficient selection using multivariate analysis techniques is needed. A tau jet can be identified through the presence of a well collimated calorimeter cluster with a small number of associated charged tracks. The identification is based on a set of variables based on Inner detector and Calorimeter information.
Tau identification is based on the multivariate selection algorithms able to separate signal from the overwhelming background. Beside the base line cut analysis also learning machine algorithms have been applied to identify tau leptons: projective likelihood estimator (LL), Probability Density Estimator with Range Searches (PDE-RS), Neural Network and Boosted Decision Tree. All these methods have similar performance, which is significantly better than the baseline cut analysis. This indicates, that the achieved background rejection is close to the maximal achievable performance.
The identification algorithms are part of the TauDiscriminant package.
Early data analysis in the following
- The inclusive W -> tau neutrino process with hadronic tau decays which will lead to a few thousands of identified taus for 100 pb-1. Observability in this channel would require excellent rejection capability with respect to fake candidates from overwhelming QCD jets background. Identified tau leptons will have to be first triggered with tau trigger. The expected signal to background ratio will be of the order of one and the observability will be proven by looking at the track multiplicity spectra of the identified candidates.
- The inclusive Z -> tau tau process will provide signature at 10 times lower rates, but with more robust prospects for analysis. It will be possible to cross-check channels with electron-tau(hadrons) and muon-tau(hadrons), to control background comparing observed number of events in the same-sign and opposite-sign samples. Moreover, events will be primarily triggered with lepton trigger, just providing unbiased sample of the hadronic tau leptons which could also serve for understanding efficiencies of the hadronic tau trigger. Measurement of the visible mass of the lepton-tau had system, for analysis with low background level will give sensitivity to the energy scale of the reconstructed tau jets.