File:Thesis aasvold.pdf

Neutrinos escaping detection is one of the main problems in mass reconstruction with tau leptons. They must be neglected or corrected for in some way. This thesis discusses two methods of handling the neutrinos in different ways. The Collinear Approximation (CA) builds upon the assumption that the neutrinos travel in the same direction as the visible tau decay products. The boost method neglects the neutrino energy contribution in the leading visible tau, as seen from the mother particle's reference system. The methods have been studied with simulated \Z°→τ+τ- , \H°→τ+τ-, and QCD samples, and with early data from ATLAS. This work shows that the weaknesses of CA is that the transverse angle of the transverse missing energy has to lie between the transverse angle of the two visible taus, and that it collapses with back-to-back taus in the transverse plane. A strength of the CA is that it uses the missing energy, which is all information available about the neutrinos. The CA works better for boosted taus, i.e. taus decaying from heavy particles, like H° and Z°. The weaknesses of the boost method are that it does not use the transverse missing energy information, and that the distribution is not easily fitted, but the method is still under development on these points. The strength of the boost method is that it works for all tau pairs, making it a good complimentary method to the CA. Both methods work in \Z°→τ+τ- and \H°→τ+τ- events, and can be potentially applied to other decay chains as well. In the future, many studies will include mass reconstruction from tau leptons, where both the CA and the boost method will be important methods.