Measurement of σ(p$\bar{p}$→Z) Br(Z→τ<sup>+</sup>τ<sup>-</sup>) and search for Higgs bosons decaying to τ<sup>+</sup>τ<sup>-</sup> at √s= 1.96 TeV

Abstract

The resonant production of tau-lepton pairs is as interesting for the study of Standard Model (SM) physics as the production of lighter leptons pairs. For new phenomena, such as Higgs boson production or in case new particles beyond the SM would arise, the detection of (resonant) pairs of tau leptons becomes much more interesting. This is due to the fact that tau leptons are much heavier than the other leptons, which increases the chance that these new phenomena would be observed first in this channel. Unfortunately their clean detection is far more difficult than that of muons or electrons. The cross section times branching ratio σ. Br for the process p$\bar{p}$ → Z → τ+τ- was measured at √s = 1.96 GeV using 1.0 fb-1 of data collected by the D0 experiment. This measurement was performed in the channel in which one of the tau leptons decays to a muon and neutrinos, while the other decays either hadronically or to an electron and neutrinos. A set of 1511 events, of which about 20% estimated background, passed all selection criteria. The trigger and muon reconstruction efficiencies, as well as the efficiency for track reconstruction were obtained from data using the 'tag and probe' method on Z → μ+μ- events. The multijet background was estimated from the sample of events which passed all selection criteria but in which the muon and the tau candidate had the same charge. The W → μv + jets background was modeled by Monte Carlo simulations, but normalized to data. All the other backgrounds, as well as the efficiency for Z → τ+τ- events were estimated using simulated events normalized to the theoretical calculations of cross sections at next-to-leading order or next-to-next-to-leading order. The energy of the tau candidates was corrected for the estimated response of the charged pions in the calorimeter, which is of the order 50-80%. Since the charged pion response in data was not well reproduced by the default simulation of hadronic interactions (Geisha), a different simulation (gCALOR) was used to obtain an estimated charged pion response consistent with the one measured in data. This tau energy correction method makes use of the superior resolution of the track momentum measurement compared to the resolution of the tau candidate energy as measured by the calorimeter, which leads to a better data--simulation agreement and a decrease of 10% in the resolution of the visible mass peak. The result of this measurement is σ(p$\bar{p}$ → Z) • Br(Z → τ+τ-) = 240 ± 8(stat) ± 12(syst) ± 15(lumi) pb, in good agreement with the theoretical predictions of 241.6$+3.6\atop{3.2}$ pb [79] or 251.9$+5.1\atop{-12}$ pb [93-95], as well as with other measurements performed by the D0 and CDF experiments in all channels in which the Z boson decays leptonically [96-100]. This is the most precise Z boson cross section measurement to date performed in the tau lepton channel at hadron colliders. The analysis demonstrates the ability of the D0 experiment to identify tau leptons decaying hadronically with good efficiency and high purity, a challenging task in p{bar p} collisions where the number of jets resembling tau leptons is very high. This achievement forms a solid basis for other analyses using hadronic tau lepton decays, such as the search for the Higgs boson decaying into tau-lepton pairs, which was performed for the last part of this thesis.

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