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Measurement of Tau hadronic branching ratios in DELPHI experiment at LEP

Measurement of Tau hadronic branching ratios in DELPHI experiment at LEP. Dima Dedovich (Dubna) DELPHI Collaboration. Final results on exclusive hadronic branchings ( π /K blind) – submitted to E.Phys.J. C

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Measurement of Tau hadronic branching ratios in DELPHI experiment at LEP

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  1. Measurement of Tau hadronic branching ratios in DELPHI experiment at LEP Dima Dedovich (Dubna) DELPHI Collaboration • Final results on exclusive hadronic branchings (π/K blind) – submitted to E.Phys.J. C • Preliminary results on inclusive single-prong branching to charged kaons D.Dedovich Tau04

  2. The DELPHI detector D.Dedovich Tau04

  3. The first stage (common for both studies):the tau pair selection • Almost full LEP-1 statistic was used (1992-1995) • Analysis was restricted to the barrel region • Standard LEP-1 tau selection based on kinematic criteria was used: low multiplicity events with large missing energy • Selection efficiency was about 52% (85% within acceptance) with background 1.5% • In total, about 80,000 tau pairs were selected D.Dedovich Tau04

  4. Exclusive hadronic branchingsTrack counting • “Track counting” – event classification into 1- , 3- , and 5-prong tau decays. Method was the same as in the published paper on topological branchings • Charged pions from Ks decays were not counted due to requirement of Vertex Detector measurement on track • The number of selected tau decay candidates was • 134421 for 1-prong • 23847 for 3-prong • 112 for 5-prong D.Dedovich Tau04

  5. Exclusive hadronic branchingscharged hadron selection DELPHI DELPHI Electron rejection Muon rejection • 3- and 5- prong decays all are hadronic • For 1-prong the leptonic decays were rejected using: dE/dx, EM calorimeter, Hadron calorimeter and muon chambers D.Dedovich Tau04

  6. Exclusive hadronic branchingsπ0counting • 4 types of reconstructed π0 were accepted: • 2 separated photon showers • Photon shower and converted e+e- pair • Single energetic shower (overlapped photons) • Neutral shower + shower wrongly assigned to charged track • Neural networks was used to separate π0 andsingle photons • Efficiency to reconstruct π0 was about 70% with purity of about 90% D.Dedovich Tau04

  7. Exclusive hadronic branchingsπ0invariant mass D.Dedovich Tau04

  8. Exclusive hadronic branchingsdecay mode identification • 2 analyses were performed for 1- and 3-prong samples: one was based on sequential cuts and the other on neural network approach • The final results were based on the NN ( trained on simulation) which provided better precision • Only sequential cuts was used for 5-prong sample • The following semi-exclusive decay mode were identified: • 1-prong: h±ν ; h±π0 ν ; h±2π0 ν ; h±≥3π0 ν • 3-prong: 3h ±ν ; 3h± π0 ν; 3h± ≥2π0 ν • 5-prong: 5h± ν; 5h±≥1π0 ν D.Dedovich Tau04

  9. Exclusive hadronic branchingsinvariant masses of hadronic systems D.Dedovich Tau04

  10. Exclusive hadronic branchingsneural network outputs h μ e h3π0 hπ0 h2π0 3h2π0 3h 3hπ0 D.Dedovich Tau04

  11. Exclusive hadronic branchingscalibration and systematic errors • Careful checks of data/simulation agreement were performed using clean test samples selected from real data : ee→ee; ee→μμ; ee→eeγ; ee→μμγ; τ→hπ0ν • When necessary, corrections were applied on simulation • Response of calorimeters, track momentum, dE/dx , secondary interactions, track and π0 reconstruction efficiency and muon chamber response were calibrated • The uncertainties of these calibrations were the main source of systematic errors D.Dedovich Tau04

  12. Exclusive hadronic branchingsRESULTS D.Dedovich Tau04

  13. Inclusive branching to kaons • DELPHI is the only LEP experiment capable to identify kaons using not only dE/dx but also with RICH detector • So far only 1992 results on τ→K±Xνwere published. • Current preliminary results cover full LEP-1 statistics (1992-1995) and are supposed to replace the old results • Only inclusive branching ratio is being presented D.Dedovich Tau04

  14. Inclusive branching to kaonshadronic sample selection • To reduce systematic effects we actually measure the ratio Br(τ→K±Xν)/Br(τ→ π±Xν). Many biases are canceled as kaons and pions are both hadrons • As a first stage, a sample of 1-prong hadronic tau decays was selected using calorimeters and muon chambers. • The efficiency of the hadronic selection was about 89%, the background was about 0.3% from non-tau events, and 3.7% from leptonic and multiprong tau decays D.Dedovich Tau04

  15. Inclusive branching to kaonsKaon identification • At LEP1 kaons from tau decays are allowed to have momentum in the range 3.6-45 GeV/c • Measurements of dE/dx in TPC provide π/K separation in the full kinematic range at the level of 1.6-2.2 σ • For momenta below 8.5 GeV/c kaons are also identified by VETO in DELPHI RICH detector • For momenta between 8.5 and about 25 GeV/c identification is based on Cherenkov angle measurement in RICH (Ring measurements) D.Dedovich Tau04

  16. Inclusive branching to kaonsKaon identification π π K K D.Dedovich Tau04

  17. Inclusive branching to kaonsPull variables The K identification was based on pull variables ΠH for hypothesis H=π/K/e/μ For Cherenkov angle measurements a similar variables ΠRING was constructed D.Dedovich Tau04

  18. Inclusive branching to kaonsdE/dx calibration • dE/dx pull position and width were carefully calibrated as a function of particle velocity and direction using test sample of pions, muons and kaons selected from real data using RICH. • Small discrepancy was found between pions and muons of same velocity. Therefore for final calibration clean pions sample was used. • dE/dX of kaons and pions of same velocity was found in agreement, and the uncertainty of this comparison (2.4% of pull width) was assigned to systematic error D.Dedovich Tau04

  19. Inclusive branching to kaonsClean sample of pions (kaons suppressed by RICH) D.Dedovich Tau04

  20. Inclusive branching to kaonsKaon-enriched sample dE/dx kaon pull D.Dedovich Tau04

  21. Inclusive branching to kaonsAll hadronic tau decay candidates D.Dedovich Tau04

  22. Inclusive branching to kaonsRing pull calibration • Unlike the case of dE/dx, the ring pull has significant non-Gaussian tails. Therefore the following calibration procedure was adopted : • Small corrections (few % of pull width) depending on velocity were applied to simulation to get agreement with the real data (clean pion samples selected using dE/dx) • The pull distribution shapes obtained for simulation were used as probability density function in further fits • The far parts of tails were combined into 2 single bins to avoid problems with shape description D.Dedovich Tau04

  23. Inclusive branching to kaonsClean sample of pions (kaons suppressed by dE/dx) D.Dedovich Tau04

  24. Inclusive branching to kaonsKaon-enriched sample D.Dedovich Tau04

  25. Inclusive branching to kaonsAll hadronic tau decay candidates D.Dedovich Tau04

  26. Inclusive branching to kaonsVETO identification • The main source of systematic is the rate of false VETO identifications • The data/simulation agreement was checked using clean samples of muons and pions D.Dedovich Tau04

  27. Inclusive branching to kaonsThe fit procedure • The measured pulls were used to construct the probability W that the particle is a kaon: W=FK/(Fπ+FK) • Here FK(ΠK) and Fπ(Ππ) are the probability density functions for a given hypothesis • Gaussian PDF was used for dE/dx and the shapes predicted by simulation in the case of RICH • Distribution of W in real data was fitted by a linear combination of simulated pions and kaons • The results of dE/dX and RICH were fitted either separately or combined into a single probability W D.Dedovich Tau04

  28. Inclusive branching to kaonsfit to dE/dx probability D.Dedovich Tau04

  29. Inclusive branching to kaonsfit to Ring probability D.Dedovich Tau04

  30. Inclusive branching to kaonscombined fit : Ring+dE/dx D.Dedovich Tau04

  31. Inclusive branching to kaonsSystematic errors • The main source of systematic errors is the uncertainties in calibration of pull position and width. Even small bias results in large error in estimation of pion background • However this error reduced dramatically if RICH and dE/dx are used in combination • Therefore our results were obtained using combined measurement when possible (RICH was not always operational) • Individual measurements were used for a cross-check D.Dedovich Tau04

  32. Inclusive branching to kaonsSystematic errors The uncertainty of residual pion background (colored) Is strongly redused if pions were already suppresed by another detector D.Dedovich Tau04

  33. Inclusive branching to kaonsSystematic errors in % Other sources of systematic errors are MC statistics (1.2%) and tau decay branchings (1.9%) D.Dedovich Tau04

  34. Inclusive branching to kaonsThe results (in %) χ2 = 3.26/3 χ2 = 1.99/2 D.Dedovich Tau04

  35. Inclusive branching to kaonsResults of Individual measurements in % D.Dedovich Tau04

  36. Inclusive branching to kaonsResults of combined measurements in % D.Dedovich Tau04

  37. Summary • We have measured tau semi-exclusive hadronic branching ratios. Some of them are at the level of world best. • We also presented preliminary result for inclusive tau to kaons branching :1.545±0.078% D.Dedovich Tau04

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