Measurement of tau hadronic branching ratios in delphi experiment at lep
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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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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

  • Preliminary results on inclusive single-prong branching to charged kaons

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The delphi detector

The DELPHI detector

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The first stage common for both studies the tau pair selection

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

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Exclusive hadronic branchings track counting

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

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Exclusive hadronic branchings charged hadron selection

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

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Exclusive hadronic branchings 0 counting

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%

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Exclusive hadronic branchings 0 invariant mass

Exclusive hadronic branchingsπ0invariant mass

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Exclusive hadronic branchings decay mode identification

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

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Exclusive hadronic branchings invariant masses of hadronic systems

Exclusive hadronic branchingsinvariant masses of hadronic systems

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Exclusive hadronic branchings neural network outputs

Exclusive hadronic branchingsneural network outputs

h

μ

e

h3π0

hπ0

h2π0

3h2π0

3h

3hπ0

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Exclusive hadronic branchings calibration and systematic errors

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

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Exclusive hadronic branchings results

Exclusive hadronic branchingsRESULTS

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Inclusive branching to kaons

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

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Inclusive branching to kaons hadronic sample selection

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

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Inclusive branching to kaons kaon identification

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)

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Inclusive branching to kaons kaon identification1

Inclusive branching to kaonsKaon identification

π

π

K

K

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Inclusive branching to kaons pull variables

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

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Inclusive branching to kaons de dx calibration

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

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Inclusive branching to kaons clean sample of pions kaons suppressed by rich

Inclusive branching to kaonsClean sample of pions (kaons suppressed by RICH)

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Inclusive branching to kaons kaon enriched sample

Inclusive branching to kaonsKaon-enriched sample

dE/dx kaon pull

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Inclusive branching to kaons all hadronic tau decay candidates

Inclusive branching to kaonsAll hadronic tau decay candidates

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Inclusive branching to kaons ring pull calibration

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

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Inclusive branching to kaons clean sample of pions kaons suppressed by de dx

Inclusive branching to kaonsClean sample of pions (kaons suppressed by dE/dx)

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Inclusive branching to kaons kaon enriched sample1

Inclusive branching to kaonsKaon-enriched sample

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Inclusive branching to kaons all hadronic tau decay candidates1

Inclusive branching to kaonsAll hadronic tau decay candidates

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Inclusive branching to kaons veto identification

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

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Inclusive branching to kaons the fit procedure

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

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Inclusive branching to kaons fit to de dx probability

Inclusive branching to kaonsfit to dE/dx probability

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Inclusive branching to kaons fit to ring probability

Inclusive branching to kaonsfit to Ring probability

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Inclusive branching to kaons combined fit ring de dx

Inclusive branching to kaonscombined fit : Ring+dE/dx

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Inclusive branching to kaons systematic errors

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

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Inclusive branching to kaons systematic errors1

Inclusive branching to kaonsSystematic errors

The uncertainty of residual pion background (colored)

Is strongly redused if pions were already suppresed

by another detector

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Inclusive branching to kaons systematic errors in

Inclusive branching to kaonsSystematic errors in %

Other sources of systematic errors are MC statistics (1.2%) and tau decay branchings (1.9%)

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Inclusive branching to kaons the results in

Inclusive branching to kaonsThe results (in %)

χ2 = 3.26/3

χ2 = 1.99/2

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Inclusive branching to kaons results of individual measurements in

Inclusive branching to kaonsResults of Individual measurements in %

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Inclusive branching to kaons results of combined measurements in

Inclusive branching to kaonsResults of combined measurements in %

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Summary

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%

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