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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

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 experiment at LEP

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The first stage common for both studies the tau pair selection
The first stage (common for both studies): experiment at LEPthe 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 branchings experiment at LEPTrack 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 branchings experiment at LEPcharged 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 branchings experiment at LEPπ0counting

  • 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 branchings experiment at LEPπ0invariant mass

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Exclusive hadronic branchings decay mode identification
Exclusive hadronic branchings experiment at LEPdecay 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 branchings experiment at LEPinvariant masses of hadronic systems

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Exclusive hadronic branchings neural network outputs
Exclusive hadronic branchings experiment at LEPneural network outputs

h

μ

e

h3π0

hπ0

h2π0

3h2π0

3h

3hπ0

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Exclusive hadronic branchings calibration and systematic errors
Exclusive hadronic branchings experiment at LEPcalibration 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 branchings experiment at LEPRESULTS

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Inclusive branching to kaons
Inclusive branching to kaons experiment at LEP

  • 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 kaons experiment at LEPhadronic 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 kaons experiment at LEPKaon 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 kaons experiment at LEPKaon identification

π

π

K

K

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Inclusive branching to kaons pull variables
Inclusive branching to kaons experiment at LEPPull 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 kaons experiment at LEPdE/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 kaons experiment at LEPClean sample of pions (kaons suppressed by RICH)

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Inclusive branching to kaons kaon enriched sample
Inclusive branching to kaons experiment at LEPKaon-enriched sample

dE/dx kaon pull

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Inclusive branching to kaons all hadronic tau decay candidates
Inclusive branching to kaons experiment at LEPAll hadronic tau decay candidates

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Inclusive branching to kaons ring pull calibration
Inclusive branching to kaons experiment at LEPRing 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 kaons experiment at LEPClean sample of pions (kaons suppressed by dE/dx)

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Inclusive branching to kaons kaon enriched sample1
Inclusive branching to kaons experiment at LEPKaon-enriched sample

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Inclusive branching to kaons all hadronic tau decay candidates1
Inclusive branching to kaons experiment at LEPAll hadronic tau decay candidates

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Inclusive branching to kaons veto identification
Inclusive branching to kaons experiment at LEPVETO 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 kaons experiment at LEPThe 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 kaons experiment at LEPfit to dE/dx probability

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Inclusive branching to kaons fit to ring probability
Inclusive branching to kaons experiment at LEPfit to Ring probability

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Inclusive branching to kaons combined fit ring de dx
Inclusive branching to kaons experiment at LEPcombined fit : Ring+dE/dx

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Inclusive branching to kaons systematic errors
Inclusive branching to kaons experiment at LEPSystematic 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 kaons experiment at LEPSystematic 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 kaons experiment at LEPSystematic 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 kaons experiment at LEPThe 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 kaons experiment at LEPResults of Individual measurements in %

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Inclusive branching to kaons results of combined measurements in
Inclusive branching to kaons experiment at LEPResults of combined measurements in %

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Summary
Summary experiment at LEP

  • 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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