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Measurement of Tau hadronic branching ratios in DELPHI experiment at LEP

Dima Dedovich (Dubna)

DELPHI Collaboration

1. Final results on exclusive hadronic branchings (π/K blind) – submitted to E.Phys.J. C E.Phys.J. C

2.2. Preliminary results on inclusive single-prong Preliminary results on inclusive single-prong branching to charged kaonsbranching to charged kaons

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The DELPHI detector

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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

• “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 branchingscharged hadron selection

• 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

Electron rejection

Muon rejection

DELPHI DELPHI

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Exclusive hadronic branchings π0 counting

• 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 and

single photons• Efficiency to reconstruct π0 was about 70% with

purity of about 90%

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Exclusive hadronic branchings π0 invariant mass

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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 branchingsinvariant masses of hadronic systems

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Exclusive hadronic branchingsneural network outputs

e μ h

hπ0 h2π0 h3π0

3h 3hπ0 3h2π0

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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 branchingsRESULTS

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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±Xν were 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 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 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 identification

ππ

K

K

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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

• 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 kaonsClean sample of pions (kaons suppressed by RICH)

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Inclusive branching to kaonsKaon-enriched sample

dE/dx kaon pull

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Inclusive branching to kaonsAll hadronic tau decay candidates

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

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Inclusive branching to kaonsKaon-enriched sample

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Inclusive branching to kaonsAll hadronic tau decay candidates

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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 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 kaonsfit to dE/dx probability

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Inclusive branching to kaonsfit to Ring probability

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Inclusive branching to kaonscombined fit : Ring+dE/dx

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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 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 kaonsSystematic errors in %

RING pull dE/dx pull VETO

Posi-tion

width Moment. depend.

Posi-tion

width Moment. depend.

K/π MIPagreem.

FalseID

dE/dx 3.3 5.3 2.0 2.4

Ring 4.6 3.1 4.2

veto 8.4

dE/dx+VETO 0.3 0.7 0.6 0.6 2.4

dE/dx+Ring 1.5 0.7 1.1 0.7 1.3 0.1 1.0

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

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Inclusive branching to kaonsThe results (in %)

χ2 = 3.26/3χ2 = 1.99/2

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Inclusive branching to kaons Results of Individual measurements in %

Ring 1.745 ± 0.170 (0.126 stat 0.115 syst)

dE/dx 1.455 ± 0.131 (0.068 stat 0.105 syst)

VETO 1.685 ± 0.272 (0.231 stat 0.144 syst)

Total 1.579 ± 0.097

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Inclusive branching to kaons Results of combined measurements in %

Ring+dE/dx 1.639 ± 0.112 (0.097 stat 0.054 syst)

VETO+dE/dx 1.594 ± 0.184 (0.172 stat 0.066 syst)

dE/dx only 1.346 ± 0.139 (0.082 stat 0.106 syst)

Ring only 1.871 ± 0.489 (0.462 stat 0.165 syst)

Total 1.545 ± 0.078

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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%