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

1. Final results on exclusive hadronic branchings
   (p/K blind) submitted to E.Phys.J.
   C
2. Preliminary results on inclusive single-prong
   branching 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

DELPHI
DELPHI

Electron rejection
Muon rejection
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Exclusive hadronic branchings p0 counting

• 4 types of reconstructed p0 were accepted
• 2 separated photon showers
• Photon shower and converted ee- pair
• Single energetic shower (overlapped photons)
• Neutral shower shower wrongly assigned to
  charged track
• Neural networks was used to separate p0 and
  single photons
• Efficiency to reconstruct p0 was about 70 with
  purity of about 90

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Exclusive hadronic branchings p0 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? h p0 ? h2p0 ? h3p0 ?
• 3-prong 3h ? 3h p0 ? 3h 2p0 ?
• 5-prong 5h ? 5h1p0 ?

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Exclusive hadronic branchingsinvariant masses of
hadronic systems
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Exclusive hadronic branchingsneural network
outputs
µ
h
e
h3p0
hp0
h2p0
3h2p0
3h
3hp0
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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?ee? ee?µµ?
  t?hp0?
• When necessary, corrections were applied on
  simulation
• Response of calorimeters, track momentum, dE/dx ,
  secondary interactions, track and p0
  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 t?KX? 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(t?KX? )/Br(t? pX? ). 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 p/K
  separation in the full kinematic range at the
  level of 1.6-2.2 s
• 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
p
p
K
K
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Inclusive branching to kaonsPull variables

• The K identification was based on pull variables
  ?H for hypothesis Hp/K/e/µ

For Cherenkov angle measurements a similar
variables ?RING 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
  WFK/(FpFK)
• Here FK(?K) and Fp(?p) 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
RingdE/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 RING pull RING pull dE/dx pull dE/dx pull dE/dx pull dE/dx pull VETO
Posi-tion width Moment. depend. Posi-tion width Moment. depend. K/p MIP agreem. False ID
dE/dx 3.3 5.3 2.0 2.4
Ring 4.6 3.1 4.2
veto 8.4
dE/dxVETO 0.3 0.7 0.6 0.6 2.4
dE/dxRing 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 )
?2 3.26/3
?2 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
RingdE/dx 1.639 0.112 (0.097 stat 0.054 syst)
VETOdE/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 1.5450.078