14 Sept 2004 D.Dedovich Tau041 Measurement of Tau hadronic branching ratios in DELPHI experiment at LEP Dima Dedovich (Dubna) DELPHI Collaboration E.Phys.J. C 1.Final results on exclusive hadronic branchings ( π /K blind) – submitted to E.Phys.J. C 2.Preliminary results on inclusive single-prong branching to charged kaons

14 Sept 2004 D.Dedovich Tau042 The DELPHI detector

14 Sept 2004 D.Dedovich Tau043 The first stage (common for both studies): the tau pair selection Almost full LEP-1 statistic was used ( ) 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

14 Sept 2004 D.Dedovich Tau044 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 K s decays were not counted due to requirement of Vertex Detector measurement on track The number of selected tau decay candidates was for 1-prong for 3-prong 112 for 5-prong

14 Sept 2004 D.Dedovich Tau045 Exclusive hadronic branchings charged 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

14 Sept 2004 D.Dedovich Tau046 Exclusive hadronic branchings π 0 counting 4 types of reconstructed π 0 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 π 0 and single photons Efficiency to reconstruct π 0 was about 70% with purity of about 90%

14 Sept 2004 D.Dedovich Tau047 Exclusive hadronic branchings π 0 invariant mass

14 Sept 2004 D.Dedovich Tau048 Exclusive hadronic branchings decay 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 ± π 0 ν ; h ± 2 π 0 ν ; h ± ≥3 π 0 ν –3-prong: 3h ± ν ; 3h ± π 0 ν ; 3h ± ≥2 π 0 ν –5-prong: 5h ± ν ; 5h ± ≥1 π 0 ν

14 Sept 2004 D.Dedovich Tau049 Exclusive hadronic branchings invariant masses of hadronic systems

14 Sept 2004 D.Dedovich Tau0410 Exclusive hadronic branchings neural network outputs e μ h hπ0hπ0 h2 π 0 h3 π 0 3h 3h π 0 3h2 π 0

14 Sept 2004 D.Dedovich Tau0411 Exclusive hadronic branchings calibration 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 γ ; ee→ μμγ ; τ →h π 0 ν When necessary, corrections were applied on simulation Response of calorimeters, track momentum, dE/dx, secondary interactions, track and π 0 reconstruction efficiency and muon chamber response were calibrated The uncertainties of these calibrations were the main source of systematic errors

14 Sept 2004 D.Dedovich Tau0412 Exclusive hadronic branchings RESULTS

14 Sept 2004 D.Dedovich Tau0413 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 ± X ν were published. Current preliminary results cover full LEP-1 statistics ( ) and are supposed to replace the old results Only inclusive branching ratio is being presented

14 Sept 2004 D.Dedovich Tau0414 Inclusive branching to kaons hadronic sample selection To reduce systematic effects we actually measure the ratio Br( τ →K ± X ν )/Br( τ → π ± X ν ). 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

14 Sept 2004 D.Dedovich Tau0415 Inclusive branching to kaons Kaon identification At LEP1 kaons from tau decays are allowed to have momentum in the range GeV/c Measurements of dE/dx in TPC provide π /K separation in the full kinematic range at the level of 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)

14 Sept 2004 D.Dedovich Tau0416 Inclusive branching to kaons Kaon identification π π K K

14 Sept 2004 D.Dedovich Tau0417 Inclusive branching to kaons Pull variables The K identification was based on pull variables Π H for hypothesis H= π /K/e/ μ For Cherenkov angle measurements a similar variables Π RING was constructed

14 Sept 2004 D.Dedovich Tau0418 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

14 Sept 2004 D.Dedovich Tau0419 Inclusive branching to kaons Clean sample of pions (kaons suppressed by RICH)

14 Sept 2004 D.Dedovich Tau0420 Inclusive branching to kaons Kaon-enriched sample dE/dx kaon pull

14 Sept 2004 D.Dedovich Tau0421 Inclusive branching to kaons All hadronic tau decay candidates

14 Sept 2004 D.Dedovich Tau0422 Inclusive branching to kaons Ring 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

14 Sept 2004 D.Dedovich Tau0423 Inclusive branching to kaons Clean sample of pions (kaons suppressed by dE/dx)

14 Sept 2004 D.Dedovich Tau0424 Inclusive branching to kaons Kaon-enriched sample

14 Sept 2004 D.Dedovich Tau0425 Inclusive branching to kaons All hadronic tau decay candidates

14 Sept 2004 D.Dedovich Tau0426 Inclusive branching to kaons VETO 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

14 Sept 2004 D.Dedovich Tau0427 Inclusive branching to kaons The fit procedure The measured pulls were used to construct the probability W that the particle is a kaon: W=F K /(F π +F K ) Here F K (Π K ) and F π (Π π ) 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

14 Sept 2004 D.Dedovich Tau0428 Inclusive branching to kaons fit to dE/dx probability

14 Sept 2004 D.Dedovich Tau0429 Inclusive branching to kaons fit to Ring probability

14 Sept 2004 D.Dedovich Tau0430 Inclusive branching to kaons combined fit : Ring+dE/dx

14 Sept 2004 D.Dedovich Tau0431 Inclusive branching to kaons Systematic 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

14 Sept 2004 D.Dedovich Tau0432 Inclusive branching to kaons Systematic errors The uncertainty of residual pion background (colored) Is strongly redused if pions were already suppresed by another detector

14 Sept 2004 D.Dedovich Tau0433 Inclusive branching to kaons Systematic errors in % RING pulldE/dx pullVETO Posi- tion width Moment. depend. Posi- tion width Moment. depend. K/ π MIP agreem. False ID dE/dx Ring veto8.4 dE/dx+VETO dE/dx+Ring Other sources of systematic errors are MC statistics (1.2%) and tau decay branchings (1.9%)

14 Sept 2004 D.Dedovich Tau0434 Inclusive branching to kaons The results (in %) χ 2 = 3.26/3 χ 2 = 1.99/2

14 Sept 2004 D.Dedovich Tau0435 Inclusive branching to kaons Results of Individual measurements in % Ring1.745±0.170(0.126 stat syst) dE/dx1.455±0.131(0.068 stat syst) VETO1.685±0.272(0.231 stat syst) Total1.579±0.097

14 Sept 2004 D.Dedovich Tau0436 Inclusive branching to kaons Results of combined measurements in % Ring+dE/dx1.639±0.112(0.097 stat syst) VETO+dE/dx1.594±0.184(0.172 stat syst) dE/dx only1.346±0.139(0.082 stat syst) Ring only1.871±0.489(0.462 stat syst) Total1.545±0.078

14 Sept 2004 D.Dedovich Tau0437 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%