Search for 7 Pion Decays of the -Lepton

Search for  7 Pion Decays of the -Lepton

Search for 7 Pion Decays of the -Lepton with BABARLepton with BABAR Richard Kass + e tag - e The Ohio State University rec Introduction 7-Lepton with BABARprongs inclusive analysis 43+ (0) exclusive: 43+ & 43+ 0 32+ 20 2-Lepton with BABAR Results and Conclusions Richard Kass TAU06 Sept 19 2006 1 Motivation for Searches decays with 7 pions have never been observed We have a lot more statistics than previous experiments ( (50-Lepton with BABAR200)) Better understanding of strong interactions in hadronic decays resonant substructure exotic resonances ? Potentially useful bound on mass if the decay is observed MeV

Richard Kass Multi-Lepton with BABARpion decays are important ! TAU06 Sept 19 2006 2 4433++ ((00)) Previous Experiments: HRS (1987): BR < 2.9 10-Lepton with BABAR4 PRD 35, 2269 OPAL (1997): BR < 1.8 10-Lepton with BABAR5 PL B404, 213 CLEO (1997): BR < 2.4 10-Lepton with BABAR6 PRD 56, 5297 Theory: 3 1 d p1 d pn 4 4 d | M |2 ... ( 2 ) ( p0 p1 ... pn ) 3 3 2E 0

(2 ) 2 E1 (2 ) 2 En 3 amplitude (dynamical factor) phase space (kinematical factor) BR( 7 ) 10 3 6 10 6 BR( 5 ) S. Nussinov, M. Purohit, Phys.Rev.D.65 034018 (2002) Richard Kass TAU06 Sept 19 2006 BR ( 7 ) 6 10 12 Provided the decay does not go via resonances 3 3322++ 2200 Previous Results: No 7: BR () < 1.110-Lepton with BABAR4 CLEO PRL, 73, 934 (1994) But 6 has been measured: BR () = (1.70.3)10-Lepton with BABAR4 CLEO PRL 86,4467 (2001) BR () = (1.20.2)10-Lepton with BABAR4 Theory: No theoretical prediction of the decay rate The decay will most likely go through meson

If meson dominates the 7-Lepton with BABARpion decays, will most likely have the largest BR The decay is likely to go through (2) channel ( 5 2 0 ) 2 1 8 18 3 3 7 5 2 3 3 2 35 35 35 10 5 R. Sobie, Phys. Rev. D. 60 017301 (1999) Richard Kass TAU06 Sept 19 2006 4 Data Collection at PEPII The analyses presented here use 232 fb-1 of data* =206.6x106 -pairs Collected at CM energy ~ 10.58 GeV *BaBar has collected a total of ~390 fb-Lepton with BABAR1 Richard Kass TAU06 Sept 19 2006 5 Event Pre-Selection Criteria

Reject background and reduce size of data samples. 2 < Nch.trk < 11 in event Event divided to 2 hemispheres perpendicular to thrust Thrust magnitude > 0.9 8 (6) well-Lepton with BABARmeasured tracks in event, with 1 track recoiling against 7: 1-Lepton with BABAR7 topology (1-Lepton with BABAR5 topology). Zero net charge 0 No conversions (Me+e< 5 MeV, XY < 2cm) No loopers (tracks trapped in B-Lepton with BABARfield) Well-Lepton with BABARmeasured photons 1 (2) reconstructed 0s on signal side s on signal side 70 (520) Richard Kass Rejected 99.95% x is of bkg., a t andth 77% rus of signal TAU06 Sept 19 2006 BR(7/7=210-Lepton with BABAR5 X e+ e

Lots more bkg. to reject 6 Background Suppression Against qq -Lepton with BABARmesons on signal side pT >100 MeV/c on signal side Residual energy on the signal side < 300 MeV DOCAXY / pT < 0.7cmc/GeV on signal side 1-Lepton with BABARprong tags: e, , , h+0 1.3 < Pseudo-Lepton with BABARMass < 1.8 GeV/c Against bkg 2 invariant mass Use Pseudo-Lepton with BABARmass instead of Invariant mass

Assume: is massless & direction is given by 7 tracks m*2= 2(Ebeam E7)(E7 P7)+m72 BR = 210-5 Signal region: 1.3 < M7 < 1.8 GeV/c2 pseudo-mass background shifts upwards with pseudo-Lepton with BABARmass Shape of pseudo-Lepton with BABARmass distribution well modeled by MC Richard Kass TAU06 Sept 19 2006 7 43+ (0) qq Background Estimate After all cuts (DATA) After preselection (DATA) BABAR fit BABAR signal region extrapolate integrate Fit from 1.8 to 2.6 GeV/c2 after pre-Lepton with BABARselection with a Gaussian function

Extrapolate the fit below 1.8 GeV/c2 Integrate from 1.3 to 1.8 GeV/c2 Use these fit parameters on the final pseudo-Lepton with BABARmass spectrum. Mean and sigma do not vary significantly throughout the cuts Richard Kass TAU06 Sept 19 2006 8 43+ (0) qq Background Estimate Validation Validation on: 18 topology data (after all cuts) 1) 1-8 data: 1.3 1.8 GeV/c2 region (pure qq background) Cuts 1-8 data 1-7 data exp. obs. exp. obs. Preselection 19.0 2.7

23 3356 79 3238 7-Lepton with BABARprong ID 12.2 1.6 10 1503 40 1415 7-Lepton with BABARprong pT 6.7 0.8 8 1092 30 1103 DOCAXY/pT 2.7 0.3 1 656 18 642 1-Lepton with BABARprong tag

0.5 0.1 0 114 4 106 Good agreement! Richard Kass TAU06 Sept 19 2006 17 topology data (after all cuts) validation region 2) 1-7 data: 1.8 2.0 GeV/c2 region validation region BABAR BABAR 9 bkgd. Signal Efficiency 43+ (0) Systematic Error Estimate Tracking efficiency Particle ID 1-Lepton with BABARprong generic BR

Limited MC statistics 5.2 % 2.7 % 0.5 % 0.7 % Total uncertainty of signal efficiency (%) 6.0 % Limited MC statistics 75 % 50branching ratio 15 % 5branching ratio Total uncertainty of background (%) 7% 77% qq bkgd. 1.3 1.0 events Fit parameters (%) Num. events fitted (%) Fit range (%) Total uncertainty of qq background (%) 3.4 % 2.0 % 0.1 % 4.0 % 20.3 0.8 events

Luminosity and cross-section Richard Kass TAU06 Sept 19 2006 2.3 % 10 43+ (0) Results 43+ (0) eff. (9.4 0.6) % Expected bkg. 1.3 1.0 Expected qq bkg. 20.3 0.8 Total expected bkg. 21.6 1.3 Observed events signal region 24 No evidence for signal ! Sensitivity (Nexp=Nobs) < 2.5 10-Lepton with BABAR7 @ 90% CL Branching ratio signal region

< 3.0 10-Lepton with BABAR7 @ 90% CL Calculated using Bayesian approach Richard Kass TAU06 Sept 19 2006 11 Exclusive 43+ Background estimate method is identical to 43+ (0) case Additional cut: no s on the 7-prong side.s on the 7-prong side. 43+ eff. (5.5 0.3) % Expected bkg. 0.8 0.8 Expected qq bkg. 3.1 0.1 Total expected bkg. 3.9 0.8 Observed events 8 BABAR signal

region signal region No evidence for signal ! Sensitivity < 2.2 10-Lepton with BABAR7 @ 90% CL Branching ratio < 4.3 10-Lepton with BABAR7 @ 90% CL Richard Kass TAU06 Sept 19 2006 12 Exclusive 43+0 Require 1 reconstructed 0 on the 7-Lepton with BABARprong side: BABAR 113 < M < 155 MeV/c2 43+ 0 eff. (3.6 0.3) % Expected bkg. 0.4 0.4 Expected qq bkg. 7.8 0.3 Total expected bkg. 8.2 0.5

Observed events 7 No evidence for signal ! Sensitivity signal region signal region < 4.2 10-Lepton with BABAR7 @ 90% CL Branching ratio < 2.5 10-Lepton with BABAR7 @ 90% CL Richard Kass TAU06 Sept 19 2006 13 2++ 200 Search for 3 Search for 3 2 2 Richard Kass TAU06 Sept 19 2006 14

Background Estimate 32+ 20 Background After all cuts the background is dominated by the mode. MC Generate a large sample (3x data) of MC events for Fit pseudo-Lepton with BABARmass with a crystal balls on signal side PDF allowing hadrons on the tag side gives more statistics MC Use the shape parameters to fit the pseudo-Lepton with BABARmass after all cuts. Scale expected bkg. to 232 fb-Lepton with BABAR1 MC Expected (fit): 3.6 0.6 events Richard Kass MC Counted : 3.2 events Other MC bkg. 0.7 0.5 events TAU06 Sept 19 2006 15 qq Background Estimate 32+ 20 MC Fit qq with a double Gaussian (uu/dd/ss, cc). data MC = qqDATA

Fit qqDATA above 1.8 GeV/c2 with qqMC PDF, allowing PDF shape parameters float Extrapolate the fit below 1.8 GeV/c2 BABAR Data Expected (fit): 2.2 1.7 1.0 MC PDF Data PDF events Validate method by requiring at least three high energy photons on the tag side. blinded Signal efficiency <0.01% Richard Kass TAU06 Sept 19 2006 16 qq bkgd. bkgd. Signal Efficiency 32+ 20 Systematic Error Estimate Richard Kass

Tracking efficiency Reconstruction of 20 Single photon Particle ID Limited MC statistics 3.9 % 6.6 % 1.8 % 1.7 % 1.8 % Total uncertainty of signal efficiency (%) 8.3 % Tracking, Neutrals, PID, 0 8.4 % BR ( 14.9 % Fitting 16.7 % background background Total background 3.6 0.9 events 0.7 0.5 events 4.3 1.0 events 1.6 1.0 0.7 0.0

PDF parameters Fit function events events Total qq background 2.2 11..07 events Luminosity and cross-section 2.3 % TAU06 Sept 19 2006 17 32+ 20 Results BABAR Signal eff. 0.66 0.05 % 2.0 Total expected bkg. 6.5 1.4 Observed events 10 No evidence for signal ! Sensitivity < 1.8 10-Lepton with BABAR6 @ 90% CL

Branching ratio < 3.4 10-Lepton with BABAR6 @ 90% CL Calculated using Cousins-Highland approach incorporating errors R. Barlow, Comput. Phys. Commun. 149, 97 (2002) Richard Kass TAU06 Sept 19 2006 18 Search for 2 2 Motivation Pre-selection should dominate the decay mode (PRD 60, 017301 (1999)) Kinematic constraints of suppress the bkg. and allows to loosen the cuts. BABAR Sensitivity of O(10-7) can be achieved. Omega Reconstruction Loosen photon and 0 selection criteria Allow hadrons on the tag side peak BABAR Significantly relax bkg. suppression cuts. Require 0.76 < M+-0 < 0.80 GeV/c2 2 efficiency: 8.2% 2 purity:

Richard Kass 87% TAU06 Sept 19 2006 19 2Results Kinematics of this decay greatly suppresses the background BABAR Only contribution is from the Signal eff. 1.53 0.13 % Total exp. bkg. 0.4 10..04 background Observed events 1 No evidence for signal ! Sensitivity < 4.3 10-Lepton with BABAR7 @ 90% CL BR ( 2 -Lepton with BABAR ) < 5.4 10-Lepton with BABAR7 @ 90% CL Richard Kass TAU06 Sept 19 2006

20 Summary and Conclusions Multi-Lepton with BABARpion mode Previous < 2.4 10-Lepton with BABAR6 N/A N/A < 1.1 10-Lepton with BABAR4 BABAR < 3.0 10-Lepton with BABAR7 < 4.3 10-Lepton with BABAR7 PRD72:012003, 2005 < 2.5 10-Lepton with BABAR7 < 3.4 10-Lepton with BABAR6 PRD74:011103, 2006 N/A < 5.4 10-Lepton with BABAR7 More details: Search for Rare Multi-Lepton with BABARPion Decays of the Tau Lepton Using the BABAR Detector PhD thesis, Ruben Ter-Lepton with BABARAntonyan, Ohio State University, 2006. Tau decays to 7 or more particles (+ neutrino) have not been observed yet. Unlikely to be observed with < 1 ab-Lepton with BABAR1 of data Challenge to theory/theorists to predict these decay rates Richard Kass TAU06 Sept 19 2006 21

Backup Slides Richard Kass TAU06 Sept 19 2006 22 PEPII-Asymmetric e+e- Collider Stanford Linear Accelerator Center, Stanford, California SLAC is an asymmetric e+e collider: 9 GeV (e-Lepton with BABAR)/3.1 GeV (e+) Center of Mass has =0.56 Richard Kass TAU06 Sept 19 2006 23 The BABAR Detector Electromagnetic Calorimeter (EMC) 1.5 T Solenoid Detector of Internally Recflected Cherenkov Light (DIRC) Ge e (9 -

.1 e (3 + V) Instrumented Flux Return (IFR) ) G eV Drift Chamber (DCH) Silicon Vertex Tracker (SVT) BABAR features: Charged particle tracking (silicon+drift chambers+1.5T Bfield) Electromagnetic calorimetry (CsI) and electron ID /K/p separation up to the kinematic limit (dE/dx+DIRC) Muon/KL identification Richard Kass TAU06 Sept 19 2006 24 Pseudo-mass Data-MC Comparison 32+ 20 43+ (0) Pseudo-Lepton with BABARmass shapes of qq data and MC in agreement Gaussian function is a good fit To estimate qq bkg. in signal region, fit data with a PDF extracted

from MC Richard Kass TAU06 Sept 19 2006 25 Bayesian Approach P( | x) L( x | ) P ( ) L( x | ' ) P( ' )d ' Bayess on signal side theorem P ( | x) -Lepton with BABAR probability of unknown given observed vector of data x L( x | ) -Lepton with BABAR likelihood function for the data x given a certain up 1 P( | x)d -Lepton with BABAR fraction of probability in a given interval [up, low] low ( s b) n ( s b ) -Lepton with BABAR likelihood for Poisson distributed n observed events L(n | s ) e with expected background b, and a certain signal s n! sup L(n | s ) P( s )ds 1 P( s | n)ds L(n | s) P(s)ds sup

sup upper limit at confidence level 1-Lepton with BABAR Particle Data Group 2004 Richard Kass TAU06 Sept 19 2006 26 Bayesian Approach In our case, we want the upper limit on the branching ration: b B 2 N = is the mean of the number of observed events (Poisson) b number of expected background with error b (Gaussian) 2Nf scale factor with error f (Gaussian) Likelihood vs. BR Maximize the likelihood wrt. f, b to obtain B: B( 43+ (0) ) = 0.7 11..34 10 7 Integrate over all permitted values of f and b to obtain the likelihood function in the branching fraction: B( 43+ (0) ) < 3.0 10-Lepton with BABAR7 @ 90%CL Richard Kass

TAU06 Sept 19 2006 BR 27 43+ (0) Consistency Check DATA Cuts bkg. qq bkg. Observed Topology 128 13 574 21 695 Particle ID 28 6 244 Conv. Veto 104 2.4 1.3 119 5 1-Lepton with BABARprong Tag 24 Cuts

1.3 1.0 20.3 0.8 Richard Kass 241 10 free fit Agreement between exp. and obs. is consistent throughout cuts fixed fit Topology 574 21 574 21 Particle ID 241 10 222 19 Conv. Veto 119 5 126 18 TAU06 Sept 19 2006 Agreement between fixed and free fits is consistent throughout cuts

28 32+ 0 Background Error Estimate Generate Toy MC for 2D gaussian of and (fit parameters) and their errors. Correlation between and is taken into account. Using accept/reject method, plot the estimated bkg. for accepted toy MC fit parameters. 1 from the central value is the error. Richard Kass TAU06 Sept 19 2006 29 qq Background Validation 32+ 20 Require 3 high-Lepton with BABARenergy (E > 300 MeV) photons on 1-Lepton with BABARpr. side, not associated with 0 Signal efficiency <0.01% Suppresses tau events, leaving clean qq sample in the data, which can be studied unblinded BABAR Data fit of the qq pseudo-Lepton with BABARmass spectrum with a PDF taken from MC nicely describes the tail of the spectrum and gives a valid estimate of expected bkg. events in the signal region Observe: 12 events Predict: 11.8 events

Richard Kass TAU06 Sept 19 2006 30 2Background Estimate blinded blinded blinded blinded bkg. qq bkg. generic : 0 events 1.0 : 0.4 0.4 events Poisson err. 68% CL pseudo mass shapes of data and scaled MC agree no tail expected due to kinematical constraints extrapolation expected qq bkg: 0 0.1 0

Richard Kass TAU06 Sept 19 2006 31 Measurement Measurement of of BR( BR()) as as aa Cross Cross Check Check BABAR Preliminary Total bkg. Repeat the event selection of (reconst. 1 0) Repeat the background estimate method. Validate qq bkg. estimate with signal-Lepton with BABARfrees on signal side data sample (3 high-Lepton with BABARenergy photons on the tag side) Signal efficiency (%) 2.16 0.16 % Expected tau bkg. 67 ZZ Expected qq bkg. 84 YY Observed Data 1742 Observed Signal

1591 XX XX Branching Ratio (x10-4) 1.79 XXX XXX BR(50) ALEPH, 96 1.8 0.7 1.2 OPAL, 99 2.7 1.8 0.9 CLEO, 01 1.7 0.2 0.2 PDG, 04 1.81 0.27 Nice cross-check of the event selection and background estimate methods Richard Kass TAU06 Sept 19 2006 32

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