Search for 7-prong Decays Ruben Ter-Antonyan on behalf of the BaBar Collaboration Tau04 Workshop, Sep 14, 2004, Nara, Japan Outline: Introduction Event Selection Data - Monte Carlo Comparison Background Estimate Systematic Uncertainties Preliminary Results
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Search for 7-prong Decays Ruben Ter-Antonyan on behalf of the BaBar Collaboration Tau04 Workshop, Sep 14, 2004, Nara, Japan Outline: Introduction
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Search for 7-prong Decays
Ruben Ter-Antonyan
on behalf of the BaBar Collaboration
Tau04 Workshop, Sep 14, 2004, Nara, Japan
Outline:
Introduction
Event Selection
Data - Monte Carlo Comparison
Background Estimate
Systematic Uncertainties
Preliminary Results
1.5 T Solenoid Electromagnetic Calorimeter
(EMC)Detector of Internally
Recflected Cherenkov
Light (DIRC)
Instrumented Flux Return
(IFR)
Silicon Vertex Tracker (SVT)
Drift Chamber (DCH)
BaBar and Physics
BaBar is a great place for physics(e+ e- + -) = 0.89 nb at 10.58 GeV Recorded luminosity: 244 fb-1 220 million pairs!
Analyzed luminosity: 124.3 fb-1 110 million pairs
e- (9 GeV)
e+ (3.1 GeV)
PEP-II Delivered 253 fb-1
BaBar Recorded 244 fb-1
7-prong decays
MC 1-7 event
1-prong side
7-prong side
Experiment:
BR()< 2.4 10-6
(CLEO, 1997, PRD 56, 5297) Theory:
BR( ) < 6 10-11 (assuming no substructure)(S. Nussinov, M. Purohit, 2002, PRD 65)
Motivation: With 25 times CLEO’s statistics we hope for a first observation
More stringent bound on the neutrino mass if the decay is observed
Search for possible substructure in decay products.
Very rare – no observation to date.
e-
e+
tag
rec
MC Studies of Signal and Background
Signal 7(0):
generated using phase space
Background:
generic :
-- generated using TAUOLA
biggest contribution from 50 mode (- conversions)
hadronic: uds, cc, bb
-- continuum qq simulated with JETSET
Bhabha, -pair, 2-photon: negligible
Signal region
BABARpreliminary
Mass (GeV/c2)
Analysis proceeds “blinded”: events below 2 GeV/c2 are removed from the data.
Pseudo-Mass
Pseudo-mass was introduced by ARGUS in 1992 to measure thelepton mass.
Assume neutrino is mass-less and takes zero energy
direction is approximated by 7 ch. tracks
m*2=2(Ebeam – E7)(E7 – P7)+m7
2
MC 7-prong Invariant and Pseudo-Mass
Advantage of pseudo-mass:
Sharp cut-off at the mass (1.777 GeV/c2).
significant improvement of signal-background separation
BABARpreliminary
BR=2.4×10-6
BABARpreliminary
Invariant Mass (GeV/c2) Pseudo-Mass (GeV/c2)
BABARpreliminary
Mass (GeV/c2)E
ven
ts /
0.01
GeV
/c2
Eve
nts
/ 0.
01 G
eV/c
2
Eve
nts
/ 0.
005
GeV
/c2
All plots on this slide show Monte Carlo simulated events
1-prong tags:
electron ID + 0 or 1
muon ID + 0 or 1
, 0
h, 0
Pre-Selection:
Up to 10 charged tracks and 12 neutrals in event
Thrust magnitude > 0.90
Reject -conversions
Select 8 “good” tracks in event: distance of closest approach to the beam spot in XY-plane DOCAXY < 1.5 cm distance of closest approach to the beam spot in Z-plane DOCAZ < 10 cm 5 tracks with ≥12 drift chamber hits and transverse momentum pT >100 MeV/c
Topology cut: event is divided into two hemispheres perpendicular to thrust axis with 1 “good” track recoiling against 7 “good” tracks and zero net charge
Event Selection
Event and 7-prong cuts:
Thrust magnitude > 0.93
Particle ID for -mesons
pT >100 MeV/c
DOCAXY / pT < 0.7cmc/GeV
1.3 < Pseudo-Mass (7-prong) < 1.8 GeV/c2
Data-MC comparison
Both data and MC have smooth pseudo-mass distributions
Both can be fitted with a Gaussian function
MC simulated qq events will be used as a check of bkg. estimate method.
Pseudo-Mass (GeV/c2)
BABARpreliminary
MC qq is scaled to data qq above 2 GeV/c2. Background from events is small and is determined from MC.
signal region
Quantitative disagreement between data and MC throughout the analysis
Data after all cuts contain 5 times larger sample of qq events than MC simulation predicts
MC simulated qq events will not be used for bkg. estimate in data.
Eve
nts
/ 0.
025
GeV
/c2
Data above 2 GeV/c2 will be used to estimate qq bkg. in signal region
Background Estimate Scenario
Fit from 2 to 2.5 GeV/c2 after thrust cut with a Gaussian function
Extrapolate the fit below 2 GeV/c2
Integrate from 1.3 to 1.8 GeV/c2
Use these fit parameters on the pseudo-mass spectrum after all cuts.
extrapolate
integrate
fit
Pseudo-Mass (GeV/c2)Pseudo-Mass (GeV/c2)
DATA After thrust cut DATA After all cuts
Mean and sigma do not vary significantly after thrust cut.
BABARpreliminary
Eve
nts
/ 0.
025
GeV
/c2
Eve
nts
/ 0.
025
GeV
/c2
thrust cut
Sig
ma
Mea
n
Cuts
BABARpreliminary
Background Estimate Validation: MC
Pseudo-Mass (GeV/c2)
MC Hadronic Bkg. (75 fb-1)
Pseudo-mass is fitted after thrust cut and fit parameters are used for bkg. estimate after each cut.
Good agreement between expected and observed number of bkg. events throughout the cuts. After all cuts (1.3-1.8 GeV/c2): -- expected: 1.8 ± 0.7 -- observed: 1
Pseudo-Mass (GeV/c2)
BABARpreliminary
Eve
nts
/ 0.
025
GeV
/c2
Pre-selection Thrust cut
1-prong tagsDOCAXY/ PT cut
PT cut7-prong ID
Background Estimate Validation: 1-8 data
Pseudo-Mass (GeV/c2)
1-8 Data after thrust cut 1-8 Data after all cuts
BABARpreliminary
1-8 Topology Data. (91 fb-1)
Pure hadronic bkg.
Good agreement between expected and observed number of bkg. events in the signal region throughout the cuts.
Eve
nts
/ 0.
025
GeV
/c2
Cuts Expected bkg. Observed evt.
Thrust mag. 41 ± 10 57
7-prong ID 29 ± 7 32
pT 19 ± 5 22
DOCAXY/pT7.7 ± 2.3 8
1-prong tag 2.0 ± 0.6 1B A B A R p r e l i m i n a r y
Eve
nts
/ 0.
025
GeV
/c2
Pseudo-Mass (GeV/c2)
BABARpreliminary
Preliminary Results
Events in signal region
-- expected bkg.: 11.9 ± 2.2
-- observed: 7
Pseudo-Mass (GeV/c2)Pseudo-Mass (GeV/c2)
No evidence for signal !
After all cutsAfter thrust cut
BABARpreliminary
BABARpreliminary
signal region
Signal efficiency:
-- 7 mode: 8.05%
-- 70mode: 8.04%
extrapolation of fit
Eve
nts
/ 0.
025
GeV
/c2
Eve
nts
/ 0.
025
GeV
/c2
Systematic Uncertainties
Signal Efficiency (both modes have equivalent uncertainties)
Tracking efficiency 5.2 %
Particle ID 2.7 %
1-prong generic BR 0.5 %
Limited MC statistics 2.6 %
Luminosity and cross-section 2.3 %
background
Limited MC statistics (3 events out of 621 fb-1) 58 %