Search for 7-prong ? Decays

1
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

2
BaBar and ? Physics
1.5 T Solenoid
Electromagnetic Calorimeter (EMC)
Detector of Internally Recflected Cherenkov Light
(DIRC)
e (3.1 GeV)
PEP-II Delivered 253 fb-1 BaBar Recorded 244 fb-1
e- (9 GeV)
Drift Chamber (DCH)
Instrumented Flux Return (IFR)
Silicon Vertex Tracker (SVT)

• BaBar is a great place for t physics!
• s(e e- ? t t-) 0.89 nb at 10.58 GeV
• Recorded luminosity 244 fb-1 ? 220 million
  t pairs!
• Analyzed luminosity 124.3 fb-1 ? 110
  million t pairs

3
7-prong t decays
Very rare no observation to date.
e
ttag

• Experiment
• BR(t ? 7p(p0)nt ) lt 2.4 ? 10-6
• (CLEO, 1997, PRD 56, 5297)
• Theory
• BR(t ? 7p(p0)nt ) lt 6 ? 10-11 (assuming no
  substructure)
• (S. Nussinov, M. Purohit, 2002, PRD 65)
• Motivation
• With 25 times CLEOs statistics we hope for a
  first observation
• More stringent bound on the t neutrino mass if
  the decay is observed
• Search for possible substructure in decay
  products.

trec
e-
MC 1-7 event
1-prong side
7-prong side
4
MC Studies of Signal and Background

• Signal 7p(p0)nt
• generated using phase space

• Background
• generic t
• -- generated using TAUOLA
• - biggest contribution from 5pp0nt mode (g -
  conversions)
• hadronic uds, cc, bb
• -- continuum qq simulated with JETSET
• Bhabha, m-pair, 2-photon negligible

BABAR preliminary
Mass (GeV/c2)
Signal region
Analysis proceeds blinded events below 2
GeV/c2 are removed from the data.
5
Pseudo-Mass
MC 7-prong Invariant and Pseudo-Mass

• Pseudo-mass was introduced by ARGUS in 1992 to
  measure the t-lepton mass.
• Assume neutrino is mass-less and takes zero
  energy
• t direction is approximated by 7 ch. tracks
• mt22(Ebeam E7p)(E7p P7p)m7p2

BABAR preliminary
Events / 0.005 GeV/c2
Mass (GeV/c2)

• Advantage of pseudo-mass
• Sharp cut-off at the t mass (1.777 GeV/c2).
• significant improvement of signal-background
  separation

BABAR preliminary
BABAR preliminary
Events / 0.01 GeV/c2
Events / 0.01 GeV/c2
BR2.410-6
All plots on this slide show Monte Carlo
simulated events
Invariant Mass (GeV/c2)
Pseudo-Mass (GeV/c2)
6
Event Selection

• Pre-Selection
• Up to 10 charged tracks and 12 neutrals in
  event
• Thrust magnitude gt 0.90
• Reject g-conversions
• Select 8 good tracks in event
• distance of closest approach to the beam spot in
  XY-plane DOCAXY lt 1.5 cm
• distance of closest approach to the beam spot in
  Z-plane DOCAZ lt 10 cm
• 5 tracks with 12 drift chamber hits and
  transverse momentum pT gt100 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

• 1-prong tags
• electron ID 0 or 1g
• muon ID 0 or 1g
• r, 0g
• h, 0g

• Event and 7-prong cuts
• Thrust magnitude gt 0.93
• Particle ID for p-mesons
• pT gt100 MeV/c
• DOCAXY / pT lt 0.7cm?c/GeV

1.3 lt Pseudo-Mass (7-prong) lt 1.8 GeV/c2
7
Data-MC comparison

• 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.

BABAR preliminary
Events / 0.025 GeV/c2
signal region
Pseudo-Mass (GeV/c2)

• 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.

MC qq is scaled to data qq above 2 GeV/c2.
Background from t events is small and is
determined from MC.
Data above 2 GeV/c2 will be used to estimate qq
bkg. in signal region
8
Background Estimate Scenario
DATA After thrust cut
DATA After all cuts
fit
BABAR preliminary
m, s
Events / 0.025 GeV/c2
Events / 0.025 GeV/c2
extrapolate
integrate
Pseudo-Mass (GeV/c2)
Pseudo-Mass (GeV/c2)
BABAR preliminary
Mean

• 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

Sigma

• Use these fit parameters on the pseudo-mass
  spectrum after all cuts.

Mean and sigma do not vary significantly after
thrust cut.
thrust cut
Cuts
9
Background Estimate Validation MC
Pre-selection
Thrust cut

• 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

PT cut
7-prong p ID
Events / 0.025 GeV/c2
1-prong tags
DOCAXY/ PT cut
BABAR preliminary
Pseudo-Mass (GeV/c2)
Pseudo-Mass (GeV/c2)
10
Background Estimate Validation 1-8 data
1-8 Data after thrust cut
1-8 Data after all cuts
BABAR preliminary
BABAR preliminary
Events / 0.025 GeV/c2
Events / 0.025 GeV/c2
Pseudo-Mass (GeV/c2)
Pseudo-Mass (GeV/c2)

• 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.

Cuts Expected bkg. Observed evt.
Thrust mag. 41 10 57
7-prong p ID 29 7 32
pT 19 5 22
DOCAXY/pT 7.7 2.3 8
1-prong tag 2.0 0.6 1
B A B A R p r e l i m i n a r y
11
Preliminary Results
After all cuts
After thrust cut
BABAR preliminary
BABAR preliminary
Events / 0.025 GeV/c2
Events / 0.025 GeV/c2
signal region
extrapolation of fit
Pseudo-Mass (GeV/c2)
Pseudo-Mass (GeV/c2)
Events in signal region -- expected bkg.
11.9 2.2 -- observed 7
Signal efficiency -- 7pnt mode 8.05
-- 7pp0nt mode 8.04
No evidence for signal !
12
Systematic Uncertainties
Signal Efficiency (both modes have equivalent
uncertainties) Tracking efficiency 5.2
Particle ID 2.7 1-prong generic t
BR 0.5 Limited MC statistics
2.6 Luminosity and tt- cross-section
2.3 t background Limited t MC
statistics (3 events out of 621 fb-1) 58
5pp0nt branching ratio 15 qq background
Fit parameters () 18 Fit range () 3
Num. events fitted () 4
Total uncertainty of signal efficiency ()
6.8
B A B A R p r e l i m i n a r y
Total uncertainty of t background ()
60
Total uncertainty of qq background ()
19
13
Preliminary Upper Limit
N tt- 1.1 108 tt- background 0.6
0.4 qq background 11.3 2.2 Total expected
background 11.9 2.2 t- ? 4p- 3p nt
efficiency (8.05 0.55) t- ? 4p- 3p p0 nt
efficiency (8.04 0.55)
B A B A R p r e l i m i n a r y
using most conservative Bayesian approach
BR (t- ? 4p- 3p (p0) nt ) _at_ 90 CL lt 2.7 10-7
Experiment CLEO (1997) BaBar Luminosity
(fb-1) 4.6
124.3 Observed (predicted) events 0
(2.8) 7 (11.9) BR (t- ? 4p- 3p (p0) nt ) _at_
90 CL lt 2.4 10-6 lt 2.7 10-7
14
Summary

• Pseudo-mass is a powerful tool for reducing qq
  background in the signal region
• Hadronic background estimate completely done
  from data
• No evidence for t- ? 4p- 3p (p0) nt found BR
  upper limit is 10 times better than previously
  set
• Will finalize the analysis with doubled
  statistics soon.

15
Backup Slides
16
1-7 Topology Event
A typical example of a MC simulated 1-7 event on
the left plot 8 tracks are counted, but the right
plot shows where the 1 additional track comes
from.
17
Looper and Photon Conversion Rejection

• Looper candidate
• A pair of tracks with SVT hits
• pT,LAB lt 200 MeV/c for each track
• cos?LAB lt 0.18 for each track
• DpT,LAB lt 100 MeV/c
• Remove tracks with largest DOCAZ

• Photon Conversion candidate
• A pair of tracks with invariant mass lt 5 MeV
• Distance between tracks in XY-plane lt 0.2 cm

18
Efficiency of the Cuts
Cuts 7pnt 7pp0nt t bkg. uds cc bb
Pre-selection () 23.6 22.8 0.0006 0.01 0.006 0.0001
Pre-selection (events) 23.6 22.8 628 26093 9786 152
7-prong cuts 13.4 12.8 5.1 725 99 2.8
1-prong tags 8.6 8.4 2.3 143 13 0
Events in signal region 8.1 8.3
0 0 1.6 0
BABAR preliminary

• After pre-selection background is always
  dominated by qq events.
• 7-prong cuts suppress the background from
  generic t events.
• Background from qq is suppressed after tagging
  the 1-prong and the pseudo-mass cut.

19
Data-MC Comparison
Data/MC ratio for various topologies

• Quantitative data-MC disagreement increasing
  with cuts for multi-prong events.
• Domination of qq bkg. in multi-prong events,
  resulting in worse data-MC agreement.
• MC simulation of qq in 1-7
• topology does not agree with data.
• MC simulation of t events is reliable for an
  estimate.

85
68
3.8
0.59
0.37
0.03
BABAR preliminary
Numbers show MC simulated qq/t ratio for
different topologies.
20
Background Estimate Validation

• 1-7 MC expected and observed qq bkg. in the
  region (1.3-1.8) GeV/c2
• 1-8 Data expected and observed events in the
  region (1.3-2.0) GeV/c2
• 1-7 Data expected background (t and qq) in the
  region (1.3-1.8) GeV/c2
• -- t bkg. is estimated using Monte Carlo
  simulation
• -- qq bkg. is estimated from the fits

Cuts 1-7 MC 1-8 Data 1-7
Data exp. obs. exp. obs. exp. obs. Thrust
mag. 89 34 95 41 10 57
257 36 298 7-prong p ID 33 11 29
29 7 32 108 18 98 7-prong pT
22 8 23 19 5 22 83
14 79 DOCAXY/pT 10 4 15 7.7
2.3 8 47 9 40 1-prong tag
1.8 0.7 1 2.0 0.6 1 11.9
2.2 7
B A B A R p r e l i m i n a r y
The agreement is quite good!
21
Upper Limit Calculation with Errors
To obtain the BR upper limit calculation
incorporating uncertainties, we integrate the
Likelihood function of the experiment
n number of events observed, sampled from
Poisson, m ltngt f B b b - number of bkg.
expected, b sample from normal N(b, sb) f 2
Ntt e, f sample from normal N(f, sf)