search for 7-prong decays ruben ter-antonyan on behalf of the babar collaboration tau04 workshop,...
Post on 15-Jan-2016
213 views
TRANSCRIPT
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 %
50branching ratio 15 %
qq background
Fit parameters (%) 18 %
Fit range (%) 3 %
Num. events fitted (%) 4 %
Total uncertainty of signal efficiency (%) 6.8 %
Total uncertainty of background (%) 60%
Total uncertainty of qq background (%) 19%
B A
B A
R
p r
e l
i m i
n a
r y
Preliminary Upper Limit
1.1 × 108
background 0.6 ± 0.4
qq background 11.3 ± 2.2
Total expected background 11.9 ± 2.2
43+ efficiency (8.05 ± 0.55) %
43+ 0 efficiency (8.04 ± 0.55) %BR ( 43+ (0) ) @ 90% CL < 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 ( 43+ (0) ) @ 90% CL < 2.4 × 10-6 < 2.7 × 10-7
B A
B A
R
p r
e l
i m i
n a
r y
using most conservative
Bayesian approach
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 4 3+ (0) found; BR upper limit is 10 times better
than previously set
Will finalize the analysis with doubled statistics soon.
Backup Slides
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.
Looper and Photon Conversion Rejection
Looper candidate:
A pair of tracks with SVT hits
pT,LAB < 200 MeV/c for each track
|cosLAB| < 0.18 for each track
|pT,LAB| < 100 MeV/c
Remove tracks with largest DOCAZ
Photon Conversion candidate:
A pair of tracks with invariant mass < 5 MeV
Distance between tracks in XY-plane < 0.2 cm
Efficiency of the Cuts
Cuts 7 7 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
After pre-selection background is always dominated by qq events.
7-prong cuts suppress the background from generic events.
Background from qq is suppressed after tagging the 1-prong and the pseudo-mass cut.
BABAR preliminary
Data-MC Comparison
68
3.8
0.59
0.37
0.03
85 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 events is reliable for an estimate.
Data/MC ratio for various topologies
BABARpreliminary
Numbers show MC simulated qq/ ratio for different topologies.
Background Estimate Validation
1-7 MC: expected and observed qq bkg. in the region (1.3-1.8) GeV/c
1-8 Data: expected and observed events in the region (1.3-2.0) GeV/c2
1-7 Data: expected background ( and qq) in the region (1.3-1.8) GeV/c2
-- 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 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
The agreement is quite good!
B A
B A
R
p r
e l
i m i
n a
r y
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, = <n> = f B + b
b - number of bkg. expected, b* sample from normal N(b, b)
f = 2 N, f* sample from normal N(f, f)