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

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/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)