Background Understanding andSuppression in Very Long

Baseline Neutrino Oscillation Experiments with Water Cherenkov

DetectorChiaki Yanagisawa

Stony Brook

Talk at NNN05, Aussois, France

April 7-9, 2005

VLBNO

Setting the stage

e ande +N e + invisible N' + (invisible n s, n0)

e e'invisible n s, n0)

Look for single electron events

Major background

econtamination in beam (typically 0.7%)

How do we find the signal for ve

~ a half megaton F.V. water Cherenkov detector, for example UNO

BNL very long baseline neutrino beam

VLB neutrino oscillation experiment

See, for example, PRD68 (2003) 12002 for physics argument

● Very long baseline neutrino oscillation

Neutrino spectra of on- and off-axis BNL Superbeams

on-axis beam

1 o off-axis beam

Neutrino energy (GeV)

s/

GeV

/m2 /P

OT

PRD68 (2003) 12002; private communication w/ M.Diwan

VLBNO

VLBNO

How is analysis done ? Use of SK atmospheric neutrino MC

Flatten SK atm. spectra and reweight with BNL beam spectra Normalize with QE events: 12,000 events for events for beame for 0.5 Mt F.V. with 5 years of running, 2,540 km baseline

Reweight with oscillation probabilities for and for e

Standard SK analysis package +

m221 =7.3 x 10- 5 eV2, m2

31=2.5 x 10- 3eV2

sin22ij(12,23,13)=0.86/1.0/0.04, CP=0,+45,+135,-45,-135o

Probability tables from Brett Viren of BNL

Oscillation parameters used:

distance from BNL to Homestake

special 0 finder

VLBNO

Selection criteria used to improve

Likelihood analysis using the following eight variables:

Initial cuts: One and only one electron-like ring with energy and reconstructed neutrino energy more than 100 MeV without any decay electron

mass, energy fraction, costh, -likelihood, e-likelihood

-likelihood, total charge/electron energy, Cherenkov angle

To reduce events with invisiblecharged pions

Traditional SK cuts only

With finder

VLBNO

QE events onlybefore likelihood cut

All CC eventsbefore likelihood cut

Erec Erec

Reconstructed energy Reconstructed energy

E EErec Erec

All CC events that survive the initial cuts are signals

What are sources of the signal (QE and nonQE) ?

Only single e-like events left after initial cut

0 finder

finder

reconstruction efficiency with standard SK software

measured opening angle vs. mass with finder

m (MeV/c2) true opening angle (deg)

effic

ienc

y

open

ing

angl

e m

easu

red(

deg)

Single e-like events from single int. All single interactionsSK atm. neutrino spectra

Always looks for an extra ring in a single ring event

inefficiencydue to overlap

inefficiency due toweak 2nd ring

● Finder

reconstruction efficiency

reconstruction efficiency with standard SK + finder ef

ficie

ncy

All the single int.

True opening angle (deg)

finder

0 mass cut:1- and 2-ring events

0 mass cut:2-ring events

With atmospheric neutrino spectra

with 0 finder

without 0 finder

with 0 finderw/o 0 finder

Variables

0.0-0.5 GeV 0.5-1.0 GeV

1.0-1.5 GeV 1.5-2.0 GeV

2.0-2.5 GeV 2.5-3.0 GeV

3.0-3.5 GeV 3.5-4.0 GeV

background

signal

0 mass• Useful Variables All the distributions of useful variables are obtained with

neutrino oscillation “on” with CPV phase angle +450

0 mass

0 mass 0 mass

0 mass

0 mass

0 mass

0 mass

0 mass

Variables

0.0-0.5 GeV 0.5-1.0 GeV

1.0-1.5 GeV 1.5-2.0 GeV

2.0-2.5 GeV 2.5-3.0 GeV

3.0-3.5 GeV 3.5-4.0 GeV

background

signal

Energy fraction of 2nd ring Fake ring has less energy than real one

Variables

Primary electron ring

An undetected weakring initially

- One algorithm optimized to find an extra ring near the primary ring (forward region)

- Another algorithm optimized to find an extra ring in wider space (wide region)

- See the difference ln -likelihood (forward) - ln -likelihood (wide)

Difference between ln of two -likelihood (wide vs. forward)

Variables

Difference between ln of two -likelihood (wide vs. forward)

0.0-0.5 GeV 0.5-1.0 GeV

1.0-1.5 GeV 1.5-2.0 GeV

2.0-2.5 GeV 2.5-3.0 GeV

3.0-3.5 GeV 3.5-4.0 GeV

background

signal

Variables

0.0-0.5 GeV 0.5-1.0 GeV

1.0-1.5 GeV 1.5-2.0 GeV

2.0-2.5 GeV 2.5-3.0 GeV

3.0-3.5 GeV 3.5-4.0 GeV

backgroundsignal

costh = cos e

Preli

minary

Likelihood Cut

ln likelihood distributionsTrained with e CC events for signal, CC/NC & e NC for bkg

0.0-0.5 GeV 0.5-1.0 GeV

1.0-1.5 GeV 1.5-2.0 GeV

2.0-3.0 GeV 3.0- GeV

likelihood

likelihood

likelihood

likelihood

likelihood likelihood

signal

background

Difference in ln likelihood between sig and bkg ln likelihood ratio

Preli

minary

Bkgs 169 (112 from +others) ( 57 from e)

ln likelihood cut (100%signal retained)

Background from CP+45 o

Signal 700 ev Bkgs 2005 (1878 from +others) ( 127 from e)

Signal

e backgroundo

CP+45

ln likelihood cut (~50%signal retained)

Effect of cut on ln likelihood

ErecErec

Preliminary Preliminary

Signal/Background

Signal 321 ev

e CC for signal ; all e NC , e beam for background

TRADITIONAL ANALYSIS

After initial cuts

ln likelihood cut (40%signal retained)

Background from

CP+45o

Signal 251 ev Bkgs 118 ( 74 from +others) ( 44 from e)

Signal

e background

oCP-45

ln likelihood cut (~40%signal retained) Effect of cut on likelihood

Erec Erec

Preliminary Preliminary

Signal/Background

Signal 142 ev

e CC for signal ; all e NC , e beam for backgrounds

Bkgs 118 ( 75 from +others) ( 43 from e)

Effect of cut on likelihood

ln likelihood cut (~40%signal retained)

Background from CP+135

o

Signal

e background

oCP-135

ln likelihood cut (~40%signal retained)

Erec Erec

Preliminary Preliminary

Signal/Background

Signal 342 ev Bkgs 126 ( 81 from +others) ( 45 from e)

Signal 233 ev Bkgs 122 ( 78 from +others) ( 44 from e)

e CC for signal ; all e NC , e beam for backgrounds

Preli

minary

Summary of BNL superbeam@UNOVariableremoved Signal Bkg Signal Bkg Effic

None e CC all, e, NC 321 112

e CC 321 59

e CC 316 56e CC

50% 311 127

333 167

310 143

pi0lh

pi0-lh

poa

e-lh

50%

50%

Beam e

57

119

126303 116 52

50%

50%

50% 50%

55

60

56

e CC

Issues

● S/B and variables

e CC

e CC

e CC 50%

efrac

pi0mass

costh 322 146 57

)(/ 0BS

2.86

1.80

2.51

2.61

2.53

1.99

2.17

2.21

Neutrino oscillation was on to define template distributionsFor analysis CPV=+45o

Some issues

all, e, NC

all, e, NC

all, e, NC

all, e, NC

all, e, NC

all, e, NC

all, e, NC

all, e, NC ange e CC 50% 321 119 55 2.70

Conclusion● Conclusion

Realistic MC simulation studies have been performed for the BNL very long baseline scenario with a water Cherenkov detector. It was found that BNL VLB combined with a UNO type detector seems to DO A GREAT JOB – Very exciting news but needs confirmation.

It was demonstrated that there is some room to improve S/B ratio beyond the standard water Cherenkov detector software even with currently available software

We may need further improvement of algorithm/software, which is quite possible

A larger detector such as UNO has an advantage over a smaller detector such as SK (we learned a lesson from 1kt at K2K)

Detailed studies on sensitivity on oscillation parameters needed