Background%20Understanding%20and%20Suppression%20in%20Very%20Long%20Baseline%20Neutrino%20Oscillation%20Experiments%20with%20Water%20Cherenkov%20Detector

1
Background Understanding andSuppression in Very
LongBaseline Neutrino Oscillation Experiments
with Water Cherenkov Detector

Chiaki Yanagisawa
Stony Brook
Talk at NNN05, Aussois, France
April 7-9, 2005
2
VLBNO

• Very long baseline neutrino oscillation

• Setting the stage

• 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

• How do we find the signal for n?? ve

• nm? ne and ne N ? e invisible N' (invisible
  n ??s, n?0)

• Look for single electron events

g (g)

• Major background

• nm,t,e N ? nm,t,e N' p0 (invisible n
  ??s, n?0)

• ne contamination in beam (typically 0.7)

3
VLBNO

• Neutrino spectra of on- and off-axis BNL
  Superbeams

PRD68 (2003) 12002 private communication w/
M.Diwan
on-axis beam
1 o off-axis beam
nms/GeV/m2/POT
Neutrino energy (GeV)
4
VLBNO

• How is analysis done ?

• Use of SK atmospheric neutrino MC

special p0 finder

• Standard SK analysis package

• Flatten SK atm. n spectra and reweight with BNL
  beam spectra

• Normalize with QE events 12,000 events for nm ,
  84 events for beam
• ne for 0.5 Mt F.V. with 5 years of running,
  2,540 km baseline

distance from BNL to Homestake

• Reweight with oscillation probabilities for nm
  and for ne

• Oscillation parameters used

• Dm221 7.3 x 10- 5 eV2, Dm2312.5 x 10- 3eV2

• sin22qij(12,23,13)0.86/1.0/0.04,
  dCP0,45,135,-45,-135o

Probability tables from Brett Viren of BNL
5
VLBNO

• Selection criteria used to improve

Traditional SK cuts only

• Initial cuts

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

To reduce events with invisible charged pions
With p0 finder

• Likelihood analysis using the following eight
  variables

• p0 mass, energy fraction, costh, p0-likelihood ,
  e-likelihood

• Dp0-likelihood, total charge/electron energy,
  Cherenkov angle

6
VLBNO

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

Only single e-like events left after initial cut
Reconstructed energy
Reconstructed energy
QE events only before likelihood cut
All CC events before likelihood cut
En
En
Erec
Erec
Erec
Erec
All CC events that survive the initial cuts are
signals
7
p0 finder

• p0 Finder

• p0 finder

Always looks for an extra ring in a single ring
event

• p0 reconstruction efficiency with standard SK
  software

• measured opening angle vs. p0 mass with p0
  finder

inefficiency due to overlap
inefficiency due to weak 2nd ring
Single e-like events from single p0 int.
All single p0 interactions SK atm. neutrino
spectra
opening angle measured(deg)
efficiency
mgg (MeV/c2)
true opening angle (deg)
8
p0 finder

• p0 reconstruction efficiency

• p0 reconstruction efficiency with standard SK
  p0 finder

All the single p0 int.
with p0 finder
w/o p0 finder
with p0 finder
p0 mass cut1- and 2-ring events
With atmospheric neutrino spectra
efficiency
without p0 finder
p0 mass cut2-ring events
True opening angle (deg)
9
Variables
All the distributions of useful variables are
obtained with neutrino oscillation on with CPV
phase angle 450

• Useful Variables

• p0 mass

0.0-0.5 GeV
0.5-1.0 GeV
2.0-2.5 GeV
2.5-3.0 GeV
background
signal
p0 mass
p0 mass
p0 mass
p0 mass
1.0-1.5 GeV
1.5-2.0 GeV
3.0-3.5 GeV
3.5-4.0 GeV
p0 mass
p0 mass
p0 mass
p0 mass
10
Variables
Fake ring has less energy than real one

• Energy fraction of 2nd ring

0.0-0.5 GeV
0.5-1.0 GeV
background
2.0-2.5 GeV
2.5-3.0 GeV
signal
1.0-1.5 GeV
1.5-2.0 GeV
3.0-3.5 GeV
3.5-4.0 GeV
11
Variables

• Difference between ln of two p0-likelihood (wide
  vs. forward)

- 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 p0-likelihood (forward) -
ln p0- likelihood (wide)
Primary electron ring
An undetected weak ring initially
12
Variables

• Difference between ln of two p0-likelihood (wide
  vs. forward)

0.0-0.5 GeV
0.5-1.0 GeV
2.0-2.5 GeV
2.5-3.0 GeV
background
signal
1.0-1.5 GeV
1.5-2.0 GeV
3.0-3.5 GeV
3.5-4.0 GeV
13
Variables

• costh cos qe

0.0-0.5 GeV
0.5-1.0 GeV
2.0-2.5 GeV
2.5-3.0 GeV
background
signal
1.0-1.5 GeV
1.5-2.0 GeV
3.0-3.5 GeV
3.5-4.0 GeV
14
Likelihood Cut
Trained with ne CC events for signal, nm CC/NC
ne,t NC for bkg
Difference in ln likelihood between sig and bkg

• D ln likelihood distributions

ln likelihood ratio
0.0-0.5 GeV
0.5-1.0 GeV
2.0-3.0 GeV
3.0- GeV
signal
background
Preliminary
Preliminary
D likelihood
D likelihood
D likelihood
D likelihood
1.0-1.5 GeV
1.5-2.0 GeV
D likelihood
D likelihood
15
Signal/Background

• ne CC for signal all nm,t,e NC , ne beam
• for background

• Effect of cut on D ln likelihood

After initial cuts
No D ln likelihood cut (100 signal retained)
D ln likelihood cut (50 signal retained)
TRADITIONAL ANALYSIS
Preliminary
Preliminary
Background from p0
o
CP45
o
ne background
CP45
Signal
Erec
Erec
Signal 700 ev
Bkgs 2005 (1878 from p 0others)
( 127 from ne)
Bkgs 169 (112 from p 0others)
( 57 from ne)
Signal 321 ev
16
Signal/Background

• ne CC for signal all nm,t,e NC , ne beam
• for backgrounds

• Effect of cut on D likelihood

D ln likelihood cut (40 signal retained)
D ln likelihood cut (40 signal retained)
Preliminary
Preliminary
Background from p0
o
o
CP45
CP-45
ne background
Signal
Erec
Erec
Signal 251 ev
Bkgs 118 ( 74 from p 0others)
( 44 from ne)
Signal 142 ev
Bkgs 118 ( 75 from p 0others)
( 43 from ne)
17
Signal/Background

• ne CC for signal all nm,t,e NC , ne beam
• for backgrounds

• Effect of cut on likelihood

D ln likelihood cut (40 signal retained)
D ln likelihood cut (40 signal retained)
Preliminary
Preliminary
Background from p0
o
o
CP135
CP-135
ne background
Signal
Erec
Erec
Signal 342 ev
Bkgs 126 ( 81 from p 0others)
( 45 from ne)
Signal 233 ev
Bkgs 122 ( 78 from p 0others)
( 44 from ne)
18
Summary of BNL superbeam_at_UNO
Issues
Some issues
Neutrino oscillation was on to define template
distributions For analysis CPV45o

• S/B and variables

Variable removed
Bkg
Beam ne
Signal
Bkg
Signal
Effic
ne CC
nm all, ne,nt NC
321
112
57
50
2.86
None
Dpi0lh
119
nm all, ne,nt NC
1.80
ne CC

321
59
50
poa
ne CC

2.51
nm all, ne,nt NC
316
56
126
50
ne CC
303
116
52
50
nm all, ne,nt NC
2.61
pi0-lh
Preliminary
50
311
127
e-lh
55
ne CC
2.53
nm all, ne,nt NC
333
167
50
60
ne CC
nm all, ne,nt NC
efrac
1.99
310
143
50
56
ne CC
pi0mass
nm all, ne,nt NC
2.17
57
146
nm all, ne,nt NC
costh
322
2.21
ne CC
50
nm all, ne,nt NC
ange
ne CC
50
321
119
55
2.70
19
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

• Detailed studies on sensitivity on oscillation
  parameters needed

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