第１９回「宇宙ニュートリノ」研究会

Search for νμ→νe oscillationin the K2K experiment
July 6, 2006

Shimpei Yamamoto (Kyoto Univ.)Outline
Motivation for νμ→νe search
The K2K experiment
Search for νμ→νe oscillation
Constraint on neutrino oscillation
parameters
Summary

July 6, 2006

第１９回「宇宙ニュートリノ」研究会

2Motivation for νμ→νe search
No measurement of θ13, but a limit of θ13 <12° (reacotorν)We can derive a constraint
on θ213 fromν →νe oscillation
222 L μP(vμ → v e ) ≈ sin 2θ13sin θ 23sin ( Δm31~0.5)
4E~2.8×10-3eV2

First experimental search with a νμ beam and a longbaselineIf θ13≠0, LBL experiments can prove CPV in lepton
sectorPioneering research toward the next generation LBLexperiment (T2K)– Neutral pion rejection (crucial background for νμ→νe search)
July 6, 2006

第１９回「宇宙ニュートリノ」研究会

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The K2K Experiment“KEK to Kamioka long baseline neutrino oscillation experiment”
compared w/ the expectation from near— νμ beam (~98% purity)
detector measurement.
— Eν= ~1.3GeV (in average) Æ ν disappearance (confirm atm-ν result)μ— 250km baselineÆ νe appearance
L = 250km,Δm2 = 3 ×10−3 eV2Super-Kamiokande(Far detector)
250kE(GeV)

0.6GeVnull oscillation
w/ oscillationm

Near detectors
July 6, 2006

Oscillation Maximum

KEK

第１９回「宇宙ニュートリノ」研究会

E(GeV)
4

Data Sample
2 experimental phases– 1999.6-2001.6 with SK-1
Æ K2K-1– 2002.12-2004.11 with SK-2 Æ K2K-2

Protons on target (POT): 9.2x1019 (entire data sample)

Delivered POT (x1018)– 4.8×1019POT in K2K-1– 4.4×1019POT in K2K-2

1999
July 6, 2006

200020012002

2003

第１９回「宇宙ニュートリノ」研究会

2004 (year)
5

Search for νμ→νe oscillation
Look for the excess of νe events at SK– Find ve signal candidates in SK– Predict # of background events
Prediction of SK events

MC event rate of ν e signal
NSK ×
MC total event rate
– NSK: total number of ν events estimated from ND
event rate.
• Cancellation of syst. uncertainties of cross-section andabsolute flux

July 6, 2006

第１９回「宇宙ニュートリノ」研究会

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Estimate of SK event rateint(NNeutrino interaction rate meas. with 1KT KT )The same detector technology as Super-K.

Total num. of SK events:
NSK = Φ SK × σ × ε SK
Φ SK(Eν )σ(Eν )dEν MSK∫int= NKT ××× ε SK
∫ Φ ND(Eν )σ(Eν )dEν MKT≡Far/Near Ratio ~1×10-6
M: Fiducial mass MSK=22.5kt, MKT=25t
εSK: detection efficiency =77.0 (78.2)% for K2K-I (K2K-II)

N SK = 158.4
July 6, 2006

+ 9.4
− 8,7

第１９回「宇宙ニュートリノ」研究会

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Observation of beam neutrinos
GPSΔT = TSK-Tspill-TOF±500μsec

Tspill TOF=0.83msec

SKTSK(BG: 1.6events within ±500ms
2.4x10-3 events in 1.5μs )±5μsecObserve: 112 events
112 events

July 6, 2006

第１９回「宇宙ニュートリノ」研究会

8Signature of νμ→νe oscillation
νe CC quasi-elasticinteraction– Dominant process at
E~0.6GeV.
– Only an electron is visible.

Event display of νe CC-QE event

Select single-ringshower-type events

July 6, 2006

第１９回「宇宙ニュートリノ」研究会

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Background for νμ→νe search
Dominant background– π0 production event with
one reconstructedgamma-ray

π0Remaining backgroundoriginates from νe in the beam+μ + → e+ + νe + νμ (μe3)+ (0)K+(0) → π0(-) + e+ + νe (Ke3)– Intrinsically irreducible.
– A fraction of 0.9% at SK– Beam- νe contribution isestimated with MC• MC is validated by the neardetector measurements.
νe flux at SK

July 6, 2006

第１９回「宇宙ニュートリノ」研究会

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Neutral pion background
Dominant process: single π0 production such as Δ
resonance via NC interaction (NC1π0)ν + N → ν + Δ(→ π 0 + N ′)Characteristics of π0 background– Small opening angle of 2 gamma-rays– Small energy of 2nd gamma-rayOverlapping ofCherenkov photons
from 2 gamma-rays

July 6, 2006

2nd gamma ray is not rec.
due to small Eγ2

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π0 reconstruction algorithm
Assume 2 emitted gamma-rays from a vertex
Inputs to the fitter– Event vertex– Visible energy (Evis, electron-equivalent energy)– Direction of 1st ring (rγ1)

Outputs– Momentum vectors of 2 gamma-rays (Eγ1, Eγ2, rγ2)

Fitting method– Share Evis with 2 gamma-ray (no kinematical constraint of π0decay)– Compare the observed charge dist. on PMTs with the MCqiobs2exp
obs
obs
2
expectations based on Poisson χ χ = ∑ 2(qi − qi ) + 2qi ln exp
qiPMT
July 6, 2006

第１９回「宇宙ニュートリノ」研究会

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e/π0 separation
e/π0 separation using
invariant mass of 2
gamma-rays:
Minv = 2Eγ1Eγ 2 (1− cosθ γγ )– Minv~π0 mass for π0 events– Minv <100MeV/c2 for singleelectron events

Reconstruct 2nd fake ringfor single-electron events:– Small θγγ ~20°
– Small Eγ2(Eγ2/(Eγ1+Eγ2)~0.2)
Æ Minv depends on electronenergy (Ee)July 6, 2006

第１９回「宇宙ニュートリノ」研究会

13Validation of π0 fitter
Atm. ν 1-ring shower-typeevents– ~94% fraction of electron(νe)– ~6% fraction of π0 via NC

Minv~π0 mass for π0events– Events clustered around
Minv~π0 mass in data, as
we expecte according to
MC NC interaction events.

July 6, 2006

第１９回「宇宙ニュートリノ」研究会

14Validation of π0 fitter
SK-1

Good agreements forboth νe and π0compositions
Minv peak for π0’s:– 122(118)MeV/c2 for SK1(2) data
– 116(117)MeV/c2 for MCevents
Æ Good agreement btw/ data& MCelectronπ0SK-2electron• Energy scale error:~3MeV• Stat. error: 2~3MeV

π0MC events are normalized by live timeSK-1: 1492 days, SK-2: 627 days
July 6, 2006

第１９回「宇宙ニュートリノ」研究会

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300

200

Apply a π0 rejection cut inK2K data sample
3⎧⎪Minv <Ee + 40MeV for Ee < 400MeV⎨
20⎪⎩ Minv < 100MeVfor Ee ≥ 400MeV

100Minv(MeV/c2)

νe signal

500

1000

νμ originatedEe(MeV)

K2K-1
K2K-2

100

200

300

0Minv(MeV/c2)

Neutral pion rejection cut0July 6, 2006

Optimized to maximize
the sensitivity on sin22θμe
4 of 5 remaining events
are rejected.
– Rejection capability is
consistent w/ MC predictionof 70%

500

1000Ee(MeV)

第１９回「宇宙ニュートリノ」研究会

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Numbers of observed & expected
events― K2K-1 ―Dataν MC
beam ν
μe

Fully contained

81.1

0.8155Single ring

50.92

0.47

33

Shower-type ring

2.66

0.40

3

Visible energy cut

2.47

0.40

2

No decay electron

1.90

0.35

1

Non-π0 like

0.58

0.17

0

― K2K-2 ―νμ MCbeam νe

Data

Fully contained

77.40.8657

Single ring

49.41

0.5234Shower-type ring

3.21

0.44

5Visible energy cut

2.930.44

5No decay electron

2.17

0.39

4

Non-π0 like

0.74

0.21

1July 6, 2006

第１９回「宇宙ニュートリノ」研究会

Signal selection eff:
•47% for K2K-1•55% for K2K-2
νμ BG rejection:•0.55% for K2K-1•0.74% for K2K-2#observed=1#expected B.G. =1.70
Consistent w/ theBG expectation 17

Calculating upper limits
Assume 2-flavor mixing:P(v → v ) = ⎧⎪sin2 2θ μesin2 (1.27Δm2 L ) for CCμe⎨⎪⎩0Efor NC

Confidence interval with Poisson dist. using Nobs and22NNexp(sin2θ,Δm)
exp
μe= Sν e (sin2 2θ μe , Δm2 ) + Bν μ (sin2 2θ μe , Δm2 ) + Bν e
νe signal

PDF

July 6, 2006

νμ-originated BG

beam-νe BG

Syst. uncertainties are implemented in prob.
densities. (~±30%)–Many virtual exp. with random syst.
parameters–Observe Nobs events in Poisson regime
Confidence interval calc. with Feldman &Cousins method
第１９回「宇宙ニュートリノ」研究会

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Constraint in Δm2-sin22θμe space
K2K-1+2Δm2atm region

Nobs = 1event
Nexp(no osc.) = 1.7events
Constraint on oscillation parameters(90% C.L.):
sin22θμe < 0.13 at Δm2=2.8×10-3 eV2July 6, 2006

第１９回「宇宙ニュートリノ」研究会

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Constraint on the mixing parameter θ13
sin2 2θ μe ≈1 2
sin 2θ13
2

Comparison w/ reactor
neutrino experiments:
Δm2atm region– sin22θ13<0.13 (CHOOZ)– sin22θ13<0.20 (Palo-Verde)ÆConsistent results
The most strict limit in νeappearance mode
The only result using a νμbeam at Δmatm2 region
July 6, 2006

第１９回「宇宙ニュートリノ」研究会

20Future prospects
K2K
The K2K sensitivity reaches
sin22θμe=0.1 with 10 times stat.
(90%CL, Δm2=2.8×10-3eV2)
The projected sensitivity of T2K:
sin22θμe~O(10-3)– Intense neutrino beam

Æ×50 statistics for 5 years– Narrow band, lower energy(<1GeV)

T2K

ÆSuppress π0 production
Demonstrated the π0 rejection asthe pioneering work
POT (×1021)
July 6, 2006

第１９回「宇宙ニュートリノ」研究会

21Summary
We have searched for νμ→νe oscillation using the entire
K2K data sample of 9.2×1019 POT.
We have developed a new reconstruction algorithm forπ0’s.
– Validated the performances with atmospheric neutrinos.
– Achieved νμ background rejection by 70%.
– Pioneering works for the future long-baseline experiment.
Æ Improved sensitivity for νμ→νe oscillation

New constraints on neutrino oscillation parameters– sin22θμe < 0.13 (90% C.L.) at Δm2=2.8×10-3 eV2• Most strict limit in νe appearance mode
– sin22θ13 < 0.26 (90% C.L.) at Δm2=2.8×10-3 eV2 in 3-flavor
framework• Consistent with the results from the reactor neutrino
July 6, 2006
第１９回「宇宙ニュートリノ」研究会
experiments (CHOOZ,
Palo-Verde)22