June 10th, 2004
@Kyoto U.

M. Yokoyama (Kyoto University)for K2K collaboration

11. Introduction

K2K experimentsince 1999

First accelerator-based long baseline (250km) neutrino experiment.

Search for νµ disappearance and νe appearance

250km
Pure νµ beam (99%)
w/ <Eν>~1.3GeV
50 kton Water
Cherenkov detector

12GeV PS νµ beamline
 Beam monitor
 Near detector
2Flavor mixing in lepton sector
 Flavor eigenstates ≠ mass eigenstatesνeνµντ

m1m2m3

Sij:sinθij, Cij;cosθij
Parameters: 3 mixing angle (θ12,θ23,θ13)
31 complex phase (δ)

Neutrino Oscillation
 Time evolution of neutrino
 Consider mixing b/w two flavors

Consider neutrino generated as pure νµ (=cosθ|ν2>+sinθ|ν3>)2−im−im 22 t /4 p
−im
3 t /4 p
3
ν ,t,0 = cos θ e
ν 2 + sinθ ee
ν3µ

If m2≠m3 , ν is in mixed state after time evolution!

€€
P = ν ν ,tµ µ2€2
ΔmL(km) 2
2= 1− sin 2θsin 1.27 2
eV E(GeV ) 

If there is neutrino oscillation →Δm2≠0→m≠0!

€4

Super-Kamiokande atmospheric ν

1.3x10-3<Δm2<3.0x10-3eV2(@ sin22θ=1, 90% CL)
νµ→ντ oscillation?

5

Brief history of K2K
1995
 Proposed to study neutrino oscillation for atmospheric
neutrinos anomaly.
 1999
 Started taking data.
 2000 Detected the less number of neutrinos than the expectation
at a distance of 250 km. Disfavored null oscillation at the 2σlevel.
 2002
 Observed indications of neutrino oscillation. The probabilityof null oscillation is less than 1%.
 2004 This result!
6K2K Collaboration

JAPAN: High Energy Accelerator Research Organization (KEK) / Institute for Cosmic Ray
Research (ICRR), Univ. of Tokyo / Kobe University / Kyoto University / Niigata
University / Okayama University / Tokyo University of Science / Tohoku University
KOREA: Chonnam National University / Dongshin University / Korea University / Seoul National
University
U.S.A.: Boston University / University of California, Irvine / University of Hawaii, Manoa /Massachusetts Institute of Technology / State University of New York at Stony Brook /University of Washington at Seattle
POLAND: Warsaw University / Solton InstituteSince 2002
JAPAN: Hiroshima University / Osaka University
CANADA: TRIUMF / University of British ColumbiaItaly: Rome France: Saclay Spain: Barcelona / Valencia Switzerland: Geneva
RUSSIA: INR-Moscow

7

2. K2K experiment overview~1 event/2days~1011 νµ/2.2sec~106 νµ/2.2sec(/10m×10m) νµ (/40m×40m)

12GeV protons

π+SK ντµ+Target+Hornπ monitor

100m200m
decay pipe

µ monitor~250kmNear ν detectors
2 1.27 Δm

210x10prob. = sin 2θ ⋅ sin (ν Energy（ＭＣ）

12250kmx1082LEν4)（ＭＣ）
no oscillation

684Extrapolation
π monitor + simulation

40123E(GeV)?

45Near detectors at KEKoscillation

20

123E(GeV)?

4Super-K

58

Neutrino beamline @ KEK
Bird’s Eye Neutrino Beam Line

200m
100 m

9

Neutrino beam and the directional control
The beam direction
monitored by muons

Y centerwith 250kA.

X center ~1GeV neutrino beam by a dual horn system<±1mrad99 Jun~5 years≤±1 mrad04 10Feb

protons/pulse Accumulated POT( 1018)( 1012)

Accumulated POT (Protons On Target)
8.9 1019 POT for Analysis(previous results: 4.8 1019 POT)Jan 99K2K-IIK2K-I

Jan 00

Jan 01

Jan 02

Jan 03

Jan 0411

Near detector system at KEK
 1KT Water Cherenkov Detector (1KT)
 Scintillating-fiber/Water sandwich Detector (SciFi)
 Lead Glass calorimeter (LG) before 2002
 Scintillator Bar Detector (SciBar) from 2003
 Muon Range Detector (MRD)

Muon range detector

12

EM

SciBar Detector

eretrimlocadetector made of
Scintillator Bars

3m

Extruded
scintillator(15t) Full-active, fine-segment

PMT (64 ch.)3m

ν2.5x1.3x300cm3 cell
 ~15000 channels
Multi-anode WLS fiber+MAPMT readout

Detect short (~10cm) track p/π separation using dE/dx
Precise ν spectrum
measurement
 ν interaction study

1.7m



Wave-length
shifting fiber13

Detector Photos
Scintillators (64layers)

EM calorimeter
Fibers and front-end elec.14

Just Completed!

Aug. 22, 2003

Far detector : Super-Kamiokande 1996.4

Start data taking (SK- I)
C Scientific AmericanSK- I

˜5years

K2Kstartν
42m

 1999Water Cherenkov
detector

 1000 m underground 50,000 ton(22,500 ton fid.) 11,146 20 inch PMTs
 1,885 anti-counter PMTs

39mSK- IIK2K-II

 2001.7 Stop data taking for detector upgrade 2001.11 Accident (˜7000 inner PMTs, 1100 outer PMTs were destroyed)
partial reconstruction of the detector16 2002.10 resume data taking (SK- II) 2002.12 resume K2K beam (K2K-II)SK is back !
Full water on 10-Dec.-2002
Jan.-2003, fully contained event

Acrylic + FRP
vessel

Sep.-2002, before water filling

17

GPSSK Events
Tspill TOF=0.83msec

SKTSK

Decay electron cut.

±500µsec≥20MeV Deposited Energy
No Activity in Outer DetectorEvent Vertex in Fiducial Volume
More than 30MeV Deposited Energy±5µsec Analysis Time Window

108 events (K2K-1:56)-0.2<TSK-Tspill-TOF<1.3µsec(BG: 1.6 events within ±500µs
2.4 10-3 events in 1.5µs)

TDIFF. (µs)

18

3. Analysis Overview
KEKObservation#ν, pµ and θµMeasurement
Φ(Eν), ν int.
ν interaction MC

Far/Near Ratio(beam MC with π mon.)SKObservation#ν and Eν rec.

(sin22θ, Δm2)

Expectation#ν and Eν rec.
19

4. Near Detector measurements
 Event rate measurement (#of ν int.)
 Measurement w/ 1KT
 Cross-checked by other detectors

 Spectrum shape measurement
 1KT, SciFi, SciBar (pµ, θµ) Measure spectrum and nQE/QE (ν interactionmodel)

Predict number of event and spectrum
shape at SK

20

Neutrino Interaction @~1 GeV&Eν reconstruction
νµ + n → µ + p
νµ-p

νµ + n → µ + p + π

θµ

ν

µ-

νµ + n → ν + p + π’s
νCC QE ~100% efficiency for NSK
 can reconstruct Eν ←(θµ,pµ)CC nQE (1pi, multi-pi, coherent,DIS)
 ~100% efficiency for NSK
 Bkg. for Eν measurementNC ~40% efficiency for NSK(Eµ, pµ)π’s pσ/E (10-38cm2/GeV)

ν1π’s pm N − E µ + p µ cos θ µ(Eµ, pµ)θµE ν rec =m N E µ − m µ2 2Total (NC+CC)
CC Total
CC quasi-elasticDIS
CC single π
NC single π0521

4.1 Event rate measurement @1KT
 The same detector technology as Super-K.
 Sensitive to low energy neutrinos.

Nexp
SK=NobsKT

Φ ( E )σ ( E )dE M∫••MΦ(E)σ(E)dE∫
SKνννSKνννε SK•
ε KTKTSK≡Far/Near Ratio (by MC)~1 10-6
M: Fiducial mass MSK=22,500ton, MKT=25ton
ε: efficiency εSK-I(II)=77.0(78.2)%, εKT=74.5%

NSK

exp=150.9+11.6-10.0

NSKobs=10822

4.2 Measurement with SciBarFull Active Fine-Grained detector (target: CH).
Sensitive to a low momentum track.
 Identify CCQE events and other interactions (non-QE)
separately.
CCQE Candidate

νCCQEpnon-QEDATACC QECC 1π
CC coherent-π
CC multi-πµΔθp2 track events

pµ25°

Δθp (degree)23

4.3 Near Detector Spectrum Measurements
 1KT
 Fully Contained 1 ring µ (FC1Rµ) sample.

 SciBar 1 track, 2 track QE (Δθp≤25°), 2 track nQE(Δθp>25°) where one track is µ.

 SciFi
 1 track, 2 track QE (Δθp≤25°), 2 track nQE(Δθp>30°) where one track is µ.

24

A hint of K2K forward µ deficit.
K2K observed forward µ deficit.
A source is non-QE events.
 For CC-1π, Suppression of ~q2/0.1[GeV2] at
q2<0.1[GeV2] may exist.

For CC-coherent π,
 The coherent π may not exist.

We do not identify which processcauses the effect. The MC CC-1π(coherent π) model is correctedphenomenologically.
Oscillation analysis is insensitive to thechoice.

q2rec

Preliminary

DATA
CC 1π
CC coherent-π

q2rec (GeV/c)2(Data-MC)/MCSciBar
non-QE Events
25

q2rec (GeV/c)2

4.4 Near Detectors combined measurements(pµ,θµ) for 1track, 2trackQE and 2track nQE samples→ Φ(Eν), nQE/QE

Fitting parameters Φ(Eν), nQE/QE ratio
Detector uncertainties on the energy scale and the track
counting efficiency.
The change of track counting efficiency by nuclear effectuncertainties; proton re-scattering and π interactions in anucleus …
Strategy① Measure Φ(Eν) in the more relevant region of θµ≥20° for 1KTand θµ≥10° for SciFi and SciBar.
② Apply a low q2 correction factor to the CC-1π model (orcoherent π).
③ Measure nQE/QE ratio for the entire θµ range.
26θµ (MeV/c)

EνKT data

QE (MC)nQE(MC)MC templates

0-0.5 GeV

0.5-0.75GeV

0.75-1.0GeV

Pµ (MeV/c)• ν flux ΦKEK(Eν) (8 bins)• ν interaction (nQE/QE)

1.0-1.5GeV

••

• 27
•

Flux measurements
χ2=638.1 for 609 d.o.f








Φ(Eν) at KEK
preliminaryΦ1 (Eν < 500) = 0.78 ± 0.36
Φ2 ( 500≤ Eν < 750) = 1.01 ± 0.09
Φ3 ( 750≤ Eν <1000) = 1.12 ± 0.07
Φ4 (1500≤ Eν <2000) = 0.90 ± 0.04
Φ5 (2000≤ Eν <2500) = 1.07 ± 0.06
Φ5 (2500≤ Eν <3000) = 1.33 ± 0.17
Φ6 (3000≤ Eν) = 1.04 ± 0.18
nQE/QE= 1.02 ± 0.10
The nQE/QE error of 10% is assigned
based on the variation by the fitcondition.
 θ>10°(20 °) cut:
nQE/QE=0.95 ±0.04
 standard(CC-1π low q2 corr.):
nQE/QE=1.02 ±0.03
 No coherent: π=nQE/QE=1.06 ±0.03

Eν281KT: µ momentum and angular distributions.
with measured spectrum

0800

pµ (MeV/c)

16000

θµ (deg.)9029

SciFi (K2K-IIa with measured spectrum)Pµ 1trkθµ 1trkPµ 2trkQE

θµ 2trk
QE

Pµ 2trk
non-QEθµ 2trk
non-QE30

02(GeV/c)040(degree)

SciBar (with measured spectrum)Pµ 1trkθµ 1trkPµ 2trk QE

θµ 2trk QE

Pµ 2trk nQE

θµ 2trk nQE

31

5. Super-K oscillation analysis
 Total Number of events
 Eνrec spectrum shape of FC-1ring-µ events
 Systematic error term

L(Δm 2 , sin 2θ , f x )= Lnorm (Δm 2 , sin 2θ , f x ) ⋅ Lshape (Δm 2 , sin 2θ , f x ) ⋅ Lsyst ( f x )f x : Systematic error parameters
Normalization, Flux, and nQE/QE ratio are in fx
Near Detector measurements, Pion Monitor
constraint, beam MC estimation, and Super32
K systematic uncertainties.

K2K-SK events
K2K-alll(K2K-I, K2K-II)
FC 22.5kt

preliminary

DATA(K2K-I, K2K-II)
108(56, 52)
1ring
66(32, 34)µ-like57 (56)for Eνrec(30, 27)e-like9(2, 7)
Multi Ring
42
(24, 18)
Ref; K2K-I(47.9MC(K2K-I, K2K-II)
150.9(79.1, 71.8)
93.7(48.6, 45.1 )
84.8(44.3, 40.5)
8.8(4.3, 4.5)
57.2
(30.5, 26.7)

1018POT), K2K-II(41.21018POT) 33

2x

Lnorm (Δm , sin 2θ , f )2xLshape (Δm , sin 2θ , f )KS probability=0.11%#SK Events
Toy MC

Expected shape(No Oscillation)

CC-QE assumption

108

150.9

Eν

rec=Eνrec[GeV](mN − V ) Eµ − mµ2 2 + mNV − V 2 2(mN − V ) − Eµ + pµ cos θ µ 34
V: Nuclear potential

6. Results

preliminary

 Best fit values.
 sin22θ = 1.53
 Δm2 [eV2] = 2.12×10-3 sin22θ = 1.00
 Δm2 [eV2] = 2.73×10-3

ΔlogL=0.64

Δm2

 Best fit values in the physical region.

A toy MC
14.4%2.73

sin22θ=1.53 can occur by statisticalfluctuation with 14.4% probability.

1.00 1.53

sin3522θΔm2[eV2]

Data are consistent with the oscillation.
 NSKobs=108

preliminary NSKexp (best fit)=104.8

Best Fit
KS prob.=52%Based on ΔlnL

sin22θ

Eνrec[GeV]36νµ disappearance versus Eν shape distortionEν shape

NSK (#νµ)Δm2[eV2]Δm2[eV2]sin22θ

Both disappearance of νµ and the distortion ofEν spectrum have the consistent result.

sin22θ
37

Null oscillation probability

preliminary

The null oscillation probabilities are calculated based on ΔlnL.

K2K-I

K2K-II

K2K-allνµ disappearance

2.0%

3.7%

0.33%(2.9σ)

Eν spectrum distortion

19.5%

5.4%

1.1% (2.5σ)

Combined

1.3%

0.56%

0.011%(2.5σ)(2.8σ)(3.9σ)

Disappearance of νµ and distortion of the energy
spectrum as expected in neutrino oscillation.

K2K confirmed neutrino oscillation
discovered in Super-K atmospheric neutrinos.
38

8. SummaryK2K has confirmed
neutrino oscillations at 3.9σ.
With 8.91019 POT,

Disappearance of νµ Distortion of Eν spectrum

2.9σ
2.5σ



Δm2[eV2]

preliminary

1.7x10-3<Δm2<3.5x10-3eV2@sin22θ=1 (90%C.L.)

0.006
0.004
0.002

0.0 0.2 0.4 0.6 0.8 1.0

sin22θ

39

Backup40

7. Other Physics in K2K (based on K2K-I data)
νµ +H2O→NC1π0νµ→νe search

not NC1π0Mγγ(MeV)σ (ν µ → NC1π 0 )=0.065±0.001±0.007σ (ν µ → CCall )Δm2[eV2]

90%CL limit
90%CL sensitivity

PRL accepted
sin22θµe=0.064 (prediction)
preliminary41

1KT: µ momentum and angular
distributions.
with measured spectrum

flux measurement

low q2 corr.
0800

1600

pµ (MeV/c)020

θµ (deg.)4290

SciFi (K2K-IIa with measured spectrum)θµ 1trkPµ 1trk

flux measurementPµ 2trkQEθµ 2trk
QE

Pµ 2trk
non-QEθµ 2trk
non-QE
4302(GeV/c)

0 1040(degree)

SciBar (with measured flux)
θµ 1trkPµ 1trkflux measurementPµ 2trk QE

θµ 2trk QE

Pµ 2trk nQE

θµ 2trk nQE

4410

Log Likelihood difference from theminimum.
ΔlnL

ΔlnL
- 68%
- 90%
- 99%Δm2[eV2]


- 68%
- 90%
- 99%

sin22θ

Δm2<(1.7~3.5) 10-3 eV2 at sin22θ=1.0 (90% C.L.)

45

The change of NSKexp in K2K-I (Bugs)
 The detector position
 295m → 294m

-1%294m 295m
 MC difference between KT and SK
 KT; MA(QE)=1.1

 SK; MA(QE)=1.0

σ (NCel)KT=1.1 σ(NCel)SK⇒ Efficiency change! -1%NSKexp Change ~2%46

47

CC-1π suppression versus coherent π48

Systematic Bias without the MC correction.
ND (SciBar) measurement
DATA; MC w/ CC-1π suppression
MC template; Default MC

Oscillation Results
Default MC for ND and SK
sin22θ=1.00
Δm2=2.73 _10-3 eV2Prob.(null oscillation)=0.0049%

Toy MCCorrected MC
sin22θ=1.00
Δm2=2.65 10-3eV2
Prob.(null oscillation)=0.011%

systematic bias

There is a small bias in nQE/QE
and the low energy flux.
fluxnQE/QE49Oscillation result with a default MC
Without low q2 MC correction

The result w/o low q2 MC correction gives the better (biased)
50
measurement due to the more low energy flux and the smaller nQE/QE.

K2K-I vs K2K-II

51

K2K-I vs K2K-II52

νµ Disappearance Result (K2K-I)
PRL 90(2003)041801

Null oscillation probability: less than 1%.
Δm2=1.5~3.9 x 10-3 eV2 @sin22θ=1(90%CL)
Normalized by area
Expectation
w/o oscillation
Best Fit

Best fit:Δm2=2.8x10–3eV2

Reconstructed EνAllowed region53Extrapolation from Near to Far sites
x10x10101612128250km

ν beamx1010x10

Energy1211

Radius16128840FD (<2m)

41x10 4884401010

23E ν(GeV)

450x104812
Radius (m)x10 48SK

20

123E ν(GeV)

450

123
Radius (km)23450x10

Energy64812
Radius (m)512Radius8 SK

43mrad420(insensitive to primaryprotons)
3mrad
Measure, N(pπ, θ4π)
just after the horns.

41104 E ν(GeV)

512PION moniter
6Gas Cherenkov8 detector:

1112

3E ν(GeV)

4

50

1.0

10-6

123
Radius (km)54

55