June 24th, 2004
@ ICRR
ニュートリノ研究会

M. Yokoyama (Kyoto University)for K2K collaboration

1

Introduction

2

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
3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!
4

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
UniversityU.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 Columbia
Italy: Rome France: Saclay Spain: Barcelona / Valencia Switzerland: Geneva
RUSSIA: INR-Moscow

5Overview6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 monitor100m200m
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

20123E(GeV)?

4Super-K

57

Neutrino beamline @ KEK
Bird’s Eye Neutrino Beam Line

200m
100 m

8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 mrad904 Feb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-II

K2K-I

Jan 00

Jan 01

Jan 02

Jan 03

Jan 0410

Near detector system at KEK
 1KT Water Cherenkov Detector (1KT)
 Scintillating-fiber/Water sandwich Detector (SciFi)
 Lead Glass calorimeter (LG) before 2002 (K2K-I)
 Scintillator Bar Detector (SciBar) from 2003 (K2K-II) Muon Range Detector (MRD)

Muon range detector

11

3m

ν

2.5x1.3x300cm3 cell
 ~15000 channels
Multi-anode(dead-ch:<10)
PMT (64 ch.)
 WLS fiber+MAPMT readout

Detect short (~10cm) track p/π separation using dE/dx

3m

detector made of
Scintillator Bars

Extruded
scintillator(15t)

eretrimloca

 Full-active, fine-segment

EM

SciBar Detector

Precise ν spectrum
measurement
 ν interaction study

1.7m
Wave-length
shifting fiber
12Detector Photos
Scintillators (64layers)

EM calorimeterFibers and front-end elec.13

Just Completed!

Aug. 22, 2003

Event Display

Area of red circles
Proportional to energypµ

Area of circle
proportional to energy

CCQE (ν+n→µ–+p) candidate

3track event(ν+p→µ–+p+π?)
15Far Detector -Super-Kamiokande50,000 ton water Cherenkov detector(22.5 kton fiducial volume)

Optically separated INNER and OUTER detector

SK is back!

41.4m

w/ half PMT density

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

40m

Sep.-2002, before water filling16

GPSSK Events
Tspill TOF=0.83msec

SKTSK

Decay electron cut.

±500µsec≥20MeV Deposited Energy
No Activity in Outer Detector
Event 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)

17Near Detector
Measurements18

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

NEUT: K2K Neutrino interaction MCCC quasi elastic (CCQE) Smith and Moniz with MA=1.1GeVCC (resonance) single π(CC-1π)
Rein and Sehgal’s with MA=1.1GeVDIS GRV94 + JETSET with Bodek andYang correction.
CC coherent π

σ/E (10-38cm2/GeV)






Rein&Sehgal with the cross section
rescale by J. Marteau

NC+ Nuclear Effects

Total (NC+CC)
CC Total
CC quasi-elasticDIS
CC single π
NC single π0Eν (GeV)21

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θµ (MeV/c)

4.2 Near DetectorEνSpectrum Measurements
KT 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••• 23•

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

ν

CCQE

p

non-QE

DATA
CC QE
CC 1π
CC coherent-π
CC multi-πµΔθp2 track events

pµ25°

Δθp (degree)24

A hint of K2K forward µ deficit.
K2K observed forward µ deficit in all
ND (KT, SciFi and SciBar).
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.

q2recPreliminaryDATA
CC 1π
CC coherent-π

q2rec (GeV/c)2(Data-MC)/MCWe do not identify which processcauses the effect. The MC CC-1π(coherent π) model is corrected tobe consistent with data.
Oscillation analysis is insensitive to thechoice.

SciBarnon-QE Events
25

q2rec (GeV/c)2

Near Detector Data
 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.

With the low q2 suppression of nQE in SciBar,
angular distributions of all other samples arereasonably reproducible with the correction.

26

Near Detectors combined measurements(pµ,θµ) for 1track (1Rµ), 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) in MC.
③ Measure nQE/QE ratio for the entire θµ range.
27

1KT: µ momentum and angular distributions
with measured spectrum

0800

pµ (MeV/c)

1600

0

θµ (deg.)9028

SciFi (K2K-IIa with measured spectrum)Pµ 1trkθµ 1trkPµ 2trkQEθµ 2trk
QE

Pµ 2trk
non-QEθµ 2trk
non-QE
2902(GeV/c)

040(degree)

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

θµ 2trk QE

Pµ 2trk nQE

θµ 2trk nQE30

ND measurement results
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 fit
condition.
 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 ±0.03Eν
31

Oscillation Analysis

32

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

preliminaryDATA(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

Compared with null-oscillation…
22xx

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

Expected shape(No Oscillation)

CC-QE assumption

108

150.9Eν

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

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 Super35
K systematic uncertainties.

6. Likelihood fit 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

sin3622θ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]37

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.)38

νµ disappearance versus Eν shape distortionEν shape

NSK (#νµ)Δm2[eV2]

Δm2[eV2]

sin22θ

Both disappearance of νµ and the distortion ofEν spectrum give consistent results.

sin22θ39K2K-I vs K2K-II

Consistent results from both sub-samples.

40

Null oscillation probabilitypreliminary

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.
418. SummaryWith 8.91019 POT,K2K hasconfirmed
neutrino
oscillation
with 3.9σ.

Δm2[eV2]

preliminary

0.006
0.004

Disappearance of νµ0.002
2.9σ
 Distortion of Eν spectrum
0.02.5σ


0.2

0.4

0.6

sin22θ

0.8

1.042