Recent results from the K2K experiment

Yoshinari Hayato (KEK/IPNS)

for the K2K collaboration

• Introduction

• Summary of the results in 2001• Overview of the new analysis

(Number of events + Spectrum)

• Flux measurements at KEK• Oscillation analysis and results• Summary

Contents

K2K CollaborationHigh Energy Accelerator Research Organization(KEK)

Institute for Cosmic Ray Research(ICRR), University of TokyoKobe UniversityKyoto UniversityNiigata University

Okayama UniversityTokyo University of Science

Tohoku University

Chonnam National UniversityDongshin University

Korea UniversitySeoul National University

Boston UniversityUniversity of California, IrvineUniversity of Hawaii, Manoa

Massachusetts Institute of TechnologyState University of New York at Stony Brook

University of Washington at Seattle

Warsaw University Solton Institute

Introduction Principle of the long baseline experiment

Oscillation Probability=

1)Reduced number of events

2)Distorted energy spectrum

Compared to 　 the NULL oscillation case,

(2 flavor)

Fixed distancePro

bab

ility 1

0.5

00.5 1 2 2.5 31.50 3.5

0.5 1 2 2.5 31.50 3.5

# of

inte

ract

ion

s

No oscillationOscillated

dip around 0.6 ~ 0.7 GeV

clear dip

Oscillationmaximum

long baseline experiment

The K2K experiment

250km

Super Kamiokande (far detector)50kt water Cherenkov detector

12GeV PS@KEKbeamlinebeam monitors near detectors

Neutrino beamGeV3.1~ E

%)98(~ pure almost

Beam monitors & near detectors

• beam direction

Far detector

disappearancespectrum distortion

• monitor

• flux & spectrum

• flux & spectrum

200m

Front (Near) Detector direction () spectrum , rate

12 GeV PSfast extraction every 2.2secbeam spill 1.1s ~6x1012 protons/spill

Double Horn(250kA) ~20 x flux

Neutrino beamline

Pion monitor

（ P, after

Horn ）　Near to Far flux ratio RFN

-monitor direction (p)

Al target

1kt water Cherenkov detector

Fine grained detectors• Scintillating fiber tracker

• Muon range detector

same type as SK(25t fid. vol.)

water target

(330t fid. vol.)Iron target

(6t fid. vol.)

beam monitor (mom. & dir.)

Muon range detector

CCQE identification

water target

(SciFi)

(MRD)

(1kt)

Near neutrino detectors

Super-Kamiokande (Far detector)

39m

41.4

m50kt water Cherenkov detector

Outer detector

Inner detector

1885 8” PMTs

11146 20” PMTs

Atmospheric ~8 events/day (FCFV)

1000m under the ground

Fiducial volume 22.5kt

accidental coincidence events/day~10-5

Event selection at Super-Kamiokande

Tspill TSK

GPS

TOF=0.83ms

T SKT spillT TOF- -

s3.120.- T

w/o pre-activity

High energy trigger

(FC = no activityevents in thefiducial volume (FCFV)

in the outer detector)

requiring

s5.1Number of FCFV eventsNsk=56 events

Expected number of atmospheric BG

<10-3 events(1ring -like=26)

From June ‘99 to July ’01 (4.8 x 1019 protons on target)

Fully contained

Summary of K2K results in 2001

From June ’99 to July ‘01accumulated number of protons

4.8x1019 POT

Neutrino beam was very stable

• direction of the beam : controlled less than 1mrad.

confirmed by profile monitor

muon range detector (MRD)( interaction vertex)

• energy spectrum of

1kt water Cherenkov detector (1kt) & MRDconfirmed by

for the analysis

( decay)

# of FCFV events in Super-K7.38.080

Observed : 56 Expected :

Probability of null oscillation < 3%

Full and Improved error estimations

and spectrum shape analysis

Neutrino flux @ SKestimated by Monte-Carlo

monitorconfirmed bynormalization was done by 1kt water Cherenkov detector

[number of protons was measured by Current Transformer (CT)]

Next step

Summary of K2K results in 2001

Neutrino interactions around 1GeVCharged current quasi-elastic scattering (QE)

pion productions

p: muon momentum

: muon angle

proton

p

nucleon

p

1kt

SciFi

: single ring -like FCFV events

or single track events

: QE-like 2track events

(when protons are not observed)

(when protons are identified)

SciFi : non-QE-like events

1kt : FCFV events(single-ring or multi-rings)

E can be reconstructed from P and .

Flow of the oscillation analysis

Observed quantities at the near detectors

Neutrino interaction models

(P, ) for each event category

Obtain neutrino spectrum at near detectors

Near to Far extrapolation (RFN(E))

Predict neutrino spectrum without oscillation (SK(E))

Fit the observed results at SK

Use maximum likelihood fit

(sin22,m2 )

Observables at SK • number of events (NSK )

• Reconstructed energy of

with

)(Erec

non-QE-like

QE/non-QE selection1. Select 2track events2. Select muon track3. Calculate expected direction of proton exp

(assuming QE interaction)4. Compare with the observed direction of 2nd track obs

track

2nd track

p<25 deg. : QE-like p>30 deg. : non-QE-like

p= |exp- obs|

QE -like

QE and non-QE eventsin SciFi 2track events

Used data for near(E)

KT Fully Contained Fiducial Volume (FCFV) events(0) No. of events (Evis >100MeV) (1) Single -like events

SciFi (2) 1-track events(3) 2-track QE-like events(4) 2-track non QE-like events

flux near(E) (8 bins) interaction model (parameterized as QE/non-QE ratio)

Pion monitor & Beam simulation distribution in (p)

flux estimation near(E) w. error

4 sets of (p, ) distributions

Fitting method ( flux at KEK)

2=227 for 197 d.o.f. (90 from 1kt, 137 from FGD)for fitted (p) dist. of 1KT and FGD (124 data points)

0 ~0.5 GeV

0.5 ~0.75GeV

0.75 ~1.0GeV

E QE(MC) non-QE(MC)

neutrino spectrum (E) 　 and interaction model (QE/non-QE ratio).

Fit the parameters.

Measured (p)

7bins•••

•••

8bins

p

p

Prepare MC templates

(E) , QE/non-QE ratio …

Fitted results (1kt)

0 20 40 60 100 (deg.)100800 400 800 p(MeV/c)1200

pand distributions of 1kt 1ring -like FCFV events

Both distributions agree well with the fitted MC.

Fitted results (SciFi)

p(GeV/c) (deg.)

1track

2track

non-QE2track

QE like

like

0 1 2 0 25 50

Very good agreements

Reconstruction of neutrino energy at SK

p: muon momentum

: muon angle

proton

p

cos

2/2

PEm

mEm

n

n

recE

Use single-ring -like FCFV (1R) events

Assuming QE interaction and reconstruct E

( ~50% of K2K 1Revents are CCQE)

E: muon energy

Oscillation analysis

1.Maximum Likelihood method

Normalization term (Nexp= 80.1+6.2-5.4 )

Shape term (for 1R)

Constraint term for systematic parameters. (error matrices)

Ltot = Lnorm(f) Lshape(f) Lsyst(f)

2.different treatment of systematic term. Generate many MC samples. (changing systematic parameters within the error.)

Ltot = weighted mean of L for the MC samples

1) Used data sets1. Number of events June ’99 - July ’01

FCFV events (56 events)

2. Spectrum shape Nov. ’99 - July ’01

1Revents (29 events)

2) Analysis methods

Allowed m2 by NSK and shape analysis

NSK and shape analysis indicate the same m2 region for sin22=1

Number of Events only

Spectrum Shape only

NSK + Shape

Allowed regions

m2=1.5~3.9 x 10-3 eV2

@sin22(90%CL)

Best fit parameterssin22,m2

=(1.0 , 2.8x10-3 eV2 )

=(1.0 , 2.7x10-3 eV2 )[Method-1]

[Method-2]

90%CL

dashed: method1solid : method2

Best fit resultsReconstructed energy of neutrino for 1R

and NSK

Without OscillationBest fit

Best fit parameterssin22,m2

=(1.0 , 2.8x10-3 eV2 )

• NSK

Observed = 56= 54

• Shape KS test 79%

Both shape and NSK

Very good agreements

Expected (W/osc.)

Null oscillation probability

method-1 method-2NSK only 1.3% 0.7%Shape only 15.7% 14.3%NSK + Shape 0.7% 0.4%

Probability of null oscillation is less than 1%.

Use log(likelihood) from best fit point in the physical region

SummaryK2K Oscillation analysis on June’99 ~ July ’01 data

Use both Number of events + Spectrum shape

Full and Improved error estimationsand spectrum shape analysis

(June ’99 – July ’01) (Nov. ’99 – July ’01)

• Null oscillation probability is less than 1%.• Spectrum shape distortion

m2=1.5~3.9 x 10-3 eV2 @sin22(90%CL)

• Oscillation parameters (sin22 and m2) are

c.f. m2=1.6~3.9 x 10-3 eV2 @sin22(90%CL)

and observed # of events @SK indicates consistent oscillation parameter regions.

consistent with the atmospheric results.