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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.