C. Mariani INFN Rome

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May 19th, 2005 _at_LNF Spring School Bruno Touschek
New K2K Results

• C. Mariani (INFN Rome)

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K2K Collaboration
250km
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
Institute Since 2002 JAPAN Hiroshima
University / Osaka University U.S.A. Duke
University CANADA TRIUMF / University of
British Columbia ITALY Rome FRANCE Saclay
SPAIN Barcelona / Valencia SWITZERLAND Geneva
RUSSIA INR-Moscow
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1. Introduction and history of K2K

• 1995
• Proposed to study neutrino oscillation for
  atmospheric neutrinos anomaly.
• 1999
• Started taking data.
• 2000
• Detected smaller number of neutrinos than the
  expectation at a distance of 250 km. Disfavored
  null oscillation at the 2s level.
• 2002
• Observed indications of neutrino oscillation. The
  probability of null oscillation is less than 1.
• 2004
• Confirm neutrino oscillation with both a deficit
  of nm and the distortion of the En spectrum.

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2. K2K experiment
1 event/2days
1011 nm/2.2sec (/10m?10m)
106 nm/2.2sec (/40m?40m)
nm
12GeV protons
nt
p
SK
m
TargetHorn
100m
200m decay pipe
250km
p monitor
Near n detectors (ND)
m monitor
(monitor the beam center)

• Signal of n oscillation at K2K
• Reduction of nm events
• Distortion of nm energy spectrum

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Accumulated POT (Protons On Target)
10.5x1019 POT, 8.91019 POT for Analysis
Accumulated POT (1018)
K2K-I
K2K-II
K2K-II
K2K-I
protons/pulse (1012)
Oct-Nov 04
Jan 03
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SK Events
Decay electron cut.
?500msec
?20MeV Deposited Energy
No Activity in Outer Detector Event Vertex in
Fiducial Volume More than 30MeV Deposited Energy
107 events
Analysis Time Window
?5msec
-0.2ltTSK-Tspill-TOFlt1.3msec
(BG 1.6 events within ?500ms
2.410-3 events in 1.5ms)
TDIFF. (ms)
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3. Analysis Overview
KEK
Observation n, pm and qm
Measurement F(En), n int.
n interaction MC
Far/Near Ratio (beam MC with p mon.)
SK
Observation n and En rec.
Expectation n and En rec.
(sin22q, Dm2)
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4. Near detector measurements

• 1KT Water Cherenkov Detector (1KT)
• Scintillating-fiber/Water sandwich Detector
  (SciFi)
• Lead Glass calorimeter (LG) before 2002
• Scintillators Bar Detector (SciBar) after 2003
• Muon Range Detector (MRD)

LG calorimeter
Muon range detector
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4.1 1KT Flux measurement

• The same detector technology as Super-K.
• Sensitive to low energy neutrinos.

?Far/Near Ratio (by MC)110-6
M Fiducial mass MSK22,500Kton, MKT25ton e
efficiency eSK-I(II)77.0(78.2), eKT74.5
exp SK
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4.2 SciBar neutrino interaction study

• Full Active Fine-Grained detector (target CH).
• Sensitive to a low momentum track.
• Identify CCQE events and other interactions
  (non-QE) separately.

CCQE Candidate
2 track events
CCQE
n
p
non-QE
m
Dqp qobs -qCCQE
p
m
25?
Dqp (degree)
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4.3 Near Detectors combined measurements

• (pm, qm) for 1track, 2trackQE and 2track nQE
  samples ? F(En), nQE/QE
• Fitting parameters
• F(En), nQE/QE ratio
• Detector uncertainties on the energy scale and
  the track counting efficiency.
• The change of track counting efficiency by
  nuclear effect uncertainties proton
  re-scattering and p interactions in a nucleus
• Strategy
• Measure F(En) in the more relevant region of
  qm?20? for 1KT and qm?10? for SciFi and SciBar.
• Apply a low q2 correction factor to the CC-1p
  model (or coherent p).
• Measure nQE/QE ratio for the entire qm range.

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5. Super-K oscillation analysis

• Total Number of events
• Enrec spectrum shape of FC-1ring-m events
• Systematic error term

f x Systematic error parameters
Normalization, Flux, and nQE/QE ratio are in fx
Near Detector measurements, Pion Monitor
constraint, beam MC estimation, and Super-K
systematic uncertainties.
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K2K-SK events
(56)
for Enrec
Updated from the previous analysis
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KS probability0.08
SK Events
Toy MC
Expected shape (No Oscillation)
EnrecGeV
CC-QE assumption
107
150.9
V Nuclear potential
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Data are consistent with the oscillation.

• NSKobs107
• NSKexp (best fit)150.9

preliminary
Dm2eV2
Best Fit KS prob.36
sin22q
EnrecGeV
Based on DlnL
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Null oscillation probability
The null oscillation probabilities are calculated
based on DlnL.
The value is changed from the previous one.
Disappearance of nm and distortion of the energy
spectrum as expected in neutrino oscillation.
K2K confirms neutrino oscillation discovered in
Super-K atmospheric neutrinos.
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6. Summary

• With 8.91019 POT, K2K has confirmed neutrino
  oscillations at 4.0s (Phys.Rev.Lett.94081802,2005
  ).
• Disappearance of nm 3.0s
• Distortion of En spectrum 2.6s

Dm2eV2
0.006
- 68 - 90 - 99
0.004
0.002
0.0 0.2 0.4 0.6 0.8 1.0
sin22q
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Backup slide
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Neutrino beam and the directional control

• 1GeV neutrino beam by a dual horn system with
  250kA.

The beam direction monitored by muons
X center
?1 mrad
Y center
lt?1mrad
5 years
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Neutrino spectrum and the far/near ratio
Far/Near Ratio
nm energy spectrum _at_ K2K near detector
beam MC w/ PION Monitor
10-6
1.0
2.0
En (GeV)
En (GeV)
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NEUT K2K Neutrino interaction MC

• CC quasi elastic (CCQE)
• Smith and Moniz with MA1.1GeV
• CC (resonance) single p(CC-1p)
• Rein and Sehgals with MA1.1GeV
• DIS
• GRV94 JETSET with Bodek and Yang correction.
• CC coherent p
• ReinSehgal with the cross section rescale by J.
  Marteau
• NC
• Nuclear Effects

s/E (10-38cm2/GeV)
En (GeV)
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Neutrino energy reconstruction
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4.3 Near Detector Spectrum Measurements

• 1KT
• Fully Contained 1 ring m (FC1Rm) sample.
• SciBar
• 1 track, 2 track QE (Dqp25?), 2 track nQE
  (Dqpgt25?) where one track is m.
• SciFi
• 1 track, 2 track QE (Dqp25?), 2 track nQE
  (Dqpgt30?) where one track is m.
• After applying the low q2 suppression of nQE
  observed in SciBar, the angular distributions of
  all other samples are reasonably reproduced.

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A hint of K2K forward m deficit.

• K2K observed forward m deficit.
• A source is non-QE events.
• For CC-1p,
• Suppression of q2/0.1GeV2 at q2lt0.1GeV2 may
  exist.
• (0.1GeV2 value comes from fitting 2 track nonQE
  sample in SciBar)
• For CC-coherent p,
• The coherent p may not exist.
• We do not identify which process causes the
  effect. The MC CC-1p (coherent p) model is
  corrected phenomenologically.
• Oscillation analysis is insensitive to the choice.

Preliminary
q2rec
q2rec (GeV/c)2
(Data-MC)/MC
SciBar non-QE Events
q2rec (GeV/c)2
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Flux measurements

• c2638.1 for 609 d.o.f
• F1 ( En lt 500) 0.78 ? 0.36
• F2 ( 500? En lt 750) 1.01 ? 0.09
• F3 ( 750? En lt1000) 1.12 ? 0.07
• F4 (1000? En lt1500) 1.00
• F5 (1500? En lt2000) 0.90 ? 0.04
• F6 (2000? En lt2500) 1.07 ? 0.06
• F7 (2500? En lt3000) 1.33 ? 0.17
• F8 (3000? En ) 1.04 ? 0.18
• nQE/QE 1.02 ? 0.10
• The nQE/QE error of 10 is assigned based on the
  sensitivity of thefitted nonQE/QE value by
  varying the fit criteria.
• qgt10?(20 ?) cut nQE/QE0.95 ?0.04
• standard(CC-1p low q2 corr.) nQE/QE1.02 ?0.03
• No coherent pnQE/QE1.06 ?0.03

F(En) at KEK
preliminary
En
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1KT m momentum and angular distributions.with
measured spectrum
flux measurement
low q2 corr.
1Kt m-like sample Quasi elastic Single pion
20
0
90
qm (deg.)
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En QE (MC) nQE(MC)
MC templates
0-0.5 GeV
KT data
qm (MeV/c)
0.5-0.75GeV
0.75-1.0GeV
1.0-1.5GeV
Pm (MeV/c)

• n flux FKEK(En) (8 bins)
• n interaction (nQE/QE)

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SciFi (K2K-IIa with measured spectrum)
qm 1trk
Pm 1trk
flux measurement
Pm 2trk QE
qm 2trk QE
Pm 2trk non-QE
qm 2trk non-QE
10
0
2
(GeV/c)
0
40
(degree)
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SciBar (with measured flux)
qm 1trk
Pm 1trk
flux measurement
Pm 2trk QE
qm 2trk QE
qm 2trk nQE
Pm 2trk nQE
10
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nm disappearance versus En shape distortion
En shape
NSK (nm)
Dm2eV2
Dm2eV2
sin22q
sin22q
Both disappearance of nm and the distortion of En
spectrum have the consistent result.
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Results

• Best fit values.
• sin22q 1.51
• Dm2 eV2 2.19?10-3
• Best fit values in the physical region.
• sin22q 1.00
• Dm2 eV2 2.79?10-3
• DlogL0.75

A toy MC
Dm2
12.6
2.79
sin22q1.51 can occur due to a a statistical
fluctuation with 12.6.
sin22q
1.51
1.00
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Log Likelihood difference from the minimum.
DlnL
DlnL
- 68 - 90 - 99
- 68 - 90 - 99
sin22q
Dm2eV2

• Dm2lt(1.873.58)10-3 eV2 at sin22q1.0 (90 C.L.)

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K2K-I vs K2K-II
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The change of NSKexp in K2K-I (Bugs)

• The detector position
• 295m ? 294m -1
• MC difference between KT and SK
• KT MA(QE)1.1 s (NCel)KT1.1s(NCel)SK
• SK MA(QE)1.0 ? Efficiency change! -1
• NSKexp Change 2

294m
295m