Measurement of inclusive po production in charged current neutrino interactions at K2K

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Measurement of inclusive po production in charged
current neutrino interactions at K2K
C. Mariani (INFN Roma and University of Rome La
Sapienza)
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K2K collaboration
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Outline

• Motivations
• K2K experiment
• neutrino beam
• near detectors
• far detector (SK)
• MC of neutrino interaction
• CC inclusive po measurement (PRD in preparation)

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Motivations

• Background to the identification of ne
  interaction in SK during T2K
• Nuclear effects are not well known (proton
  re-interaction with p0 production)
• First measurement of CC p0 neutrino cross
  section on Carbon
• Improving the precision of previous CC p0 cross
  section measurement

5
Poor knowledge on neutrino cross section, very
few experimental results, background to the
oscillation ne ? nm Nuclear reinteractions are
not well known (theoretical model are under
development, test of QCD at low Q2). Actual
precision on the neutrino cross sections is a
limit on the systematics of future experiments
together with the uncertainties on the knowledge
of the neutrino fluxes (process of
hadro-production in the simulation of the
neutrino beam).
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• K2K is the first long baseline neutrino
  experiment
• Proposed to confirm atmospheric neutrino
  oscillations
• Data taken from June 1999 to November 2004.

• T2K is the next generation long baseline
  neutrino experiment
• 50 GeV high intensity proton beam
• Off-axis neutrino beam
• Data taking expected in 2009.

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KEK
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K2K(KEK to2 Kamioka)
Super-K
Average neutrino energy 1.3 GeV
250km
neutrino beam
nt
nm
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Super-Kamiokande
Worlds largest water Cherenkov detector
cylindrical shape, 41 m in height, 39 m in
diameter, housing 50 kton of water Separated
into ID and OD parts ID 11,416 (5,182) 20-inch
PMTs during K2K-I (K2K-II) phase Reconstruct
interaction vertex, number of tracks producing
Cherenkov rings, e-like/m-like track properties,
fully or partially-contained interactions FV
22.5 ktons GPS system syncronize timing of
beam spill between KEK and SK, to distinguish
beam neutrino interactions from cosmic ray
induced activity.
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Neutrino Beam
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Decay volume Beam Dump

• 200 m decay volume p ? m nm
• Cylindrical in shape and separated in three
  sections of different radius
• filled with helium gas at 1 atm
• a beam dump stops all particles except
  neutrinos
• a 3.5 m thick iron, 2m thick concrete, and a
  region of soil 60 m long.

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Neutrino flux _at_ near detector
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K2K Near Detector

• 1 Kiloton Water Cerenkov Detector
• Fine Grain Detector
• Scintillating-Fiber/water target tracker
• Originally Lead-Glass, since summer 2003, fully
  active carbon Scintillator-Bar tracker
  Electromagnetic Calorimeter
• Muon Range Detector (MRD)

EC
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1KT

• Primary role to measure the nm interaction rate
  and the nm energy spectrum
• 10.8 m diameter and height
• 1 kton pure water
• Optically separated into ID (680 PMTs, 40
  coverage) and OD (68 PMTs)
• From Cherenkov light, reconstruct
• Neutrino interaction vertex
• Number of tracks producing Cherenkov rings
• Showering (e, g) or non-showering (m, p)
  track type
• Fully contained or partially contained event
  tracks.
• p0 neutral current neutrino interaction
  measurement

Phys.Lett.B619255-262,2005
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SciFi
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SciBar and EC

• Extruded scintillator bars with WLS fiber
  readout
• Neutrino target is the scintillator itself
• Volume 3 x 3 x 1.7 m3 (16 tons)
• 2.5 x 1.3 x 300 cm3 cell
• 15.000 channels
• Light yield 10p.e./MeV
• Detect tracks down to 8cm
• Distinguish protons from pions by using dE/dx
• Wrong ID lt 5 (lt1GeV/c proton)
• High efficiency for CCQE 2-track

• Spaghetti calorimeter 1mm scintillating fibers
  in grooves of lead foils 1mm thick
• 4x4 cm2 cell readout from both ends
• 2 planes (11X0)
• Hor 30 modules
• Ver 32 modules
• Energy resolution13/?E

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MRD
EC
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Neutrino energy spectrum
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K2K final oscillation result
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K2K neutrino interaction MC
CC Coherent pion production neglected according
to the SciBar measurement (Hasegawa et al. PRL
95252301, 2005)
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CC p0 Inclusive signal definition
??
??
anything else (if any)
Target nucleus
?

• We define signal an event with
• 1 or more pos from the neutrino interaction
  vertex
• 1 or more pos from a re-interaction inside the
  target nucleus
• 1 or more h (decaying either in pos or photon
  pairs).

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Signal ratios in CC

• 5.4 single pion production
• 4 single???s produced via resonances decay
  (mainly D)
• 1.4 ??s produced by nuclear reinteractions
  in the target nucleus following a
  resonance production
• 6.2 multi pion production (DIS with 1.3GeV lt W lt
  2.0 GeV, np gt 2)
• 5.6 ??s produced in multi pion interactions
• 0.6 ??s produced by nuclear reinteractions
  in the target nucleus following a
  multi-pion neutrino interaction
• 1.5 from ? decay
• ? ?- ?? (22.6 of total ? b.r.)
• 3?? (32.5 of total ? b.r.)?
• 2 gamma (39.4 of total ? b.r.)
• 0.4 from DIS

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Normalization sample
The total number of selected CC interaction are
11606 (20.2x1018 POT ), the simulated neutrino
interactions are 1500000.
CC selection cut (eff.49.5, pur.97.5)
At least one track reconstructed in SciBar Track
should be in FV 10.9 m3 Track should be matched
with MRD
CCQE samples
1T-QE a single reconstructed track 2T-QE 2
reconstructed tracks with Dqplt 20 2T-nQE-p 2
reconstructed tracks with Dqp? 20, pion
like 2T-nqe-p 2 reconstructed tracks with Dqp?
20, proton like.
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Dqpdistribution
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Experimental signature
Muon ID one track matched to a MRD track
corresponding to the muon produced in the CC
interaction CC po features A muon track to
define the neutrino interaction vertex, plus two
photon conversions (tracks disconnected from the
neutrino interaction vertex and pointing to it in
SB, or/and a narrow cluster in the EC Possibly
other tracks and/or EC clusters.
g
g
Vertex
m
CC po selection Photon tracks should be
reconstructed in FV 13.3 m3 Not matched with a
MRD 3D reconstructed track In time within 10 ns
with the muon track Pointing within 25 cm and
disconnected gt20 cm (in both views) to the
interaction vertex EC Evert. Cluster gt 50 MeV
EC Ehor.
Cluster gt 25 MeV
Vertex
g
g
m
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Disconnection from neutrino interaction vertex of
candidate photon tracks
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Events with at least 2 photon candidates
The events with at least two photons
reconstructed in data(MC) are 479(380). The
excess is of 26 4 between data and MC
considering all the reconstructed sample.
So the excess is consistently seen in
both SciBar and EC which have different
efficiencies and systematics.
1.26
SBSB
SBEC
ECEC
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p0 combinatorial

• Photon candidates per event should be 2
• In 21 of the events selected we have gt2
  candidates, in this case we reconstruct the po
  mass as follows
• If all photons reconstructed in SB ?o vertex
  closest to ? vertex (15)
• If 1 or more photons are reconstructed in the
  EC best mass (6).

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p0 invariant mass
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479 pos are reconstructed in data and 380 in
MC. The overall efficiency is 7.6 and the
purity is 59.

True po signal (59)

• Source
• Single pion from resonances 45
• Multi pion from resonances 49
• DIS 6

• Composition
• Prompt po 82
• po from reinteractions 11
• h decay 7

• The Background (41) is made of
• Reinteractions outside target nucleus 54
• NC interactions 13
• Others 33

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Neutrino energy reconstruction
The neutrino energy in CC can be reconstructed
from the muon energy and angle using the
formula Where V nuclear potential 27 MeV
and pm and qm are the muon momentum and angle.
W2 true for single pion final state
Our selected sample is mostly non-QE (98), for
the sub-sample of the single pion final state
there is a broad spectrum of W2 corresponding to
the 18 different resonances taken into account in
our reference MC simulation. We use the average
value of W1.483GeV to reconstruct the neutrino
energy, the resolution in the neutrino energy
reconstruction is 600MeV.
Mean 1483 MeV
MeV
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Reconstructed neutrino energy before fit
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Likelihood Fit description

• We perform a maximum likelihood fit of
• where Poiss(n, ?) is the poissonian for n events
  with expectation value ?
• We fit the 1-dimensional reconstructed neutrino
  energy distribution Hk
• Hk is the number of events in the bin k of
  1-dimensional histogram
• k1,30 labels 30 bins between 0-5 GeV in the
  reconstructed neutrino energy
• datak number of events observed in bin k
• predk number of events expected in bin k
• ns and preds are respectively the number of
  events observed in data and expected by MC for
  the different normalization sub-samples.
• Lsyst is a term used in the systematic errors
  evaluation.

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sCC po cross-section
The inclusive po production shows an excess of
(3915) (statistical and systematic error added
in quadrature). MC prediction is strongly
dependent from the ratio RnQE.
K2K MC 0.1630.001
A result with the MC prediction almost
independent from the ratio RnQE is the inclusive
po production normalized to the non quasi elastic
charged current cross section
K2K MC 0.2200.043
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The resulting double ratio nQE/QE is
Fixing the CC single pion contribution to the
value measured by K2K-SciBar (PRD in preparation)
we can read our po cross-section measurement in
terms of the CC multi pion cross section
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Reconstructed po mass after fit
Reconstructed neutrino energy after fit
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po momentum after fit
po cosq
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Result energy dependent
CC p0 sample
In order to measure the p0 production cross
section as a function of the neutrino energy we
consider four energy bins in the reconstructed
neutrino energy distribution. Due to resolution
effect an event with reconstructed energy in bin
i can have a true neutrino energy (MC) in a
different bin j. We use a migration matrix to
take into account for this effect.
CC QE
Measured cross section ratio as function of the
neutrino energy
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Comparison with other experiment
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Summary
K2K-SciBar/EC measures CC inclusive po
production in C8H8 (C12) to be larger than NEUT
expectations The larger po yield in CC,
together with the agreement of the single-pion in
the CC1p analysis, translates in an excess of
(30?18) of the NEUT multi-pion
cross-section The po production in CC is
larger but consistent with previous results
(exclusive 1po and ppo on Deuterium) with
improved precision. At present level of precision
the CC inclusive po production is significantly
larger than our MC prediction.
(NEUT 0.358)
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Near future SciBooNE
Spokespeople T.Nakaya, Kyoto University M.O.
Wascko, Imperial College

• New experiment at Fermilab
• K2K SciBarEC already installed and taking data
  on the BNB
• Better at distinguish photons from electrons
• Check MiniBooNEs background estimates
• Measure neutrino and anti-neutrino cross section
  at the same energy as T2K.

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Thank You
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Backup
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Reconstructed energy for muon (data/MC)
Muon energy resolution
Dqpdistribution
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Resolution on reconstructing neutrino interaction
vertex
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p0 combinatorial

• Photon candidates per event should be 2
• In 21 of the events selected we have gt2
  candidates, in this case we reconstruct the po
  mass as follows
• If all photons reconstructed in SB ?o vertex
  closest to ? vertex (15)
• If 1 or more photons are reconstructed in the
  EC best mass (6).

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• The energy resolution for photons is
• 50 MeV for the higher energy photon
• 60 MeV for the lower energy photon.
• The 3D angle reconstruction resolution is
• 0.15 rad for the higher energy photon
• 0.18 rad for the lower energy photon.

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We allow individual reweighting factors Wi for
the individual contribution in our MC. Wi 1 ?
standard reference MC.
Bkg
Signal
i interaction type fnorm is the absolute
normalization factor between data and MC and is a
free likelihood parameter. The double ratio
nQE/QE is defined as
The Lsyst term is
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Cross-section measurement
For the inclusive po production normalized to the
quasi elastic charged current cross section, our
measurement shows an excess of the inclusive po
production of (4013) (statistical and
systematic error added in quadrature). The MC
prediction is strongly dependent from the ratio
RnQE. For the inclusive po production
normalized to the non quasi elastic charged
current cross section, our measurement shows an
excess of (2011). The MC prediction is almost
independent from RnQE which is the main source of
the systematic error. The excess taking
into account the measurement of CC pi cross
section presented by Lisa could be interpreted as
an excess of the CC multi pion cross section
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Systematic error evaluation
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• To get the measurement as a function of the
  neutrino energy we take 4 energy bins
• En lt 1.5 GeV
• 1.5 GeV lt Enlt 2.0 GeV
• 2.0 GeV lt En lt 2.5 GeV
• En ? 2.5 GeV
• The bin is chosen to have approximately the same
  statistics.
• Due to resolution effect an event with a
  reconstructed energy in the bin I can have a real
  energy in different bin j. We use a migration
  matrix to take into account for this effect
• The migration matrix Mij result

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From the events observed in data in bin i,
NiDATArec, subtracting the background events
expected from MC NibkgMC and using the inverse of
the migration matrix above we compute the number
of events background subtracted Where
eiefficiency. Same procedure is applied also to
the CCQE samples.
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Comparison with other experiments

• The ratio between po cross section and CCQE cross
  section is computed dividing the experimental
  result from old experiment (Barish, Kitagaki,
  Radencky and Day) by the CCQE cross section
  measured by Barish. To compare we have to rescale
  these results taking into account the different
  neutrino target

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Efficiency as a function of ...
po cos(q)
po cos(q)
po momentum
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Target Horn
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