Search for coherent charged pion production
in neutrino-carbon
interactions
F.Sánchez for the K2K collaboration
UAB/IFAE
K2K Experiment
• disappearance• Energy spectrum distortion
250km
GeV3.1~ E %)98(~ pure almost
K2K near detectors
1kt Water Čerenkov detector (1kt) Scintillating Fiber Detector (Scifi) Scintillator Bar Detector (SciBar) (from 2003) Muon Range Detector (MRD)
SciBar Detector
• Extruded Scintillator Bar • WLS fiber readout.• Active target.• 2.5 x 1.3 x 300 cm3 cell.• Order of 15000 channels.• Light yield ~8 p.e./MeV.• Detect 10 cm track.• Distinguish protons from by
using dE/dx. Miss ID < 5% (<1GeV/c proton).
Extrudedscintillator
(15t)
1.7m
3m
3m
EM calorim
eter
Motivation of SciBar• Assuming Charged Current Quasi-
Elastic interaction the energy is reconstructed
• Also q2 can be computed the same way.
• CC-nonQE interactions are backgrounds for E measurement
• Neutrino flux at <1GeV and neutrino interactions around 1GeV are important since the oscillation maximum at K2K is around 600 MeV.
proton
p
n
cosPEm
2/mEm
n
2n
recECC-QE Interaction
nucleon
N
CC-nonQE Interaction
VME board
Detector Components
64 chMA-PMT
Frontendboard
Electron Catcher
4 cm
8 cm
262 cm
Readout Cell
Beam
Scintillating Fibers and Lead Plates
• Energy tail catcher. • Measure electron neutrino flux.• Study 0 production.• “spaghetti” calorimeter re-used
from CHORUS. • 1mm diameter scintillating fibers
in the grooves of lead foils.• 4x4cm2 cell is read out from both
ends.
• 2 planes with a total of 11X0
Horizontal: 30 modules Vertical : 32 modules• Expected resolution 14%√E.
CC Event Selection• SciBar MRD 3D track matching (3D)
– ~35% of all interactions. CC-QE fraction is ~55% (MC)• MRD 1st layer stopping (1L)
– ~9% of all interactions. CC-QE fraction is ~31% (MC)
• Momentum of is computed from its range in SciBar, EC and MRD.
Vertex
MRD(Iron plates
and drift tubes)SciBar
Vertex
3D matching (3D) 1st layer stopping (1L)
Particle Id.
Mu-like(purity ~ 99.6%)
Proton-like(purity 90%)
Range (cm)
Total deposit energy vs. Range
DATA
Proton like-Like
sample
protonsample
Proton Efficiency
Muon mis-ID
Excellent p/ using dE/dx
For 90% proton efficiency we get 1.7% of miss-ID probability.
Muon C.L.
En
erg
y (
mip
s)
p
CCQE candidateCCQE candidate
1
23
CCnQE candidateCCnQE candidate
SciBar Event examples
• CC quasi elastic (CCQE)– Smith and Moniz with
MA=1.1GeV
• CC (resonance) single (CC-1p)– Rein and Sehgal’s with
MA=1.1GeV
• DIS– GRV94 + JETSET with Bodek
and Yang correction at low q2.• CC coherent p
– Rein&Sehgal model based on PCAC.
• NC+ Nuclear Effects
/E (10-38cm2/GeV)
Total (NC+CC)
CC Total
CC quasi-elastic
DIS
CC single
NC single 0
E (GeV)
NEUT: K2K Neutrino interaction MC
Coherent + production
• K2K and MiniBoone has reported in the past a deficit of events at low q2: nuclear effects?, coherent production?,…
K2K
MiniBoone
Coherent + signature
proton
• In the coherent + production the neutrino scatters off the entire nucleus with small energy transfer.
• Rein&Sehgal based on the Partially Conserved Vector Current (PCAC) model has been tested at higher energies.
• The coherent + signature consists on:– a and a + in the final state. – No activity in the area of the vertex beyond the outgoing
tracks.
Coherent + analysis
This selection gives 5 independent samples: • 1 track, 2 tracks QE, 2 tracks nQE proton and 2 tracks nQE
pion are used to characterize the background.• The coherent sample is used to set the measurement. • The overall CC sample is used to define the normalization:
MRD sampl
e 2 tracks
1 track
non QE
QE
Pion like
Proton
like
Coherentsample
nQE/QE selectionQE Non QE
QE Efficiency
QE Purity
80% Efficiency70% Purity
QE enriched sample
p < 25deg
nQE enriched samplep > 25deg
CC1
Multi Pi
Others
CCQE
Coherent Pip is the angle between
the second track and the predicted proton direction in CCQE reactions.
p
(deg)
(deg)
Proton- Id
Proton efficiency
Proton Purity
85% Efficiency80% Purity
enrichedProton
enriched
Muon confidence level
Proton
Pion
Others
Muon Confidence Level (2nd Track : nQE sample)
1 track
2 tracks QE
2 tracks NQE like
2 tracks NQE proton-like PION
PROTON
QE
NQE
tracks
tracks fff 1
2
QE
NQE
tracks
tracks ff 1
2
tracks
tracksf1
2
1
Constrain of MC uncertainties• Control samples are fitted simultaneously for
q2>0.10(GeV/c)2 to constrain MC uncertainties.
• Simultaneously the MRD momentum scale and the MC true ratio between NQE and QE are fitted.
Constrain of MC uncertainties
Parameter Deviation from nominal
Error
Rnqe/qe 0.071 0.074MRD momentum
scale -0.0118 0.0030
f2trac/1track 0.014 0.026
fnqe/qe 0.043 0.054
fproton/ 0.079 0.051
•These values are used to re-weight the MC.
•Errors are propagated to the final sample.
Selected samples after the fit
2/d.o.f = 73.2 / 80 for q2 > 0.10 (Gev/c)2
Defi
cit
of
even
ts
q2<
0.1
0(G
ev/c
)2
No e
vid
en
ce
for C
oh
ere
nt
even
ts
Vertex activity• Highest energy deposited among cells
close to the vertex.• The CC-QE is used as a control sample.
Coherent selection cut. CC-QE
Final coherent enriched sample
cut @ q2=0.10 (Gev/c)2
Reduction summary
Cut Data Efficiency (%)Purity(%
)
MRD 10049 77.9 3.6
2 tracks 3396 35.5 5.1
NQE pion 843 27.7 14.8
Forward track 773 27.3 15.8
Activity 297 23.9 28.2
Q2<0.10(Gev/c)2 113 21.1 47.1
Final efficiency
<E> ~ 1.3GeV MRD sample shifts the distributions to high energies due to an
effective threshold of 450 GeV/c.
Results
0.211
4.111
113 exp
coherent
backg
obs
N
N
POTtstatNN
coherentNcoh
otherobs 19
exp
107.138.9.)(40.5064.7 ) (
N (coherent )
0.569
980.0
10049.0 C)(exp
cc
CC
obs
Purity
CN
POTtstatPurityN
NCC
obs 194 107.138.910.)(02.073.1 CC)(
N (CC)
Systematic Errors• Nuclear effects: and proton
cross-section is varied by 30%.• Interaction model for CCQE and
CC1p cross-section is changed varying the axial mass by 10%. In deep Inelastic we vary the Bodek-Yang correction by 30%.
• CC1 suppression is estimated from the 20% deficit for q2<0.10 (GeV/c)2 observed in the non-QE proton sample.
• Event selection is dominated by the 2nd track efficiency.
• Detector response is dominated by scintillator quenching.
• Energy spectrum error comes from the uncertainties in the K2K near detector flux shape.
Source +Error -Error
Nuclear Effects
+0.23 -0.24
Interaction model
+0.10 -0.09
CC1 suppresio
n+0.14 --
Event Selection
+0.11 -0.17
Detector response
+0.09 -0.16
Energy spectrum
+0.03 -0.03
Total +0.32 -0.35
Results
CLCC
CohCC%[email protected]
)(
) ( 2
232.035.0 10.))(.)(29.0(0.04
)(
) (
syststat
CC
CohCC
~30% of MC expectation
Comparison with other results
• No data available for CC-Coherent. • To compare with NC with should do two assumptions: (NC) ~ 2(CC) A1/3 nuclear dependency.
CLCC
CohCC%[email protected]
)(
) ( 2
( CC) = 1.07 x 1032 cm2/nucleon
Rein&Sehgal
Aachen(NC)
GGM(NC)
(1
04
0 c
m2/n
ucle
on
)
Conclusions • We have reported the search for CC coherent
production by with a mean energy of 1.3 GeV.
• The data corresponds to 1.7x1019 pot recorded with the new SciBar detector at K2K neutrino beam line.
• No evidence for this channel has been found and an upper limit on the cross-section has been derived:
• (CC-Coh)/(CC) < 0.6 10-2 @ 90 % C.L. • This is the first experimental measurement of
this channel in the region around 1.3 GeV.