1. F.Sánchez for the K2K collaboration UAB/IFAE
Search for coherent charged pion production in neutrino-carbon interactions
F.Sánchez for the K2K collaboration
UAB/IFAE

2. K2K Experiment
250km
nm disappearance
Energy spectrum distortion

3. K2K near detectors 1kt Water Čerenkov detector (1kt)
Scintillating Fiber Detector (Scifi)
Scintillator Bar Detector (SciBar) (from 2003)
Muon Range Detector (MRD)

4. SciBar Detector n EM calorimeter 3m 3m 1.7m Extruded
scintillator
(15t)
EM calorimeter
Extruded Scintillator Bar
WLS fiber readout.
Active target.
2.5 x 1.3 x 300 cm3 cell.
Order of channels.
Light yield ~8 p.e./MeV.
Detect 10 cm track.
Distinguish protons from p by using dE/dx. Miss ID < 5% (<1GeV/c proton). 3m n 3m
1.7m

5. 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 En measurement
Neutrino flux at <1GeV and neutrino interactions around 1GeV are important since the oscillation maximum at K2K is around 600 MeV. nm m
proton qm pm n
CC-QE Interaction m nm N p
nucleon
CC-nonQE Interaction

6. Detector Components
VME board
64 ch
MA-PMT
Frontend
board

7. Electron Catcher n Energy tail catcher.
4 cm
8 cm
262 cm
Readout Cell
n Beam
Scintillating Fibers and Lead Plates
Electron Catcher
Energy tail catcher.
Measure electron neutrino flux.
Study p0 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. n

8. (Iron plates and drift tubes)
CC Event Selection
SciBar MRD 3D track matching (3D)
~35% of all n interactions. CC-QE fraction is ~55% (MC)
MRD 1st layer stopping (1L)
~9% of all n interactions. CC-QE fraction is ~31% (MC)
Momentum of m is computed from its range in SciBar, EC and MRD.
Vertex
MRD
(Iron plates and drift tubes)
SciBar m m
Vertex
3D matching (3D)
1st layer stopping (1L)

9. Particle Id. DATA Muon C.L. m sample proton sample
Total deposit energy vs. Range
m sample
DATA
Energy (mips)
Proton-like(purity 90%)
Mu-like(purity ~ 99.6%)
proton sample
Range (cm)
Excellent p/m(p) using dE/dx
For 90% proton efficiency we get 1.7% of m miss-ID probability.
Proton Efficiency
Muon mis-ID
m-Like
Proton like

10. SciBar Event examples
CCQE candidate
CCnQE candidate m p n 1 2 3 n

11. NEUT: K2K Neutrino interaction MC
s/E (10-38cm2/GeV)
Total (NC+CC)
CC Total
CC quasi-elastic
DIS
CC single p
NC single p0
CC quasi elastic (CCQE)
Smith and Moniz with MA=1.1GeV
CC (resonance) single p(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
En (GeV)

12. Coherent p+ production
K2K and MiniBoone has reported in the past a deficit of events at low q2: nuclear effects?, coherent production?,…
K2K
MiniBoone

13. Coherent p+ signature n p m n m p
proton
In the coherent p+ 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 p+ signature consists on:
a m- and a p+ in the final state.
No activity in the area of the vertex beyond the outgoing tracks.

14. Coherent p+ analysis 1 track MRD sample QE 2 tracks Proton like non QE
Pion like
Coherent
sample
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:

15. nQE/QE selection m Dqp Dqp < 25deg Dqp > 25deg QE Non QE
CCQE
CC1p
Dqp is the angle between
the second track and the predicted proton direction in CCQE reactions.
Coherent Pi
Multi Pi
Others
(deg)
QE Efficiency
QE enriched sample
Dqp < 25deg
nQE enriched sample
Dqp > 25deg
QE Purity
80% Efficiency
70% Purity
(deg)

16. Proton-p Id p enriched Muon Confidence Level (2nd Track : nQE sample)
Pion
Others
Proton efficiency
Proton Purity
85% Efficiency
80% Purity
Muon confidence level

17. Constrain of MC uncertainties
Control samples are fitted simultaneously for q2>0.10(GeV/c)2 to constrain MC uncertainties.
1 track
2 tracks QE
2 tracks NQE p like
2 tracks NQE proton-like
Simultaneously the MRD momentum scale and the MC true ratio between NQE and QE are fitted.

18. Constrain of MC uncertainties
Parameter
Deviation
from nominal
Error
Rnqe/qe
0.071
0.074
MRD momentum scale
0.0030
f2trac/1track
0.014
0.026
fnqe/qe
0.043
0.054
fproton/p
0.079
0.051
These values are used to re-weight the MC.
Errors are propagated to the final sample.

19. Selected samples after the fit
No evidence for Coherent events
Deficit of events q2<0.10(Gev/c)2
c2/d.o.f = 73.2 / 80 for q2 > 0.10 (Gev/c)2

20. 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

21. Final coherent enriched sample
q2=0.10 (Gev/c)2

22. 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

23. Final efficiency <En> ~ 1.3GeV
MRD sample shifts the distributions to high energies due to an effective threshold of 450 GeV/c.

24. Results
N (nmCC)
N (coherent p)

25. Systematic Errors
Nuclear effects: p 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%.
CC1p 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
CC1p suppresion
+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

26. Results
~30% of MC expectation

27. Comparison with other results
No data available for CC-Coherent.
To compare with NC with should do two assumptions:
s(NC) ~ 2s(CC)
A1/3 nuclear dependency.
s(nm CC) = 1.07 x 1032 cm2/nucleon
Rein&Sehgal
Aachen(NC)
GGM(NC)
s(1040 cm2/nucleon)

28. Conclusions
We have reported the search for CC coherent p production by nm 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:
s(CC-Coh)/s(CC) < % C.L.
This is the first experimental measurement of this channel in the region around 1.3 GeV.