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%90@1060.0

)(

) ( 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%90@1060.0

)(

) ( 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.