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

K2K Experiment 250km nm disappearance Energy spectrum distortion

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

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 15000 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

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

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

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

(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)

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

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

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)

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

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.

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:

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)

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

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.

Constrain of MC uncertainties Parameter Deviation from nominal Error Rnqe/qe 0.071 0.074 MRD momentum scale -0.0118 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.

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

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

Results N (nmCC) N (coherent p)

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

Results ~30% of MC expectation

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)

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) < 0.6 10-2 @ 90 % C.L. This is the first experimental measurement of this channel in the region around 1.3 GeV.