Neutrino interaction measurements in K2K SciBar RCCN Neutrino Workshop December 10, 2004, Kashiwa M.Hasegawa (Kyoto-Univ.) For the K2K SciBar Group

Table of Contents Introduction SciBar Detector Charged current analysis Flux fitting 　(σ(nQE)/σ(QE)) Low Q2 deficit On-going analysis and future prospects Summary

Introduction n K2K near detector Muon range detector n Main role is To measure the neutrino property at production To study the neutrino interaction ( useful for SK analysis) SciBar was installed in the summer 2003.

SciBar Detector n Preliminary Just constructed in last summer! Extruded scintillator (15t) Multi-anode PMT (64 ch) Wave-length shifting fiber EM calorimeter 1.7m 3m Fine segmented , Full Active Scintillator-Bar Tracker Neutrino target is scintillator itself 2.5 x 1.3 x 300 cm3 cell (15000ch) (Fairly) large volume (7000 int. / month/10t) Tracking Threshold : 8cm (=0.4GeV/c Proton) Track-finding efficiency >99% (Single Track) Preliminary Excellent p/m(p) using dE/dx misID(m P) = 1.7% @ PEff=90% High 2-track CC-QE efficiency Identify ν interaction mode clearly Just constructed in last summer!

SciBar Shot! (1) CCQE candidate CCnQE candidate p m 3 2 1 Area of circle is proportional to ADC. Hits along the proton track are larger CCQE candidate CCnQE candidate p n m 3 2 n 1

SciBar Shot! (2) NC p0 candidate ne candidate Large energy deposit in Electron Catcher e p Vertex! p0 e n g NC elastic scattering cand. p

Physics output from SciBar What can SciBar measure? To estimate SK number of events and En spectrum (single ring m-like) En spectrum at near site from CC-QE (n+nm–+p) CC-QE form factors CC resonance production form factors s(n+pm–+p+p+)/s(CC-QE) as a function of En s(n+nm–+p+p0)/s(CC-QE) as a function of En s(n+nm–+n+p+)/s(CC-QE) as a function of En CC coherent pion production (n+Cm–+C+p+) CC multi-pion production NC resonance production (n+Nn+N’+p) ne/nm flux ratio as a function of En (ne appearance)  nucleon Ds measurement s(n+pn+p)/s(CC-QE) as a function of q2

Charged current analysis Main Motivation is to determine En spectrum shape @ near site and σ(nQE)/σ(QE) CC Quasi elastic CCQE 1Rm in W-ch can reconstruct En (pm, qm) undetected in W-ch nonQE non Quasi elastic Background for En meas.

Flux measurement (strategy) (F(i) : Flux weight , RnQE/QE : cross section ratio) En QE (MC) nQE(MC) pm qm [GeV/c] [degree] MC templates *F(1) 0-0.5 GeV *F(2) *F(2)*RnQE/QE 0.5-0.75GeV *F(3) *F(3)*RnQE/QE 0.75-1.0GeV We change Flux and nonQE/QE and search for best combination by comparing with data. *F(4) *F(4)*RnQE/QE 1.0-1.5GeV • •

Used event for flux fitting SciBar Dqp QE nonQE DATA CC QE CC 1p CC coherent-p CC multi-p Dqp (deg) m Observed second track Dqp Expected proton direction assuming CCQE (1) 1 Track ( QE fraction 58%) (2) 2 Track QE enriched ( QE fraction 72%) 2 Track nonQE enriched (nQE fraction 83%) We fit these three samples simultaneously.

Basic distribution (1track) q2 is reconstructed w/ assuming CCQE (dq2 ～0.01GeV2 @0.10GeV2) pm Agreement is well except for low q2(forward) region. DATA CC QE CC 1p CC coherent-p CC multi-p Used for flux fitting q2(rec) qm * MC prediction is normalized by the total number of CC candidate events.

Basic distribution (2track QEenriched) pm Good Agreement (No deficit can be found) DATA CC QE CC 1p CC coherent-p CC multi-p qm q2(rec)

Basic distribution (2track nQEenriched) Clear deficit can be seen. pm  Main source is nonQE CC1p ? CC coherent p ? (Next topics) DATA CC QE CC 1p CC coherent-p CC multi-p qm q2(rec)

MC correction in low q2 region Low q2 suppression in CC-1p {Q2/0.1 for Q2<0.1(GeV/c)2} No coherent p interactions. We checked the two kinds of corrections CC1p phenomenological suppression No coherent p These correction do not change the flux , but nonQE/QE.

Flux fit result (combined all near detector) F(En) at KEK c2=638.1 for 609 d.o.f F1 ( En < 500) = 0.03  0.02 F2 ( 500 En < 750) = 0.32  0.03 F3 ( 750 En <1000) = 0.73  0.04 F4 (1000 En <1500) = 1.00 F5 (1500 En <2000) = 0.69  0.03 F6 (2000 En <2500) = 0.34  0.02 F7 (2500 En <3000) = 0.12  0.02 F8 (3000 En ) = 0.05  0.01 nQE/QE = 1.02  0.10 preliminary small angle cut 0.955 CC1p suppression 1.020 No coherent p 1.059 7% 3.8% (Fitting error ～5%) nonQE/QE (each situation) En 10% error of nonQE/QE is due to lowQ2 uncertainty nonQE/QE and low energy spectrum are strongly correlated. LowQ2 problem is key issue for nonQE/QE measurement

SciBar Low q2 study

Low q2(forward m) deficit (Problem has existed over 3 years) Scifi 2track nonQE enriched event DATA CC QE CC 1p CC coherent-p CC multi-p MiniBooNE From J. Raaf (NOON04) (GeV2) Same phenomena is observed by other detectors.  It is not due to the detector systematics. This discrepancy cannot be explained by uncertainty of the neutrino spectrum shape.  Neutrino interaction From K2K (SciBar/Scifi) data , source is nonQE int. (CC1p or coherent p or both)

CC1p / coherent p n m n m p* p p t ~ 0 CC 1p (n+Nm+N+p) Coherent p < p* < < N * < < p p N t ~ 0 N When pi is absorbed inside nucleus or Proton momentum < 400MeV/c, it looks 2track event.  Half of 2nd tracks in 2track-nonQE sample are proton. 2nd track is completely pion. We apply PID to 2nd track and make CC1p enriched/Coherent p enriched samples.

CC1p / coherent p Preliminaly Muon Confidence Level (2nd Track : nQE sample) Proton Proton Eff. = 85% Purity = 80% Pion Others non Muon-like Muon-like Preliminaly 2nd track = Proton like 2nd track = Pion like Coherent Pi Coherent Pi CC1pi CC1pi Others (CC) Others (CC) NC NC CCQE CCQE (GeV)2 (GeV)2 Clear deficit can be seen only in Coherent p enriched sample.

CC coherent p cross section measurement SciBar measurement is not only the first experimental result in the few GeV neutrinos for the charged current production, but also use target with ‘A’ less than 20. K2K Coherent p is most suspicious mode of low q2 deficit. Scibar can select this mode with(fairly) high efficiency (20～30%) and purity (>～50%) by using info. of activity around vertex region. ～160 candidate is expected (MC). 20%～30%(stat.+sys.) measurement is expected. Preliminaly Analysis stage is very closed to final result.

On-going analysis and future prospects

Current situation & goal NC π0 , elastic NC/CC ～20%  NC/CC 10～20 % Key : nuclear effect neutron BG. MA measurement ～10%  < 10% ?? Key : Eμ scale ne measurement related with K2K analysis. NEUT prediction will be more reliable with SciBar data.

Summary First physics result will be released A brand new neutrino detector ‘SciBar’ was installed in K2K near detector and took ～ 20k neutrino data. Basic distributions for muon agree with MC prediction well except for lowq2 region. We confirmed a main cause of lowq2 deficit was nonQE int, especially coherent p is suspicious with observed data. We already started the following analysis, nonQE/QE and MA /NC pi0 / ..... First physics result will be released in the begging of next year.