Neutrino interaction studyNeutrino interaction studyin the K2K near detectorsin the K2K near detectors

M. Hasegawa (Kyoto Univ.)for the K2K collaborators

RCNP workshop, Feb. 24, RCNP Osaka

Outline

Introduction - K2K experiment and Near detectors

Current activities- Charged current analysis

for Eν spectrum measurement- Low q2 deficit - Neutral current π0 production- MA measurement

On going analysis and future prospectsSummary

K2K ExperimentKEK to Kamioka –– first accelerator-based

long (>100km) baseline neutrino experiment

KEKν beam lineBeam monitorNear detectors

Super-K (far detector)

L=250km<Eν>~1.3GeV

Goal:Goal:Confirmation of Confirmation of ννµµ→→ννxx oscillationoscillation

reported by Superreported by Super--K K atmatm--ννSearch for Search for ννµµ→→ ννee oscillatonoscillaton

Started in 1999

PRL 93, 051801

Neutrino beam-line

Target/Horn12GeV PS

Decay volume(200m, π+→µ++νµ)

µ monitor

Near Detector

To Kamioka

1.1µs spill/2.2s~5.5x1012 p/spill

Eν (GeV)

νµ energy spectrum @ K2K near detector

<Eν> ~ 1.3GeV

Neutrino interaction ~1GeV

Total (NC+CC)

CC Total

CC quasi-elastic

DISCC single π

NC single π0

Eν(GeV)

σ/E

(10-

38cm

2 /GeV

)

Cross-sectionsNEUT (Nucl.Phys.Proc.112,p171)

Charged current quasi-elastic scattering (CCQE)Neutral current elastic scatteringCC/NC Single π,(η,K ) production via resonancesCC/NC Coherent pion productionsCC/NC Deep inelastic scattering

ν+ n l+ p

ν+ N l + N’ + πν+ N ν + N

(η,K)

ν+ 16O l +16 O + πνµ+ N l + N’ + hadrons

(l : lepton, N,N’ : nucleon, m : integer)

It is possible to investigatevarious neutrino interactionsw/ the K2K near detectors.

(In fact, more precise knowledgeof ν-nucleus interaction is vital to improve the accuracy of (future) oscillation measurements)

K2K Near detectors

Main roleTo measure

- flux- energy spectrum- direction

To study- neutrino interaction

1kt Water Cherencov detector (1kt) Scintillating Fiber Detector (Scifi) Scintillator Bar Detector (SciBar) (2003-) Muon Range Detector (MRD)

inside view

1Kt water cherencov detector (1Kt)

pµ−

νCherenkovlight

• 1000t cylindrical water tank w/ 680 20inch PMTs• Use ring pattern for PID (µ/e)

Flux & low energy (<1GeV) spectrumσ(NCπ0) /σ(total CC)

Physics output

Scintillating Fiber tracker (SciFi)SciScintillating FiFiber sheet / Water

Sandwich detector260cm 173cm

260cm

Spectrum shape (>1GeV)MA measurement

SciBar detector

ν

Extrudedscintillator(15t)

Multi-anodePMT (64 ch)

Wave-lengthshifting fiber

EM calorimeter

1.7m

3m

3m

Neutrino target is scintillator itself2.5 x 1.3 x 300 cm3 cell (15000ch)(Fairly) large volume

(7000 int. / month/10t)

Just constructed in last summer!

Fine segmented , Full ActiveSciScintillator-Bar Bar Tracker

Tracking Threshold : 8cm(=0.4GeV/c Proton)

Track-finding efficiency>99% (Single Track)

Excellent p/µ(π) using dE/dxmisID(µ P) = 1.7% @ PEff=90%

High 2-track CC-QE efficiencyIdentify ν interaction mode clearly

SciBar Shot! (1)

µ

pν

CCQE candidateCCQE candidate

1

23ν

CCnQECCnQE candidatecandidate

Area of circle is proportional to ADC.Hits along the proton track are larger

SciBar Shot! (2)

Large energy deposit in Electron Catcher

e

p

NC π0 candidate νe candidate

Vertex!

π0

e

e

νγ

γ NC elastic scattering cand.

p

• What can SciBar measure?To estimate SK number of events andEν spectrum (single ring µ-like)– Eν spectrum at near site from CC-QE (ν+n µ–+p) – CC-QE form factors– CC resonance production form factors

σ(ν+p µ–+p+π+)/σ(CC-QE) as a function of Eνσ(ν+n µ–+p+π0)/σ(CC-QE) as a function of Eνσ(ν+n µ–+n+π+)/σ(CC-QE) as a function of Eν

– CC coherent pion production (ν+C µ–+C+π+)– CC multi-pion production– NC resonance production (ν+N ν+N’+π)

νe/νµ flux ratio as a function of Eν (νe appearance)nucleon ∆s measurement

σ(ν+p ν+p)/σ(CC-QE) as a function of q2

Physics output from SciBar

Current activities

Main Motivation is to determine Eν spectrum

shape @ near site and σ(nQE)/σ(QE)

CCQE

nonQE

undetected in W-ch

CC Quasi elastic

1Rµ in W-chcan reconstruct Eν (pµ, θµ)

µµµ

µµν θ+−

−=

cospEm2mEm

EN

2Nrec

non Quasi elastic

Background for Eν meas.

charged current analysis

[ ]∑ ∗+= nQEQEnQEQE RiFPData σσθµµ /*)(),(

)1(F×

[ ]∑ ∗+= nQEQEnQEQE RiFPData σσθµµ /*)(),(

0-0.5 GeV

0.5-0.75GeV

Eν QE (MC) nQE(MC)

MC templates*F(1)

*F(2) *F(2)*RnQE/QE

(F(i) : Flux weight , RnQE/QE : cross section ratio)

We change flux weightand nonQE/QE and search for best combinationby comparing with data.

••

••

0.75-1.0GeV

1.0-1.5GeV

*F(3)

*F(4)

*F(3)*RnQE/QE

*F(4)*RnQE/QE

0

20

40

60

0 1 2 3 2550

0500

10001500

pµ

θ µ

[GeV/c]

[degre

e]

Eν spectrum measurement (strategy)

Event sample

Vertex

MRD(Iron plates and drift tubes)SciBar

µ

ν

μ

1kt水チェレンコフ検出器

CC fraction ~ 98%(SB) 100%(Scifi)Event Sample (CCQE candidates)

1kt : 1-Ring μ-like FC eventsScifi , SciBar : MRD matching events

(1Track , 2Track QE,nQE)

QE/nQE

QE nonQE

DATACC QECC 1πCC coherent-πCC multi-π

∆θp

(deg)

(1) 1 Track 60% QE (2) 2 Track QE enriched 60%(SF) / 70%(SB) QE (3) 2 Track nonQE enriched 85% nQE

µ

Expected proton direction assuming CCQE

∆θp

Observed second track

SciBar ∆θp

Use proton track direction to enhanceCCQE / non QE

We fit all samples of near detectors simultaneously.

Low q2 correction

SciBarnon-QE Events

K2K observed forward µ deficit in all ND (KT, SciFi and SciBar).– A source is non-QE events.– For CC-1π,

• Suppression of ~q2/0.1[GeV2] at q2<0.1[GeV2] may exist.

– For CC-coherent π,• The coherent π may not exist.

We do not identify which process causes the effect. The MC CC-1π(coherent π) model is corrected to be consistent with data.

Oscillation analysis is insensitive to the choice.

Study of source ongoing with SciBar.next topics

q2rec

(Data-MC)/MC

DATACC QECC 1πCC coherent-π

Preliminary

q2rec (GeV/c)2

q2rec (GeV/c)2

Φ(Eν) at KEK

Eν

preliminaryχ2=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

10% error of nonQE/QE is due to lowq2 uncertaintynonQE/QE and low energy spectrum are strongly correlated.

Low q2 problem is key issue for nonQE/QE measurement

small angle cut 0.955CC1π suppression 1.020No coherent π 1.059

7%

3.8%

(Fitting error ～5%)

nonQE/QE (each situation)

Flux fit result (combine all near detectors)

Low q2 study in SciBar

Low q2 (forward µ) deficit

MiniBooNEFrom J. Raaf(NOON04)

(Problem has existed over 3 years)

(GeV2)

Scifi 2track nonQE enriched event

DATACC QECC 1πCC coherent-πCC multi-π

Pure QE sample

Same phenomena is observed by all K2K near detectorsin non-QE sample and other exp. in both QE and non-QE sample.

It is not due to detector systematicsThis discrepancy cannot be explained by the uncertainty of neutrino

spectrum shape. neutrino interaction + nuclear effects

(As for K2K), source is cc1π or coherent π or both.

CC1π vs coherent π

ν µ

ππ∗

t ~ 0

ν µ

πΝ ∗

N

N<< <<<

CC 1π (ν+N µ+N+π) Coherent π

When pi is absorbed inside nucleusor 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 makeCC1π enriched/Coherent π enriched samples.

CC Coherent π cross section measurement

Rein-Sehgal

Eν (GEV)

σ(10

-40 C

M2 )/

NU

CL

EO

N

σ(νµCC) = 1.07 x 10-38 cm2/nucleon

Aachen(NC)

GGM(NC)

0123456789

10

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

K2K (CC)

This measurement is the first experimental resultin a few GeV region for CC int.

providing the test of the theoretical model(σ(Eν), σ(CC)/σ(NC)=2 , σ(A) …)

SciBar can detect the int. with high efficiency (20~30%) and purity (~50%)thanks to the fully activity.

ν µ

ππ∗

t ~ 0

<<<

Coherent π

no activity

Analysis is very close to final stage.release the result in a few months

NC pi0 measurement in 1kt

ν

ν

π0

γ

γ

Data setPeriod : 2000/Jan-Mar. 2001/Jan~Jul. (~ 3x1019 pot)

dataMC

0

100

200

300

400

500

600

700

0 50 100 150 200 250 300Mγγ (MeV/c2)

Nu

mb

er

of

ev

en

ts

Mass peak

(MeV/c2)

data :

147.4MC : 144.1

non-π0 BG very clean π0 sample

Selection Criteria of π0 events 25t fiducial volumeFully-Containednumber of rings = 2both e-like PIDMγγ : 85 ~ 215 MeV/c2

FC 2ring π0 : 2496 events

π0 momentum (FC 2ring π0 sample)#

of F

C 2

ring

π0ev

ents

0

50

100

150

200

250

300

0 200 400 600 800 1000

NC resonant πNC coherent πNC multi πothers

π0 momentum (MeV/c)

( MC is normalizedby area )

large NC fraction ~ 87 %

NFC2Rπ0obs x rpure x corrfid

NNC1π0 =eff

N(NC1π0) : 3.69 ± 0.07 ± 0.37 x 103

stat. sys.

in 25 ton

= 0.065 ± 0.001 ± 0.007stat. sys.

at the K2K beam energy, <Eν>~1.3 GeV

cf. σ(NC1π0) / σ(νµCC) = 0.064 from NEUT

σ(NC1π0)σ(νµCC)

Hep-ex/0408134Submitted to PLB

On-going analysis

On going analysis

MA measurement (SciFi, SciBar)

NC π0 , elastic (SciBar)

NC/CC ～20% NC/CC 10～20 %Key : nuclear effect

neutron BG.

Key : Eμ scale

νe measurement (SciBar, 1Kt)

related with K2K νe analysis.

SciFi-MA study (first measurement for water) is almost done.results appear soon.

SciBar-MA analysis is just started.

CC1pi production (SciBar)

Energy dependent cross section (ratio to CCQE)

SummaryThe recent discovery of neutrino oscillation has

renewed interest in the ν-nucleus scattering in thesub to a few GeV region.

In the K2K near detectors, the following topics are studied.- CC coherent pion production (appear soon)- NC pi0 production (submitted in PLB)- MA measurement for water target (appear soon)

On-going analysis-σ(CC1p)/σ(CC) , σ(NC elastic)/σ(QE elastic)- MA (carbon) …..

Many physics results will be released around this summer. Stay tuned!

Supplement

NEUT: K2K Neutrino interaction MC • CC quasi elastic (CCQE)

– Smith and Moniz with MA=1.1GeV

• CC (resonance) single π(CC-1π)– Rein and Sehgal’s with

MA=1.1GeV• DIS

– GRV94 + JETSET with Bodekand Yang correction.

• CC coherent π– Rein&Sehgal with the cross

section rescale by J. Marteau• NC+ Nuclear Effects

σ/E (10-38cm2/GeV)

Total (NC+CC)

CC Total

CC quasi-elastic

DISCC single π

NC single π0

Eν (GeV)

(Nucl.Phys.Proc.112,p171)