K2K NC p0 productionShoei NAKAYAMA (ICRR, Univ. of Tokyo)for the K2K CollaborationJuly 28, 2004 @ NuFact04

OutlineIntroductionMotivation for p0 analysesK2K neutrino beam, 1kt water Cherenkov detectorSelecting p0 events and data/MC comparisonFC 2ring p0 sampledata / MC comparisonRelative cross section measurementefficiency corrected N(NC1p0)taking ratio to N(nmCC)Summary

Introduction

Motivation for p0 analysesMain background to nm ne search p0 gg can mimic an electronasymmetric decayring overlap

Usable to distinguish nm nt from nm ns in atmospheric nm oscillation single p0 events : good NC sample (see Nakayamas talk @ICRC2003)ggggnm ntnm nsnmCC NCno changeattenuatedneed to know accurately p0 production cross section, p0 momentum and angular distributionno osc.

K2K experimentJAPAN: High Energy Accelerator Research Organization (KEK) /Institute for Cosmic Ray Research (ICRR), Univ. of Tokyo / Kobe University / Kyoto University /Niigata University / Okayama University / Tokyo University of Science / Tohoku UniversityKOREA: Chonnam National University / Dongshin University / Korea University /Seoul National UniversityU.S.A.: Boston University / University of California, Irvine / University of Hawaii, Manoa /Massachusetts Institute of Technology / State University of New York at Stony Brook /University of Washington at SeattlePOLAND: Warsaw University / Solton Institute

Since 2002JAPAN: Hiroshima University / Osaka UniversityCANADA: TRIUMF / University of British ColumbiaEUROPE: Rome / Saclay / Barcelona /Valencia / Geneva RUSSIA: INR-Moscow

K2K neutrino beamalmost pure nm beam (~98%)En ~ 1 GeVNn / cm2 / 0.1GeV / 1020 potnm energy spectrum@ front detector sitex101012GeV PSpAluminum targetFront detector200m decay tunnel

p+ m+ + nm

1kt water Cherenkov detector 1000t cylindrical water tank a smaller version of Super-Kamiokande (~1/50 volume) 680 20inch PMTs with 70cm spacing (same as SK) the same detection mechanism, analysis algorithmsand interaction MC as SKn beam25t fiducial volume2m4m

Selecting p0 events and data/MC comparison

Data set and selection criteriap0 events single-event spill Fully-Contained number of rings = 2 both e-like PID Mgg : 85 ~ 215 MeV/c2Observed data : 2000/1~3, 2001/1~7(~ 3x1019 pot)

Nring and PID cutsNringPIDe-likem-likee-likem-like1st ring2nd ringhatch = NC where single p0 and no other mesons get out of 16O nucleus( MC is normalized by area )PreliminaryPreliminary

p0 mass distributionvery clean p0 sample( MC is normalized by area )FC 2ring p0 : 2496 eventsthe number of events after each cutnon-p0 BGMass peak (MeV/c2) data : 147.4 0.5 MC : 144.1 0.3PreliminaryPreliminary

Neutrino interaction MC ( NEUT version4.5 )(quasi-) elastic n N N ( : lepton ) MA = 1.1 GeV/c2resonant meson production n N N p (h, K) based on Rein&Sehgal MA = 1.1 GeV/c2coherent p production n 16O 16O pRein&Sehgal J.Marteau et al., NIM A451(2000)cross section rescaled by ~0.7 p and p rescattering in 16O nucleus absorption inelastic scattering charge exchangePauli blockingFermi motionnuclear potential

deep inelastic scattering n N N ppp GRV94 + JETSET A.Bodek et al., hep-ex/0203009rescaled by q2/(q2+0.188)n interaction vertexnuclear effects

The fraction of each interaction channelNC resonant meson production60.4 %NC coherent p0 production10.4 %NC multi p production10.8 %NC elastic scattering5.0 %CC resonant meson production8.8 %CC multi p production4.1 %large NC fraction ~ 87 %FC 2ring p0 sample

Graph1

3390.01

582.16

605.63

282.56

16.98

505.78

1.13

222.42

Sheet1

1pi resonant3390.01

1pi coherent582.16

multi pi605.63

elastic282.56

quasi elastic16.984.8221.8

1pi resonant505.7813.32519.1

1pi coherent1.130.571.7

multi pi222.425.24227.66

Sheet1

Sheet2

Sheet3

p0 momentum distribution100 MeV/c bin25 MeV/c binstat. error onlyThe observed data is reproduced fairly well by our neutrino MC.# of FC 2ring p0 events# of FC 2ring p0 eventsPreliminaryPreliminary( MC is normalizedby area )

p0 production angle distribution10 bins40 binsstat. error onlyAgreement between data and neutrino MC is good.backwardforwardPreliminary# of FC 2ring p0 events# of FC 2ring p0 events( MC is normalized by area )Preliminary

Relative cross section measurements(NC1p0) / s(nmCC)

What can be measured ?p0s from 16O are largely modified by nuclear effects. Probability of each p0-16O interaction in NEUT MC We measure the p0 production cross section after nuclear effects.

NC1p0 definition np0NC?only one p0 and no other mesons get out ofa 16O nucleus via NC interactions.(after nuclear re-scatterings)p0 invisible mCCp0n, invisible mNC or CCp+ invisiblep0NCRecoil pBG interactionsp0 w/ invisible mp0 w/ invisible p,mp0 produced outside nuclei

NC1p0 estimationNFC2Rp0obs x rpure x corrfidNNC1p0 = effnon-NC1p0subtractionrec truefiducial correctiondetection and reconstructionefficiencyFC 2ring p0 mom.raw NC1p0 mom.true/rec = 1.03 0.02sys.Preliminary

non-NC1p0 BG subtractionNC1p0 fraction curveestimated by neutrino MCNC1p0BGinner : stat.outer : stat.+sys.NC 1p071 %fractionp0 mp0 mp0 nppp0n or mCC w/ invisible m6 %CC w/ invisible p, m3 %NC w/ invisible p7 %p0 produced outside the nuclei 10 %Preliminary

systematic error on BG subtractionn cross section6.3 %MA(QE) 1.11.0 && MA(1p) 1.11.00.2 %QE cross section +-10%

efficiency correctionp0 detection efficiency curveestimated by MCinner : stat.outer : stat.+sys.overall efficiency : ~46%FADC peak cut(effectively 1000p.e. cut)FC cutNring = 21st ring is e-like2nd ring is e-likeinvariant mass cutsystematic errors for overall efficiencyring counting5.4 %PID3.9 %escale(+-3%)0.3 %

total6.8 %PreliminaryPreliminary

NC1p0 production in true 25t (eff corrected)N(NC1p0) : 3.69 0.07 0.37 x 103stat. sys.data (inner: stat, outer: stat+sys)MC true (inner: MC stat, outer: MC stat + sys error on shape from our MC model uncertainties)normalized by the number of all events in 25t fiducialin 25 ton# of NC1p0 interactionsPreliminaryPreliminary

nmCC interactions as a normalizationnmCC enriched sample :FC single-ring m-like + FC multi-ring m-like + PCmmmFC single-ring m-likeFC multi-ring m-likePCHigh nmCC fraction, High efficiencynmCC fraction 96.5 % 91.2 % 98.5 %

N(nmCC) estimationN(nmCC) = N(FCm+PC)25tobs x (1 BGnon-n) x purity x corrfid / eff = 50226 x (1 0.015) x 0.960 x 1.02 / 0.854

stat. sys.= 5.65 0.03 0.26 x 104BG: NCinefficiency: multi-ring PID FADC cuteffenergy scale1.2 %PID1 %(1/eff) x rpureE spec.0.5 %cross section1.1 %corrfidfiducial volume4 % total 4.6 % NobsFC1Rm 22612FCmRm 12386PC 15228systematic error on N(nmCC)in 25 tonPreliminaryPreliminary

Resultss(NC1p0) / s(nmCC) = 0.065 0.001 0.007cf. s(NC1p0) / s(nmCC) = 0.064 from NEUT

cf. s(nmCC) ~ 1.1 x 10-38 cm2 / nucleon from NEUT(K2K beam spectrum averaged)stat. sys. ~ 1.3 GeVat the K2K beam energy, ~1.3 GeVPreliminary

Summary 2496 FC 2ring p0 events are observed.

Our neutrino MC reproduces data quite well.(p0 momentum, p0 direction)

Efficiency corrected number of NC1p0 interactions and their momentum distribution are measured.

s(NC1p0) / s(nmCC) = 0.065 0.001 0.007 at the K2K beam energy.