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K2K NC p 0 production. Shoei NAKAYAMA (ICRR, Univ. of Tokyo) for the K2K Collaboration July 28, 2004 @ NuFact04. Outline. Introduction Motivation for p 0 analyses K2K neutrino beam, 1kt water Cherenkov detector Selecting p 0 events and data/MC comparison FC 2ring p 0 sample

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K2k nc p 0 production

K2K NC p0 production

Shoei NAKAYAMA (ICRR, Univ. of Tokyo)

for the K2K Collaboration

July 28, 2004 @ NuFact04


Outline
Outline

  • Introduction

    • Motivation for p0 analyses

    • K2K neutrino beam, 1kt water Cherenkov detector

  • Selecting p0 events and data/MC comparison

    • FC 2ring p0 sample

    • data / MC comparison

  • Relative cross section measurement

    • efficiency corrected N(NC1p0)

    • taking ratio to N(nmCC)

  • Summary



Motivation for p 0 analyses
Motivation for p0 analyses

  • Main background to nm ne search

    p0  gg can mimic an electron

    • asymmetric decay

    • ring overlap

  • Usable to distinguish nm  nt from nm  ns in atmospheric nm oscillation

    single p0 events : good NC sample

    (see Nakayama’s talk @ICRC2003)

  • g

    g

    g

    g

    nm nt

    nm ns

    no osc.

    nmCC

    NC

    no change

    attenuated

    need to know accurately p0 production cross section, p0 momentum and angular distribution


    K2k experiment
    K2K experiment

    JAPAN: 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 University

    KOREA: Chonnam National University / Dongshin University / Korea University /

    Seoul National University

    U.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 Seattle

    POLAND: Warsaw University / Solton Institute

    Since 2002

    JAPAN: Hiroshima University / Osaka University

    CANADA: TRIUMF / University of British Columbia

    EUROPE: Rome / Saclay / Barcelona /

    Valencia / Geneva

    RUSSIA: INR-Moscow


    K2k neutrino beam

    En (GeV)

    K2K neutrino beam

    • almost pure nm beam (~98%)

    • En ~ 1 GeV

    Aluminum target

    nm energy spectrum

    @ front detector site

    x1010

    Front detector

    200m decay tunnel

    p+ m+ + nm

    Nn / cm2 / 0.1GeV / 1020 pot

    p

    12GeV PS


    1kt water cherenkov detector
    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 algorithms

    • and interaction MC as SK

    2m

    n beam

    4m

    25t fiducial volume


    Selecting p 0 events and data mc comparison
    Selecting p0 events and data/MC comparison


    Data set and selection criteria
    Data set and selection criteria

    p0 events

    • single-event spill

    • Fully-Contained

    • number of rings = 2

    • both e-like PID

    • Mgg : 85 ~ 215 MeV/c2

    Observed data : 2000/1~3, 2001/1~7 (~ 3x1019 pot)


    Nring and pid cuts

    data

    MC

    data

    MC

    Nring and PID cuts

    Nring

    PID

    Preliminary

    Preliminary

    m-like

    e-like

    1st ring

    m-like

    e-like

    2nd ring

    ( MC is normalized by area )

    hatch = NC where single p0 and no other mesons get out of 16O nucleus


    P 0 mass distribution

    data

    MC

    p0 mass distribution

    Preliminary

    ( MC is normalized by area )

    the number of events after each cut

    Mass peak (MeV/c2)

    data : 147.4 ± 0.5

    MC : 144.1 ± 0.3

    Preliminary

    non-p0 BG

    FC 2ring p0 : 2496 events

    very clean p0 sample


    Neutrino interaction mc neut version4 5

    (quasi-) elastic

    n N  ℓ N’ ( ℓ: lepton )

    MA = 1.1 GeV/c2

    resonant meson production

    n N  ℓ N’ p (h, K)

    based on Rein&Sehgal

    MA = 1.1 GeV/c2

    coherent p production

    n16O  ℓ16O p

    Rein&Sehgal

    J.Marteau et al., NIM A451(2000) cross section rescaled by ~0.7

    Neutrino interaction MC ( “NEUT” version4.5 )

    n interaction vertex

    nuclear effects

    • p and p rescattering in 16O nucleus

      • absorption

      • inelastic scattering

      • charge exchange

    • Pauli blocking

    • Fermi motion

    • nuclear potential

    • deep inelastic scattering

      • n N  ℓ N’ ppp…

      • GRV94 + JETSET

      • A.Bodek et al., hep-ex/0203009 rescaled by q2/(q2+0.188)


    The fraction of each interaction channel
    The fraction of each interaction channel

    CC multi p production

    4.1 %

    CC resonant meson production

    8.8 %

    NC resonant meson production

    60.4 %

    NC elastic scattering

    5.0 %

    NC multi p production

    10.8 %

    FC 2ring p0 sample

    NC coherent p0 production

    10.4 %

    large NC fraction ~ 87 %


    P 0 momentum distribution

    data

    MC

    p0 momentum distribution

    100 MeV/c bin

    25 MeV/c bin

    Preliminary

    Preliminary

    stat. error only

    # of FC 2ring p0 events

    # of FC 2ring p0 events

    ( MC is normalized

    by area )

    The observed data is reproduced fairly well by our neutrino MC.


    P 0 production angle distribution

    data

    MC

    p0 production angle distribution

    10 bins

    40 bins

    Preliminary

    Preliminary

    backward

    forward

    # of FC 2ring p0 events

    # of FC 2ring p0 events

    stat. error only

    ( MC is normalized by area )

    Agreement between data and neutrino MC is good.



    What can be measured
    What can be measured ?

    p0’s 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.


    Nc1 p 0 definition
    “ NC1p0 “ definition

    n

    NC

    only one p0 and no other mesons get out of

    a 16O nucleus via NC interactions.

    (after nuclear re-scatterings)

    ?

    p0

    BG interactions

    CC

    NC

    NC or CC

    invisible m

    n, invisible m

    Recoil p

    p0

    p0

    p0

    invisible

    p+

    p0 w/ invisible m

    p0 w/ invisible p,m

    p0 produced outside nuclei


    Nc1 p 0 estimation
    NC1p0 estimation

    sys.

    true/rec = 1.03 ± 0.02

    rec  true

    fiducial correction

    non-NC1p0

    subtraction

    FC 2ring p0 mom.

    NFC2Rp0obs x rpure x corrfid

    NNC1p0 =

    eff

    Preliminary

    detection and reconstruction

    efficiency

    raw NC1p0 mom.


    Non nc1 p 0 bg subtraction
    non-NC1p0 BG subtraction

    fraction

    NC1p0fraction curve

    estimated by neutrino MC

    NC 1p071 %

    CC w/ invisible p, m

    3 %

    CC w/ invisible m

    6 %

    Preliminary

    m

    m

    BG

    p0

    p0

    p

    NC1p0

    NC w/ invisible p

    7 %

    p0 produced outside

    the nuclei 10 %

    inner : stat.

    outer : stat.+sys.

    n

    nor m

    p0

    p0

    p


    Systematic error on bg subtraction
    systematic error on BG subtraction

    total: 6.9 %

    • n cross section 6.3 %

    • MA(QE) 1.11.0 && MA(1p) 1.11.0 0.2 %

    • QE cross section +-10% <<1 %

    • 1p cross section +-10% 0.9 %

    • DIS cross section +-5% 0.5 %

    • w/o Bodek reweighting(DIS) 5.1 %

    • w/o Marteau reweighting(coherent p) 1.6 %

    • CC/NC +-20% 3.2 %

    • nuclear rescattering 1.6 %

      absorption probability +-30% 1.5 %

      inelastic scattering probability +-30% 0.7 %

      p0 from nucleon(or p) interaction 2.3 %

      in water (2nd interaction)

      total interaction probability +-20% 2.3 %

    estimated by

    reweighting

    estimated from

    different MC sets

    generated with

    varied cross section


    Efficiency correction
    efficiency correction

    FADC peak cut

    (effectively

    1000p.e. cut)

    p0 detection efficiency curve

    estimated by MC

    FC cut

    Preliminary

    Preliminary

    Nring = 2

    1st ring is e-like

    2nd ring is e-like

    invariant mass cut

    systematic errors for overall efficiency

    ring counting 5.4 %

    PID 3.9 %

    escale(+-3%) 0.3 %

    total 6.8 %

    inner : stat.

    outer : stat.+sys.

    overall efficiency : ~46%


    Nc1 p 0 production in true 25t eff corrected
    NC1p0 production in true 25t (eff corrected)

    Preliminary

    data (inner: stat, outer: stat+sys)

    MC true

    (inner: MC stat,

    outer: MC stat + sys error on shape

    from our MC model

    uncertainties)

    # of NC1p0 interactions

    stat. sys.

    N(NC1p0) : 3.69 ± 0.07 ± 0.37 x 103

    Preliminary

    in 25 ton

    normalized by the number of all events in 25t fiducial


    N m cc interactions as a normalization
    nmCC interactions as a normalization

    nmCC enriched sample :

    FC single-ring m-like + FC multi-ring m-like + PC

    m

    m

    m

    FC single-ring m-like

    FC multi-ring m-like

    PC

    nmCC fraction 96.5 % 91.2 % 98.5 %

    High nmCC fraction, High efficiency


    N n m cc estimation
    N(nmCC) estimation

    N(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 104

    BG: NC

    inefficiency: multi-ring PID

    FADC cut

    Nobs

    FC1Rm 22612

    FCmRm 12386

    PC 15228

    in 25 ton

    Preliminary

    Preliminary

    systematic error on N(nmCC)

    eff energy scale 1.2 %

    PID 1 %

    (1/eff) x rpure E spec. 0.5 %

    cross section 1.1 %

    corrfid fiducial volume 4 %

     total 4.6 %


    Results
    Results

    Preliminary

    stat. sys.

    s(NC1p0) / s(nmCC) = 0.065 ± 0.001 ± 0.007

    at the K2K beam energy, <En>~1.3 GeV

    cf. s(NC1p0) / s(nmCC) = 0.064 from NEUT

    cf. s(nmCC) ~ 1.1 x 10-38 cm2 / nucleon from NEUT

    (K2K beam spectrum averaged)

    <En> ~ 1.3 GeV


    Summary
    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.


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