ChargedParticle Multiplicities in Neutrino Interactions

1
Charged-Particle Multiplicities in Neutrino
Interactions

• Murat GULER, Umut KOSE
• METU-Ankara
• (for the CHORUS Collaboration)

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Charged-Particle Multiplicities

• The multiplicty of charged particles is an
  important global parameter reflecting the
  dynamics of the interaction.
• The characteristics of charged-particle
  multiplicity distributions have been studied in
  detail in high energy hadronic collisions and in
  electron-positron annhilation.
• Also, it has been studied in neutrino and
  anti-neutrino interactions on nucleons and light
  nuclei. In particular, Bubble Chamber experiments
• However, data on neutrino interactions in nuclear
  emulsion are relatively scare. No data on
  anti-neutrino-nucleus interactions in nuclear
  emulsion.
• Such data are useful in tuning interaction models
  in Monte Carlo event
• generators.

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The n beam
West Area Neutrino Facility at CERN SPS

• Wide Band Beam
• 4-years of operation (1994-1997)
• 5.06 ? 1019 POTs
• ltEnmgt 27 GeV

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CHORUS detector
T5C
Nucl. Instr. Meth A 401 (1997) ,7
? -
Calorimeter
h-

• Events are classified as
• 0m sample
• 1m sample
• based on electronic detector
• information

Muon spectrometer
Air core spectrometer
and emulsion tracker
Veto plane
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Nuclear Emulsion Target

• 60 years of RD.
• The sub-micron spatial resolution of nuclear
  emulsion allows both investigation of the event
  topology and the mesurement of the angular
  distribution of the charged particles.
• 770 kg nuclear emulsion was exposed to the
  neutrino beam in CHORUS.
• Massive automatic scanning (pioneered by Nagoya
  Univ. group of CHORUS)

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Automatic Scanning
Tracks reconstructed by a hardware video
processor frame to frame emulsion grains
coincidence
microscope stroke
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Event Location

• The location of the emulsion plate containing the
  interaction vertex
• Tracks reconstructed by the electronic detectors
  are followed upstream into the emulsion stacks
• A track is followed upstream in the target
  emulsion stack using track segments reconstructed
  in the most upstream 100µm of each plate.
• If the track is not found in two subsequent
  plates the first plate with no track hit is
  defined as the plate containing the vertex.

1
2
3
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Event Location

• 150,000 n interactions have been located events
  with at least one reconstructed muon in the
  spectometer.
• A sample of 1200 events was randomly selected
  to measure the parameters associated with
  charged-particle tracks at the (anti-) neutrino
  vertex.
• We use the highest-mometum muons charge in
  the event to determine whether the interaction
  was induced fom neutrino or anti-neutrino.
• n sample 627 events
• n sample 581 events

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Data Sample

• To evaluate the genuine anti-neutrino events,
  contaminations which are mainly punch through
  hadrons and miss-identified µ- events must be
  evaluated.
• In order to reduce this background a set of
  selection criteria imroving the realibity of the
  muon reconstruction were applied.
• After contamination cleaning the number of
  anti-neutrino events becomes to be 529
• Finally, we further require that square of the
  invariant mass of the hadronic system,W2, of
  (anti-) neutrino events is greater than 1 GeV2/c4
• n sample 496 events
• n sample 369 events

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Analysis

• Tracks are classified accordingly grain density
  as shower, grey and black.
• Black prong glt1.4g0
• Grey prong 1.4g0glt10g0
• Shower prong g10g0 where go is grain density
  of higly relativistic a singly charged particle
  which equals 30 grains in 100mm for the CHORUS
  emulsion
• The criteria used in old nuclear emulsion
  experiments is difficult to apply in the CHORUS
  analysis since the emulsion sheets were exposed
  prependicular to the beam direction.
• In CHORUS, once the neutrino vertex had been
  located, the event was checked and measured
  extensively with a manual controlled microscope
  system.

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Analysis

• The black prongs are due to heavily ionizing
  particles have short path lengths and stop within
  one emulsion plate.
• For the grey and shower prongs, the particle
  direction is measured. These particles are mainly
  pions with a small contamination of protons.
• Ambiguity in classifying shower and grey prongs
  can be arise either due to the difficulty
  strickly applying the ionization criteria or due
  to variations in the quality of optics of the
  microscope.

Black prongs
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Analysis

• To overcome these limitations, we decided to
  classify these prongs by using the
  pseodo-rapidty variable
• This has advantage of being independent of
  scanner and of the microscope optics, allowing us
  to compare in a straight manner the multiplicty
  measurements with theoretical models

The pseodo-rapidty distributions for the track
classified as shower and grey by the scanner
shower
grey

• All prongs with ? 1 are classified as shower
  particles

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Efficiency Estimation
Neutrino events

• Reconstruction and detection efficiencies were
  evaluated making a detailed simulation of
  detector response using a program based on GEANT3
• The simulated response of electronic detectors
  was processed through reconstruction program
  as that used for experimental data
• The reconstrcution efficiency as function of W2
  and nch is given in tables
• The samples are normalized in such a way that
  the efficiency at first bin is taken as 1.00

Anti-neutrino events
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Multiplicty Distributions

• The average number track multiplicity (nchns-1)
  in neutrino interactions.
• ltnch(n A)gt 3.40.1
• ltnh(n A)gt 4.70.2
• In anti-neutrino inte-ractions
• ltnch(n A)gt 2.80.1
• ltnh(n A)gt 3.50.2

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Multiplicity Distributions

• The mean multiplicities are in good agreement
  with a linear depen- dence on lnW2

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(QERES)-like events

• For the first time a sample of (anti-) neutrino
  is large enough to study (QERES)-like topologies
  
• Event is defined as being (QERES)-like
• if the number of shower prongs is zero or one and
  the number of grey prongs zero or one for n-A
  interactions regardless of the number of black
  prongs.
• In order to obtain (QERES)-like enriched sample
  in n-A interactions
• the sum of shower and grey prongs is required to
  be zero or one regardless of the number of black
  tracks.
• In order supress the background from DIS
  interactions we further require W2lt10 GeV2/c4 for
  both sample. After this selection the number
  (QERES)-like events is

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(QERES)-like events

• The number of background events that mimic
  (QERES)-like topology is obtained from the MC
  simulation to be
• 83.4 (73.5)for n-A(n-A) interactions
• The ratio of reconstruction and location
  efficiency of (QERES)-like events to that of all
  CC events is found to be
• 1.22(1.13) for n-A(n-A) interactions
• After applying efficiency and background
  corrections, the fraction of (QERES)-like events
  are found to be
• (13.40.10.2) for n-A interactions
• (26.31.43.9) for n-A interactions
• The sub-sample of events with neither black or
  grey prongs is important for the understanding of
  nuclear mechanism involving hadrons in the
  nucleus. The fraction of this type of topology is
  measured as
• (1.20.40.2) for n and (9.51.01.4) for n

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Dispersion of track multiplicity

• The linear dependence of Dch on nch would imply
  that multiplicity is independent of W2

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KNO Scaling

• One can observe that our data are not compatible
  with A0. Non-zero A is due to heavy nuclear
  targets in nuclear emulsion. In that case one
  introduce
• Where a is the extrapolation point where the
  fitted dispersion line crosses average
  multiplicity line

The superimposed curve represent a fit to pp
data. The data approximately agree with KNO
scaling. The data points at different W2 lie
approximately on a single curve.
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Conclusion

• The multiplicity feature of (anti-) neutrino
  interactions in emulsion have been investigated.
• The average number of shower and heavy prongs in
  (anti-) neutrino events are measured as
• The dependence of the average multiplicity ltnchgt
  on lnW2 for n-A and n-A interactions is
  compatible with being linear with similar slope.
• The dispersion Dch of the multiplicity
  distribution shows a linear dependence on the
  mean multiplicity ltnchgt.
• The emulsion data is consistent with the KNO
  scaling as a function of
• an appropriate multiplicity variable z.

ltnch(n A)gt 3.40.1 ltn h(n A)gt 4.70.2
ltnch( n A)gt 2.80.1 ltnh( n A)gt 3.50.2