BG studies with LAT CU Beam Test

1
BG studies with LAT CU Beam Test

• Tsunefumi Mizuno, Hiromitsu Takahashi, Hideaki
  Katagiri and Yasushi Fukazawa (Hiroshima Univ.)
• Hiro Tajima (SLAC)

• Objective
• Dominant Particle
• DC2 BG data analysis/BG property (1)(4)
• Whats necessary to estimate BG?
• Toy MC of e run
• Summary

2
Objective

• Bill's talk at DC2 kickoff showed larger than
  expected BG level.
• BG exceeds 1/10 of extragalactic ?-rays with E lt
  a few 100 MeV.
• BG level is close to extragalactic ?-ray flux
  for E lt 100 MeV.
• Reliable BG modeling is required
• Monitor the flux/estimate the contamination to
  photon events.
• Minimize impact on the diffuse/faint source study
  by GLAST.
• Beam Test for BG study
• Establish a way to estimate BG in orbit
• Minimum impact on calibration runs.

primary proton
secondary e/e-
secondary proton
primary alpha
atmospheric gamma
3
What contributes to BG?

• Proton primary, positron reentrant/splash and
  Earth10 (atmospheric gamma) are dominant.
• Most of e reentrant background are due to
  annihilation.

Correlated BG
https//confluence.slac.stanford.edu/display/SCIGR
PS/Residualbackgroundanddiffuseemission
From Bills talk at kickoff meeting
4
Analysis of DC2 data

• Analyze DC2 BG data to study BG property
  (particle type, direction and energy of those
  which contribute to BG).
• Use DC2 BG data (v7r3p5) backgndDC2-GR-v7r3p5-mer
  it.root (total 1.5day)
• Select GoodEvent1. (further cuts with CTBBestZDir
  and FT1ZenithTheta were applied for DC2)

5
BG property (1) e reentrant

• Most of e reentrant backgrounds are due to
  annihilation at blanket or meteorite shield or
  top ACD tiles.
• Particle energy peak at 100 MeV. Significant
  contribution from e of 1GeV

Intersect position of particle trajectory and LAT
(imaginary box which encloses LAT).
hitz
hity
McEnergy (MeV)
hitx (mm)
6
BG property (2) proton primary
hitz
hity
hitz(mm)
hitx (mm)

• Interaction in CAL contributes to BG.
• Measured energy is much less than proton energy

FT1Energy (MeV)
McEnergy (MeV)
7
BG property (3) atmospheric gammas

• Most of atmospheric gamma-ray BGs are coming
  downward relative to LAT and can be distinguish
  from celestial gammas. Significant fraction of
  gammas from back of CAL.
• Measured energy is close to gamma-ray energy.

CTBBestZDir
FT1Energy (MeV)
McZDir
upward
McEnergy (MeV)
sideway
8
BG property (4) positron splash
hitz
hity
hitz(mm)
hitx (mm)

• Interaction in CAL contributes to BG.
• Measured energy is close to positron energy.

FT1Energy (MeV)
McEnergy (MeV)
9
What is necessary to estimate BG?

• Positron Reentrant
• Large uncertainty in positron secondary flux
  needs to be monitored.
• They could be monitored by observing two photon
  events.
• BG estimation heavily relies on MC.
• positron annihilation process of G4 needs to be
  validated.
• Proton Primary
• Primary proton flux is well known. (in 10)
• Interaction in CAL needs to be understood.
• Atmospheric gamma
• Flux is uncertain but can be monitored by LAT.
• Interaction in CAL needs to be understood.
• Positron Splash
• Upward e also contributes to BG. So does the
  upward e-.
• Interaction in CAL needs to be understood.

BG run with CU (in priority order)

• Positron runs with blanket and meteorite shield
  or equivalent
• Proton runs from back/side of CAL
• Gamma runs from back/side of CAL
• Electron runs from back/side of CAL

10
Do we have enough two gamma?
To see how many gammas will be generate in e
run, we ran a very simplified LAT.
e (normal incidence) 500 MeV and 1 GeV (1M each)

• Equivalent to Blanket meteorite shield
• Carbon of 0.39 g cm-2

• ACD
• Plastic scintillator of 1cm

• Perfect Tracker which is made of vacuum but has
  100 detection efficiency

11
of gammas expected
All events (1M)
1GeV e
no hits in ACD (162)
50 events with eGamMingt30 MeV
Lower Energy of gamma
of gammas in LAT
higher Energy of gamma (MeV)
500 MeV e
102 events with eGamMingt30 MeV
343 events.

• Lower energy is preferred.
• At least a few million triggers is required for
  sufficient statistics. (See also CERN Beam Test
  Plan by Benoit and Eduardo)

12
Summary

• Positron annihilation and proton interaction in
  CAL is the dominant component of LAT BG. Upard
  gammas/positrons/electrons also contribute to BG.
• Positron flux is fairly uncertain needs to be
  monitored. Annihilation process needs to be
  validated.
• Positron runs with blanket/meteorite shield or
  equivalent.
• Lower energy is preferred.
• At least a few million triggers are necessary
• Significant fraction of 2 gamma events in
  annihilation events.
• Proton flux is well known. Interaction in CAL
  needs to be validated.
• Proton runs from back/side of CAL.
• Upward gammas/positrons/electrons also
  contributes to BG
• Gammas/electron runs from back/side of CAL.
• Calibration Unit simulation is necessary to
  estimate the sufficient number of triggers.