Konstantin Belov

1
ltXmaxgt measurements by fluorescence experiments.

• Konstantin Belov
• High Resolution Flys Eye (HiRes) Collaboration

GZK-40. INR, Moscow. May 17, 2006.

• Columbia University
• University of Adelaide
• University of New Mexico
• Rutgers University
• University of Montana
• Los Alamos National Laboratory
• University of Tokyo

2
Motivation

• Compare the ltXmaxgt measurements from the three
  fluorescence experiments Flys Eye, HiRes/MIA,
  and HiRes Stereo.
• Only fluorescence experiment SEE Xmax.
• In HiRes energy range (1017.2-1020.5 eV) expect
  to see a transition between galactic and
  extragalactic sources
• Expect highest energy galactic cosmic rays to be
  heavy nuclei (iron)
• Expect extragalactic cosmic rays to be light
  (protons)
• Expect transition from mixed to light
  composition.

3
Xmax distribution for p, CNO, Fe

• Heavier primaries develop earlier in the
  atmosphere.
• RMS of the Xmax distribution is different.

QGSJet
4
Elongation rate p, CNO, Fe
5
Flys Eye detector.

• Two fluorescence detectors were positioned 3.5 km
  apart
• The effective energy range extended from 1017 eV
  to 3?1019 eV
• 5?5 degree pixel size
• Geometry of the event was determined by the
  intersection of the two event-detector planes
• Elevation angle range from 0 to 90 degrees
• FE II had only partial azimuth coverage

6
HiRes prototype/MIA

• Hybrid fluorescence - muon array detector
• Fourteen mirror prototype of the current HiRes
  detector had a 1?1 degree pixel size looked from
  3 to 71 degrees in elevation.
• Was located 3.5 km from the center of the MIA
  muon array
• The muon array sampled a part of muon lateral
  distribution. 3 km effective area
• Energy range from 5?1016 eV to just above 1018
  eV
• Event geometry was determined by using the HiRes
  event-detector plane and timing from both
  detectors.

7
HiRes fluorescence detector.
8
Stereo observations.

• HiRes 1 has 20 operational mirrors in one ring.
  S/H electronics. FOV 3-17º in elevation, 322º in
  azimuth
• HiRes 2 has 42 operational mirrors in two rings.
  FADC electronics. FOV 3-31º in elevation, 336º in
  azimuth.
• Detectors are 12.6 km apart forming a stereo
  pair.
• Stereo fixes shower geometry improving resolution.

Idealistic view of a stereo event
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An event of the HiRes displays.
HiRes1 one ring detector
HiRes2 two ring detector
10
Measured shower profile.
Measured shower parameters.

• Event by event
• Xmax in g/cm2
• Total energy of the primary particle
• Arrival direction

• Statistically
• p-air inelastic cross-section
• Mass composition.

11
HiRes stereo and HiRes/MIA data.

• HiRes/MIA - hybrid experiment. Resolution
  function estimated from simulations 44 gm/cm2
• HiRes stereo - Xmax resolution measured, 30 gm/cm2

12
Flys Eye stereo data.

• Flys Eye Stereo - Xmax resolution 45 gm/cm2
  measured resolution function

13
Systematic errors

• All three experiments quote systematic errors
  25 gm/cm2.
• Dominant contributions
• Mirror survey
• Cherenkov subtraction
• Atmospheric profile
• Aerosol corrections
• Highest energy data is best measurements
• - most complete profile
• - minimum Cherenkov subtraction

14
Comparison of Flys Eye Stereo and HiRes energy
scales

• Energy scale for all three experiments is
  consistent (location of ankle and second knee).

15
Elongation rate

• Use HiRes stereo average Xmax and Flys Eye
  stereo average Xmax above 1018 eV.
• Require a 13 gm/cm2 upward shift for Flys Eye to
  bring means into agreement
• Shift all Flys eye Xmax data points by the same
  amount.

Filled circles, HiRes Triangles, HiRes/MIA, Open
circles, Flys Eye (shifted).
16
Elongation Rate

• Simple shift of Flys Eye data brings all data
  into reasonable agreement.
• Flys Eye and HiRes data are in excellent
  agreement above 1018 eV.
• HiRes/MIA shows earlier transition to protons,
  but point by point discrepancy is small.
• HiRes/MIA systematics are better understood,
  however.

17
What about Xmax distributions?

• Are Xmax distribution widths consistent?
• Two overlap regions
• - Flys Eye and Hires/MIA in 3x1017 to 5 x
  1017 eV bin.
• - Flys Eye and HiRes stereo in gt 1018 eV
  bin.
• No evidence of discrepancy in distribution
  widths.

18
Flys Eye vs HiRes/MIA Xmax at 3-5 x 1017 eV
Triangles Flys Eye Stereo Circles HiRes/MIA.
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Flys Eye vs HiRes Xmax gt 1018 eV
Triangles Stereo Flys Eye Circles HiRes.
20
Sequence of Xmax distributions in three
increasing energy bins 3-5?1017, 5-10?1017 and gt
1018eV.
Red-Flys Eye Blue-HiRes/MIA Purple-HiRes stereo
21
Conclusions

• A simple Xmax shift brings all three experiments
  into reasonable agreement.
• Widths of Xmax distributions are in agreement.
• Normalized Xmax distribution show change to wider
  distribution above 1018, consistent with change
  to protons.
• Interpretation of elongation rate over limited
  energy range is problematic - Need large dynamic
  range in a single experiment!