UHECRs in the Northern Hemisphere: A Status Report

1
UHECRs in the Northern Hemisphere A Status
Report
Recent Results from HiRes

• Douglas Bergman
• University of Utah
• CCAPP Inaugural Symposium
• 12 October 2009

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Introduction

• The High Resolution Flys Eye (HiRes) experiment
  has recently finished its 10 year data taking
  run.
• Good chance to summarize our knowledge of
  ultra-high energy cosmic rays as seen from the
  northern hemisphere
• The recent results from HiRes, final analyses,
  cover all three of the basic types of cosmic ray
  measurements
• Spectrum (now in stereo)
• Composition
• Anisotropy (in particular, correlation with the
  local mass structure of the universe)

3
The HiRes Experiment

• HiRes was a stereo fluorescence detector,
  operated from 1997-2006 on Dugway Proving Grounds
  in Utah
• Observe the air-showers created by CRs by
  collecting fluorescence light

4
The HiRes Experiment

• Light collected by 5 m2 mirrors onto an array of
  256 (1616) of PMTs
• Each PMT sees 1 cone
• Each PMT records time and amount of light seen
• Reconstruct shower geometry by stereo

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Sample HiRes Event
Nmax (7.1 0.5) 109 Xmax 779 26 g/cm2 E
8.6 0.6 EeV ?2/DOF 19.5/17
Nmax (6.14 0.13) 109 Xmax 812 5 g/cm2 E
8.4 0.2 EeV ?2/DOF 100/54
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Stereo Spectrum Measurement

• To find spectrum
• Collect data
• Find energy of each event
• Bin events in energy bins
• Calculate the aperture (thats the hard part)
• Calculate aperture by simulation of detector
• Verify by data/simulation comparisons
• Reduce systematic by finding fully efficient
  area at each energy

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Stereo Spectrum Measurement

• To find spectrum
• Collect data
• Find energy of each event
• Bin events in energy bins
• Calculate the aperture (thats the hard part)
• Calculate aperture by simulation of detector
• Verify by data/simulation comparisons
• Reduce systematics by finding fully efficient
  area at each energy

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The UHECR Energy Spectrum

• The geo-constrained spectrum is not
  systematically different than the full spectrum,
  so we use the full spectrum

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The UHECR Energy Spectrum

• The stereo spectrum confirms the observation of
  the GZK we observed with out monocular analyses
• In the southern hemisphere, Auger see a similar
  (but with perhaps slightly different slopes and a
  different cutoff energy)

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UHECR Composition Measurement

• Xmax grow logarithmically with energy as the
  shower branches more
• Heavier CRs (more nucleons) act like a
  superposition of lower energy proton showers

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UHECR Composition Measurement
Protons in QGSJetII

• Measure composition by finding average Xmax vs
  energy
• Not gaussian mean subject to biases
• Different models give different averages, but
  similar slopes (elongation rate)

Iron in QGSJetII
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UHECR Composition Measurement

• Heres the HiRes data
• Looks mostly like protons

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UHECR Composition Measurement

• Heres the HiRes data
• Looks mostly like protons

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UHECR Composition Measurement

• Heres the HiRes data
• Looks mostly like protons
• Make acceptance correction based on QGSJetII
  protons

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UHECR Composition Measurement

• Heres the HiRes data
• Looks mostly like protons
• Make acceptance correction based on QGSJetII
  protons
• Compare to other results
• Combined with HiRes/MIA, heavier at low energies,
  mostly light by 1 EeV

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UHECR Composition Measurement

• Heres the HiRes data
• Looks mostly like protons
• Make acceptance correction based on QGSJetII
  protons
• Compare to other results

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UHECR Composition Measurement

• Heres the HiRes data
• Looks mostly like protons
• Make acceptance correction based on QGSJetII
  protons
• Compare to other results
• Note that uncorrected average is very close to
  Auger

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UHECR Composition Measurement

• Also look at the width of showers
• HiRes width agrees with predicted width for
  protons

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UHECR Composition Measurement

• Also look at the width of showers
• HiRes width agrees with predicted width for
  protons
• Compare to Auger (without detector resolution
  removed)

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UHECR Correlation with LSS

• HiRes data indicates
• UHECRs are protons
• Many come from far away
• Otherwise no GZK
• Beyond 50 Mpc
• Trajectories rigid enough to point back to origin
• Look for correlations with various objects (say
  AGN as Auger has done)
• Or look for correlation with mass structure out
  to 250 Mpc using flux limited samples (2MASS)

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UHECR Correlation with LSS

• Start with 2MASS to create LSS model
• Smear by variable angle
• Limit distance by energy
• Convolve with HiRes exposure
• Perform K-S test based on density of LSS model

57 EeV
40 EeV
10 EeV
Smearing angle of 6
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UHECR Correlation with LSS
10 EeV
40 EeV
57 EeV
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UHECR Correlation with LSS

• Plot K-S probability for both isotropic and LSS
  models
• Choose 95 CL a priori
• Good agreement with isotropy
• Poor agreement at small scattering angles for LSS
• No correlation at 95 CL for E gt 40 EeV and ?s lt
  10

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Conclusions

• HiRes has observed the GZK cutoff in both
  monocular and stereo modes
• HiRes finds the composition of UHECRs above 1
  EeV to be predominantly light, as one might
  expect from the presence of the GZK cutoff
• HiRes observes no correlation with the local,
  large-scale structure of the universe
• The lack of correlations is surprising since
  magnetic field smearings are only expected to be
  at the 5 level
• The Telescope Array is currently operating in the
  North, and will provide much more anisotropy data