Results from the HiRes Experiment

1
Results from the HiRes Experiment

• Gordon Thomson
• University of Utah

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Outline

• Introduction
• Some recent results
• Spectrum
• Composition
• Anisotropy
• Summary

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High Resolution Flys Eye (HiRes) Collaboration

• J. Boyer, B. Connolly, C.B. Finley, B. Knapp,
  E.J. Mannel, A. ONeill, M. Seman, S. Westerhoff
• Columbia University
• J.F. Amman, M.D. Cooper, C.M. Hoffman, M.H.
  Holzscheiter, C.A. Painter, J.S. Sarracino, G.
  Sinnis, T.N. Thompson, D. Tupa
• Los Alamos National Laboratory
• J. Belz, M. Kirn
• University of Montana
• J.A.J. Matthews, M. Roberts
• University of New Mexico
• D.R. Bergman, G. Hughes, D. Ivanov, S.R.
  Schnetzer, L. Scott, B.T. Stokes, S. Stratton,
  G.B. Thomson, A. Zech
• Rutgers University
• N. Manago, M. Sasaki
• University of Tokyo

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The Two HiRes Fluorescence Detectors

• HiRes1 atop Five Mile Hill
• 21 mirrors, 1 ring (3ltaltitudelt17 degrees).
• Sample-and-hold electronics (pulse height and
  trigger time).
• HiRes2 Atop Camels Back Ridge
• 12.6 km SW of HiRes1.
• 42 mirrors, 2 rings (3ltaltitudelt31 degrees).
• FADC electronics (100 ns period).

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Mirrors and Phototubes

• 4.2 m2 spherical mirror
• 16 x 16 array of phototubes, .96 degree pixels.

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Reconstruction
1.)

• The trajectory of the EAS can be determined in
  one of two ways
• Monocular reconstruction using the arrival time
  of light signal at the detector.
• Stereo by intersecting the shower-detector planes
  (SDP) seen from the two detector sites.

2.)
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Spectrum and Composition Method

• Spectrum aperture varies with energy. Must
  calculate with Monte Carlo method.
• Trigger and thresholds very important.
• Excellent accuracy is achievable.
• Composition Monte Carlo simulations are
  essential to understand Xmax studies.

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Compare Data to Monte Carlo Judge success of
simulation and acceptance calculation.
Inputs to Monte Carlo Flys Eye stereo
spectrum HiRes/Mia and HiRes Stereo composition
Library of Corsika showers. Detailed nightly
information on trigger logic and thresholds, live
mirrors, etc.
Result excellent simulation of the data,
and an accurate aperture calculation.
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5s Observation of the Break in the Spectrum

• Broken Power Law Fits
• Two BP with extension to test hypothesis that a
  break is present.
• Expect 43 events
• Observe 13 events
• Poisson probability P(1343) 7x10-8, which
  is 5.3s
• The break is statistically significant.

E-5.1
Break is at (5.6 0.5) x 1019 eV GZK expected
between 5 and 6 x 1019 eV. The break is the true
GZK cutoff.
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Use Berezinskys E½ Method to Test

• E½ is the energy where the integral spectrum
  falls below the power-law extension by a factor
  of 2.
• Berezinsky et al. log10E½ 19.72, for a wide
  range of spectral slopes.
• Use 2 Break Point Fit with Extension for the
  comparison.
• log10E½ 19.73 0.07
• Passes the test.

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Local Density of Sources

• Theoretical predictions for spectrum shape agree
  with HiRes measurements.
• Compare HiRes spectrum slope above the GZK energy
  to Berezinsky et al. predictions
• Line 1 constant density.
• Line 5 no sources within 10 Mpc.
• Line 2 double density within 30 Mpc.

Berezinsky, Gazizov, and Grigorieva, Phys. Rev.
D74, 043005 (2006) (uses older HiRes spectrum)
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Local Density of Sources

• Compare HiRes spectrum slope above the GZK energy
  to Berezinsky et al. predictions
• Line 1 constant density.
• Line 5 no sources within 10 Mpc.
• Line 2 double density within 30 Mpc.
• Line 3 triple density within 30 Mpc.
• HiRes E-5.1 fall-off.
• More work is needed to make a better comparison,
  but...
• Constant density of sources is favored.

E-5.1
Berezinsky, Gazizov, and Grigorieva, Phys. Rev.
D74, 043005 (2006) (uses older HiRes spectrum)
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HiRes/Auger Spectra Comparison
Auger confirms all spectral features of HiRes
spectrum
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Spectrum Summary

• The GZK cutoff is present. The first observation
  was by the HiRes experiment.
• All details of the spectrum indicate the
  composition is protons.
• The energy of the GZK cutoff is as expected for
  protons.
• Highest energy extragalactic cosmic rays travel gt
  50 Mpc.
• The fall-off above the cutoff is evidence for a
  constant density of sources. CRs travel a long
  distance. Spallation breaks up all nuclei at
  high energies ? proton flux results.
• The ankle has been observed by HiRes, at 1018.65
  eV. The spectral index changes from -3.2 to -2.8
• Shape and energy of the ankle are consistent with
  ee- production in collisions between
  extragalactic protons and photons of the CMBR.

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More Direct Composition Measurement
use ltXmaxgt

• HiRes result on ltXmaxgt, and on s(Xmax), indicate
  light composition.
• ltXmaxgt dependence is logarithmic halfway is Be.
  Spallation at high energies breaks up light
  elements.
• ? Composition is essentially all protons.

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QGSJetII Protons Look Like Our Data(pointsdata,
histogramMC)
MC protons
MC iron
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Data (points) vs. QGSJet-II MC (histogram)
Energy Bins 18.3, 18.5
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Energy Bins 18.7, 18.9
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Energy Bins 19.1, 19.3
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Energy Bins 19.5, 19.7
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Composition Summary

• The most direct indicator of composition is
  ltXmaxgt.
• Our ltXmaxgt data indicate a light composition, and
  favor protons.
• This is consistent with all of our spectrum
  information.

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The Search for Anisotropy

• Aim is to identify UHECR sources.
• Two methods
• Make a sky plot and look for bumps.
• Look for correlations with known astronomical
  object types.
• All cosmic ray results are of marginal
  significance
• Sky plots AGASA doublets/triplet at 40 EeV
• Correlations BL Lacs at 10 EeV AGNs at 57
  EeV

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AGN Correlations

• Auger events correlations with AGNs (south)
• Early data set scanned in (Emin, ?, zmax) using
  Veron-CettyVeron catalog, found best
  correlations at (57 EeV, 3.1, .018).
• Tested correlation with later data set, found
  8/13 events correlated, chance probability of
  0.002 (2.9s).
• Not an observation PRL requires 5s. Require
  confirmation.
• Auger test using later data
• 42 events, 12 correlated,
• expect 8.8 random, 1s.
• ? no effect.

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HiRes Test of AGN Correlations (north)

• Choose Auger optimum point, same catalog.
• 2/13 events correlated (expect 3 randomly)
• chance probability of 0.23 ? no effect.
• Scan the HiRes data
• best point is (15.8 EeV, 2.5, 0.016), 46/198
  correlated.
• chance probability of 0.29 ? no effect.

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Test of Correlations with Local Large Scale
Structure

• All results extragalactic, point source energies
  of 10 (BL Lac), 40 (clusters), 57 EeV (AGN).
• Matter is not distributed uniformly within the
  horizon of 57 EeV protons
• ? Search for correlations with local large scale
  structure for high energy events with relatively
  close horizons. A priori choice 10, 40, 57
  EeV. Also choose, a priori, to quote 95 CL.

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HiRes Stereo Data

• HiRes complete stereo data set, angular
  resolution 0.8
• Events not within 10 of galactic plane
• 10 above 57 EeV
• 27 above 40 EeV
• 310 above 10 EeV
• 6636 events in all

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Local LSS Model

• Based on 2 Micron All-Sky Redshift Survey
  (2MRS)1, a flux-limited sample of galaxies with
  m 11.25
• Remove galactic plane (b lt 10) and objects
  within 5 Mpc.
• Transform to a volume-limited sample by
  weighting.
• Result is 15,508 galaxies between 5 and 250 Mpc.
• Assume distribution is isotropic beyond 250 Mpc.
• 1. J. Huchra, L. Macri, T. Jarrett, et al., in
  preparation.

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Procedure set of MC events coming from LSS

• Start with local LSS model.
• Modify using HiRes aperture.
• Simulate the data set find average predicted
  event density.
• Two parameters
• Minimum energy
• Angular smearing to simulate magnetic fields
• Expect 1, extragalactic fields, for E40 EeV1
• Expect 2-4, galactic fields.
• Perform K-S test between data and expectation
  from LSS.
• Repeat starting with an isotropic galaxy
  distribution.
• 1. T. Kashti and E. Waxman, JCAP 0805, 006
  (2008).

57 EeV
40 EeV
10 EeV
Smearing angle of 6
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Sky Plots
10 EeV
40 EeV
57 EeV
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Results

• Choose 95 c.l. exclusion to quote, a priori.
• For isotropic model, get good agreement.
• For local LSS model get poor agreement.
• Exclude correlation at 95 c.l. for ?s lt 10,
  E 40 EeV

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LSS Conclusions

• We have searched for correlations between the
  pointing directions of HiRes stereo high energy
  events and local large scale structure.
• There are none at the 95 confidence level for
  magnetic smearing angles lt 10.
• This is very surprising.
• One expects to see correlations.
• One expects magnetic field smearing at the 4-5
  level.
• With limited statistics we are able to place very
  significant limits.
• Are the sources not in galaxies? Are magnetic
  field estimates wrong?
• This result should be confirmed by a future
  experiment (north).
• The Telescope Array Experiment is now collecting
  data. It has statistics equal to HiRes stereo
  (see TA talk, Yuichiro Tameda).

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Summary

• HiRes made the first observation of the GZK
  cutoff.
• It occurs at (5.6 0.5 0.9) x 1019 eV.
  
• Flux a E-5.1 0.7 above the cutoff.
• We see the ankle at 1018.6 eV.
• E-3.3 below and E-2.8 above.
• The composition is very light, probably protons
• No AGN correlations are observed.
• Correlations with local large scale structure are
  very weak magnetic fields may be larger than
  previously thought.