The High Resolution Fly’s Eye ( HiRes ) Experiment

May 31, 2004 Benjamin Stokes CRIS2004

The High Resolution Fly’s Eye (HiResHiRes) Experiment Collaboration:

Columbia University University of Adelaide University of New

Mexico Rutgers University University of Montana Los Alamos National

Laboratory University of Tokyo Beijing Institute for High

Energy Physics


HiRes-I Monocular Anisotropy Several potential anisotropies have been suggested by

previous experiments and predicted by theorists. We will descope our search to the following: Global Dipole Effects: =1+cos Galactic and Supergalactic Plane Enhancements Small-Scale Clustering (<5degrees) Discrete Point Sources (such as Cygnus X-3…)


HiRes-I: Largest Current Exposure in UHECR Physics:

Monocular Aperture similar to stereo: – ~ 9100 km2-Sr @ 1020 eV.– ~ 4730 hours on-time (May 1997 to April 2004).

~3665 hours of good weather data: equivalent integrated exposure:– ~ 3320 km2-Sr-Yr @ 5x1019 eV – ~ 3735 km2-Sr-Yr @ 1020 eV.

Array Integrated Exposure @~5x1019 eV (km2-Sr-Yr)

AGASA (100 km2)Fly’s Eye (Stereo)Fly’s Eye (Mono)Haverah Park (12 km2)Yakutsk (25 km2)

~1050~150~930~270~490

Total: ~2900


HiRes-I Monocular Analysis (cont.)Events reconstructed using profile-constrained time-fit. Phys. Rev. Lett. 92 151101 (2004) Astro-ph/0208301 (submitted to Astroparticle Physics) Assume Gaisser-Hillas shower profile.

– Shown to be in good agreement with data at lower energies previously by the original Fly’s Eye, and by the MIA-HiRes prototype studies.

– Air shower width (~500-550 gm/cm2) has been shown to be largely independent of energy or composition in Corsika studies.

Set X0=40 gm/cm2.– average value of first-interaction depth obtained from Corsika studies.

Allow Xmax to vary in the range of 680-900 gm/cm2.– Results from original Fly’s Eye and others.

Energy Resolution better than ~20% above 3x1019 eV.


Mono-Stereo Event Comparison

Profile-constrained reconstructed energy was compared with energy estimated by HiRes-I with the aid of stereo geometry

Results confirmed Monte Carlo energy resolution estimates


Monocular Analysis (cont.)Reconstruction assumes average

observed atmosphere – Vertical Aerosol Optical Depth

(VAOD): 0.04 ± 0.02


Atmospheric Effect on Flux…


Monocular Angular Resolution While data has the

advantage of providing superior statistics…

Monocular data introduces the additional complication of asymmetric angular resolution


HiRes-I Arrival Directions with 1 Error Ellipses (>1019.5eV)


Monocular Angular Resolution (cont.) Each event arrival direction is treated as a 2-d

asymmetric, normalized Gaussian distribution about the nominal reconstructed arrival direction

At 1018.5 eV, the 1 error is:– ~ 0.5 degrees in the determination of the

plane of reconstruction– ~ 14 degrees in the determination of the

arrival direction within the plane of reconstruction


Monocular Angular Resolution (cont.)

Angular resolution parameterization is confirmed by mono-stereo comparison

±7.5% systematic uncertainty


HiRes-I Arrival Directions (>1018.5eV) with Angular Resolution Probability Densities…


Exposure Estimation…

Detector “good weather” observations times are tabulated

Mirror-by-mirror correction is utilized to correct for asymmetric running conditions.






HiRes-I Relative Exposure (@ >1018.5eV)


HiRes-I Relative Exposure (@ >1019.5eV)


Global Dipole Effects = 1 + cos Measured observable: <cos> Comparison of real data set with

simulated sets with dipole effect inserted

SagA: =0.005 ± 0.055 CenA: =-0.005 ± 0.065 M87: =-0.010 ± 0.045 Consistent with Isotropy Astropart. Phys. 21 111 (2004)


Galactic Plane Enhancement?


Galactic Plane Enhancement?<|bG|>° (bG)rms°

>E(eV) #evts data MC Pu data MC Pu

1019.75 17 33.00 32.12 0.557 43.05 39.35 0.750

1019.50 56 32.52 31.90 0.591 40.13 38.50 0.717

1019.25 178 32.23 31.91 0.496 39.66 38.30 0.815

1019.00 418 30.49 31.69 0.1230.123 37.36 38.06 0.248

1018.75 871 30.99 31.58 0.190 37.69 37.96 0.339

1018.50 1683 31.46 31.44 0.521 37.97 37.85 0.607


Supergalactic Plane Enhancement?


Supergalactic Plane Enhancement?

<|bSG|>° (bSG)rms°>E(eV) #evts data MC Pu data MC Pu

1019.75 17 37.23 31.08 0.876 46.48 38.40 0.905

1019.50 56 32.87 30.92 0.758 40.22 37.74 0.813

1019.25 178 31.46 30.83 0.662 38.40 37.46 0.506

1019.00 418 31.62 30.59 0.848 38.42 37.16 0.889

1018.75 871 30.25 30.31 0.513 37.07 36.83 0.638

1018.50 1683 30.02 30.12 0.405 36.58 36.62 0.459


Small Scale Anisotropy The Akeno Giant Air Shower

Array (AGASA) reported statistically significant clustering in its highest energy events.

However, the AGASA events do not possess an asymmetric angular resolution like the HiRes-I events

A method was developed to directly compare the sensitivity of the two experiments.


Autocorrelation Function

Sample each event’s angular resolution Calculate opening angle for every

possible pair of events (with inclusion of angular resolution)

Histogram. Repeat 106 times. Normalize Measure sharpness of peak @ opening

angle = [0,10] degrees


Autocorrelation Function


Autocorrelation FunctionHiRes-I AGASA


HiRes and AGASA Clustering Signals:


HiRes-AGASA Sensitivity Comparison…HiRes-I AGASA


Small Scale Anisotropy The global sensitivity of the two experiments to small scale

anisotropy was shown to be comparable. However, HiRes-I saw no evidence of clustering HiRes Stereo, which contains superior angular resolution

but lower statistics has also observed no evidence of autorcorrelation

Mono result: astro-ph/0404366 (submitted to Astropart. Phys.)

Stereo result: astro-ph/0404137 (submitted to ApJ Lett.)


Point Source Searches

No evidence for any discrete point sources Efforts to place an upper limit on the

luminosity of unknown point sources in the observable sky are ongoing…

One interesting tactic for doing so involves measuring the entropy of the arrival direction distribution using a technique borrowed from fractal dimensionality…


Information Dimension

The information dimension is tool used in fractal dimensionality analysis that is analogous to entropy

In general:

– Where P is the probability of finding an event in the i-th bin of “edge-size”


Application to UHECR arrival directions: In our specific case, the

sky can be divided into latitudinal bins and P can simply be thought of as the event density in a single bin divided by the integrated event density over the entire sky

Astropart. Phys. 21 95 (2004)


Example: Seven Source Model

We will assume that there are seven bright sources in the sky (corresponding to the reported AGASA clusters) superimposed on an isotropic background.

We will further assume that these source are subject to magnetic smearing on the order of 5 degrees


Seven Source Model Exposure


Seven Source Model: Simulated Data Sets


How well can we exclude the seven source model for a particular isotropic simulated set?


Distributions of Dl-values for large numbers of simulated sets with a fixed parameter:


Information Dimension (cont.)

It is possible to discriminate between different source models…

However this isn’t as effective as a direct measurement…


How do the different methods compare for the HiRes-I data?


Advantages to Dl method

Single parameter negates the need to apply statistical penalties to findings

Completely “blind” measurement that is not geometry-dependent

Can discern anisotropies at any scale from global to the intrinsic angular resolution of the given data


Disadvantages to Dl method

Not as sensitive as a direct measurement and it is “blind”

The effect that a particular anisotropy will have on the value of Dl is not always intuitively obvious

This is still novel method which might manifest other problems if it comes in greater use.


More answers will be forthcoming…More answers will be forthcoming…

Pierre Auger Observatory Large international collaboration Southern Site in Argentina currently under

construction. Northern Site proposed for Millard County,

Utah (~2007?)

Telescope Array (~2005) U.S.- Japan–Taiwan collaboration Proposed site: 3 stations w/ground array in

Millard County, Utah


www.cosmic-ray.org

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The High Resolution Fly’s Eye ( HiRes ) Experiment