# Presentation: 340444

##  Observation of the GZK Cutoffby the HiRes Experiment

- Gordon Thomson
- Rutgers University

- BNL, January 25, 2007

## Outline

- Introduction
- The HiRes experiment
- Calculating the spectrum
- Results (plus composition and anisotropy)
- Future studies

## Cosmic Rays over a Wide Energy Range

- At lower energies, spectrum of cosmic rays is almost featureless.
- Only the “knee” at 3x1015 eV
- The knee is due to a rigidity-dependent cutoff, seen in composition.
- Kascade experiment:  measures electron and muon components of showers.
- Model dependent, but indicative.
- Is it Emax or containment?
- Low energy (Ec=3x1017 eV) and            sharp elemental cutoffs  limit comes from Emax, rather than containment.
- Learn about galactic sources.
- Structure  Physics

## Big Change Expected at High Energies

- Expect two spectral features due to interactions between CR protons and CMBR photons.
- GZK cutoff due to pion production.
- Dip in spectrum due to e+e- pair production (the ankle).
- A third spectral feature is seen (second knee).
- Galactic/extragalactic transition.
- Learn about extragalactic sources; and propagation over cosmic distances.

## What Causes the Ankle?Two Interpretations

- 1.  Extragalactic protons, losing energy by e+e- production, are pushed to the left, excavating the ankle.
- 2.  The (heavy) galactic spectrum is giving way to the (light) extragalactic one.
- Composition should provide the answer.

## Second Knee at ~1017.6 eV

- Yakutsk, Akeno, Fly’s Eye Stereo, HiRes Prototype/MIA all saw flat spectrum followed by a steepening in the power law.  The break is called the second knee.
- Correct for varying energy scales:  all agree on location of the second knee.
- There are THREE spectral features in the UHE regime.  Location of second knee is not known accurately.
- Cause:  galactic or extragalactic?                         source effect or propagation?
- Cause of second knee is unknown.

## GZK Cutoff

- Predicted in 1966 by K. Greisen, G. Zatsepin, and V. Kuzmin.
- Photons of CMBR interact with cosmic ray protons of extragalactic origin.
- Photoproduction of pions; Δ resonance is near threshold.
- Pion carries away 20% of proton’s energy  strong energy-loss mechanism for protons, that travel > 50 Mpc.
- Causes a strong break in the spectrum.
- Should occur at about 6x1019 eV (10J).

## Previous Experiments

- Several smaller experiments saw one super-GZK event each:  Volcano Ranch, Haverah Park, Yakutsk, Fly’s Eye.
- Akeno Giant Air Shower Array (AGASA) was the first experiment to be large enough to measure the spectrum at the GZK energy;  they didn’t see the cutoff.
- AGASA main evidence:         11 events above 1020 eV.
- This led some to question whether the GZK cutoff exists, and how it might be evaded.

## High Resolution Fly’s Eye (HiRes) Experiment:Has the World’s Highest Exposure (5xAGASA)

- HiRes is a fluorescence experiment.
- Fluorescence yield:
- Charged particles in cosmic ray air shower excite N2 molecules.
- Emit ~5 UV photons/mip/meter.
- 300-400 nm wavelength.
- High energy showers are bright.
- HiRes has two detectors located atop desert mountains in west-central Utah.  Operated from May, 1997 to April, 2006.
- Collected data on moonless nights:  about 10% duty factor.
- Mono:  wider energy range (1017.2  < E < 1020.5 eV), best statistics.
- Stereo: best resolution, 1018.5 < E < 1020.5 eV, fewer events.

## High Resolution Fly’s Eye (HiRes) Collaboration

- J. Boyer, B. Connolly, C.B. Finley, B. Knapp, E.J. Mannel, A. O’Neill, 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, S. Stratton, G.B. Thomson, A. Zech
**Rutgers University**
- N. Manago, M. Sasaki
**University of Tokyo**
- R.U. Abbasi, T. Abu-Zayyad, G. Archbold, K. Belov, A. Blake, Z. Cao, W. Deng, W. Hanlon, P. Huentemeyer, C.C.H. Jui, E.C. Loh, K. Martens,
-  J.N. Matthews, D. Rodriguez, J. Smith, P. Sokolsky, R.W. Springer, B.T. Stokes, J.R. Thomas, S.B. Thomas, L. Wiencke
**University of Utah**

## Mirrors and Phototubes

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

## The Two HiRes Detectors

- HiRes1:  atop Five Mile Hill
- 21 mirrors, 1 ring (3<altitude<17 degrees).
- Sample-and-hold electronics (pulse height and trigger time).
- HiRes2:  Atop Camel’s Back Ridge
- 12.6 km SW of HiRes1.
- 42 mirrors, 2 rings (3<altitude<31 degrees).
- FADC electronics (100 ns period).

## Two Calibrations 

-           Photon scale
- Absolute calibration by Xenon flasher
- Referenced to NIST- traceable photodiodes
- Checked by HPD, laser shots.
**Achieve 10% absolute calibration.**

-          Atmospherics
- Molecular:  density checked by radiosonde balloons from nearby airports.
- Aerosols:  measured *in situ* by laser systems.
- Very clear, stable skies.
- <VAOD> = 0.04
- 1/10 of molecular optical depth
- Change by ±0.02 changes energy by 10% at 25 km.
- Aerosols vary slowly:  typically constant over a night or two.
**HiRes has an excellent site.**

## Measurement of Fluorescence Yield

- Three published results:  Kakimoto *et al*., Nagano *et al*., and T461.
- Ratio of fit to (Kakimoto, Nagano, and T461) to fit to Kakimoto
-      = 1.00 ± 0.06

## Monocular Data Analysis

- Pattern recognition.
- Fit SDP.
- Time fit (HiRes2),       5o resolution.
- Profile plot.
- Gaisser-Hillas fit.
- Profile-Constrained time Fit (HiRes1 PCF),                   7o resolution.

## HiRes1 Energy Reconstruction 

- Test HiRes1 PCF energy reconstruction using events seen in stereo.
- Reconstructed energy using mono PCF geometry vs. energy using stereo geometry.
- Get same answer.

## Stereo Analysis

- Intersection of shower-detector planes determines geometry, 0.60 resolution.
- Timing does as well for parallel SDP’s.
- Two measurements of energy, Xmax.   Allows measurement of resolution.

## Back of Envelope Energy Calculation

- Energy determination is robust.
- Based on center of shower, not tails.
- Easy to Monte Carlo.

## Systematic Uncertainties

- Energy scale:  total = 17%
- Photon scale         10%
- Mean dE/dx          10%
- Fluorescence yield   6%
- Missing energy       5%
- Atmosphere             5%
- Spectrum:  total = 30%

## The Monte Carlo Technique in Cosmic Ray Physics

- Two –step process:  Corsika or Aires shower codes, using QGSjet or Sibyll hadronic generators, to generate showers.  Followed by a detector simulation.
- Success is limited for ground arrays, due to “thinning” and poor prediction of tails of shower, particularly for muons.
- Success is good in the center of the shower, the part seen by fluorescence detectors.
- Techniques from HEP:
- Shower libraries: every event is an actual Corsika event.
- Simulation using previous measurements of the spectrum and composition.
- Simulation using exact detector conditions as a function of time.
- The data/MC comparison method for judging success of simulation.
- Development of model-independent acceptance calculation.
- Result for HiRes is an excellent calculation of the acceptance.

## Aperture Calculation

- Need complete simulation of detector:  create MC sample identical to the data.
- Put in spectrum, composition, as measured by Fly’s Eye, HiRes-MIA, HiRes stereo experiments; use actual Corsika showers.
- Shower development
- Light emission, transmission, and collection
- Trigger and readout electronics
- Write out MC in same format as data.
- Analyze both with same program.
- Compare histograms of data and MC to judge success (or failure) of simulation.

- acceptance

## Compare Data to Monte Carlo: Judge success of simulation and acceptance calculation.

- Inputs to Monte Carlo:
- Fly’s 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.

##  (Steeply Falling) Spectrum Calculation

- If spectrum + resolution correctly modeled, D(E)/A(E) = constant.
- First order correction for resolution.
- Possible bias:  GZK appears in data, but not in MC.
- Second order correction:
- Bias is smaller than statistical uncertainties; correction reduces J(E).

## Testing the Aperture

- Test the aperture calculation by limiting distances to the region to where the detector is fully efficient.
- Spectrum is invariant.
- Histogram of events’ energies shows ankle, high energy suppression.

## Monocular Spectra

- We observe:  GZK cutoff;
-                       ankle;
-                       second knee?

- HiRes1: 7/97-5/05
- HiRes2: 12/99-8/04

## Spectrum with Systematic Uncertainty from Composition

- Composition determines whether <Xmax> is in HiRes’ field of view, or above.
- Different apertures for Corsika/QGSJet protons and iron; leads to systematic uncertainty below 1018 eV, which is larger than statistical uncertainty.
- HiRes can’t say much about the second knee.
**The field needs an experiment, with wide enough energy range, which would see all three UHE cosmic ray features with good statistics!**

## Add the HiRes Stereo Spectrum(absolutely normalized)

## HiRes and Other Experiments

- HiRes, Fly’s Eye Stereo, and HiRes/MIA

- HiRes, AGASA, Auger(2005)

## 5σ Observation of the Break in the Spectrum

- Broken Power Law Fits
- No Break Point
- Chi2/DOF = 162/39
- One BP
- Chi2/DOF = 68.2/37
- BP = 18.63
- Two BP’s
- Chi2/DOF = 34.7/35
- 1st BP = 18.63
- 2nd BP = 19.75
- Difference in chi2 is equivalent to 5.6 σ observation.
- Two BP with extension to test hypothesis that a break is present.
- Expect 51.1 events
- Observe 15 events
- Poisson probability:  P(15;51.1) = 3x10**-9 **(5.8)
- Independent statistics:  P(14;44.9)=7x10-8 **(5.2****σ****)**
**The break is present.**

- Break is at (5.6 ± 0.7) x 1019 eV;
- GZK expected at 6 x 1019 eV.
**The break is at the GZK energy.**

## The Break is at the GZK Energy!Use Berezinsky’s Integral Spectrum Test

*E*1⁄2  is the energy where the integral spectrum falls below the power-law extension by a factor of 2.
- Berezinsky *et al*.:  log10*E*1⁄2 = 19.72, for a wide range of conditions.
- Use 2 Break Point Fit with Extension for the comparison.
- log10*E*1⁄2 = 19.73 ± 0.07
**Suppression is the GZK cutoff.**

## “Test Beam” of High Energy Events

- Laser at Terra Ranch
- 35 km from HiRes-2, at edge of aperture.
- Vertical, 355 nm
- Fires at five energies, as bright as 40-125 EeV showers.
- Efficiency for good-weather nights.
- Excellent trigger + reconstruction efficiency above GZK energy.
**The lack of high energy events is not an instrumental effect.  It is due to physics.**

**GZK Cutoff**

## <Xmax> Indicates Composition

- <Xmax>  Composition  Galactic/Extragalactic Transition
- There is a **model-independent** break in slope at about 1018 eV.
- Heavy (galactic) nuclei decrease, give way to light (extragalactic) composition.
- Galactic/extragalactic transition is complete by about 1018 eV.
- The ankle is **not** the transition.

## HiRes Anisotropy Results: BL Lac Correlations

- Correlations between UHE cosmic rays’ pointing directions, and BL Lac sources have been found individually in AGASA, Yakutsk, and HiRes stereo data, by P.Tinyakov, I.Tkachev, D.Gorbunov, S. Troitsky *et al*.
- HiRes stereo result:
- BL with m<18:  10-4 chance probability.
- Add HP sources:  10-5 chance probability.
- The HiRes result should not exist, since the ~0.5o resolution is smaller than expected magnetic deflections.
- This is a northern-hemisphere effect, since many fewer sources are known in the southern hemisphere.

## More Northern Hemisphere Anisotropy

- The Quartet:  AGASA triplet + HiRes stereo high energy event; in Ursa Major.
- Dip near galactic anticenter, observed by AGASA and HiRes at lower energies.

## Upcoming Experiment:The Telescope Array (TA) and TA Low Energy Extension (TALE)

- TA surface detector:  576 scintillation counters, 1.2 km spacing.
- 3 TA fluorescence detectors overlook SD, 108o in azimuth each.
- TA will cover E > 1018.5 eV
- TA will be running in spring, 2007.
- 2 TALE fluorescence detectors plus infill array:
- Stereo detector to observe the ankle with flat aperture.
- Tower detector + infill array to cover lower energies.
- Cover 1016.5 – 1020.5 eV.

## TA Detectors

- SD in Millard County, Utah

- FD being deployed

## TA/TALE Layout

- Two 6-km stereo pairs:  observe the ankle with flat aperture.
- Tower detector with 3 times larger mirrors:             reach down to 1016.5 eV.
- Infill array for hybrid observation at the lowest energies.
**Cover 10****16.5**** – 10****20.5**** eV.**
- TALE doubles the high energy aperture.

- 1020 eV

- 1018 eV

## TA/TALE Aims

- Apertures:
- High energy aperture: 3000 km2 ster (3x HiRes)
- half SD events,
- half FD events (in mono, stereo, hybrid, stereo hybrid).
- 10x HiRes stereo aperture at 1018 eV.
- 10x HiRes/MIA hybrid aperture, E < 1018 eV.
- Extend Emin down to 1016.5 eV.
- Measure all three spectral features in one experiment.
- Perform correlated spectrum-composition study at the second knee.
- Study the galactic-extragalactic transition:
- Mixed composition at low energies:  Watch the heavy elements die away (~1017.5 eV).
- Observe light composition above 1018 eV.
- Study anisotropy in the northern hemisphere.

## Site Configuration and Aperture Calculation

- Two very similar configurations: (3-31 elevation & 220 azimuthal) + ***Tower mirrors****
- Stereo trigger
- At least 6 track “length” in each “eye”
- Xmax is seen

- Black Rock

- Long Ridge

## HiRes (plus Auger and TA) Lower-energy Limitations

- HiRes observes elongation above 1018.0 eV clearly.
- HiRes looks up to 31o, can’t see Xmax for close-by (low energy) events.
- Makes spectrum measurements difficult below 1017.5 eV.
- Composition bias for E < 1018.0 eV.

- Before bracketing and Cerenkov cuts

## Observe the Second Knee in Hybrid Mode with a Tower Detector

- Two improvements
- Use bigger mirrors.
- Look higher up.
- Tower detector with 3x mirrors:
- 750 cm radius of curvature.
- Use HiRes-type phototubes with Winston cones.

## TA FD, Tower, Infill Array

- E>1018 eV

- E<1017 eV

- 1017<E<1018

- 15 mirrors, 3xHiRes area, in rings 3,4,5.
- 111 AGASA counters, spacing of 400m, shown in red.  Can see events hitting outside also.
- 10 x HiRes/MIA hybrid aperture.

## TA/TALE Anisotropy

- HiRes + AGASA see correlations with BL Lac’s.
- Point source figures of merit at 1019 eV:
-   (HiRes has 31 events above bkg correlated with BL Lac’s)

| Experiment | Aperture 
(km2 ster) | Resolution | Figure of Merit (A/Resolution2) |
| --- | --- | --- | --- |
| HiRes stereo |     300 (avg) |    0.5 deg |      1200 |
| TA/TALE stereo |     340 |    0.5 |      1360 |
| TA SD |   1500 |    1.5 |        667 |
| Auger SD |   6600 |    1.5 |      2933   (1000) |
| TA/TALE hybrid stereo |     260 |    0.1 |    26000 |

- Multi-energy observations are important!

## TA/TALE – Auger Comparison

| Item | TA/TALE | Auger |
| --- | --- | --- |
| Hemisphere | Northern  | Southern |
| Energy range  | 1016.5 – 1020.5 eV | 1018.5 – 1020.5 eV |
| Total Aperture  | 3000 km2 ster | 8000 km2 ster |
| Good Resolution Aperture | 1500 km2 ster |   800 km2 ster |
| Poor Resolution Aperture | 1500 km2 ster | 7200 km2 ster |
| Galaxy Center/Anticenter | No/Yes | Yes/No |
| BL Lac’s | Yes  | No  |

## Structure  Physics

- We have a good idea what causes the
- Knee
- Ankle
- GZK cutoff
- We have a pretty good idea where the galactic/extragalactic transition is.
- The biggest unanswered question is:  What is the second knee?
- Study it using correlated spectrum/composition measurements.

## Summary

**HiRes has observed the GZK cutoff.                   **It occurs at (5.6 ± 0.7 ± 0.9) x 1019 eV.
- We see the “ankle” of the cosmic ray spectrum.
- We have evidence for the galactic/extragalactic transition.
- We have hints of interesting anisotropy.
- We will continue these studies with TA and TALE.