The Telescope Array Low Energy Extension (TALE) ‏

1. Pierre Sokolsky University of Utah The Telescope Array Low Energy Extension(TALE)‏

2. Spectrum: Ankle and Cutoff Ankle Pair production? Galactic to extragalactic transition? Cutoff GZK feature Sources getting tired?

3. Spectrum: Second Knee Left: Cosmic ray spectra measured by the Fly’s Eye, HiRes/MIA, Akeno, and Yakutsk experiments. Right: Aligning the flat parts of the spectra from the four experiments, a “second knee” appears The energy and origin of the feature is unknown – needs to be measured with other features

4. Composition Changes in composition and how they correlate with changes in spectral shape tell us about the sources – width and <Xmax> HiRes-MIA data imply a change from heavy to light from 1017 – 1018 eV HiRes data imply a constant light composition for E>1018 eV

5. TALE Goal: Reach 1016.5 eV • Study the Transition Region from Galactic to Extra-galactic cosmic ray flux • Extend the coverage of the TA experiment to include all three cosmic ray spectral features in the ultrahigh energy regime: • The GZK Suppression • The Ankle • The Second Knee It is important than we establish a single unified energy scale for the measurement of all three features

6. Telescope Array – High Energy The High Energy component of Telescope Array – 507 scintillator surface detectors and 28 fluorescence telescopes at 3 stations is complete and operational as of 1/2008.

7. TA-FD From HiRes Middle Drum New FDs Long Ridge Black Rock Mesa

8. Example stereo hybrid event BRM CAMERA8 LR CAMERA7 BRM CAMERA7

9. Absolute Energy Calibration in situ by 40 MeV electron beam released vertically into the sky.

10. TA Stage-1 • The energy region > 1019 eV is well-covered by the existing TA detectors • Ground Array becomes fully efficient at ~5x1018 eV • The three FD stations • TA-FD0 at Black Rock Mesa • TA-FD1 at Long Ridge • TA-FD2 at Middle Drum provide ~100% coverage of the ground array at 1019 eV and above

11. Below 1019 eV • However, Stage-1 of TA was not designed for physics below 1019 eV. • There is no overlap at all in the aperture of the three fluorescence detectors at 1018 eV • The ground array efficiency drops quickly in the 1018-1019 eV decade

12. Low Energy Extension to TA 4th Fluorescence Station - 6 km separation 24 telescopes (3-31o elevation) – “ring 1 & 2” 15 large area Tower telescopes (31-73o elevation)‏ Infill scintillator array 111 detectors at 400 m Graded muon array – 25 detectors, buried 3 m

13. Lessons from HiRes Stereo • HiRes Stereo aperture falls too rapidly through the ankle region to extend flux measurements much below ~31018eV. • There are two primary reasons for this: • The 12.6 kmseparation of the two stations is too large: the overlap between the two shrinks very quickly below 31018eV • HiRes-1 only covers elevation angles up to 17, which further limits the aperture near and below the ankle itself

14. 6 km Stereo and Tower • 24 low elevation (ring 1 & 2) telescopes; mirrors the same effective area as HiRes and Auger • 15 higher elevation (rings 3-5) telescopes; mirrors 3x larger area

15. Aperture • The 6 km stereo provides • a much flatter stereo aperture than HiRes • a 10x increase in aperture at 1018 eV

16. HiRes Prototype 92-96 • 14 (HiRes-1) + 4 (HiRes-2) mirror prototype detector operated between 1992 and 1996 • HiRes-1 field of view up to ~70. • HiRes-1 operated in hybrid mode with the MIA muon array (16 patches64 underground scintillation counters each):

17. HiRes Prototype/MIA Hybrid

18. TALE Tower Fluorescence Detector: 3 additional rings of mirrors, 31° – 72º Each mirror 3x HiRes mirror area • Left: TALE-1 site, showing 3rd 4th and • 5th rings • Right: Prototype 4th ring detector

19. Improved Sensitivity • The increased mirror size will improve substantially the sensitivity of TALE in the 1016.5-1017.5 eV energy decade • Note the gain in sensitivity comes from the improvement in signal.

20. Tower helps with <Xmax> Bias Comparison of <Xmax> (solid line) measured by the HiRes and HiRes-MIA experiments and the distribution of Xmax of HiRes events (2-D box histogram), demonstrating the trigger bias (towards light composition) inherent in a “2-ring” fluorescence detectors like those used by HiRes and PAO. need the Tower to cover this region, which contains the Second Knee Structure

21. Infill Array Will place 111 additional surface array counters overlapping with main ground array: 4km x 4km 16 of the counters in the main ground array will form part of the infill

22. TALE Methods: ‏ 1017 eV showers, 30° zenith at TA ground (CORSIKA/EPOS)‏

24. TALE: Underground Muon Detectors • Preferred design, if BLM permits • Reduced cost • Simpler • Reclamation, reseeding required

25. /e ratio signal for transition in log(E) = 17 decade

26. Conclusion: TA/TALE will bring together four different detector systems with overlapping energy ranges to give continuous coverage from 1016.5 eV to the highest energies. The cost will be shared between U.S., Japan, South Korea, and Russia. TA/TALE will be able to study all three spectral features in the UHE regime. TALE will have two orthogonal composition measurements in galactic/extragalactic transition region: FD shower profile and /e ratio.