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

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 HiRes-MIA data imply a change from heavy to light from 10 17 – 10 18 eV HiRes data imply a constant light composition for E>10 18 eV

5. TALE Goal: Reach 10 16.5 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. 7 Long Ridge Black Rock Mesa Middle Drum TA-FD From HiRes New FDs

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

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

10. TA Stage-1 The energy region > 10 19 eV is well-covered by the existing TA detectors Ground Array becomes fully efficient at ~5x10 18 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 10 19 eV and above

11. Below 10 19 eV However, Stage-1 of TA was not designed for physics below 10 19 eV. There is no overlap at all in the aperture of the three fluorescence detectors at 10 18 eV The ground array efficiency drops quickly in the 10 18 -10 19 eV decade

12. Low Energy Extension to TA 4 th Fluorescence Station - 6 km separation  24 telescopes (3-31 o elevation) – “ring 1 & 2”  15 large area Tower telescopes (31-73 o 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  10 18 eV. There are two primary reasons for this: – The 12.6 km separation of the two stations is too large: the overlap between the two shrinks very quickly below 3  10 18 eV – 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 10 18 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. Left: TALE-1 site, showing 3 rd 4 th and 5 th rings Right: Prototype 4 th ring detector 3 additional rings of mirrors, 31° – 72º Each mirror 3x HiRes mirror area TALE Tower Fluorescence Detector:

19. Improved Sensitivity The increased mirror size will improve substantially the sensitivity of TALE in the 10 16.5 -10 17.5 eV energy decade Note the gain in sensitivity comes from the improvement in signal.

20. Tower helps with Bias Comparison of (solid line) measured by the HiRes and HiRes-MIA experiments and the distribution of X max 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. 10 17 eV showers, 30° zenith at TA ground (CORSIKA/EPOS)‏ TALE Methods:  ‏

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 10 16.5 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.