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

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

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

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 – eV HiRes data imply a constant light composition for E>10 18 eV

TALE Goal: Reach 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

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

Example stereo hybrid event BRM CAMERA7 BRM CAMERA8 LR CAMERA7

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

TA Stage-1 The energy region > 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 eV and above

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

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

Lessons from HiRes Stereo HiRes Stereo aperture falls too rapidly through the ankle region to extend flux measurements much below ~3  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  eV – HiRes-1 only covers elevation angles up to 17 , which further limits the aperture near and below the ankle itself

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

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

HiRes Prototype (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):

HiRes Prototype/MIA Hybrid

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:

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

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

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

10 17 eV showers, 30° zenith at TA ground (CORSIKA/EPOS)‏ TALE Methods:  ‏

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

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

Conclusion: TA/TALE will bring together four different detector systems with overlapping energy ranges to give continuous coverage from 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.