29th International Cosmic Ray Conference Pune (2005) 00, 101–104

The Telescope Array Low Energy Extension
G. B. Thomson , and the TA/TALE Collaboration(a) Department of Physics and Astronomy, Rutgers University, Piscataway, NJ 08854, USA

Presenter: G. B. Thomson (thomson@physics.rutgers.edu) usa-thomson-G-abs2-he15-oral
The Telescope Array Low Energy Extension (TALE) is a series of detectors to be added to the Telescope Array(TA) experiment to increase its aperture at high energies and extend its energy coverage to lower energies.
The combination of TA and TALE will be able to measure the spectrum of ultrahigh energy cosmic rays from  toeV. At each energy in this range, TA/TALE will be able to measure, in a model independent
way, the average depth of shower maximum - and hence the composition of cosmic rays. TA/TALE will
have excellent pointing resolution for anisotropy studies. The TALE detectors will consist of two fluorescence
detectors located 6 km from two TA fluorescence detectors (6 km spacing produces a flat stereo aperture in
the region of the ankle), a third fluorescence detector with larger mirrors observing at higher elevation angles(which will observe cosmic rays at lower energies), and an infill array.

1.

Introduction

The TA and TALE experiments are being built by a collaboration of research groups from Japan and theUnited States. The experiment is designed to study the spectrum, composition, and anisotropy of ultrahighenergy cosmic rays, over a wide energy range, with a large aperture, and with good control of systematic
uncertainties.
The experiment will be located in Millard County, Utah. It will consist of TA detectors (a ground array withthree fluorescence detectors overlooking it), two TALE fluorescence detectors, and an infill array. The TALE
fluorescence detectors will be located a distance of 6 km from two of the TA fluorescence detectors. This
spacing is optimum for studying the ankle of the cosmic ray spectrum with a nearly flat stereo aperture. All
these fluorescence detectors will observe at elevation angles from 3 to 31 degrees above the horizon. Associatedwith one of the TALE detector sites will be a “tower detector” which will observe at higher elevations, from31 to 72 degrees. The tower detector will have larger mirrors and be able to detect cosmic rays down to about eV. A ground array of about 100 scintillation counters, of 400 m spacing, will be deployed in front of
the tower detector for hybrid coverage at low energies.
Together the TA and TALE detectors will make an experiment that has good sensitivity over four orders ofmagnitude in energy, where the same events will seen by different detectors and cross normalization of energy
scales will be possible.
Addition of the TALE detectors will enhance the high-energy aperture of TA by about a factor of 2. Thisaperture will be about 3000 km ster, and the events collected will be about half surface array events and half
fluorescence events (where the fluorescence events are seen in mono, stereo, hybrid, or hybrid stereo).
This suite of detectors will study cosmic rays over a very wide energy range, from well below the second knee
through the area of the GZK suppression [1].

2.

Physics of TA/TALE

The aim of the TA/TALE experiment is to study ALL the physics in the ultrahigh energy region. Measuring thespectrum over four orders of magnitude will allow the experiment to see the three features in this region: the102

G. B. Thomson et LL.

second knee, the ankle, and the GZK suppression. No experiment has seen all three features, and the energy at
which the second knee occurs is not known, even relatively, much less absolutely.
The ankle is very important for learning about extragalactic sources of the highest energy cosmic rays. Many
physicists now think that the ankle is made by  pair production when photons of the cosmic microwave
background radiation strike cosmic ray protons. Studies of the detailed shape of the ankle allow one to determine the average spectral index of these sources, and also their average evolution.
At the highest energies, the HiRes experiment has an indication that a simple model of extragalactic sources  does not fit the data in the region ofeV: a model with a constant power-law spectrum at thesource predicts a higher flux than the data will allow [2]. So the spectrum in the highest energy region, in addition to shedding light on the GZK suppression, may contain information on extragalactic sources’ distributionof maximum energies.
In the region of the second knee and below, the light component of cosmic rays are presumably of extragalactic
origin. The sources that produce these particles are a long distance away: they reach out to redshifts of twoto three. Beyond a redshift of about 1.5 some candidate sources, like QSO’s and AGN’s, show a break intheir evolution [3]. One interpretation of this break is that at earlier times the large black holes that power these  ! sources were just forming. The possibility exists of doing cosmology with cosmic rays of energyeV; i.e., observing the evolution of cosmic ray sources.
Composition studies are very important, and will continue to be so in the future. The Fly’s Eye stereo measurement of mean depth of shower maximum, "$#&%('*),+ , and those of HiRes (the HiRes/MIA hybrid experimentand HiRes stereo), are consistent, and show an elongation rate of 93 g/cm at lower energies giving way to  one of 55 g/cm at abouteV. The heavier composition giving way to the lighter is considered a sign ofthe transition from galactic cosmic rays to ones from extragalactic sources. TA/TALE is designed to measure"-# %('*) + in hybrid and stereo modes over its entire four decade wide energy range.
At the lowest energies a mix of heavy elements of galactic origin is expected to be seen. The direct observation of ".# %('*) + by the fluorescence detectors of TA/TALE is a model-independent measurement. Other
experiments will be carrying out model-dependent measurements of the composition in this energy range,e.g., Kascade and Kascade-Grande measuring the electron and muon component of showers. Checking theirobservations with TA/TALE data will be very important.
 In theeV decade the heavy component disappears, and at higher energies a light composition dominates.
Observing this transition all in one experiment will be very important. There is growing consensus that theextragalactic cosmic rays should be almost all protons. So the HiRes measurement of "/#0%('*)1+ is likely
very close to the value for protons (perhaps protons are 10-15 g/cm higher). Confirmation of this by future2fluorescence experiments like TA/TALE, and Auger above
eV, will be an important step forward.
In the search for the origins of ultrahigh energy cosmic rays, anisotropy studies play an important role. In
the northern hemisphere, the AGASA experiment observed clusters of events, and a hint of galactic extended
anisotropy, and HiRes data seems to confirm some of these results [4].
HiRes has also seen correlations between arrival directions of events (seen in stereo, over a wide energy range)
with BL Lac sources. There are also events that point back to sources seen in TeV gamma ray experiments [5].
The HiRes collaboration plans to collect more data to confirm or reject these correlations. But if true, thesecorrelations would be startling since these are extragalactic sources and one would expect magnetic fields towash out these correlations at the level of the 0.5 degree angular resolution of HiRes stereo events. It may
be that there are regions of extragalactic space with very small magnetic fields - or that some cosmic rays areneutral. The BL Lac correlations are also a northern hemisphere effect since most identified BL Lac sources
are in the northern hemisphere.

103

Telescope Array Low Energy Extension

TA/TALE will have excellent angular resolution. The stereo aperture will be larger than that of HiRes, andabout 85% of stereo events will also be seen in hybrid mode, improving the angular resolution from 0.5 to0.1 degrees. If the BL Lac correlations are confirmed by HiRes then TA/TALE, which will collect the largest
number of hybrid stereo events in the world, will be just the experiment to study anisotropy of this kind.

3.

TALE Detectors

In 2006, we plan to move the HiRes detectors from the U.S. Army Dugway Proving Ground about 150 km
south to Millard County, Utah, to become TA and TALE detectors. Fourteen HiRes mirrors will become oneof the TA fluorescence detectors overlooking the ground array.
The remaining 50 mirrors will be redeployed to two locations near the other TA fluorescence detectors to form
stereo pairs with the TA detectors. The spacing for stereo will be about 6 km, which is optimized for studies ofthe ankle region. These detectors will be observing partly in directions away from the TA detectors, and will
increase the aperture of TA at high energies by about a factor of 2. The energy range covered by these detectors  toeV.
will be from about
We plan to build a tower detector to extend the coverage to lower energies. This will consist of 15 mirrors thatwill cover elevation angles from 31 to 72 degrees, and cover about 90 degrees in azimuth. Looking higher in elevation with HiRes mirrors would lower the minimum energy observed to abouteV. The tower mirrors
will be about 12.6 m in area, three times larger than HiRes mirrors, which will lower the minimum energy to abouteV.
We plan to deploy an infill array of about 110 scintillation counters in front of the tower detector. With about3
400 m separation (1/3 of the separation of the TA scintillation counters) they will have good efficiency ateV. The infill array will be used for hybrid observation with the tower detector, and will improve its energy
resolution, its # %('*) resolution, and its pointing resolution. The infill array will reach up in energy to about 4eV.
 4Events between
andeV will be collected by the tower detector, TALE fluorescence detector, andinfill array. Many will be seen in stereo with one of the TA fluorescence detectors and in hybrid with the main
TA ground array. Cross calibration of all of these detectors will be performed using these events. At higher
energies, events will be seen monocularly, in stereo, in hybrid, and in hybrid and stereo, between the TA andTALE fluorescence detectors and the TA ground array. Thus cross calibration will be possible throughout theentire energy range of the experiment.

4.

Conclusions 

The physics of ultrahigh energy cosmic rays in the energy range above
eV is very rich. Three spectralfeatures are present, in contrast to the almost featureless spectrum at lower energies. In addition the galactic extragalactic transition occurs here.
The composition of the primaries at the lower end of this energy range should be heavy and of galactic origin,then at higher energies make a transition to become light and of extragalactic origin. This will occur justwhere the TALE detectors are at their optimum efficiency. The highest energy events should continue this lightcomposition.
By making composition-tagged measurements of the spectrum we will be able to study both galactic and extragalactic sources independently. We will determine the distribution of maximum energies of galactic sources,

104

G. B. Thomson et LL.

and how their composition varies. For extragalactic sources we will measure the spectral index at the source,
the distribution of end-point energies, and the detailed evolution out to a redshift z=3.
The TA/TALE detector will be very powerful for anisotropy studies. The experiment has a large apertureand excellent pointing accuracy at all energies. The large fraction of events seen in stereo and hybrid modes
simultaneously, with pointing accuracies about 0.1 degrees, is an unprecedented advantage of the TA/TALE
design. The northern hemisphere sky is a very rich field for anisotropy studies. The possible observation ofthe first point sources by AGASA and HiRes will be pursued by TA/TALE, and the correlation with BL Lac
sources will be investigated.

5.

Acknowledgements

We thank colleagues from the AGASA, HiRes, and TA collaborations for valuable discussions. The Telescope
Array is being constructed by the support of Grant-in-Aid for Scientific Research (Kakenhi) on the Priority
Area ”The Highest Energy Cosmic Rays” by the Ministry of Education, Culture, Sports, Science, and Technology of Japan. We also thank the University of Utah, the State of Utah, and the Utah Economic Development
Board for their generous support.

References[1] K. Griesen Phys. Rev. Lett. 16, 748 (1966).
G.T. Zatsepin and V.A. Kuzmin, JETP. Lett. 4, 78 (1966).
[2] D.R. Bergman (HiRes Collaboration), to appear in the Proc. of the 29th ICRC, Pune (2005).
[3] B.J. Boyle and Roberto Terlevich, astro-ph/9710134.
A.J. Barger et al., submitted to Astron. J., astro-ph/0410527.
[4] S. Westerhoff (HiRes Collaboration), to appear in the Proc. of the 29th ICRC, Pune (2005).
[5] C. Finley (HiRes Collaboration), to appear in the Proc. of the 29th ICRC, Pune (2005).