Recent Results from the Telescope Array Project, Hanlon 2016

Available online at www.sciencedirect.com
Nuclear and Particle Physics Proceedings 279–281 (2016) 15–22
www.elsevier.com/locate/nppp
Recent Results from the Telescope Array Project
William F. Hanlon
High Energy Astrophysics Institute & Department of Physics and Astronomy, University of Utah, 201 James Fletcher Building, 115 South 1400
East, Salt Lake City, UT, USA 84112-0830
Abstract
The Telescope Array Project (TA) is the largest cosmic ray observatory in the northern hemisphere and has entered
its eighth year of data collection exploring astrophysical phenomena at the highest ends of the cosmic ray energy
spectrum. New additions to TA have expanded its reach down to lower parts of the energy spectrum, thus allowing
it to probe over an unprecedented 4.5 decades of energy via hybrid detection techniques. Recent results suggestive
of anisotropy in the arrival direction of cosmic rays are presented as well as updated measurements of the spectrum,
primary source composition, new measurement of the inelastic proton-air cross section, and the first ever quantitative
radar cross section upper limit measurement.
Keywords: UHECR, Cosmic Rays, Telescope Array, Proton-Air Cross Section, Proton-Proton Cross Section,
Anisotropy, Composition, Spectrum, Radar Cross Section
1. Introduction
The Telescope Array Project (TA) is the largest cosmic ray observatory in the Northern Hemisphere, covering approximately 700 km2 in Millard County, Utah
(centered at 39.3◦ N and 112.9◦ W, 1400 m above sea
level). It is a joint international collaboration of 33 institutions located in Belgium, Japan, Russia, South Korea, and the United States and is the successor to the
AGASA and HiRes experiments. Expertise from the
AGASA ground array and HiRes air fluorescence techniques have been combined in TA to build a hybrid cosmic ray detector designed to probe the properties of cosmic rays with primary energies near the “ultra high energy” regime (E 1018 eV). The TA observatory is
composed of 507 scintillation surface counters sensitive
to muons and electrons spaced 1.2 km apart in a gridlike manner which make up the surface detector (SD)
array, and 48 fluorescence detector (FD) telescopes distributed in 3 separate detector stations which are spaced
Email address: whanlon@cosmic.utah.edu (William F.
Hanlon)
http://dx.doi.org/10.1016/j.nuclphysbps.2016.10.004
2405-6014/© 2016 Elsevier B.V. All rights reserved.
in a roughly equiangular manner on the perimeter of the
SD array looking towards its center. This configuration
allows for independent cosmic ray detection by either
just the SD or FD array as well as coincident detection
by both, also called hybrid detection. While the fluorescence detectors are restricted to running during clear
moonless hours reducing their duty cycle to about 10%,
the surface array runs continuously day and night under
all weather conditions. Data collection started during
the first quarter of 2008 and TA has entered it’s 8th year
of data collection. Figure 1 shows the physical layout of
the TA observatory.
Each surface detector is made up of 2 layers of plastic scintillator material measuring 3 m2 x 1.2 cm. Embedded in grooves in each layer of scintillator is 5 m
of wavelength shifting fiber, which transmits light to a
photomultiplier tube, 1 for each scintillator layer, and
associated electronics to record the passage of charged
particles through the detector. The analog PMT signal is
digitized by FADC electronics with 50 MHz clock rate
and stored in a local buffer. Onboard electronics implement a triggering system to determine if detected signals represent the passage of particles by an air shower
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W.F. Hanlon / Nuclear and Particle Physics Proceedings 279–281 (2016) 15–22
Figure 1: Location of the Telescope Array SDs and FD stations. Each
red diamond is one of 507 surface counters and the blue hexagons
show the locations of the FD stations which all look in toward the SD
array. Yellow stars indicate SD communication towers and the CLF is
a calibration laser located equidistant from each FD station.
based upon the signal expected from a minimum ionizing particle. When the trigger threshold for a single
SD station is passed, the SD communicates via wireless LAN to one of three communication towers placed
around the ground array. Event level triggers are generated by electronics in the communication towers which
can direct all SDs that have detected a low level trigger to send their data for storage and event construction.
Event data is sent from the communications towers to a
central computing facility located nearby and the data is
then collected and written to computer hard drives for
further analysis. GPS timing is used by each SD station
to record the time of each trigger and to record the relative timing of triggers between other SDs and the FD
stations as well in the event of a hybrid event. The SD
array trigger efficiency is nearly 100% for air showers
with energies in excess of 1019 eV and zenith angles below 45◦ [1].
Three FD stations, named Middle Drum FD, Black
Rock FD, and Long Ridge FD, overlook the SD array.
Each FD station is located about 35 km away from each
other around the perimeter of the array. Near the center
of the SD array is a central laser facility (CLF) and each
FD station is located 21 km away from it. The CLF is
used for energy scale calibration between the different
stations and to study aerosol distributions in the atmo-
sphere.
Middle Drum FD uses the mirrors and electronics of
the HiRes1 fluorescence detector from the HiRes experiment, while Black Rock FD and Long Ridge FD were
constructed as new FD stations for the TA project. Reutilizing the HiRes1 equipment allows TA to relate the
energy scale of previous measurements from the HiRes
experiment to new results measured by TA. There are
14 telescopes viewing 112◦ in azimuth arranged in 2
rings of zenith angle coverage. Ring 1 telescopes observe between 3◦ - 17◦ and ring 2 telescopes observe
between 17◦ - 31◦ in zenith angle. Each telescope’s
5.2 m2 mirror collects and reflects light onto a cluster of
256 PMTs arranged into a tightly packed 16x16 hexagonal array with each pixel viewing about a 1◦ cone of
the sky. Black Rock FD and Long Ridge FD utilize 12
telescopes at each station with similar sky coverage as
Middle Drum. Black Rock and Long Ridge FDs utilize
FADC electronics to digitize and record light they observe, while Middle Drum employs a sample and hold
electronics design.
Operation of the Telescope Array Low Energy Extension (TALE) began in May 2013. TALE uses 10 FD
telescopes pointed up into a higher zenith angle configuration than the other TA FD stations. 5 mirrors each
make up a ring 3 and ring 4 viewing a total zenith angle
range of 31◦ - 57◦ and 100◦ in azimuth. TALE is colocated with the Middle Drum FD effectively giving that
station a zenith angle coverage of 3◦ - 57◦ zenith angle
coverage of some low energy events. A closeby infill
array of SD counters with 400 m spacing is also part of
the TALE detector design. There are 31 counters are in
place with 16 counters currently operational. TALE was
designed to record events in the cosmic ray spectrum
down to the region just above the knee (∼ 1016.5 eV)
and as high as the ankle (∼ 1018.5 eV). TALE can also
be operated as a hybrid detector combining coincident
events measured by the infill array as well as the FD
station or independently as only an FD station.
2. Spectrum
The cosmic ray energy spectrum can be observed by
TA either independently by the SD array or the FD array. Observations can also be combined between the
SD array and the FD array (hybrid observation) or can
also be combined between multiple independent FD stations (so-called stereo or triple events). Measurements
made by combining the different elements can greatly
improve the determination of important air shower parameters such as event timing and geometry, thereby
W.F. Hanlon / Nuclear and Particle Physics Proceedings 279–281 (2016) 15–22
improving the resolution of other observables important to characterizing the properties of an air shower
(e.g., primary particle energy, depth of shower maximum, shower arrival direction). The greatest statistical
power in the energy spectrum comes from the independent SD array measurement since it has near 100% duty
cycle. The TA SD energy spectrum has been reported
in the past for 4 years of observation[2] and updates
have been presented at International Cosmic Ray Conferences. At the recent 2015 ICRC held in the Hague,
the Netherlands the SD energy spectrum was updated
to show the measurement using seven years of collected
data.
Figure 2 shows the preliminary TA energy spectrum using 7 years of data collected using the SD array. The 2 break points indicate changes in the spectral indices and mark the locations of the ankle and
the GZK cutoff at log10 (Eankle ) = 18.70 ± 0.02 and
log10 (EGZK ) = 19.78 ± 0.06 respectively. The spectral
indices are measured to be −3.30 ± 0.03 below the ankle, −2.68 ± 0.03 between the ankle and the GZK break,
and −4.55 ± 0.56 above the GZK break. The significance of the GZK cutoff is measured by calculating the
significance of the difference in the number of events of
the integral spectrum assuming there is no break in the
spectrum (Nexpect ) and the actual number of events observed (Nobserve ). Above EGZK , Nexpect is calculated to
be 99.3 events and Nobserve is measured to be 44 events
leading to a chance probability of measuring this deficit
to be 4 × 10−4 , or ∼ 5.5 standard deviations. HiRes reported the first observation of the GZK cutoff using 9
years of monocular FD data with a significance of 5.3
standard deviations[3]. A second way to measure the
location of the GZK cutoff is to find the energy where
the predicted integral spectrum exceeds twice the number of events in the observed integral spectrum[4]. This
energy is called E1/2 . The 7 year SD energy spectrum
measures log10 (E1/2 ) = 19.77 ± 0.06 in agreement with
that found with the broken power law fit.
With the addition of TALE, TA now has the ability to
probe the spectrum near the knee. TA can go even lower
in energy by taking advantage of a new technique to
observe highly inclined air showers by measuring their
direct Cherenkov light. As an air fluorescence detector, TALE’s energy threshold is about 1016.5 eV. By observing showers with very small viewing angles, TALE
can achieve an energy threshold of about 1015.5 eV. This
technique allows TALE to act as a small imaging atmospheric Cherenkov telescope (IACT). By combining
these spectrum measurements, TA can measure the primary energy of cosmic rays spanning over 4.5 decades
of energy. This is particularly important because we
17
Figure 2: The preliminary 7 year energy spectrum measured by the TA
surface detector array. The solid line shows a broken power law fit,
where the spectral indices and break points are each fit as independent
parameters to the spectrum.
have one single experiment in which we can understand
our systematics and energy scale to measure the energy
of air showers in the energy region from the galatic to
extra-galactic transition in cosmic ray flux all the way to
the GZK cutoff. By going to lower energies, we are also
closing the gap between accelerator energies reachable
on earth, such as at the LHC, which will improve our understanding of hadronic physics in these energy ranges.
This will, in turn, improve hadronic modeling which is
very important to understanding the composition of cosmic rays above the knee. Figure 3 shows four different spectrum measurements using the SD, Black Rock
and Long Ridge FD monocular, TALE FD (called TALE
bridge), and TALE Cherenkov. This one measurement
spans over four decades in energy and observes four distinct features of the cosmic ray spectrum. When fit with
a broken power law fit features observed are: a low energy ankle at log10 (E/eV) = 16.34 ± 0.04, a 2nd knee
at log10 (E/eV) = 17.30 ± 0.05, the high energy ankle
at log10 (E/eV) = 18.72 ± 0.02, and the GZK cutoff at
log10 (E/eV) = 19.80 ± 0.05.
3. Composition
TA measures primary particle composition via FDs or
via hybrid FD-SD observations by measuring the depth
of air shower maximum, called Xmax . Because we only
observe the secondary particles produced after the primary particle inelastically collides with an air molecule,
primary particle composition can only be inferred statistically by observing many showers. By comparing the
distribution of Xmax as a function of energy (called Xmax
elongation) to Monte Carlo simulations we can estimate
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W.F. Hanlon / Nuclear and Particle Physics Proceedings 279–281 (2016) 15–22
Figure 3: Combined spectrum of TA using 4 different measurements:
7 year SD, 7 year BR-FD monocular, TALE bridge which measures
below 1018.5 eV, and TALE Cherenkov which observes low energy
showers dominated by Cherenkov light. The spectrum found by combining all 4 spans over 4 decades in energy.
the distribution of primary particles observed. Composition is very important to understanding the sources of
cosmic rays as well as their distances from Earth.
A TA composition measurement using 5 years of hybrid Middle Drum data has been previously reported[5]
and updates have been presented at International Cosmic Ray Conferences. At the recent 2015 ICRC new
preliminary composition results were shown using different techniques.
The Middle Drum hybrid composition analysis first
presented in [5] was updated to include 7 years of
data and further refinements to the pattern recognition
method were done. This method ensures showers of the
highest quality are kept for analysis while also reducing bias introduced by cuts. Figure 4 shows the updated
elongation plot. The mean Xmax for each energy bin is
shown for data as well as for three models consisting of
three different pure primary particles: protons, nitrogen,
and iron. As the data shows, by using the means of the
Xmax distributions in each energy bin, the composition
appears to be dominated by light primary particles.
Figure 5 shows the Xmax elongation measured using
2 or more FDs. Each event must be well observed by
multiple FD stations for this measurement. The highest
statistics measurement of composition by TA is shown
in Figure 6 and the results look compatible with the
Middle Drum hybrid and stereo measurements. For
the 3 different measurements shown Xmax resolution is
about 20 g/cm2 with reconstruction biases less than a
few grams.
Measuring the composition is typically done by examining the first and second moments of the Xmax observed distributions. These distributions are not nor-
Figure 4: Xmax elongation using the first 7 years of Middle Drum
hybrid data. Data is compared to models consisting of pure protons,
nitrogen, and iron. As the mean Xmax in each energy bin shows, the
composition appears to be dominated by light primary particles.
Figure 5: FD stereo composition measurement from the first 7 years
of data. Data is compared to a pure proton and pure iron model. Xmax
measured using this technique was required to be well observed by
multiple FDs.
Figure 6: Black Rock/Long Ridge hybrid composition measurement.
Data is compared to a pure proton and pure iron model. Shown are
two different hadronic models for each primary species.
W.F. Hanlon / Nuclear and Particle Physics Proceedings 279–281 (2016) 15–22
mal distributions due to fluctuations in how the primary
particles first interact in the atmosphere and how the
cascade of secondary particles develop with increasing
depth. Different ways of defining functions to describe
Xmax distributions are in use. One example found in [6]
describes the distribution as a convolution with a exponential function and a Gaussian function for example. Because of the non-normal nature of Xmax distributions TA has tried a different approach to compare data
to models. Instead of collapsing all information of the
Xmax distribution into 2 test statistics (mean and width),
we instead propose to use a test which uses the entire
distribution of both model and data. The Cramér-von
Mises criterion is such a test which measures the square
of the differences in the cumulative distribution functions being compared[7]. For 2 distributions which have
similar shape this squared difference is smaller than for
distributions with different shapes. To compare data to
models, we apply the Cramér-von Mises test by measuring the overall shift in the whole data distribution
that is required to find the minimum value of the test
statistic. Figure 7 shows the Cramér-von Mises test applied to data and Monte Carlo models for the Middle
Drum hybrid composition measurement shown in Figure 4. For each energy bin and each Monte Carlo model
the Xmax distribution data is shifted and the Cramér-von
Mises test is applied to find the shift required to maximize the p-value of the test. Because in the true 2
sample test one does not shift either distribution, this
measure is used to quantify the agreeement between the
2 distribution shapes and called an “s-value”. Figure 7
shows that the best agreement between data and models
is found for a pure proton primary distribution below
∼ log10 (E/eV) = 19.5. Above this energy statistics in
the data are too low to make a better measurement. By
this test we rule out pure iron as a source of primary
particles and nitrogen is disfavored as well.
4. Anisotropy
TA has reported recently about indications of medium
scale anisotropy in the arrival directions of cosmic rays
with energies > 57 EeV south of the supergalactic plane
near the Ursa Major cluster[8, 9, 10, 11]. This region
has been dubbed the “TA Hotspot”. The highest statistics for the search of cosmic ray sources is provided
by the SD array. For this analysis, events with zenith
angle up to 55◦ with a loose border cut on the SD array are accepted. This results in an angular resolution of 1.7◦ and energy resolution of about 20%. The
search method uses oversampling in circles of 20◦ radius across the sky and measures the significance of the
19
Figure 7: Cramér-von Mises applied to hybrid composition data and
3 different primary particle models. The ordinate shows the amount
the entire model distribution needs to be shifted to find the best result.
The color key on the left shows the maximum value of the “s-value”
obtained. Large s-values indicate good agreement in the 2 distributions after shifting. The colored bands around the proton and iron
model lines show the range of shifts required for 4 different hadronic
models.
excess of events observed compared to events expected
for an isotropic source distribution. Details of how the
analysis is performed are described in [10]. The previous search reported Non = 19 events recorded over 5
years of data collection in one such circle centered at
(α, δ) = (146.7◦ , 43.2◦ ) in equatorial coordinates. The
expected background given an isotropic source distribution for a circle this size is Nbg = 4.49 events. This
results in a global significance of 3.4σ in the excess of
events expected for a distribution of isotropic sources.
A new preliminary analysis has been performed to
extend the search for anisotropy to 7 years of data. By
folding in an additional 2 years of data, 37 new events
that pass cuts are added in, giving a total of 109 events.
The hotspot signal grows to 24 events with the addition
of the new data with 6.88 background events expected.
The maximum pre-trial significance of this signal is
5.1σ. Because this measurement does not take into account random clustering, the significance of finding Non
events distributed over the isotropic sky is calculated.
This is done by throwing 1 million Monte Carlo data
sets each containing Non randomly distributed events in
the TA field of view for 5 different oversampling radii.
The maximum significance for each MC set is recorded.
The global significance is found by counting the number
of MC sets with maximum significance greater than or
equal to the pre-trial test. Doing this it is found that the
global excess chance probability is 3.7 × 10−4 or 3.4σ
one-sided significance. The 7 year excess significance
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W.F. Hanlon / Nuclear and Particle Physics Proceedings 279–281 (2016) 15–22
Figure 8: Preliminary locations of events in the TA search of
anisotropy. Blue points indicate events found in the previously reported 5 year search and red points show new events by adding in 2
new years of data. 109 events in the 7 year search have be found.
The hotspot global excess significance is measured to be 3.4σ for 24
events and expected background of 6.88
Figure 9: Preliminary excess significance map in the TA search for
anisotropy using 7 years of data. Oversampling is employed using circles of 20◦ radius and the excess significance compared to
an expected isotropic source distribution is shown. The maximum
pre-trial significance is measured to be 5.1σ for a circle located at
(α, δ) = (148.4◦ , 44.5◦ ). 24 events are found there with 6.88 expected
as background. The global excess chance probability is found to be
3.7 × 10−4 or 3.4σ.
therefore remains the same as the 5 year significance.
5. Proton-air cross section
The first measurement of the TA proton-air cross
section has been made[12]. Assuming the Xmax distribution for a given energy bin has a exponential tail
with the functional form exp(−Xmax /Λm ), Λm is proportional to the interaction length λp−air [13] which can
be related to the cross section by a model dependent
K factor, Λm = Kλp−air = K · 14.45mp /σinel
p−air . The
proportionality factor K is measured by Monte Carlo
simulation and is dependent upon the hadronic model
used. For this analysis TA determined the K-factor values for 4 of the latest hadronic models available and
Figure 10: The first measurement of the TA proton-air cross section. By measuring the depth of Xmax and using the K factor method
= 567.0 ± 70.5(stat)+25
σinel
−29 (sys) mb at log10 (E/eV) = 18.68
p−air
√
( s = 95 TeV).
found a modest 3% model uncertainty among them.
The data used for this measurement was 5 years of
Middle Drum hybrid. Figure 10 shows the measure+25
ment to be σinel
p−air = 567.0 ± 70.5(stat)−29 (sys) mb at
log10 (E/eV) = 18.68
√ which corresponds to a center-ofmomentum energy s = 95 TeV.
The total proton-proton cross section can be related
to the proton-air cross section via Glauber formalism
and the Block, Halzen, Stanev (BHS) fit. Using these
methods σinel
p−air and its errors are propagated to find
√
+19
+48
=
170
s = 95 TeV. Figσtotal
p−p
−44 (stat)−17 (sys) mb at
ure 11 shows the TA measurement of σtotal
p−p and compares it to accelerator based measurements, previous
cosmic ray based measurements, and to the prediction
of the BHS fit.
6. Radar Cross Section
Telescope Array also has deployed a radar facility
(TARA) to test the ability to measure the properties of
air showers assuming the plasma density around the
core can specularly reflect radio waves of sufficient
power, polarization, and frequency. The signal of radio
waves reflected from a fast moving highly dense mass of
charge would be a “chirp”, which is detected as a rapid
change in the sounding frequency over a brief period
of time. Simulations of air showers over TA indicate a
chirp slope of order ∼-1 MHz/μsec are expected. TARA
uses a 25 kW, 54.1 MHz transmitter employing an array
of 8 high gain antennae in a bi-static radar fashion to
illuminate and detect extended air showers passing over
the TA SD array[14]. A radio receiving station located
W.F. Hanlon / Nuclear and Particle Physics Proceedings 279–281 (2016) 15–22
21
7. Summary
Figure 11: The TA measured proton-proton total cross section as
a function of center-of-momentum energy. Previous measurements
from cosmic ray experiments, the Tevatron,
√ as well as the most recent
from the TOTEM experiment at LHC at s = 7 TeV are shown. The
dashed line indicates the prediction of the BHS fit. TA measures the
+19
+48
cross section to be σtotal
p−p = 170−44 (stat)−17 (sys) mb.
at the Long Ridge FD records any potential EAS detections by looking for signals that match predetermined
chirp rates. Background noise is also recorded through
periodic snapshot triggers. The radio receiver may also
be forced to synchronously trigger with the local FD.
Using 6 months of data, forced FD triggered events
in the radio receiver were time matched with events well
reconstructed with the Long Ridge FD resulting in 1292
matches. A threshold level of 3 × RMS of background
determined from snapshot triggers was used to select
those events that had match filter value exceeding this
threshold and were considered candidate EAS events.
This resulted in 17 events of the 1292 being considered
as positive detections. By analyzing the match filter distribution of snapshot events (noise), 17.8 events were
expected as background. This is a 60% chance probability of detecting 17 or more events in the signal region,
therefore no detection of EAS by radar using this analysis method is yet claimed.
An upper limit on the radar cross section (RCS) was
measured by simulated echo waveforms on a large set
of noise triggers, then simulating the detection response
of the radio receiver. The simulated waveforms are
scaled by a parameter Γ90 until 90% exceed the detection threshold and the RCS is calculated for the 90%
confidence level upper limit. The event with the lowest
Γ90 = 0.00077 was an air shower with reconstructed energy of 11.04 EeV corresponding to a RCS of 42 cm2 .
This is the first ever quantitative measurement made of
an EAS radar cross section upper limit.
Telescope Array continues to play a leading role in
the field of particle astrophysics by making new important measurements that help broaden our understanding
of the cosmos. Our reach in measuring the spectrum
now spans over 4.5 decades in energy, allowing one experiment to probe from the just above the knee to the
GZK cutoff. One single experiment with the ability to
measure 4 distinct features of the PeV and higher range
spectrum represents an unprecedented milestone that allows us to examine the transition in the properties of
cosmic rays produced in the galaxy to those produced
by extra-galactic sources. With only a few months
of TALE data and new analysis methods, combined
with techniques previously used, we have made preliminary measurements of the energies of a low energy
ankle (1016.3±0.04 eV), a second knee (1017.3±0.05 eV),
as well as the previously observed high energy ankle
(1018.7±0.02 eV), and GZK cutoff (1019.8±0.05 eV). A preliminary high statistics 7 year spectrum using the TA
SD array has also improved our measurement of the
UHECR spectrum, including a ∼ 6σ confirmation of
the GZK cutoff.
Composition measurements have been updated to
preliminary 7 years of data using Middle Drum hybrid. New analysis has been undertaken by measuring composition using TA FDs in multi-FD mode and
a hybrid measurement using the Black Rock and Long
Ridge FDs. These multiple methods of measuring composition allows TA to understand the uncertainties in
a measurement that is dominated by systematics below the GZK cutoff. TA has also introduced a new
method to quantify agreement between data and models by moving beyond the traditional method of reporting the first 2 moments of the Xmax distributions, which
can be misleading due to effects in the tails. By using
a nonparametric goodness-of-fit test, such as the 2 sample Cramér-von Mises test, a more robust determination
of the agreement of data with a particular composition
model can be made. Using this method TA rules out
pure iron as a source of primary cosmic rays, and TA
sees a predominantly light composition below 1019.5 eV.
Preliminary new data of the TA hotspot continue to
provide hints of possible medium scale anisotropy in
the arrival directions of cosmic rays. By extending the
search using 2 additional years of data 37 new events
over the enitre TA field of view have been collected
but the significance of the hotspot in the region of the
Ursa Major cluster remains the same at 3.4σ. The first
measurement of the proton-air cross section using TA
data has been performed. Using 5 years of hybrid data
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W.F. Hanlon / Nuclear and Particle Physics Proceedings 279–281 (2016) 15–22
+25
σinel
p−air is measured to be 567.0 ± 70.5(stat)−29 (sys) mb
√
at 1018.68 eV corresponding to s = 95 TeV. The total
proton-proton cross section can also be calculated using
+19
total
+48
σinel
p−air and it is found to be σp−p = 170−44 (stat)−17 (sys)
mb.
Using the TARA facility, a preliminary measurement
of the radar cross section of an EAS has been made.
Using 6 months of FD triggered triggered data, TARA
found a possible 17 events corresponding with possible
EAS candidates, but 17.8 events are expected as background (60% chance probability), so no detection is yet
claimed. The first ever preliminary measurement of the
upper limit of an EAS cross section was performed and
found to 42 cm2 (90% c.l.).
With the recent approval of the expansion of the TA
surface array (TA×4), TA will soon begin deployment
of an additional 500 new scintillator SDs. This will provide 20 years of current TA level statistics by the year
2020. Large improvement of statistics of this sort are
needed to rapidly answer the question about the true nature of the TA hotspot and to further improve composition measurements especially above 1019.5 eV where
statistics rapidly fall.
8. Acknowledgements
The Telescope Array experiment is supported
by the Japan Society for the Promotion of Science through Grants-in-Aid for Scientific Research
on Specially Promoted Research (21000002) “Extreme Phenomena in the Universe Explored by Highest Energy Cosmic Rays” and for Scientific Research (19104006), and the Inter-University Research Program of the Institute for Cosmic Ray Research; by the U.S. National Science Foundation
awards PHY-0307098, PHY-0601915, PHY-0649681,
PHY-0703893, PHY-0758342, PHY-0848320, PHY1069280, PHY-1069286, PHY-1404495 and PHY1404502; by the National Research Foundation of Korea (2007-0093860, R32-10130, 2012R1A1A2008381,
2013004883); by the Russian Academy of Sciences,
RFBR grants 11-02-01528a and 13-02-01311a (INR),
IISN project No. 4.4502.13, and Belgian Science Policy under IUAP VII/37 (ULB). The foundations of Dr.
Ezekiel R. and Edna Wattis Dumke, Willard L. Eccles,
and George S. and Dolores Doré Eccles all helped with
generous donations. The State of Utah supported the
project through its Economic Development Board, and
the University of Utah through the Office of the Vice
President for Research. The experimental site became
available through the cooperation of the Utah School
and Institutional Trust Lands Administration (SITLA),
U.S. Bureau of Land Management, and the U.S. Air
Force.
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