SciPost Phys. Proc. 13, 041 (2023)

Highlights from the Telescope Array Experiment
Hiroyuki Sagawa⋆ for the Telescope Array Collaboration
Institute for Cosmic Ray Research, the University of Tokyo, Japan⋆ hsagawa@icrr.u-tokyo.ac.jp

21st International Symposium on Very High Energy Cosmic Ray Interactions(ISVHECRI 2022)
Online, 23-28 May 2022
doi:10.21468/SciPostPhysProc.13

Abstract
The Telescope Array (TA) is the largest hybrid cosmic ray detector in the Northern Hemisphere, which observes primary particles in the energy range from 2 PeV to 100 EeV.
The main TA detector consists of 507 plastic scintillation counters on a 1.2-km spacing
square grid and fluorescence detectors at three stations overlooking the sky above thesurface detector array. The TA Low energy Extension (TALE) detector, which consists often fluorescence telescopes, and 80 infill surface detectors with 400m and 600 m spacing, has continued to provide stable observations since its construction completion in
2018. The TAx4, a plan to quadruple the detection area of TA is also ongoing. About halfof the planned surface detectors have been deployed, and the current TAx4 continues tooperate stably as a hybrid detector. I review the present status of the TA experiment andthe recent results on the cosmic-ray anisotropy, mass composition and energy spectrum.
Copyright H. Sagawa.
This work is licensed under the Creative Commons
Attribution 4.0 International License.
Published by the SciPost Foundation.

Received 30-09-2022
Accepted 22-06-2023
Check forupdatesPublished 29-09-2023
doi:10.21468/SciPostPhysProc.13.041

Contents
1 Introduction

22 Energy spectrum

3

3 Composition

4

4 Anisotropy

6

5 Conclusion

7

References

9

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1

Introduction

The Telescope Array (TA) is the largest ultra-high-energy cosmic-ray (UHECR) observatory in
the northern hemisphere. The main goal is to explore the origin and nature of UHECRs by
measuring the energy spectrum, arrival direction distribution and mass composition.
The TA detector is located in Utah in the U.S.A. and consists of a surface array of 507
plastic scintillator detectors (SD) [1], which is overlooked by three stations of fluorescence
detectors (FD). The SDs are deployed on a square grid with 1.2-km spacing, and the SD arraycovers an area of approximately 700 km2 . Each individual SD has two layers each of a 1.2-cmthick scintillator with an area of 3 m2 . The full operation of the SDs started in March 2008,
and the duty cycle is greater than 95%. Two FD stations are located at the Black Rock Mesa
(BR) and Long Ridge (LR) sites [2], respectively. At each station, 12 fluorescence telescopes,
each with 256 photomultipliers (PMTs), cover a total field of view of 3−31◦ in elevation angle
and 108◦ in azimuthal angle. The northern FD station situated at the Middle Drum (MD) site
consists of 14 telescopes refurbished from the HiRes-1 telescopes [3], which were arranged
to view ∼120◦ in azimuthal angle. All three FD stations started the observation in November
2007, and they have duty cycles of approximately 10%. Hybrid cosmic-ray events, which aredetected simultaneously by FD and SD, are used to cross-check the SD energy by using the
FD energy measurements and to improve mass composition identification from longitudinal
shower profiles measured with the FD by the inclusion of SD information that better determines
the directions of air-shower axes.
The TALE enables detailed studies of the energy spectrum and composition at energies
over ∼1016 eV. The main goal of the TALE is to clarify the expected transition from galactic
to extragalactic cosmic rays. The TALE FD station is located at the MD site and consists of 10
telescopes refurbished from HiRes-2 [4] and has a field of view of 31−59◦ in elevation angle. A
total of 80 TALE SDs are operating. The data acquisition is ongoing with the hybrid trigger from
FD. Additionally, information on timing around the cores of cosmic-ray air showers measured
by SDs is expected to improve the event reconstruction accuracy of the FD measurement.
Consequently, mass composition measurements from the longitudinal shower profile with FD
are expected to be improved. Further low energy extension with hybrid mode is planned with
45 SDs with 100-m spacing and 9 SDs with 200-m spacing. The target energy range is E >1015 eV. The counters were assembled in October 2021.They will be deployed in 2022.
With enhanced statistics, we expect to verify the hotspot [5] along with other anisotropy
results. We intended to quadruple the TA aperture (TAx4), including the TA SD array by
installing 500 SDs at 2.08-km spacing [6]. The construction started in 2015 by reviewing
the TA scintillator detector components. A total of 257 SDs were deployed in February andMarch 2019 [7]. The array is 2.5 times larger than the TA SD array. The additional arraystarted stable data acquisition in November 2019. The new array requires two FD stations
overlooking the SD array to increase the number of detected hybrid events and to calibrate
the energy measured by SD. These FDs are formed using refurbished HiRes telescopes. The
first light was observed by the FDs at the northern site and southern site in February 2018 andOctober 2019, respectively. The layout of the TAx4 SD and FD including TA and TALE is shownin Fig. 1.
In this report, the TA results on spectrum, composition and anisotropy are described in
Section 2, Section 3 and Section 4, respectively. Section 5 concludes this report [8].

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Figure 1: The layout of TA, TALE and TAx4. The symbols for the layout are explainedon the right side of the plot. Numbers in parentheses indicate the number of detectors.

2

Energy spectrum

A preliminary result of the cosmic-ray spectrum for 11 years of TA SD data is shown above
1018.2 eV in Fig. 2 (left) [9]. The TA confirmed the ankle at 1018.69±0.01 eV and the flux
suppression above 1019.81±0.03 eV, which is consistent with the Greisen-Zatsepin-Kuzmin cutoff
prediction [10,11]. The statistical significance of the difference between the observed cosmicray flux above the cutoff and the expected one from the extrapolation of the flux with thespectral index between the ankle and the cutoff (no suppression) is ∼8.4σ.
Fig. 2 (right) [12] shows the preliminary result of TAx4 SD spectrum, which is consistent
with the TA SD spectrum. Hereafter, when referring to TAx4 results, it means results using
only data from 257 SDs deployed for TAx4 in 2019. The monocular energy spectrum using theadditional TAx4 FD [13] and the energy spectrum using the hybrid analysis with TAx4 [14]are also consistent with the TA SD spectrum.
The energy spectra measured by the Pierre Auger Observatory (hereafter Auger) [15] inthe southern hemisphere and by the TA [9] in the northern hemisphere agree well for energies (0.1−2.5) × 1019 eV after rescaling the energies by +4.5% for Auger and −4.5% for
TA, whereas there is a significant discrepancy between the two results at the suppression [9].
When we compare the energy spectra in the common declination band (−15◦ < δ < +24.8◦ )after the ±4.5% rescalings, the differences are smaller, but the persistent differences require
an additional energy rescaling in an energy-dependent way (±10%/decade for E > 1019 eV)
to get an agreement [16].
As shown in Fig 3, the TA SD data over 11 years yield the cutoffs at 1019.64±0.04 eV and
19.84±0.02
10eV for declinations of −16◦ < δ < 24.8◦ and 24.8◦ < δ < 90◦ , respectively. The
pretrial significance of the difference of these two cutoffs is 4.7σ. The chance probability ofexceeding the pretrial significance for an isotropic distribution is 8.5×10−6 or 4.3σ [9]041.3

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Figure 2: (Left) The TA cosmic-ray flux multiplied by E3 measured using 11 yearsof data from the TA SD [9]. The solid line shows the fit of the TA data to a broken
power law. (Right) The preliminary spectrum using the first one year of the TAx4
SD data [12] in black together with the Auger spectrum [15] in blue and the TA SD
spectrum [9] in green.
The TALE FD measures a spectrum with mixed Cherenkov and fluorescence signals. Theenergy spectrum collected by the TALE FD monocular measurement over 22 months was published in [18]. Fig. 4 (left) [17] shows the TA combined spectrum, which is made by combining
the TA SD and TALE FD spectra. The range of energies below 1018.2 eV is covered by the TALE
FD, while the energy range above 1018.2 eV is covered by the TA SD. We see three features in theenergy spectrum: the knee at approximately 1015.5 eV, the low-energy ankle at 1016.22±0.02 eV
and the second knee at 1017.04±0.04 eV. Fig. 4 (right) shows the preliminary spectrum using 2.5
years of TALE hybrid data, which is consistent with other TA/TALE spectra within systematic
uncertainties [19].
A new feature above 1019 eV, called the shoulder or instep, was first reported by Auger, of
which field of view is concentrated in the southern sky [20]. We performed a joint fit of TA
SD, TA Black Rock (BR) - Long Ridge (LR) monocular FD and HiRes I monocular spectra in
the northern hemisphere into a thrice broken power law. To ensure statistical independence,
the TA monocular FD observation period was removed from the TA SD spectrum measurement. The shoulder feature as shown in Fig. 5 was found at 1019.25±0.03 eV with a statisticalsignificance of 5.3σ [17].

3Composition

The depth of shower maximum, X max , of extensive air-shower profiles is the key estimate of themass composition of UHECRs. The results of X max measurements in the energy range between
1018.2 eV and 1019.1 eV based on the TA hybrid events observed over 10 years are shown in
Fig. 6 together with MC predictions of QGSJET II-04 [21] for proton, helium, nitrogen, andiron primaries [22]. We need more statistics to clarify the feature of X max above 1019 eV, forexample, to more accurately measure the values of the mean X max and the width of X max .
The model predictions for proton and helium primaries appear to agree with the data within
systematic uncertainties.
The X max above 1015.3 − 1018.3 eV was measured using the TALE FD data and a change inthe elongation rate was observed at an energy of ∼1017.3 eV [23]. The X max was measured
using the TALE hybrid data collected over a period of 2.5 years in the energy range 1016.6 −041.4

SciPost Phys. Proc. 13, 041 (2023)

Figure 3: The TA cosmic-ray flux multiplied by E3 measured using 11 years of dataobtained by the TA SD for the upper (24.8◦ < δ < 90◦ ) and lower (−16◦ < δ < 24.8◦ )declination bands with black open circles and red closed circles, respectively [9].
Superimposed lines correspond to the fits of the data to broken power-law functions.

Figure 4: (Left) The TA combined spectrum in black, which is made by combining
the TALE FD monocular data in red and the TA SD data in blue [17]. (Right) The
preliminary TALE hybrid spectrum in black together with other measurements fromTA and TALE [19].
1018.4 eV [19]. Figure 7 shows the mean X max as a function of the cosmic-ray energy. The
systematic uncertainty on X max is less than 16 g/cm2 . The result is not corrected for a biasof about −10 g/cm2 . We can see a break point at ∼1017 eV, which is likely correlated withthe observed softening (∼1017 eV) in the TALE energy spectrum [18]. The values of the mean
X max and the slope of the X max elongation ratio for the TALE hybrid analysis are consistent
with those for the TA hybrid analysis at around 1018.2 eV.
The composition information was also derived by the Boosted Decision Trees (BDT)
method using 16 composition sensitive signals from 12 years of TA SD data [25]. The measured
mean logarithmic cosmic-ray atomic number < l nA > shows no significant energy dependence
above 1 EeV and < l nA > = 0.90 ± 0.05(stat.) ± 0.30(syst.) with the QGSJET II-04 model.

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Figure 5: A joint fit [17] to the TA SD (left), TA BR - LR monocular FD (center),and HiRes I monocular energy spectra (right) together with a red fit line with three
break points. The significance of the shoulder is obtained by comparing the numberof events expected in the absence of the feature (blue line) and the number of events
observed by the experiments.

Figure 6: The results of the mean X max (< X max >) (left) and the width of X max(σ(X max )) (right) as a function of the cosmic-ray energy measured from TA hybrid
events [22].

4Anisotropy

The TA has previously reported an indication of an intermediate-scale cluster of cosmic rays
with energies over 5.7×1019 eV for five years of observation with the TA SD [5]. This TA
hotspot result has since been updated using 12 years of data from the TA SD [26]. Figure 8
shows the significance maps of 179 UHECR events with E > 5.7×1019 eV. We found the maximum significance of 5.1σ at R.A.=144.0◦ and Dec.=40.5◦ for the oversampling circle with aradius of 25◦ after searching for the maximum significance in circles with all grid directions andfive different oversampling radii (40 events were observed where 14.6 would be expected).
The chance probability of the 12-year hotspot in an isotropic sky is estimated to be 6.8×10−4
or 3.2σ.
Now we lower the energy threshold slightly. The significances of the TA 11-year data
at energies log10 (E/eV) > 19.4, 19.5, and 19.6 by the oversampling analysis with 20◦ -radius
circle are 4.4σ, 4.2σ, and 4σ, respectively (see Fig. 9). A new excess was found in the directionof the Perseus-Pisces Supercluster. For E > 1019.6 eV, the chance probability of the excess within
6.8◦ from the supercluster center is 3.5σ.

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Figure 7: The preliminary result of < X max > of the reconstructed TALE hybrid events
as a function of energy [19]. The < X max > values for proton and iron MC primaryelements, using QGSJET II-04, are also shown.

Figure 8: The statistical significance of UHECR events with E > 5.7×1019 eV in
equatorial coordinates [26]. The TA events observed over 12 years are smoothedby the 25◦ oversampling radius circle. The solid red and blue curves indicate thegalactic (GP) and supergalactic (SGP) planes, respectively.

5

Conclusion

TA confirmed the ankle at 1018.69 eV and the flux suppression above 1019.81 eV in the energy
spectrum of UHECRs. The shoulder feature in the energy spectrum in the northern sky was
found at 1019.25 eV. We confirmed the breaks at 1016.22 eV and 1017.04 eV in the energy spectrum
measured using the data observed with the TALE FD.
The TA hybrid X max measurements for the energy band 1018.2 −1019.1 eV are consistent
with a light composition, in particular, with predictions of QGSJET II-04 proton and helium.
We need more statistics to clarify the feature of X max above 1019 eV. For the TALE FD X max
results in the energy range 1015.3 − 1018.3 eV, a change in the elongation rate was observed at
an energy of ∼1017.3 eV, which is likely correlated with the observed break at 1017.04 eV in the
TALE FD energy spectrum.
We obtained 179 events above 5.7×1019 eV in 12 years of observation with the TA SD. We
found a maximum pretrial significance of 5.1σ when using a circle with a 25◦ oversamplingradius. The post-trial significance of detecting such clustered events by chance in the isotropic
arrival distribution is estimated to be 3.2σ. Evidences for some features of anisotropy arefound, for example, as a declination dependence of spectrum cutoff in the energy spectrum,
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Figure 9: The statistical significance of UHECR events with E > 4×1019 eV in equatorial coordinates [26]. The TA events observed over 11 years are smoothed by the20◦ oversampling radius circle. The solid red and blue curves indicate the galactic(GP) and supergalactic (SGP) planes, respectively.
and a new excess in the direction of Perseus-Pisces Supercluster using a slightly lower energy
threshold.
To confirm the TA hotspot and understand its features, we proposed a plan, which we callthe TAx4, to quadruple the TA SD aperture and add two FD stations. The 257 TAx4 SDs were
deployed in 2019, and currently the total area is 2.5 times the TA SD. Two TAx4 FD stations
are operating.
The 80 TALE SDs were deployed at the TALE site. The preliminary result of the spectrum
and X max using the TALE hybrid data was presented in this conference. The TALE infill SD
array is planned to further lower the energy threshold for the TALE hybrid analysis.
The TA, TAx4, and TALE will provide important measurements of the energy spectrum,
composition, and arrival directions of UHECRs from the knee region up to the highest-energy
region spanning five to six decades in energy.

Acknowledgements
The Telescope Array experiment is supported by the Japan Society for the Promotion of
Science (JSPS) through Grants-in-Aid for Priority Area 431, for Specially Promoted Research JP21000002, for Scientific Research (S) JP19104006, for Specially Promoted Research JP15H05693, for Scientific Research (S) JP19H05607, for Scientific Research (S)JP15H05741, for Science Research (A) JP18H03705, for Young Scientists (A) JPH26707011,
and for Fostering Joint International Research (B) JP19KK0074, by the joint research programof the Institute for Cosmic Ray Research (ICRR), The University of Tokyo; by the Pioneering Program of RIKEN for the Evolution of Matter in the Universe (r-EMU); by the U.S. National Science Foundation awards PHY-1607727, PHY-1712517, PHY-1806797, PHY-2012934,
and PHY-2112904; by the National Research Foundation of Korea (2017K1A4A3015188,
2020R1A2C1008230, & 2020R1A2C2102800) ; by the Ministry of Science and Higher Education of the Russian Federation under the contract 075-15-2020-778, IISN project No.
4.4501.18, and Belgian Science Policy under IUAP VII/37 (ULB). This work was partially supported by the grants of the joint research program of the Institute for Space-Earth Environmental Research, Nagoya University and Inter-University Research Program of the Institutefor Cosmic Ray Research of the University of Tokyo. The foundations of Dr. Ezekiel R. and
Edna Wattis Dumke, Willard L. Eccles, and George S. and Dolores Doré Eccles all helped with

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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 (BLM), andthe U.S. Air Force. We appreciate the assistance of the State of Utah and Fillmore offices of the
BLM in crafting the Plan of Development for the site. Patrick A. Shea assisted the collaboration
with valuable advice and supported the collaboration’s efforts. The people and the officials of
Millard County, Utah have been a source of steadfast and warm support for our work which
we greatly appreciate. We are indebted to the Millard County Road Department for their efforts to maintain and clear the roads which get us to our sites. We gratefully acknowledge
the contribution from the technical staffs of our home institutions. An allocation of computer
time from the Center for High Performance Computing at the University of Utah is gratefully
acknowledged.

References[1] T. Abu-Zayyad et al., The surface detector array of the Telescope Array experiment, Nucl.
Instrum. Methods Phys. Res. A 689, 87 (2012), doi:10.1016/j.nima.2012.05.079.
[2] H. Tokuno et al., On site calibration for new fluorescence detectors of the Telescope Array experiment, Nucl. Instrum. Methods Phys. Res. A 601, 364 (2009),
doi:10.1016/j.nima.2008.12.210;
H. Tokuno et al., New air fluorescence detectors employed in the Telescope Array experiment,
Nucl. Instrum. Methods Phys. Res. A 676, 54 (2012), doi:10.1016/j.nima.2012.02.044.
[3] R. U. Abbasi et al., Measurement of the flux of ultrahigh energy cosmic rays from monocular observations by the High Resolution Fly’s Eye experiment, Phys. Rev. Lett. 92, 151101(2004), doi:10.1103/PhysRevLett.92.151101.
[4] J. H. Boyer, B. C. Knapp, E. J. Mannel and M. Seman, FADC-based DAQ for HiRes
Fly’s Eye, Nucl. Instrum. Methods Phys. Res. A 482, 457 (2002), doi:10.1016/S01689002(01)01517-0.
[5] R. U. Abbasi et al., Indication of intermediate-scale anisotropy of cosmic rays with energy greater than 57 EeV in the northern sky measured with the surface detector of theTelescope Array experiment, Astrophys. J. Lett. 790, L21 (2014), doi:10.1088/20418205/790/2/L21.
[6] E. Kido et al., The
doi:10.22323/1.301.0386.

TAx4

experiment,

Proc.

Sci.

301,

386

(2018),[7] E. Kido et al., Status and prospects of the TAx4 experiment, Proc. Sci. 358, 312 (2021),
doi:10.22323/1.358.0312.
[8] H. Sagawa, Results of ultra-high-energy cosmic rays from the Telescope Array, J. Instrum.
15, C09012 (2020), doi:10.1088/1748-0221/15/09/C09012.
[9] D. Ivanov et al., Energy spectrum measured by the Telescope Array, Proc. Sci. 358, 298(2021), doi:10.22323/1.358.0298.
[10] K. Greisen, End to the cosmic-ray spectrum?, Phys. Rev. Lett. 16, 748 (1966),
doi:10.1103/PhysRevLett.16.748.
041.9

SciPost Phys. Proc. 13, 041 (2023)[11] G. T. Zatsepin and V. A. Kuz’min, Upper limit of the spectrum of cosmic rays, Sov. Phys. J.
Exp. Theor. Phys. Lett. 4, 78 (1966).
[12] H. Jeong et al., Reconstruction of air shower events measured by the surface detectors of theTAx4 experiment, Proc. Sci. 395, 331 (2022), doi:10.22323/1.395.0331.
[13] M. Potts et al., Monocular energy spectrum using the TAx4 fluorescence detector, Proc. Sci.
395, 343 (2022), doi:10.22323/1.395.0343.
[14] M. Potts, Ultra-high energy cosmic ray energy spectrum using hybrid analysis with TAx4,
American Physical Society April Meeting (2022), https://meetings.aps.org/Meeting/
APR22/Session/W13.5.
[15] A. Aab et al., Measurement of the cosmic-ray energy spectrum above 2.5 ×
1018 eV using the Pierre Auger Observatory, Phys. Rev. D 102, 062005 (2020),
doi:10.1103/PhysRevD.102.062005.
[16] Y. Tsunesada et al., Joint analysis of the energy spectrum of ultra-high-energy cosmic rays
as measured at the Pierre Auger Observatory and the Telescope Array, Proc. Sci. 395, 337(2022), doi:10.22323/1.395.0337.
[17] D. Ivanov et al., Recent measurement of the Telescope Array energy spectrum and observation of the shoulder feature in the northern hemisphere, Proc. Sci. 395, 341 (2022),
doi:10.22323/1.395.0341.
[18] R. U. Abbasi et al., The cosmic ray energy spectrum between 2 PeV and 2 EeV observed with
the TALE detector in monocular mode, Astrophys. J. 865, 74 (2018), doi:10.3847/15384357/aada05.
[19] K. Fujita et al., The Telescope Array low-energy extension hybrid detector, In preparation.
[20] A. Aab et al., Features of the energy spectrum of cosmic rays above 2.5 × 1018
eV using the Pierre Auger Observatory, Phys. Rev. Lett. 125, 121106 (2020),
doi:10.1103/PhysRevLett.125.121106.
[21] S. Ostapchenko, Monte Carlo treatment of hadronic interactions in enhanced Pomeron scheme: QGSJET-II model, Phys. Rev. D 83, 014018 (2011),
doi:10.1103/PhysRevD.83.014018.
[22] W. Hanlon et al., Telescope Array 10 year composition, Proc. Sci. 358, 280 (2021),
doi:10.22323/1.358.0280.
[23] T. AbuZayyad et al., TALE FD cosmic rays composition measurement, Proc. Sci. 358, 169(2021), doi:10.22323/1.358.0169.
[24] T. Pierog, I. Karpenko, J. M. Katzy, E. Katsenko and K. Werner, EPOS LHC: Test of collective
hadronization with data measured at the CERN Large Hadron Collider, Phys. Rev. C, 92,034906 (2015), doi:10.1103/PhysRevC.92.034906.
[25] Y. Zhezher et al., Cosmic-ray mass composition with the TA SD 12-year data, Proc. Sci.
395, 300 (2022), doi:10.22323/1.395.0300.
[26] J. Kim, D. Ivanov, K. Kawata, H. Sagawa and G. Thomson, Hotspot update, and a newexcess of events on the sky seen by the Telescope Array experiment, Proc. Sci. 395, 328(2022), doi:10.22323/1.395.0328.

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