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Research Result Report
ICRR Inter-University Research Program 2023
Research Subject: Ultra High Energy Cosmic Ray Research with the Telescope Array
Principal Investigator:

John Matthews

Participating Researchers: John Matthews, Charles Jui, JiHyun Kim, Gordon Thomson,
Dmitri Ivanov, Jeremy Smith for the University of Utah group
Summary of Research Result：
The University of Utah group received an Inter-University award to assist with datacollection and analysis, especially with respect to comparison to the Pierre Auger
Observatory. The Telescope Array is located about 39o N in the United States near Delta,Utah. The Auger Observatory is located in the southern hemisphere near Malargue, inArgentina. One of the differences between the data of the two experiments is the spectra
observed by the Telescope Array observatory and the Pierre Auger Observatory. This haslong been a source of controversy.
For some time it has been known that there is a ~9% difference in the normalizationof the two spectra and by shifting the Telescope Array data down 4.5% in energy and theAuger data up 4.5% the two spectra could be made to agree over much of the energy
region. Professor Shoichi Ogio and Dr. Keitaro Fujita of the Telescope Array showed that
this difference could be accounted for by the differences in the fluorescence yield andmissing energy correction assumptions that the two experiments used. If the Telescope
Array used the Auger assumptions, the two spectra could be made to agree in the lower
energy range (E < ~1019.5 eV). See Figure 1. For energies greater than this, the two
spectra diverge. The Auger spectra begins to drop off, while the Telescope Array data
continues and has a higher cutoff energy.

Figure 1. The UHECR spectra of Telescope Array (black full circles) andthe Pierre Auger (open blue circles).

When Telescope Array data is

shifted down 4.5% and Auger data is shifted down 4.5%, the two spectra
agree for E < 1019.5 eV. For energies greater than this, the two spectra
diverge. The Auger spectra begins to drop off, while the Telescope Arraydata continues and has a higher cutoff energy.

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Part of the reason for this difference originates in the regions of anisotropy that theTelescope Array observes. The Telescope Array first observed an excess of events E >5.7x1019 eV.

This is known as the Telescope Array Hot Spot.

Later, while trying tounderstand the difference in the cut-off energies of the experiments, an excess of highenergy events was found in the direction of the Perseus-Pisces Super-Cluster and then asmaller excess in the direction of NGC-1068 (See Figure 2.)sees no such anisotropies in the southern hemisphere.

The Auger Observatory

This raises the question ofwhether these excesses are significant or simple statistical fluctuations.

Figure 2. Anisotropy/Excess regions in the Telescope Array14 year data.

Left: The Telescope Array Hot Spot showing the excess for E>1019.75 eV.
There is a 3.2 sigma post-trial excess.

Right: The excesses for thePerseus-Pisces Super Cluster and NGC 1068 for E>1019.6 eV. These havea significance of 3.5 and 3.7 sigma.
If one compares the spectra inside to that outside the three excesses. They are quite
different, See Figure 3. The spectrum of data INSIDE the excesses has a much bigger
bump for E>1019.5 eV. The spectrum of data OUTSIDE the excesses looks much more
like the spectrum observed by the Auger observatory in the southern hemisphere. Thereremains a discrepancy between the TA and Auger spectra above E>1019.5 eV but it issignificantly smaller, perhaps indicating that there is yet another source.

Figure 3.

Telescope Array spectra compared to that of the AugerObservatory.

Left: The Telescope Array spectrum of data INSIDE theForm 8

three excesses (black full circles) compared to the Auger spectrum (openblue circles). Right: The Telescope Array spectrum OUTSIDE the threeexcesses (black) compared with Auger (blue).

The three excesses

contribute the vast majority of the difference from Auger for E>1019.5 eV.
We have been working with the Auger collaboration to better understand thedifference between the Telescope Array and Auger data.

Part of this includes joint

working groups to dig through the data and try to identify potential issues in analysis orassumptions. Another effort involves bringing a small array of Auger detectors to theTelescope Array and incorporating it inside the Telescope Array scintillator array. TheAuger array incorporates seven Auger North prototype detectors instrumented with asingle PMT as well as one Auger South detector instrumented with three PMTs just likethe detectors in Argentina. The Auger group has also sent scintillator detectors of thetype that they are adding in Argentina. (See Figure 4.) With these detectors we will beable to compare pulse heights and shapes on an event by event basis.

The Inter-University award supports this effort in particular by helping to “harden” the Telescope
Array detectors near the Auger mini array to insure that they participate in joint data.

Figure 4.

Auger at Telescope Array.

The Telescope Array iscollaborating with the Auger collaboration to compare the detector
response of the differing detectors. While the Telescope Array surface
detectors are two layers of scintillator, the Auger detectors are water tanks
utilizing Cerenkov light. Recently the Auger collaboration is adding asingle layer scintillator on top of the water tanks. A small array of Auger
detectors has been deployed within the Telescope Array scintillator array.
The Auger array incorporates seven Auger North prototype detectors
instrumented with a single PMT as well as one Auger South detector
instrumented with three PMTs just like the detectors in Argentina. Left:At the center of the test array are an Auger South tank, an Auger Northtank, and a Telescope Array scintillator detector. Right: maps showing
the deployment locations of the Auger mini array.
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