The results on ultra-high-energy cosmic rays (UHECR) mass composition obtained with the Telescope Array surface detector are presented. The analysis employs the boosted decision tree (BDT) ...multivariate analysis built upon 14 observables related to both the properties of the shower front and the lateral distribution function. The multivariate classifier is trained with Monte-Carlo sets of events induced by the primary protons and iron. An average atomic mass of UHECR is presented for energies \(10^{18.0}-10^{20.0}\ \mbox{eV}\). The average atomic mass of primary particles shows no significant energy dependence and corresponds to \(\langle \ln A \rangle = 2.0 \pm 0.1 (stat.) \pm 0.44 (syst.)\). The result is compared to the mass composition obtained by the Telescope Array with \(\mbox{X}_{\mbox{max}}\) technique along with the results of other experiments. Possible systematic errors of the method are discussed.
Astroparticle Physics, 110 (2019) 8-14 We present the results of the search for ultra-high-energy photons with nine
years of data from the Telescope Array surface detector. A multivariate
classifier ...is built upon 16 reconstructed parameters of the extensive air
shower. These parameters are related to the curvature and the width of the
shower front, the steepness of the lateral distribution function, and the
timing parameters of the waveforms sensitive to the shower muon content. A
total number of two photon candidates found in the search is fully compatible
with the expected background. The $95\%\,$CL limits on the diffuse flux of the
photons with energies greater than $10^{18.0}$, $10^{18.5}$, $10^{19.0}$,
$10^{19.5}$ and $10^{20.0}$ eV are set at the level of $0.067$, $0.012$,
$0.0036$, $0.0013$, $0.0013~\mbox{km}^{-2}\mbox{yr}^{-1}\mbox{sr}^{-1}$
correspondingly.
The Pierre Auger Collaboration (Auger) recently reported a correlation between the arrival directions of cosmic rays with energies above 39 EeV and the flux pattern of 23 nearby starburst galaxies ...(SBGs). In this Letter, we tested the same hypothesis using cosmic rays detected by the Telescope Array experiment (TA) in the 9-year period from May 2008 to May 2017. Unlike the Auger analysis, we did not optimize the parameter values but kept them fixed to the best-fit values found by Auger, namely 9.7% for the anisotropic fraction of cosmic rays assumed to originate from the SBGs in the list and 12.9{\deg} for the angular scale of the correlations. The energy threshold we adopted is 43 EeV, corresponding to 39 EeV in Auger when taking into account the energy-scale difference between two experiments. We find that the TA data is compatible with isotropy to within 1.1{\sigma} and with the Auger result to within 1.4{\sigma}, meaning that it is not capable to discriminate between these two hypotheses.
The Telescope Array observatory utilizes fluorescence detectors and surface detectors to observe air showers produced by ultra high energy cosmic rays in the Earth's atmosphere. Cosmic ray events ...observed in this way are termed hybrid data. The depth of air shower maximum is related to the mass of the primary particle that generates the shower. This paper reports on shower maxima data collected over 8.5 years using the Black Rock Mesa and Long Ridge fluorescence detectors in conjunction with the array of surface detectors. We compare the means and standard deviations of the observed \(X_{\mathrm{max}}\) distributions with Monte Carlo \(X_{\mathrm{max}}\) distributions of unmixed protons, helium, nitrogen, and iron, all generated using the QGSJet~II-04 hadronic model. We also perform an unbinned maximum likelihood test of the observed data, which is subjected to variable systematic shifting of the data \(X_{\mathrm{max}}\) distributions to allow us to test the full distributions, and compare them to the Monte Carlo to see which elements are not compatible with the observed data. For all energy bins, QGSJet~II-04 protons are found to be compatible with Telescope Array hybrid data at the 95% confidence level after some systematic \(X_{\mathrm{max}}\) shifting of the data. Three other QGSJet~II-04 elements are found to be compatible using the same test procedure in an energy range limited to the highest energies where data statistics are sparse.
One of the uncertainties in interpretation of ultra-high energy cosmic ray (UHECR) data comes from the hadronic interaction models used for air shower Monte Carlo (MC) simulations. The number of ...muons observed at the ground from UHECR-induced air showers is expected to depend upon the hadronic interaction model. One may therefore test the hadronic interaction models by comparing the measured number of muons with the MC prediction. In this paper, we present the results of studies of muon densities in UHE extensive air showers obtained by analyzing the signal of surface detector stations which should have high \(\it{muon \, purity}\). The muon purity of a station will depend on both the inclination of the shower and the relative position of the station. In 7 years' data from the Telescope Array experiment, we find that the number of particles observed for signals with an expected muon purity of \(\sim\)65% at a lateral distance of 2000 m from the shower core is \(1.72 \pm 0.10{\rm (stat.)} \pm 0.37 {\rm (syst.)}\) times larger than the MC prediction value using the QGSJET II-03 model for proton-induced showers. A similar effect is also seen in comparisons with other hadronic models such as QGSJET II-04, which shows a \(1.67 \pm 0.10 \pm 0.36\) excess. We also studied the dependence of these excesses on lateral distances and found a slower decrease of the lateral distribution of muons in the data as compared to the MC, causing larger discrepancy at larger lateral distances.
We report on a measurement of the cosmic ray energy spectrum by the Telescope Array Low-Energy Extension (TALE) air fluorescence detector. The TALE air fluorescence detector is also sensitive to the ...Cherenkov light produced by shower particles. Low energy cosmic rays, in the PeV energy range, are detectable by TALE as "Cherenkov Events". Using these events, we measure the energy spectrum from a low energy of \(\sim 2\) PeV to an energy greater than 100 PeV. Above 100 PeV TALE can detect cosmic rays using air fluorescence. This allows for the extension of the measurement to energies greater than a few EeV. In this paper, we will describe the detector, explain the technique, and present results from a measurement of the spectrum using \(\sim 1000\) hours of observation. The observed spectrum shows a clear steepening near \(10^{17.1}\) eV, along with an ankle-like structure at \(10^{16.2}\) eV. These features present important constraints on galactic cosmic rays origin and propagation models. The feature at \(10^{17.1}\) eV may also mark the end of the galactic cosmic rays flux and the start of the transition to extra-galactic sources.
An intermediate-scale energy spectrum anisotropy has been found in the
arrival directions of ultra-high energy cosmic rays of energies above
$10^{19.2}$ eV in the northern hemisphere, using 7 years ...of data from the
Telescope Array surface detector. A relative energy distribution test is done
comparing events inside oversampled spherical caps of equal exposure, to those
outside, using the Poisson likelihood ratio. The center of maximum significance
is at $9^h$$16^m$, $45^{\circ}$. and has a deficit of events with energies
$10^{19.2}$$\leq$$E$$<$$10^{19.75}$ eV and an excess for $E$$\geq$$10^{19.75}$
eV. The post-trial probability of this energy anisotropy, appearing by chance
anywhere on an isotropic sky, is found by Monte Carlo simulation to be
$9$$\times$$10^{-5}$ ($3.74$$\sigma_{global}$).
The Telescope Array (TA) experiment is located in the western desert of Utah, USA, and observes ultra high energy cosmic rays (UHECRs) in the Northern hemisphere. At the highest energies, ...\(E>10\)~EeV, the shape of cosmic ray energy spectrum may carry an imprint of the source density distribution along the line of sight different in different directions of the sky. In this study, we search for such directional variations in the shape of the energy spectrum using events observed with the Telescope Array's surface detector. We divide the TA field of view into two nearly equal-exposure regions: the "on-source" region which we define as \(\pm 30^\circ\) of the supergalactic plane containing mostly nearby structures, and the complementary "off-source" region where the sources are further away on average. We compare the UHECR spectra in these regions by fitting them to the broken power law and comparing the resulting parameters. We find that the off-source spectrum has an earlier break at highest energies. The chance probability to obtain such or larger difference in statistically equivalent distributions is estimated as \(6.2\pm1.1\times10^{-4}\) (\(3.2\sigma\)) by a Monte-Carlo simulation. The observed difference in spectra is in a reasonable quantitative agreement with a simplified model that assumes that the UHECR sources trace the galaxy distribution from the 2MRS catalogue, primary particles are protons and the magnetic deflections can be neglected.
We present the results of the search for ultra-high-energy photons with nine years of data from the Telescope Array surface detector. A multivariate classifier is built upon 16 reconstructed ...parameters of the extensive air shower. These parameters are related to the curvature and the width of the shower front, the steepness of the lateral distribution function, and the timing parameters of the waveforms sensitive to the shower muon content. A total number of two photon candidates found in the search is fully compatible with the expected background. The \(95\%\,\)CL limits on the diffuse flux of the photons with energies greater than \(10^{18.0}\), \(10^{18.5}\), \(10^{19.0}\), \(10^{19.5}\) and \(10^{20.0}\) eV are set at the level of \(0.067\), \(0.012\), \(0.0036\), \(0.0013\), \(0.0013~\mbox{km}^{-2}\mbox{yr}^{-1}\mbox{sr}^{-1}\) correspondingly.
An intermediate-scale energy spectrum anisotropy has been found in the arrival directions of ultra-high energy cosmic rays of energies above \(10^{19.2}\) eV in the northern hemisphere, using 7 years ...of data from the Telescope Array surface detector. A relative energy distribution test is done comparing events inside oversampled spherical caps of equal exposure, to those outside, using the Poisson likelihood ratio. The center of maximum significance is at $9^h$$16^m\(, \)45^{\circ}\(. and has a deficit of events with energies \)10^{19.2}$$\leq$$E$$<$$10^{19.75}\( eV and an excess for \)E$$\geq$$10^{19.75}\( eV. The post-trial probability of this energy anisotropy, appearing by chance anywhere on an isotropic sky, is found by Monte Carlo simulation to be \)9$$\times$$10^{-5}\( (\)3.74$$\sigma_{global}$).
