Spherical harmonic moments are well-suited for capturing anisotropy at any scale in the flux of cosmic rays. An unambiguous measurement of the full set of spherical harmonic coefficients requires ...full-sky coverage. This can be achieved by combining data from observatories located in both the northern and southern hemispheres. To this end, a joint analysis using data recorded at the Telescope Array and the Pierre Auger Observatory above $10^{19}$ eV is presented in this work. The resulting multipolar expansion of the flux of cosmic rays allows us to perform a series of anisotropy searches, and in particular to report on the angular power spectrum of cosmic rays above $10^{19}$ eV. No significant deviation from isotropic expectations is found throughout the analyses performed. Upper limits on the amplitudes of the dipole and quadrupole moments are derived as a function of the direction in the sky, varying between 7% and 13% for the dipole and between 7% and 10% for a symmetric quadrupole.
We report an investigation of the mass composition of cosmic rays with
energies from 3 to 100 EeV (1 EeV=$10^{18}$ eV) using the distributions of the
depth of shower maximum $X_\mathrm{max}$. The ...analysis relies on ${\sim}50,000$
events recorded by the Surface Detector of the Pierre Auger Observatory and a
deep-learning-based reconstruction algorithm. Above energies of 5 EeV, the data
set offers a 10-fold increase in statistics with respect to fluorescence
measurements at the Observatory. After cross-calibration using the Fluorescence
Detector, this enables the first measurement of the evolution of the mean and
the standard deviation of the $X_\mathrm{max}$ distributions up to 100 EeV. Our
findings are threefold:
(1.) The evolution of the mean logarithmic mass towards a heavier composition
with increasing energy can be confirmed and is extended to 100 EeV.
(2.) The evolution of the fluctuations of $X_\mathrm{max}$ towards a heavier
and purer composition with increasing energy can be confirmed with high
statistics. We report a rather heavy composition and small fluctuations in
$X_\mathrm{max}$ at the highest energies.
(3.) We find indications for a characteristic structure beyond a constant
change in the mean logarithmic mass, featuring three breaks that are observed
in proximity to the ankle, instep, and suppression features in the energy
spectrum.
We present measurements of the atmospheric depth of the shower maximum
$X_\mathrm{max}$, inferred for the first time on an event-by-event level using
the Surface Detector of the Pierre Auger ...Observatory. Using deep learning, we
were able to extend measurements of the $X_\mathrm{max}$ distributions up to
energies of 100 EeV ($10^{20}$ eV), not yet revealed by current measurements,
providing new insights into the mass composition of cosmic rays at extreme
energies. Gaining a 10-fold increase in statistics compared to the Fluorescence
Detector data, we find evidence that the rate of change of the average
$X_\mathrm{max}$ with the logarithm of energy features three breaks at
$6.5\pm0.6~(\mathrm{stat})\pm1~(\mathrm{sys})$ EeV, $11\pm
2~(\mathrm{stat})\pm1~(\mathrm{sys})$ EeV, and
$31\pm5~(\mathrm{stat})\pm3~(\mathrm{sys})$ EeV, in the vicinity to the three
prominent features (ankle, instep, suppression) of the cosmic-ray flux. The
energy evolution of the mean and standard deviation of the measured
$X_\mathrm{max}$ distributions indicates that the mass composition becomes
increasingly heavier and purer, thus being incompatible with a large fraction
of light nuclei between 50 EeV and 100 EeV.
The present white paper is submitted as part of the "Snowmass" process to help inform the long-term plans of the United States Department of Energy and the National Science Foundation for high-energy ...physics. It summarizes the science questions driving the Ultra-High-Energy Cosmic-Ray (UHECR) community and provides recommendations on the strategy to answer them in the next two decades.
ApJ 933 (2022) 125 Ultra-high-energy photons with energies exceeding $10^{17}$ eV offer a wealth
of connections to different aspects of cosmic-ray astrophysics as well as to
gamma-ray and neutrino ...astronomy. The recent observations of photons with
energies in the $10^{15}$ eV range further motivate searches for even
higher-energy photons. In this paper, we present a search for photons with
energies exceeding $2{\times}10^{17}$ eV using about 5.5 years of hybrid data
from the low-energy extensions of the Pierre Auger Observatory. The upper
limits on the integral photon flux derived here are the most stringent ones to
date in the energy region between $10^{17}$ and $10^{18}$ eV.
Eur. Phys. J. C (2021) 81:966 We present a measurement of the cosmic-ray spectrum above 100\,PeV using the
part of the surface detector of the Pierre Auger Observatory that has a spacing
of 750~m. An ...inflection of the spectrum is observed, confirming the presence of
the so-called \emph{second-knee} feature. The spectrum is then combined with
that of the 1500\,m array to produce a single measurement of the flux, linking
this spectral feature with the three additional breaks at the highest energies.
The combined spectrum, with an energy scale set calorimetrically via
fluorescence telescopes and using a single detector type, results in the most
statistically and systematically precise measurement of spectral breaks yet
obtained. These measurements are critical for furthering our understanding of
the highest energy cosmic rays.
High-energy neutrinos are expected to be produced by the interaction of accelerated particles near the acceleration sites. For this reason, it is interesting to search for correlation in the arrival ...directions of ultra-high energy cosmic rays (UHECRs) and HE neutrinos. We present here the results of a search for correlations between UHECR events measured by the Pierre Auger Observatory and Telescope Array and high-energy neutrino candidate events from IceCube and ANTARES. We perform a cross-correlation analysis, where the angular separation between the arrival directions of UHECRs and neutrinos is scanned. When comparing the results with the expectations from a null hypothesis contemplating an isotropic distribution of neutrinos or of UHECR we obtain post-trial p-values of the order of \(\sim 10^{-2}\).
JINST 16 P07019 (2021) The atmospheric depth of the air shower maximum $X_{\mathrm{max}}$ is an
observable commonly used for the determination of the nuclear mass composition
of ultra-high energy ...cosmic rays. Direct measurements of $X_{\mathrm{max}}$ are
performed using observations of the longitudinal shower development with
fluorescence telescopes. At the same time, several methods have been proposed
for an indirect estimation of $X_{\mathrm{max}}$ from the characteristics of
the shower particles registered with surface detector arrays. In this paper, we
present a deep neural network (DNN) for the estimation of $X_{\mathrm{max}}$.
The reconstruction relies on the signals induced by shower particles in the
ground based water-Cherenkov detectors of the Pierre Auger Observatory. The
network architecture features recurrent long short-term memory layers to
process the temporal structure of signals and hexagonal convolutions to exploit
the symmetry of the surface detector array. We evaluate the performance of the
network using air showers simulated with three different hadronic interaction
models. Thereafter, we account for long-term detector effects and calibrate the
reconstructed $X_{\mathrm{max}}$ using fluorescence measurements. Finally, we
show that the event-by-event resolution in the reconstruction of the shower
maximum improves with increasing shower energy and reaches less than
$25~\mathrm{g/cm^{2}}$ at energies above $2\times 10^{19}~\mathrm{eV}$.
Despite deflections by Galactic and extragalactic magnetic fields, the distribution of ultra-high energy cosmic rays (UHECRs) over the celestial sphere remains a most promising observable for the ...identification of their sources. Thanks to a large number of detected events over the past years, a large-scale anisotropy at energies above 8 EeV has been identified, and there are also indications from the Telescope Array and Pierre Auger Collaborations of deviations from isotropy at intermediate angular scales (about 20 degrees) at the highest energies. In this contribution, we map the flux of UHECRs over the full sky at energies beyond each of two major features in the UHECR spectrum -- the ankle and the flux suppression --, and we derive limits for anisotropy on different angular scales in the two energy regimes. In particular, full-sky coverage enables constraints on low-order multipole moments without assumptions about the strength of higher-order multipoles. Following previous efforts from the two Collaborations, we build full-sky maps accounting for the relative exposure of the arrays and differences in the energy normalizations. The procedure relies on cross-calibrating the UHECR fluxes reconstructed in the declination band around the celestial equator covered by both observatories. We present full-sky maps at energies above 10 EeV and 50 EeV, using the largest datasets shared across UHECR collaborations to date. We report on anisotropy searches exploiting full-sky coverage and discuss possible constraints on the distribution of UHECR sources.
Phys. Rev. D 102, 062005 (2020) We report a measurement of the energy spectrum of cosmic rays for energies
above $2.5 {\times} 10^{18}~$eV based on 215,030 events recorded with zenith
angles below ...$60^\circ$. A key feature of the work is that the estimates of the
energies are independent of assumptions about the unknown hadronic physics or
of the primary mass composition. The measurement is the most precise made
hitherto with the accumulated exposure being so large that the measurements of
the flux are dominated by systematic uncertainties except at energies above $5
{\times} 10^{19}~$eV. The principal conclusions are: (1) The flattening of the
spectrum near $5 {\times} 10^{18}~$eV, the so-called "ankle", is confirmed. (2)
The steepening of the spectrum at around $5 {\times} 10^{19}~$eV is confirmed.
(3) A new feature has been identified in the spectrum: in the region above the
ankle the spectral index $\gamma$ of the particle flux ($\propto E^{-\gamma}$)
changes from $2.51 \pm 0.03~{\rm (stat.)} \pm 0.05~{\rm (sys.)}$ to $3.05 \pm
0.05~{\rm (stat.)} \pm 0.10~{\rm (sys.)}$ before changing sharply to $5.1 \pm
0.3~{\rm (stat.)} \pm 0.1~{\rm (sys.)}$ above $5 {\times} 10^{19}~$eV. (4) No
evidence for any dependence of the spectrum on declination has been found other
than a mild excess from the Southern Hemisphere that is consistent with the
anisotropy observed above $8 {\times} 10^{18}~$eV.
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