GZK Suppression Research Articles

The energy spectrum of cosmic rays measured by the Telescope Array using 10 years of fluorescence detector data

The Telescope Array (TA) Cosmic Ray Observatory is the largest cosmic ray detector in the northern hemisphere. TA was built to study ultra-high-energy cosmic rays (UHECRs), cosmic rays with energies above 1018eV. TA is a hybrid detector, employing two distinct detection methods: a surface detector array and a set of fluorescence telescopes. We will present a measurement of the cosmic ray energy spectrum for energies above 1017.5eV using only the fluorescence telescopes. A novel weather classification scheme using machine learning was used to select data parts with good weather to ensure the quality of the fluorescence data. The data from the Black Rock Mesa (BRM) and Long Ridge (LR) fluorescence telescope sites were analyzed separately in monocular mode, with the calculated fluxes combined into a single spectrum. The 10-year monocular combined cosmic ray energy spectrum is observed to be in excellent agreement with previous measurements from the northern hemisphere. We present fits of the combined spectrum to a series of broken power law models. The thrice-broken power law was observed to be the best fit considering the Poisson deviance per degrees of freedom. The three breaks suggest an additional feature of the spectrum between the previously observed Ankle feature at 1018.7eV and the GZK suppression at 1019.8eV.

Knee, ankle and GZK in historical perspective

The energy spectrum of cosmic rays in the high-energy region (E > 1014 eV) shows a few structures superimposed on a fast decreasing trend. A general discussion of these structures which are usually called knee, ankle and GZK suppression, is presented following the historical evolution of the measurements and of their interpretation.

The gluon condensation at high energy hadron collisions

We report that the saturation/CGC model of gluon distribution is unstable under action of the chaotic solution in a nonlinear QCD evolution equation, and it evolves to the distribution with a sharp peak at the critical momentum. We find that this gluon condensation is caused by a new kind of shadowing–antishadowing effects, and it leads to a series of unexpected effects in high energy hadron collisions including astrophysical events. For example, the extremely intense fluctuations in the transverse-momentum and rapidity distributions of the gluon jets present the gluon-jet bursts; a sudden increase of the proton–proton cross sections may fill the GZK suppression; the blocking QCD evolution will restrict the maximum available energy of the hadron–hadron colliders.

The JEM-EUSO mission

The JEM-EUSO mission aims at studying the origin of the extreme energy cosmic rays (EECRs) beyond the GZK suppression and exploring fundamental physics at these extreme energies, through the observations of the arrival directions and energies of these particles. It is designed to open a new particle astronomy channel. This super-wide-field (60degrees) telescope, with a diameter of about 2.5m, looks down from space observing the nighttime earth’s atmosphere to detect near UV photons at 330–400nm (both fluorescent and Cherenkov photons) emitted from the giant air showers produced by EECRs. The high statistics arrival direction map, expected to have at least more than five hundred events at the highest energies, will allow the identification of individual sources of EECRs, and their association with known near astronomical objects, uncovering therefore the origin of the EECRs. This will open the door to ultra-high energy astronomy, leading to an understanding of the acceleration mechanisms and, perhaps, producing discoveries in astrophysics and/or fundamental physics. The comparison of the energy spectra among the spatially resolved individual sources will help to clarify the acceleration/emission mechanisms, and will eventually confirm the Greisen–Zatsepin–Kuz’min process validating Lorentz invariance up to γ∼1011. Neutral components (neutrinos and gamma rays) can also be detected as well, if their fluxes are high enough.

Extreme high energy proton-gamma discrimination from space observations

The origin of the highest energy cosmic rays is still unknown. At present, the major uncertainties are located at energies above ∼10 19.5 eV, the expected beginning of the GZK suppression. This is mainly due to the low statistics available, a problem that will be addressed in unprecedented way by the upcoming orbital detectors like JEM-EUSO. The detection of very high energy photons is of great relevance for the understanding of the origin of this extreme energy cosmic rays (EECR), due to the astrophysical information content. However, their discrimination is an experimental challenge for current and future cosmic ray detectors. In this work we study the statistical separation between hadron and photon showers from space observations at energies where both, the Landau–Pomeranchuk–Migdal (LPM) effect and magnetospheric interactions are important for the development of the cascades. We base our analysis on the X max parameter, which is already a well known composition discrimination parameter for ground based fluorescence observatories. Our analysis applies to orbiting detectors in general. Nevertheless, we exemplify the practical utilization of our technique by estimating a general upper limit to the photon fraction in the integral flux, attainable by an ideal orbital detector with characteristics similar to JEM-EUSO. In the process we describe the resultant asymmetry in the photon–hadron discrimination efficiency in galactic coordinates.

The Highest-Energy Cosmic Rays

The search for the origin of cosmic rays with energies above 1018 eV has made considerable progress during the past few years. With new data from the High Resolution Fly's Eye experiment in Utah and the Pierre Auger Observatory in Argentina, a suppression of the cosmic ray flux above 5 × 1019 eV consistent with the predicted suppression arising from interaction with the cosmic microwave background (the so-called GZK suppression) has been observed. Recent results from Auger seem to indicate that the arrival direction distribution of cosmic rays with energies above the GZK suppression is not isotropic. Progress toward a better understanding of the chemical composition of the cosmic rays is under way. A new generation of detectors is providing data of unprecedented quality and quantity. In this article, we review these recent developments and briefly outline how our view of the highest-energy cosmic rays has evolved over the past several years.

Lightest Nuclei in UHECR versus Tau Neutrino Astronomy

UHECR may be either nucleons [Abraham J. et al., (Pierre Auger Collaboration), Science, vol.318, p.939-943 (arXiv:0711.2256)] or nuclei; in the latter case the Lightest Nuclei, as He4 or He3, Li, Be, explains at best the absence of Virgo signals and the crowding of events around Cen-A bent by galactic magnetic fields [Fargion D., Phys. Scr. 78 (2008) 045901, 1-4]. This model fit the observed nuclear mass composition discovered in AUGER. However UHECR nucleons above GZK produce EeV neutrinos while Heavy Nuclei, as Fe UHECR do not produce much. UHECR He nuclei at few 1019eV suffer nuclear fragmentation (producing low energetic neutrino at tens PeVs) but it suffer anyway photo-pion GZK suppression (leading to EeV neutrinos) once above one-few 1020eV. Both these cosmogenic UHE secondary neutrinos signals may influence usual predicted GZK [Greisen K., 1966 Phys. Rev. Lett. 16 748], [Zatsepin, G.T., Kuz'min, V.A., Zh. Eks. Teor. Fiz., Pis'ma Red.4 (1966) 144] Tau Neutrino Astronomy [D. Fargion, et al.: “Horizontal Tau air showers from mountains in deep valley. Traces of UHECR neutrino tau” 26th ICRC, He 6.1.09, p.396-398. 1999. (USA); Fargion D., 2002, ApJ, 570, 909; Fargion D. et al. 1999, 26th ICRC, HE6.1.10, 396-398; Fargion D., et al. 2004, ApJ, 613, 1285; Fargion D. et al., Nuclear Physics B (Proc. Suppl.) 2004; Fargion D., et al. Adv. in Space Res., 37 (2006) 2132-2138; 136, 119; D. Fargion, J. Phys. Soc. Jpn. Vol.77 (2008) Suppl. B., p.1-15; Fargion D. et al. Adv. Space Res. 37 (2006) 2132-2138] in significant and detectable way; the role of resonant antineutrino electron-electron leading to Tau air-shower may also rise.

On active galactic nuclei as sources of ultra-high energy cosmic rays

Abstract We measure the correlation between sky coordinates of the Swift Burst Alert Telescope (BAT) catalogue of active galactic nuclei (AGN) with the arrival directions of the highest energy cosmic rays detected by the Auger Observatory. The statistically complete, hard X-ray catalogue helps to distinguish between AGN and other source candidates that follow the distribution of local large-scale structure. The positions of the full catalogue are marginally uncorrelated with the cosmic ray arrival directions, but when weighted by their hard X-ray flux, AGN within 100 Mpc are correlated at a significance level of 98 per cent. This correlation sharply decreases for sources beyond ∼100 Mpc, suggestive of a GZK suppression. We discuss the implications for determining the mechanism that accelerates particles to these extreme energies in excess of 1019 eV.

Fitting the HiRes Data

We fit the recently released monocular spectra of the High Resolution Fly's Eye (HiRes) experiment with broken power laws to identify features in the ultra-high energy cosmic ray (UHECR) spectrum. These fits find the previously observed feature known as the Ankle at 1018.5 eV, as well as evidence for a suppression at higher energies, above 1019.8 eV. We use the integral spectrum along with the E½ test to identify this high energy suppression with the GZK suppression. Finally, we use a model of uniformly distributed extragalactic proton sources together with a phenomenological model of the galactic cosmic ray spectrum to compare the HiRes spectra to what should be expected from the GZK suppression, and to measure how the extragalactic sources must evolve and what the input spectral slope must be to fit the HiRes data.

Results from the HiRes Experiment

The High Resolution Fly's Eye (HiRes) Experiment has been in operation in monocular mode since 1997 and in stereo mode since late 1999. The published HiRes monocular spectrum is consistent with the GZK Suppression at 1019.8 eV and observes an ankle structure at 1018.5 eV. The preliminary stereo spectrum is consistent with the monocular spectrum. Various anisotropy studies with both monocular and stereo data have yielded null results. However, a possible correlation between HiRes stereo events taken before Jan., 2004 and BL-Lac objects has been reported, but remains to be verified or refuted by the analysis of the Hires data taken after Feb., 2004. Composition studies of Hires stereo data shows a predominantly light composition in the energy range 1018.0–1019.3 eV. We also report on the result of the proton-air cross section measurement from the tails of the XMAXdistribution.

New Results from the HiRes Experiment

The High Resolution Fly's Eye (HiRes) collaboration has measured the spectrum of ultrahigh energy cosmic rays in the energy range from 1017.2 to 1020.2 eV. Two spectral features appear. We clearly see one, called the “ankle”, at about 1018.5 eV. We also have evidence for a high-energy suppression of the cosmic ray flux at an energy of about 1019.8 eV, where the GZK suppression is expected to occur. We have also measured the composition of cosmic rays between 1018.0 and 1019.4 eV using the Xmax method. We have searched for anisotropy in the arrival directions of ultra high energy cosmic rays. Results are presented on these topics.