Edited by: Frank Franz Deppisch, University College London, United Kingdom
Reviewed by: Francis Halzen, University of Wisconsin-Madison, United States; Soebur Razzaque, University of Johannesburg, South Africa
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We review open questions and prospects for progress in ultrahigh-energy cosmic ray (UHECR) research, based on a series of discussions that took place during the “The High-Energy Universe: Gamma-Ray, Neutrino, and Cosmic-ray Astronomy” MIAPP workshop in 2018. Specifically, we overview open questions on the origin of the bulk of UHECRs, the UHECR mass composition, the origin of the end of the cosmic-ray spectrum, the transition from Galactic to extragalactic cosmic rays, the effect of magnetic fields on the trajectories of UHECRs, anisotropy expectations for specific astrophysical scenarios, hadronic interactions, and prospects for discovering neutral particles as well as new physics at ultrahigh energies. We also briefly overview upcoming and proposed UHECR experiments and discuss their projected science reach.
Cosmic rays with energy exceeding 1018 eV≡1 EeV, are referred to as ultrahigh-energy cosmic rays (UHECRs). Extensive air showers (EAS) produced when a UHECR interacts with an air nucleus in the upper atmosphere have been measured since their discovery by Pierre Auger in the 1930s. The first observation of an EAS with an energy of ~1020 eV was made at Volcano Ranch in February 1962 (Linsley,
This document summarizes the discussions that took place during the workshop “The High Energy Universe: Gamma-ray, Neutrino, and Cosmic-ray Astronomy” at the Munich Institute for Astro- and Particle Physics (MIAPP). We met for 1 month in March 2018 and had daily discussions and presentations about the status and future of the field of UHECR study. What have we learned about UHECRs in the last years? Which of the open questions can we expect to be able to address with forthcoming detector upgrades and proposed next-generation experiments? What are the requirements for probing remaining open questions and going forward in the study of UHECRs?
An overview of the current status of experimental measurements is given in section 2. Section 3 presents the open questions in the field of UHECRs. The theoretical models that successfully describe UHECR data are summarized. Predictions are given of the sensitivity of forthcoming and proposed experimental measurements to specific theoretical models and to the presented open questions in general. In section 4, upcoming and proposed Earth-based and space-based experiments are presented. We conclude in section 5, with our view of the outlook of the field, and a set of suggestions that we judge as beneficial for addressing open questions at ultrahigh energies in the coming years.
The detection of an UHECR flux excess in the direction of a (few) prominent nearby source(s) would act as a
Studies at large angular scales are often performed with ground-based observatories through a Rayleigh analysis (Linsley,
Combining the right-ascension analysis with an azimuthal one, the anisotropy signal appears to be consistent with a dipolar modulation over ~85% of the sky covered by Auger. The amplitude of the dipole,
Smoothed cosmic-ray flux for
The Pierre Auger Collaboration has performed searches for intrinsic anisotropy at small angular scales at energies exceeding 40 EeV, by comparing the observed number of events within angular windows of a specified radius with that expected from an isotropic UHECR flux. The strongest excess revealed by this search is obtained at
Local-significance maps from searches for localized UHECR excess in equatorial coordinates.
The directions with largest departures from UHECR isotropy have been compared with the position of nearby prominent objects. The two most significant excesses in the Northern and Southern hemispheres are located near the supergalactic plane, and multiple candidate sources have been discussed either within or outside from Collaborations. For example, in Fang et al. (
To reach a more complete view of the UHECR sky, cross-correlation studies against numerous astronomical catalogs have been performed within the Auger and TA collaborations, as well as by independent groups. Models often assume that the UHECR source distribution follows the distribution of luminous matter in the nearby Universe, based on radio — 3CRR catalog—or infrared—IRAS and 2MASS—or X-ray—
Measuring the energy spectrum of UHECRs at high precision is of prime importance for understanding the origin and mechanisms of CR acceleration and propagation. Data at the highest energies have been accumulated for decades by AGASA (Yoshida et al.,
Both TA and Auger are hybrid observatories comprising a set of fluorescence telescopes and a surface detector array (Tokuno et al.,
Another important question related to the UHECR energy spectrum is about the origin of the flux suppression observed at the highest energies. The GZK cut-off was predicted 50 years ago independently by Greisen (
The most reliable technique to measure the mass composition of UHECRs is the simultaneous measurement of the depth,
The current data on the average shower maximum, 〈
Measurements (Abbasi et al.,
These measurements of the first two moments (mean and standard deviation) of the
For a more quantitive insight on the mass composition of UHECRs, the Pierre Auger Collaboration fitted templates of four mass groups (p, He, N, Fe) to the
Composition fractions arriving at Earth derived from fitting templates of four mass groups to the
Neutral secondaries including neutrinos and photons are expected to be produced when UHECRs interact with extragalactic background photons during intergalactic propagation. These secondary particles are also referred to as cosmogenic or GZK neutrinos and photons in the literature. Their flux mainly depends on the chemical composition, maximum energy of UHECRs, and the source evolution model (e.g., Takami et al.,
Specifically, the orange shaded area covers the expectation of the best-fit models with 90% CL and assuming a source evolution following the AGN, star-formation rate (SFR), and γ-ray burst (GRB) redshift evolution (Alves Batista et al.,
Upper limits to the UHE neutrino flux have been obtained by the IceCube Observatory (Aartsen et al.,
The
Good understanding of hadronic multiparticle production is needed for being able to derive composition information from air-shower data. While measuring shower profiles using fluorescence and Cherenkov light allows an almost model-independent determination of the shower energy (up to a correction of the order of 10–15% for “invisible” channels Barbosa et al.,
An important aspect of the hadronic interaction models is the extrapolation of accelerator data to center-of-mass energies of up to
Air-shower measurements can also be used to derive information on hadronic interactions. Given that the primary cosmic-ray composition appears to be mixed in the energy range of relevance here, there is typically a strong correlation between the results of such measurements and the assumed primary mass composition. An exception is the measurement of the proton-air cross section. If done in an energy range in which there is a large fraction of protons in the mass composition of cosmic rays, one can select showers that develop very deep in the atmosphere to build a proton-dominated sample. Then the depth fluctuations can be related to the proton-air cross section for particle production. Recent results are shown in
There is increasing evidence for a discrepancy between the number of muons predicted by model calculations and that measured at very high energy. One of the most direct measurements demonstrating this muon discrepancy is shown in
Enormous progress has been made recently from observing simple all-particle power-law distributions with just seeing the knee and ankle of the cosmic-ray spectrum, to uncovering a much more complex structure with an additional “second knee” at about 1017 eV, an ankle-like structure between the knee and this second knee, and the steep cut-off at the highest energies. Moreover, not only all-particle spectra can be derived from the air-shower data, but also energy spectra of different mass groups. All these achievements provided new insight into the astrophysics causing those structures. This became possible only by advancing both the precision of air-shower observations and reconstructions and the statistics of the data. In fact, improving simultaneously the quality and quantity will also be the key to making progress in the future.
The disentanglement of the all-particle energy spectrum into that of individual mass groups from about 1015 eV to 1017 eV, most notably by KASCADE and KASCADE-Grande, has provided new insights into the origin of the knee and ankle and will be discussed in section 3.2.2 in the context of the transition from Galactic to extragalactic cosmic rays. The origin of the flux suppression of cosmic rays at highest energies is still debated. The two competing explanations are energy-losses of UHECR in the CMB or nearby sources of UHECR with corresponding maximum acceleration energies (see section 3.2.5).
Identifying individual sources of UHECR would answer that question and remains the ultimate goal of future studies. It can be expected that the arrival directions of light primaries at the highest energies are correlated with UHECR sources located within the GZK sphere. Identifying such sources calls for a precise shower-by-shower determination of the energy and mass of the primary particle to avoid cosmic rays of lower energy diluting the event sample and to avoid heavy primaries, suffering stronger deflections, blurring the source spots in the sky. In both cases, the experimental energy and mass resolution determine the ratio of possibly source-correlated events to background events so that compromises in experimental resolution need to be paid for by larger event statistics, i.e., by larger exposures. Obviously, the steeper the spectrum in the region of interest, the stronger is the effect of spillover. This has been studied in a simplified model in Brümmel et al. (
Effect of spillover.
As mentioned in section 2.3, the inferred cosmic-ray composition at Earth shows a peculiar dependence on energy (cf.
The factor between the maximum fraction of protons and helium in
Another important open question related to mass composition is the evolution of the rigidity
Evolution of the UHECR rigidity with energy using the composition fractions estimated from Auger data in Bellido (
The challenge of accelerating cosmic rays to 1020 eV was succinctly presented in the form of the minimum requirement for the accelerators, in what is now commonly referred to as the “Hillas condition” (Hillas,
For acceleration in a shock with velocity βsh (in units of the speed of light), the maximum achievable energy is,
where η parametrises the efficiency of acceleration, with η = 1 the maximum achievable efficiency when diffusion proceeds in the Bohm limit.
The confinement condition is not sufficient to guarantee cosmic-ray acceleration to 1020 eV. This depends on the details of the acceleration mechanism and the timescale for energy loss in the source environment. A summary of constraints on astrophysical sources based on the Hillas condition was presented in Ptitsyna and Troitsky (
Hillas diagram. Source classes are shown as function of their characteristic size,
Another condition that must be met by UHECR accelerators is that they must possess the required energy budget to produce the observed UHECR diffuse flux. The energy production rate of UHECRs has been estimated in Waxman (
Characteristic source luminosity vs. source number density for steady sources, and effective luminosity vs. effective number density for transient sources assuming a characteristic time spread, τ = 3 × 105 yr. The effective number density for bursting sources is only valid for the assumed value of τ, which corresponds to mean extragalactic-magnetic-field strength of 1 nG. Stronger magnetic fields would imply larger τ and hence, larger effective number density. The black solid line gives the best-fit UHECR energy production rate derived in Aab et al. (
The orange dashed line gives the minimum source number density constraint, which comes from the analysis of arrival directions of UHECRs detected in Auger of Abreu et al. (
In order to compare the energy budget constraint to the energy budget of transient source classes, the observed burst rate, ρ, must be converted to the effective number density for UHECRs,
where
Below, we discuss the most plausible UHECR-source candidates in turn.
Low-luminosity GRBs, which are a less-well-known source population, seem to occur with a much larger rate locally than high-luminosity GRBs. They are appealing as sources of UHECRs (Murase et al.,
Though standard supernovae are not expected to be able to accelerate cosmic rays to ultrahigh energies, the ejecta of trans-relativistic and engine-driven supernovae which typically reach mildly relativistic speeds may also be able to accelerate UHECRs (Wang et al.,
In 2017 the detection of gravitational waves from the merger of a neutron star binary, followed by a short GRB and electromagnetic emission from the remnant marked the discovery of this, long-sought-for, class of events (Abbott et al.,
A brief mention to the winds of Wolf-Rayet stars is also due here. Though inspection of
In jetted AGN, a lot of the power goes to energizing the lobes, which are very extended features with relatively small magnetic fields (
In addition, radio-quiet, low-luminosity AGN and quasar outflows have been discussed as possible sources of UHECRs (Pe'er et al.,
Intermediate-mass black holes may also tidally disrupt stars. Depending on the combination of masses of both objects, tidal squeezing may trigger nuclear burning in the core of white dwarfs, leading to a supernova and potentially accelerating cosmic rays to ultrahigh energies (Alves Batista and Silk,
The cosmic-ray spectrum features three distinct spectral breaks in the energy range between 1015 and 1018eV. In order of increasing energy, these are the “knee,” “second-knee” (or “iron-knee”) and “ankle,” illustrated in
Schematic illustration of the rate of cosmic rays incident on Earth as a function of energy, and the three distinct spectral breaks which can be seen in the cosmic-ray spectrum in this energy range, the proton knee, second knee, and ankle. A. Taylor for this review.
The physical origin of the knee feature remains unclear. Both a propagation and sources maximum energy origin of this feature remain viable candidates. An outline of these two scenarios is given below.
Assuming that cosmic rays of a given Larmor radius
It is important to note, however, that such propagation origin scenarios for the knee make a considerable implicit assumption. For these scenarios it is necessary that luminous Galactic cosmic ray sources exist, capable of accelerating particles to energies well beyond the knee energy. Within the framework of our current understanding of Galactic SNR accelerators, however, such an assumption presents a considerable challenge (Bell et al.,
where the factor η describes how close to Bohm diffusion the maximum energy particles in the source achieve, βsh is the shock velocity in units of
In either the propagation or maximum-energy scenario which describes the origin of the knee feature at 3 PeV, a family of corresponding knee features for the other nuclear species are naturally expected. Observationally, it remains unclear whether the composition of CRs at the energy of the knee feature (3 PeV) are protons, helium, or heavier species. Assuming the composition of the knee to be dominated by protons (i.e., a proton knee at 3 PeV), a corresponding iron knee feature at 100 PeV would be expected. Observational evidence for such a second knee feature was reported from the analysis of the KASCADE-Grande data (Apel et al.,
On theoretical grounds, it remains extremely challenging for known Galactic CR accelerators to accelerate protons above PeV energies. The known magnetic field amplification scenarios place a hard cap for maximum energies achievable by SNR (Bell et al.,
In addition, the low level of anisotropy of cosmic rays in the energy range 1017 − 1018 eV also disfavors a Galactic origin of any light component in this range (Abreu et al.,
At energies at/just above that of the second knee, observational evidence suggesting the onset of a new component in the light composition spectrum is found in the KASCADE-Grande data, referred to as the proton ankle. Evidence pointing in this direction is also supported by the low-energy Auger HEAT
Extragalactic cosmic-ray protons at EeV energies undergo frequent Bethe-Heitler energy loss interactions with CMB photons, losing their energy through this process on Gyr timescales. These losses give rise to electron/positron pairs,
The large-scale anisotropy discovered beyond the ankle by the Pierre Auger Observatory appears to be consistent with the distribution of extragalactic matter traced by near-infrared observations from 2MASS (Aab et al.,
Cross-correlation with catalogs of objects observed throughout the electromagnetic bands has proven a powerful means to address the question of possible associations. Such searches recently hinted (3–4σ) at a fraction of 10–15% of UHECR events being consistent with the directional and flux distributions expected from either extragalactic matter—traced by 2MASS or
Ground-based observations come with a partial view of the celestial sphere. Nonetheless, attempts at full-sky coverage by combining data from the largest Northern and Southern observatories have been performed by the Pierre Auger and Telescope Array Collaborations. Such an approach is limited by the mismatch in energy scale between the two experiments, which could cause spurious anisotropies due to an improper contrast between the flux inferred from each dataset. The collaborations have designed a method to match the flux in the declination band covered from both sites, providing a common view on the UHECR sky beyond the ankle and above the flux suppression (Aab et al.,
Most current anisotropy studies exploit the arrival directions of UHECR events above a given (or scanned) energy threshold. This information could be supplemented by spectral and composition data to perform tomography of the UHECR production rate. Propagation of nuclei of different species affects the expected composition and spectrum as detected on Earth. Combined fits of the spectral and composition data show constraining power on the evolution of the density of sources at a fixed a luminosity (see e.g., Aab et al.,
Assuming that sources of UHECRs also accelerate electrons radiating photons in a relativistic flow with speed β and bulk Lorentz factor Γ, the Hillas condition imposes a minimum photon luminosity
where the rigidity
All known non-thermal sources are transient on some timescale. For UHECR sources, what defines whether a candidate object is classified as a transient or steady source is the ratio of the mean propagation timescale between sources to the source emission timescale,
The propagation timescale depends on the mode by which UHECRs propagate, which itself depends on the distance between sources and the UHECR scattering length. For a given source density, a CR energy can be found for which the distance between sources matches the cosmic-ray scattering length in the turbulent medium it is propagating through (Kotera and Lemoine,
The source emission timescale,
For AGN, the longest timescale which may be associated to particle acceleration is the jet activity timescale, estimated to be of the order 300 Myr (Wykes et al.,
For GRBs, extensive efforts to detect VHE gamma rays have until recently failed to achieve a detection(Mirzoyan,
Adopting a fiducial distance between sources of ~ 10 Mpc, a ballistic propagation time between sources of 30 Myr sets a lower limit to the actual propagation time. Adopting the 300 Myr AGN jet activity timescales as a fiducial value for
The above example demonstrates that the flux from any source class of a similar number density whose emission timescale is significantly shorter than a Myr will almost certainly be transient, and unable to achieve steady state. Furthermore, steady-state emission at low energies eventually becomes invariably unachievable for all source classes, once the diffusive sphere of cosmic rays around each source ceases to overlap with even neighboring sources—a phenomenon referred to as the magnetic horizon.
At high energies, energy losses during propagation affect whether the flux can achieve steady-state or not through a reduction of
The cut-off at the highest energies in the cosmic-ray spectrum has been established unambiguously recently, but the origin of this most prominent and significant feature is still a matter of debate. It has been tempting to identify the flux suppression with the long-predicted GZK-effect given its close coincidence to the expected threshold energy of about 6·1019 eV. Several fits of the end of the cosmic-ray spectrum with a propagated cosmic-ray composition consisting of a single element (p, He, N, Si, or Fe) at the source are shown in
Illustration of pure GZK scenarios (free injection index, maximum energy fixed at 1022 eV, first and third column) and scenarios with a freely floating maximum energy (free injection index and maximum energy, second and fourth column) for different primary masses. The first and second columns are for the Auger measurements of the flux (Abraham et al.,
A closer look at GZK scenarios is given in
CRPropa simulations of the energy spectrum and composition at Earth for UHECR sources injecting pure beams of protons (black lines), nitrogen (7 ≤
On the other hand, a simple astrophysical model of identical UHECR sources that accelerate nuclei through a rigidity-dependent mechanism provides a perfect description of the energy spectrum and mass composition above the ankle if the maximum rigidity is at about 1018.8 V, the composition is dominated by intermediate mass nuclei, and the source spectra are harder (γ ≃ 1.6) than expected by the standard Fermi mechanism (Fang et al.,
Magnetic fields in scales comparable to and larger than the size of the Galaxy may affect the propagation of UHECRs. Little is known about extragalactic magnetic fields (EGMFs). The mechanisms whereby they originated are broadly divided into two classes, astrophysical and primordial. The latter postulates that fields in the present epoch result from the amplification of seed fields generated through a cosmological process in the early Universe, whereas in the former scenarios astrophysical processes such as feedback by active galaxies and stars would seed the intergalactic medium. Comprehensive reviews on cosmic magnetogenesis can be found in Durrer and Neronov (
The Galactic magnetic field (GMF) is understood better than EGMFs. Observationally driven models have been developed using polarized synchrotron maps, combined with Faraday rotation measurements (RMs) for the regular field and synchrotron intensity maps to derive the random field component. One of the most complete models was developed by Jansson and Farrar (
A detailed study of the deflection of UHECRs in the GMF model by JF12 has been performed (Farrar and Sutherland,
In Erdmann et al. (
Due to the lack of observationally derived models for the distribution of EGMFs in the local Universe, most studies of the kind have been done using cosmological simulations of structure formation. Early works by Sigl et al. (
In addition to magnetic deflections, EGMFs induce energy and charge dependent time-delays as discussed in section 3.2.1. If the sources of UHECRs are transient, these time-delays are expected to produce an observable distortion of the arrival direction distribution with respect to that of steady sources (Kalli et al.,
Note, however, that magnetic power spectra which contain most of the energy at large scales would completely spoil UHECR astronomy in this case, though this is an unlikely scenario. Ultimately the deflection of UHECRs in EGMFs depends on the distribution of EGMFs in the local Universe. This can be understood in terms of volume filling factors, shown in
Cumulative volume filling factors for EGMFs according to several models. Details about each model can be found in the corresponding publications: Alves Batista et al. (
The effects of EGMFs on the spectrum and composition measured at Earth depend on the characteristic lengths involved. In the limit of a continuous source distribution, the propagation theorem states that the spectrum will have a universal form regardless of the modes of propagation (Aloisio and Berezinsky,
A combined spectrum-composition fit of the Pierre Auger Observatory data including a particular model of EGMF has been presented in Wittkowski (
Knowledge about the intervening magnetic fields is important to understand the origins of UHECRs. Conversely, UHECRs may also be used to constrain cosmic magnetic fields. A number of methods have been proposed for this purpose (Golup et al.,
Some attempts to constrain EGMFs using UHECRs have been made. Yuksel et al. (
Given the considerable uncertainties in the GMF and the inconclusive results on the effects of EGMFs on cosmic-ray propagation due to the model dependencies, the unambiguous identification of individual UHECRs will require better constraints on EGMFs and improved models of the GMF. Therefore, until we have a more accurate description of GMFs and unless EGMFs are small or better understood, charged-particle astronomy will remain challenging if cosmic rays at the highest energies are heavy nuclei until new data on the RMs of Galactic pulsars and Faraday tomography [e.g., from LOFAR and SKA (Beck,
Even though the current generation of hadronic interaction models gives a good description of many properties of air showers, we are far from having reached a satisfactory level in the quality and reliability of modeling extensive air showers.
First of all, there is the muon discrepancy that is still not understood. An excess in the muon number of ~ 30% relative to simulation predictions is not accounted for (Aab et al.,
Secondly, the accuracy of the predictions of hadronic interaction models has to be improved to reduce the systematic uncertainties of composition measurements. For example, the tension between the mean depth of shower maximum and the shower-by-shower fluctuations of
Over the last decades, a rich dataset on proton-proton and proton-antiproton interactions at high energy has been accumulated in various collider experiments. In an air shower, most of the interactions are initiated by pions and kaons, except the first one. There is a severe lack of pion-proton and kaon-proton data that are needed for improving our understanding of hadronic interactions and for tuning interaction models. Of prime importance are the measurement of the pion-proton, kaon-proton, pion-light-nuclei, and kaon-light-nuclei cross sections and the corresponding distributions of leading secondary particles. Taking data at LHC and selecting events in which a beam proton becomes a neutron by emitting a π+ is one possibility to study pion interactions at energies not accessible in fixed target experiments. Similar measurements have been done at HERA (Khoze et al.,
Similarly, there is almost no data available on particle production with light nuclei in the mass range of air (〈
And, last but not least, particle detectors covering the forward direction would help significantly to reduce the needed extrapolation of collider measurements to phase space regions of relevance to air showers. Understanding the scaling of the forward particle distributions at collider energies is the key to extrapolating to higher interaction energies. LHCf (Adriani et al.,
Although it can be expected that progress in understanding hadronic multiparticle production will be mainly driven by experimental results for the next years, efforts to develop a more consistent theoretical framework will be equally important. The transition between soft and hard processes (i.e., processes with small and large momentum transfer) is not understood at all. Applying Regge parameterizations for soft processes is common practice, while hard processes are treated within the QCD-improved parton model. Closely related to this transition between two regimes is the question of non-linear effects, or even parton density saturation, expected at very high parton densities. LHC data show that proton-proton interactions of high multiplicity exhibit features previously only seen in heavy ion collisions (see e.g., Khachatryan et al.,
The center-of-mass energy of an UHECR of energy
Thus, UHECR propagation in the cosmic radiation backgrounds, which have energy ϵ≪ GeV, cannot probe Lorentz-invariant physics beyond the Standard Model, but it can still probe invariance violations under Lorentz boosts. In addition, the development of air showers can be influenced both by Lorentz-invariant new physics acting on primaries or secondaries with energy
The muon abundance is roughly proportional to the energy fraction going into hadrons. Since over one hadronic interaction depth of about
Alternatively, if high-energy neutral pions were stable or had a decay rate smaller than their interaction rate in the atmosphere, for example, due to Lorentz symmetry violation at very high Lorentz factors, then their energy could contribute to increasing the energy fraction going into the hadronic channel and thus the muon signal. Generally speaking, an increase of the fraction of air-shower energy in the hadronic channel would likely be a hint for new physics. One could imagine such effects to have energy thresholds, so one could also search for comparatively large increases of muon number over a small primary energy range.
Here, ξ, χ1 and χ2 are dimensionless constants,
Operators such as Equation (6) can manifest through modifications of dispersion relations for particles of energy
Here,
Dispersion relations of the form of Equation (7) can modify both the free propagation of particles and the kinematics and thresholds of interactions(Amelino-Camelia and Piran,
Therefore, the larger the particle mass, the higher the energy at which LIV effects become relevant.
In the relativistic limit, to first order in
For positive η this would lead to superluminal motion for
If LIV exists and its effect is non-negligible, the corresponding parameter η should naturally be of order 1; constraining it to values much smaller than unity(Gagnon and Moore,
IceCube astrophysical neutrinos have been used to measure the neutrino-nucleon cross section in the TeV–PeV range for the first time(Aartsen et al.,
Numerous new-physics models have effects that are proportional to some power of the neutrino energy
New physics of different types can affect all neutrino observables: the energy spectrum (see e.g., Baerwald et al.,
At high and ultra-high energies, there are a few challenges to detecting new physics in high-energy cosmic neutrinos:
New-physics effects might be sub-dominant; in this case, their discovery is contingent on detecting a large enough number of events and on accurately reconstructing key properties of events, like energy and arrival direction;
When extracting fundamental neutrino properties from the data, one must factor in astrophysical uncertainties (e.g., shape of the energy spectrum, redshift evolution of the number density of sources, etc.), which can be significant;
Flavor is a difficult property to measure in neutrino telescopes(Aartsen et al.,
These challenges are likely surmountable. High- and ultra-high-energy observatories are in a unique position to perform powerful tests of neutrino physics, complementing and expanding tests performed by experiments with lower energies and shorter propagation baselines.
The most promising step in the activity of ground UHECR detection is the upcoming upgrade of the Pierre Auger Observatory (Aab et. al.,
Another upgrade of the Pierre Auger Observatory, recently confirmed, is the equipment of the SD with radio antennas. Contrary to the existing AERA detector (Aab et al.,
Also TA has recently started to be upgraded (Kido,
The detection of ultrahigh energy cosmic rays is also included in the scientific program of GRAND, the most ambitious ground-based experiment proposed so far (Alvarez-Muniz et al.,
J. Linsley and R. Benson were the first to propose measurements of the fluorescent radiation of EAS using a UV telescope on-board a satellite (Benson and Linsley,
The original Airwatch concept, developed into the Extreme Universe Space Observatory (EUSO) (Catalano et al.,
The TUS experiment was the first orbital detector of UHECRs. It was launched on board the Moscow State University (MSU) satellite “Lomonosov” (Klimov et al.,
Another, much larger space instrument, KLYPVE, is being developed in close cooperation with the JEM-EUSO Collaboration and is known as KLYPVE-EUSO (K-EUSO) (Panasyuk et al.,
Evolution of the exposure of past, current, and upcoming (solid lines) UHECR experiments as a function of time for ground-based and space experiments. Proposed experiments are also shown (dashed lines). F. Oikonomou and M. Panasyuk for this review.
K-EUSO will measure about 140 UHECR events in the Northern hemisphere and 30 events in the Southern hemisphere at
The project concept of OWL, based on the simultaneous detection of UHECRs by UV telescopes placed on two satellites, was recently developed in the POEMMA project (Olinto et al.,
Currently the UHE neutrino flux is best confined by the IceCube Observatory (Aartsen et al.,
Predicted fluxes of cosmogenic neutrinos and expected sensitivities of current, upcoming and proposed UHECR and UHE neutrino experiments. Upper limits are from IceCube (Aartsen et al.,
Despite revolutionary progress, some critical, long-standing questions in the field of UHECRs remain unanswered, or only answered partially: What are the sources of UHECRs? What is the mass composition of UHECRs at the highest energies? What mechanism accelerates CRs beyond PeV energies? What is the flux of secondary messengers—neutrinos, gamma rays—associated with UHECRs, and what can we infer from them about UHECR sources?
Observations performed by current and planned ultrahigh-energy facilities have an opportunity to give definite answers to these questions. Yet, to fulfill this potential, it is necessary to undertake a number of essential steps toward experimental and theoretical progress. Below, we list what we believe are the most important of these. This list is, of course, non-exhaustive and only expresses our views.
More than five decades of experimental and theoretical progress in the field of UHECRs will soon be compounded on by upgrades of Auger and TA, and by a suite of potential next-generation detectors. On one hand, thanks to these, in the next 5–10 years the increased statistics of UHECRs alone will refine the measurement of the energy spectrum, mass composition, and anisotropies to the point where several of the open questions above could already be answered. Additional improvements in analysis techniques will only enhance these prospects. On the other hand, upcoming detectors will potentially trigger a transformative change in the field: for the first time, we could reach the sensitivity needed to discover even tiny fluxes of cosmogenic neutrinos and gamma rays. Opening up the full breadth of UHE multi-messenger observables could answer most of the remaining open questions, and finally, provide a complete picture of the Universe at the highest energies.
RB, JB, MB, RE, KF, K-HK, DK, GS, FO, MP, AT, and MU contributed to the original material and writing of the manuscript. FO and KF coordinated this review. All authors contributed to the discussions at MIAPP, read the manuscript and provided critical feedback.
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
We acknowledge the support of the Munich Institute for Astro- and Particle Physics (MIAPP) of the DFG cluster of excellence Origin and Structure of the Universe, where this work was initiated. We thank the organizers of the The High-Energy Universe: Gamma-Ray, Neutrino, and cosmic-ray astronomy MIAPP Program, Francis Halzen, Angela Olinto, Elisa Resconi, and Paolo Padovani, for the very fruitful workshop.
RB is supported by grant #2017/12828-4, São Paulo Research Foundation (FAPESP). MB is supported by the Danmarks Grundforskningsfond Grant 1041811001 and Villum Fonden project no. 13164. RE, K-HK, GS, and MU are supported by the Bundesministerium für Bildung und Forschung (BMBF) and the Deutsche Forschungsgemeinschaft (DFG). KF acknowledges support from the Einstein Fellowship from the NASA Hubble Fellowship Program. The work of KM is supported by Alfred P. Sloan Foundation and NSF grant No. PHY-1620777. FO is supported by the Deutsche Forschungsgemeinschaft through grant SFB 1258 Neutrinos and Dark Matter in Astro- and Particle Physics.
1One should note though that this sensitivity limit would go down by two orders of magnitude in the case of β~0.1 and near the ankle.