What is the Nature and Origin of the Highest-Energy Particles in the Universe? ASTRO 2020 SCIENCE WHITE PAPER w FRED SARAZIN*, Colorado School of Mines; LUIS ANCHORDOQUI†, City University of New York; JAMES BEATTY, Ohio State University; DOUGLAS BERGMAN, University of Utah; CORBIN COVAULT, Case Western Reserve University; GLENNYS FARRAR, New York University; JOHN KRIZMANIC, University of Maryland–Baltimore County; DAVID NITZ, Michigan Technological University; ANGELA OLINTO, University of Chicago; PETER TINYAKOV, Université libre de Bruxelles; MICHAEL UNGER, Karlsruhe Institute of Technology; LAWRENCE WIENCKE, Colorado School of Mines *[email protected] †[email protected] COSMOLOGY AND FUNDAMENTAL PHYSICS · MULTI-MESSENGER ASTRONOMY AND ASTROPHYSICS 23 1 The big questions and goals for the next decade 24 Ultra-High-Energy Cosmic-Ray (UHECR) astronomy in the next decade aims to answer the ques- 25 tions: What is the nature and origin of UHECRs? How are UHECRs accelerated to such extreme 26 energies? Are there multiple types of sources and acceleration mechanisms? Do UHECRs consist 27 of both protons and heavier nuclei, and how does the composition evolve as a function of energy? 28 In order to address these questions, the goals for the next decade will be to: identify one or more 29 nearby UHECR sources, refine the spectrum and composition of the highest-energy Galactic and 30 extragalactic cosmic-rays, exploit extensive air showers (EAS) to probe particle physics inaccessi- 31 ble at accelerators, and develop the techniques to make charged-particle astronomy a reality. 32 This agenda is largely achievable in the next decade, thanks to major experimental upgrades un- 33 derway and new ground observatories and space missions in development. In combination with 34 improved astrophysical neutrino statistics and resolution, the future UHECR observatories will 35 enable a powerful multi-messenger approach to uncover and disentangle the common sources of 36 UHECRs and neutrinos. 37 2 The UHECR paradigm shift 38 Synopsis: Results from the current large hybrid detectors have dispelled the pre-existing simple 39 UHECR picture. A new paradigm is emerging and needs to be clarified and understood. 40 The discoveries made over the past decade have transformed our understanding of UHECRs and 41 their sources. Prior to the development of very large, hybrid detectors, it was commonly believed 42 that UHECRs were protons, that a spectral cutoff (if indeed there was one!) should be due to 43 “GZK” energy losses on the cosmic microwave background [1,2], that the Galactic-extragalactic 44 transition would be marked by a shift from Galactic iron to extragalactic protons, and that the ankle 45 feature at a few EeV marked the Galactic-extragalactic transition. We know now that this simple 46 picture is mostly, if not entirely, wrong. spp (TeV) 50 100 500 47 This revolution in our understanding was achieved 103 48 thanks to (i) hybrid detectors with air-fluorescence 49 telescopes and surface detectors, plus improved ] -1 50 measurement of fluorescence yields, giving much sr 2 -1 10 s 51 -2 better energy calibration and providing greater sen- m 1.6 F(E) 52 sitivity to composition; (ii) large aperture and high 53 statistics, essential to reducing the systematics and [GeV 10 54 particle-physics uncertainties in composition stud- 2.6 E Telescope Array 55 ies and providing sensitivity to tiny anisotropies in Auger 56 the UHECRs arrival directions; and (iii) all-sky sen- 1 57 sitivity thanks to detectors in both hemispheres, with 1018 1019 1020 [eV]E 58 overlapping regions of the sky. Together, these Figure 1: UHECR spectra measured by TA and 59 advances enabled the spectrum, composition and Auger, located in the northern and southern hemi- 60 anisotropies to be measured with higher resolution 17 20 spheres, respectively (adapted from [3]). 61 and smaller systematics from 10 to above 10 eV. 62 UHECR spectrum – ankle and flux suppression: well established but not well explained 63 As shown in Fig.1, the UHECR energy spectrum can be roughly described by a twice-broken 64 power law [4–8]. The first break is a hardening of the spectrum, known as “the ankle.” The sec- 2 65 ond, an abrupt softening of the spectrum, may be interpreted as the long-sought GZK cutoff [1,2], 66 or else may correspond to the “end-of-steam” for cosmic accelerators [9]. The differential en- 67 ergy spectra measured by the Telescope Array (TA) experiment and the Pierre Auger Observatory 19 68 (Auger) agree within systematic errors below 10 eV. However, even after energy re-scaling, a 69 large difference remains at and beyond the flux suppression [10] and could be hinting at fundamen- 70 tal differences between the northern and southern UHECR skies, since the spectra seem to agree 71 at all energies in the common region of the sky [11, 12]; see however [13]. 72 UHECR primary composition – a more complex picture emerges 73 The atmospheric column depth at which the longitudinal development of a cosmic-ray shower 74 reaches maximum (Xmax) is a powerful observable to determine the UHECR nuclear composition. 75 The Xmax measurements of both TA [14, 15] and Auger [16–19] indicate a predominantly light 76 composition at around the ankle. At the highest energies (above 10 EeV), the Auger Collaboration 77 reported a significant slowdown of the growth of average shower maximum with energy as well as 78 a decrease of the shower-to-shower fluctuations of Xmax with energy. Both observations suggest 79 a gradual increase of the average mass of cosmic rays with energy and the interpretation of the 80 data with LHC-tuned hadronic interaction models gives a mean baryon number A ≈ 14 − 20 at 19:5 81 E ≈ 10 eV. The joint working group on composition concluded that the measurements of the 82 average shower maximum by TA and Auger are compatible within experimental uncertainties at 83 all energies [20, 21]. The observed decrease of the standard deviation of the Xmax distributions 84 reported by Auger can currently neither be confirmed nor ruled out by TA because of statistical 85 limitations. Thus, the most recent data of UHE observatories revealed a complex evolution of the 86 cosmic-ray composition with energy that challenges previous simple models of CR sources. 87 UHECR Anisotropy – where are the sources? 8988 Composition measurements have led to a 90 paradigm shift, with cosmic rays now un- 91 derstood to be light (proton dominated) near 18 92 10 eV and evolving towards heavier com- 93 position with increasing energy, spanning a 94 narrow range of atomic masses at each en- 95 ergy. Below the ankle, the arrival direc- Figure 2: Sky map, in equatorial coordinates, of local over- 96 tions are highly isotropic [23], arguing that and under-densities in units of standard deviations of UHECRs 97 these protons must be of extra-galactic ori- above 47 ± 7 EeV. Taken from [28]. 98 gin. They are consistent with being secondary products of the photo-disintegration of UHECR 99 nuclei in the environment of their sources [24]. At higher energies, Galactic and extragalactic de- 100 flections of UHECR nuclei are expected to smear point sources into warm/hot spots, for which 101 evidence is accumulating. TA has recorded an excess above the isotropic background-only ex- 19:75 102 pectation in cosmic rays with energies above 10 eV [25, 26], while Auger has reported a 103 possible correlation with nearby starburst galaxies, with a (post-trial) 4σ significance, for events 19:6 104 above 10 eV [27]. A slightly weaker association (2:7σ) with active galactic nuclei emitting 19:78 105 γ-rays is also found in Auger events above 10 eV [27]. A blind search for anisotropies com- 106 bining Auger and TA data has been recently carried out, with the energy scales equalized by the ◦ 107 flux in the common declination band [28]. The most-significant excess is obtained for a 20 search 108 radius, with a global (post-trial) significance of 2:2σ. The local (Li-Ma [29]) significance map of 109 this study is shown in Fig.2. The tantalizing visual correlation of high-significance regions with 110 the supergalactic plane is currently under study within the Auger/TA anisotropy working group. 3 111 UHECR neutrino – the missing GZK neutrinos and targets of opportunity 16 112 The non-observation of neutrino candidate events beyond background expectations above 10 eV 113 by IceCube [30], Auger [31], and ANITA [32] severely constrains the magnitude of the very high- 114 energy neutrino flux. This flux has a nearly guaranteed component from the decays of pions pro- 115 duced by UHECR protons interacting en route to Earth [33]. The accumulation of these neutrinos 116 over cosmological time, known as the cosmogenic neutrino flux, constitutes a powerful tool of the 117 multi-messenger program. IceCube, Auger, and ANITA limits already challenge models in which 118 the highest energy UHECRs are proton-dominated [34, 35]. Additionally, UHECR experiments 119 add the capability of searching for neutrinos from target-of-opportunity events [36–38]. 120 Closing the loop – Particle physics with UHECRs 121 Essential to accurate composition determination is the correct understanding of the physics of 122 UHE air showers, which requires accurate modeling of particle physics at center-of-mass ener- 123 gies up to hundreds of TeV – far beyond the 14 TeV reach of the Large Hadron Collider (LHC). 124 Internal-consistency studies of UHE showers show that state-of-the-art LHC-tuned hadronic event 125 generators do not correctly reproduce in detail the multitude of observables that can be probed 126 by UHECR detectors [39, 40]. However with the even more powerful observables which will be 127 provided by the upgraded and next-generation detectors, and the continuing intensive efforts of the 128 particle physics community to improve the models, there is every reason to believe that UHECR 129 air showers will become a potent tool to extend our understanding of hadronic interactions well 130 into the hundreds of TeV regime.