Ultra-High-Energy Cosmic Rays

Ultra-High-Energy Cosmic Rays Luis A. Anchordoqui Department of Physics & Astronomy, Lehman College, City University of New York, NY 10468, USA Department of Physics, Graduate Center, City University of New York, NY 10016, USA Department of Astrophysics, American Museum of Natural History, NY 10024, USA Abstract In this report we review the important progress made in recent years towards understanding the experimental data on ultra-high-energy (E & 109 GeV) cosmic rays. We begin with a general survey of the available data, including a description of the energy spectrum, the nuclear composition, and the distribution of arrival directions. At this point we also give a synopsis of experimental techniques. After that, we introduce the fundamentals of cosmic ray acceleration and energy loss during propagation, with a view of discussing the conjectured nearby sources. Next, we survey the state of the art regarding the high- and ultra-high-energy cosmic neutrinos which may be produced in association with the observed cosmic rays. These neutrinos could constitute key messengers identifying currently unknown cosmic accelerators, possibly in the distant universe, because their propagation is not influenced by background photon or magnetic fields. Subse- quently, we summarize the phenomenology of cosmic ray air showers. We describe the hadronic interaction models used to extrapolate results from collider data to ultra-high energies and the main electromagnetic processes that govern the longitudinal shower evolution. Armed with these two principal shower ingredients and motivation from the underlying physics, we describe the different methods proposed to distinguish the primary particle species. In the end, we explore how ultra-high-energy cosmic rays can be used as probes of beyond standard model physics models. Keywords: ultra-high-energy astrophysical phenomena – extensive air showers arXiv:1807.09645v3 [astro-ph.HE] 10 Jan 2019 Preprint submitted to Elsevier January 11, 2019 Contents 1 Introduction 3 2 Experimental observations and searches 5 2.1 Historical overview . .5 2.2 Measurements of the cosmic ray intensity and its nuclear composition . .8 2.3 Anisotropy searches . 13 3 Quest for the origin(s) of UHECRs 29 3.1 Acceleration processes . 29 3.1.1 Phenomenological considerations . 29 3.1.2 Unipolar induction . 30 3.1.3 Fermi acceleration at shock waves . 35 3.2 Energy loss . 45 3.2.1 Interaction rate of UHECRs on photon fields . 45 3.2.2 Opacity of the CMB to UHECR protons . 46 3.2.3 Photonuclear interactions . 50 3.3 Plausible sources in our cosmic backyard . 55 3.3.1 γAGNs......................................... 55 3.3.2 SBGs . 58 3.4 Fitting simultaneously the UHECR spectrum and its nuclear composition . 62 3.5 Impact of ν and γ-ray observations on UHECR models . 68 3.6 Grand unified spectrum of diffuse extragalactic background radiation . 73 4 Phenomenology of UHECR air showers 77 4.1 Nature’s calorimeter . 77 4.2 Systematic uncertainties in air shower measurements from hadronic interaction models . 78 4.3 Electromagnetic processes . 85 4.4 Paper-and-pencil air shower modeling . 89 5 UHECR as probes of particle physics beyond the electroweak scale 92 5.1 Testing models of the early universe via top-down production of cosmic rays and neutrinos . 92 5.2 Search for Lorentz invariant breaking effects . 96 5.3 Delve into the electroweak sector in search for new physics at subfermi distances . 99 2 6 Looking ahead 102 Acknowledgments 103 Appendix A 104 Appendix B 105 Appendix C 110 Bibliography 111 1. Introduction For biological reasons our perception of the Universe is based on the observation of photons, most trivially by staring at the night-sky with our bare eyes. Conventional astronomy covers many orders of 5 16 magnitude in photon wavelengths, from 10 cm radio-waves to 10− cm gamma-rays of TeV energy. This 70 octave span in photon frequency allows for a dramatic expansion of our observational capacity beyond the approximately one octave perceivable by the human eye. Photons are not, however, the only messengers of astrophysical processes; we can also observe baryonic cosmic rays, neutrinos, and gravitons (glaring as gravitational waves). On 2017 August 17, the Advanced LIGO and Advanced Virgo gravitational-wave detectors made their first observation of a binary neutron star merger, with subsequent identification of transient counterparts across the entire electromagnetic spectrum [1]. On 2017 September 22, the blazar TXS 0506+056 was observed simultaneously in neutrinos and photons [2]. These unprecedented observations have been hailed as the dawn of a new multi-frequency and multi-messenger era in astronomy. Baryon astronomy may be feasible for neutral particles or possibly charged particles with energies high enough to render their trajectories magnetically rigid. Indeed, on 2018 January 18, the Pierre Auger Collab- oration reported an indication of a possible correlation between nearby starburst galaxies and cosmic rays of 10:6 5 energy > 10 GeV, with an a posteriori chance probability in an isotropic cosmic ray sky of 4:2 10− , × corresponding to a 1-sided Gaussian significance of 4σ [3]. Should Mother Nature be so cooperative, the emission of extremely high energetic particles by starburst galaxies (predicted in the late ’90s [4]) would become the first statistically ironclad observation of point sources in cosmic rays. Although there may be some residual skepticism in the broader community about the extreme-energy cosmic ray-starburst connec- tion, we expect that the very new data and arguments – which we will summarize and clarify in this article – should soon dispel that skepticism. The history of cosmic ray studies has witnessed many discoveries central to the progress of high-energy physics, from the watershed identification of new elementary particles in the early days, to the confirmation of long-suspected neutrino oscillations. A major recent achievement is establishing the suppression of the spectrum at the highest energies [5–7], which may be the long-sought Greisen, Zatzepin, and Kuzmin (GZK) cutoff [8, 9]. The GZK effect is a remarkable example of the profound links between different regimes of physics, connecting as it does the behavior of the rarest, highest-energy particles in the Cosmos to the existence of Nature’s most abundant particles – the low energy photons in the relic microwave radiation of 3 the Big Bang – while simultaneously demanding the validity of Special Relativity over a mind-boggling range of scales. Ultra-high-energy (E & 109 GeV) cosmic rays (UHECRs) are the only particles with energies exceeding those available at terrestrial accelerators. These UHECRs carry about seven orders of magnitude more energy than the beam of the large hadron collider (LHC). Hence, with UHECRs one can conduct particle physics measurements in the center-of-mass (c.m.) frame up to about one order of magnitude higher than the LHC energy reach. In this review we concentrate on the physics of UHECRs focusing tightly on the interface between experiment and phenomenology. The layout is as follows. We begin in Sec. 2 with a brief summary of the most recent observations; guidance is given in the appendices to statistical formulae and significance tests referred to in the main text. In Sec. 3 we discuss the physics and astrophysics associated with the search for the UHECR origin. We first summarize the main acceleration mechanisms and then discuss the energy loss during propagation in the intergalactic space and in the source environment. Subsequently, we evaluate the multi-messenger relations connecting neutrino, gamma ray, and UHECR observations. In Sec. 4 we focus attention on the general properties and techniques for modeling extensive air showers initiated when UHECRs interact in the atmosphere. In Sec. 5 we examine how UHECRs can be used as probes of new physics beyond the highly successful but conceptually incomplete standard model (SM) of weak, electromagnetic, and strong interactions. Finally, in Sec. 6 we provide evidence-based guidance to set strict criteria for research and development of future UHECR experiments. Before proceeding, we pause to present our notation. Unless otherwise stated, we work with natural (particle physicist’s) Heaviside-Lorentz (HL) units with ~ = c = k = "0 = µ0 = 1 : (1) 2 The fine structure constant is α = e =(4π"0~c) 1=137. All SI units can then be expressed in electron Volt ' (eV), namely 6 1 15 1 35 1 m 5:1 10 eV− ; 1 s 1:5 10 eV− ; 1 kg 5:6 10 eV ; (2) ' × ' × ' × 2 2 5 1 A 1244 eV ; 1 G 1:95 10− eV ; 1 K 8:62 10− eV : (3) ' ' × ' × Electromagnetic processes in astrophysical environments are often described in terms of Gauss (G) units. 2 2 For a comparison of formulas used in the literature, we note some conversion factors: (e )HL = 4π(e )G, 2 2 2 2 (B )HL = (B )G=4π, and (E )HL = (E )G=4π. To avoid confusion we will present most of the formulas in terms of invariant quantities i.e., eB, eE and the fine-structure constant α. Distances are generally measured in Mpc 3:08 1022 m. Extreme energies are sometimes expressed in EeV 109 GeV 18 ' × ≡ 3 ≡ 10 eV. The following is a list of additional useful conversion factors: 1 GeV = 1:602 10− erg, 27 1 16 8 × h = 6:626 10− erg Hz− , hc = 1:986 10− erg cm = 1:986 10− erg A,˚ }c = 1:973 14 × 3 × 10 × 8 2 1 ×1 10− GeV cm, 1 sr = 3:283 10 sq deg = 4:255 10 sq arcsec, 1 WB = 10− GeV cm− s− sr− , × 23 2 1 ×1 and one Jansky or 1 Jy = 10− erg cm− s− Hz− . The Planck units of mass, length, and time are 1 1 19 M = `− = t− 1:2 10 GeV. We adopt the usual concordance cosmology of a flat universe Pl Pl Pl ∼ × dominated by a cosmological constant, with dark energy density parameter Ω 0:69 and a cold dark Λ ≈ matter plus baryon component Ωm 0:31 [10].

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