The observation of electromagnetic radiation from radio to $\gamma$-ray wavelengths has provided a wealth of information about the universe. However, at PeV (10$^{15}$ eV) energies and above, most of the universe is impenetrable to photons. New messengers, namely cosmic neutrinos, are needed to explore the most extreme environments of the universe where black holes, neutron stars, and stellar explosions transform gravitational energy into non-thermal cosmic rays. The discovery of cosmic neutrinos with IceCube has opened this new window on the universe. In this white paper, we present an overview of a next-generation instrument, IceCube-Gen2, which will sharpen our understanding of the processes and environments that govern the universe at the highest energies. IceCube-Gen2 is designed to: 1) Resolve the high-energy neutrino sky from TeV to EeV energies; 2) Investigate cosmic particle acceleration through multi-messenger observations; 3) Reveal the sources and propagation of the highest energy particles in the universe; 4) Probe fundamental physics with high-energy neutrinos. IceCube-Gen2 will increase the annual rate of observed cosmic neutrinos by a factor of ten compared to IceCube, and will be able to detect sources five times fainter than its predecessor. Furthermore, through the addition of a radio array, IceCube-Gen2 will extend the energy range by several orders of magnitude compared to IceCube. Construction will take 8 years and cost about \$350M. The goal is to have IceCube-Gen2 fully operational by 2033. IceCube-Gen2 will play an essential role in shaping the new era of multi-messenger astronomy, fundamentally advancing our knowledge of the high-energy universe. This challenging mission can be fully addressed only in concert with the new survey instruments across the electromagnetic spectrum and gravitational wave detectors which will be available in the coming years.

IceCube-Gen2: The Window to the Extreme Universe

The observation of electromagnetic radiation from radio to γ-ray wavelengths has provided a wealth of information about the Universe. However, at PeV (1015 eV) energies and above, most of the Universe is impenetrable to photons. New messengers, namely cosmic neutrinos, are needed to explore the most extreme environments of the Universe where black holes, neutron stars, and stellar explosions transform gravitational energy into non-thermal cosmic rays. These energetic particles havemillions of times higher energies than those produced in the most powerful particle accelerators on Earth. As neutrinos can escape from regions otherwise opaque to radiation, they allow an unique view deep into exploding stars and the vicinity of the event horizons of black holes. The discovery of cosmic neutrinos with IceCube has opened this new window on the Universe. IceCube has been successful in finding first evidence for cosmic particle acceleration in the jet of an active galactic nucleus. Yet, ultimately, its sensitivity is too limited to detect even the brightest neutrino sources with high significance, or to detect populations of less luminous sources. In thiswhite paper, we present an overview of a next-generation instrument, IceCube-Gen2, which will sharpen our understanding of the processes and environments that govern the Universe at the highest energies. IceCube-Gen2 is designed to: (a) Resolve the high-energy neutrino sky from TeV to EeV energies (b) Investigate cosmic particle acceleration through multi-messenger observations (c) Reveal the sources and propagation of the highest energy particles in the Universe (d) Probe fundamental physics with high-energy neutrinos IceCube-Gen2 will enhance the existing IceCube detector at the South Pole. It will increase the annual rate of observed cosmic neutrinos by a factor of ten compared to IceCube, and will be able to detect sources five times fainter than its predecessor. Furthermore, through the addition of a radio array, IceCube- Gen2 will extend the energy range by several orders of magnitude compared to IceCube. Construction will take 8 years and cost about $350M. The goal is to have IceCube-Gen2 fully operational by 2033. IceCube-Gen2 will play an essential role in shaping the new era of multimessenger astronomy, fundamentally advancing our knowledge of the highenergy Universe. This challenging mission can be fully addressed only through the combination of the information from the neutrino, electromagnetic, and gravitational wave emission of high-energy sources, in concert with the new survey instruments across the electromagnetic spectrum and gravitational wave detectors which will be available in the coming years.

IceCube-Gen2: the window to the extreme Universe

We compute the absorption cross section and the total power carried by gravitons in the evaporation process of a higher-dimensional non-rotating black hole. These results are applied to a model of extra dimensions with standard model fields propagating on a brane. The emission of gravitons in the bulk is highly enhanced as the spacetime dimensionality increases. The implications for the detection of black holes in particle colliders and ultrahigh-energy cosmic ray air showers are briefly discussed.

Hawking emission of gravitons in higher dimensions: non-rotating black holes

Neutrinos are key to probing the deep structure of matter and the high-energy Universe. Yet, until recently, their interactions had only been measured at laboratory energies up to about 350 GeV. An opportunity to measure their interactions at higher energies opened up with the detection of high-energy neutrinos in IceCube, partially of astrophysical origin. Scattering off matter inside Earth affects the distribution of their arrival directions---from this, we extract the neutrino-nucleon cross section at energies from 18 TeV to 2 PeV, in four energy bins, in spite of uncertainties in the neutrino flux. Using six years of public IceCube High-Energy Starting Events, we explicitly show for the first time that the energy dependence of the cross section above 18 TeV agrees with the predicted softer-than-linear dependence, and reaffirm the absence of new physics that would make the cross section rise sharply, up to a center-of-mass energy $\sqrt{s}\ensuremath{\approx}1\text{ }\text{ }\mathrm{TeV}$.

Extracting the Energy-Dependent Neutrino-Nucleon Cross Section above 10 TeV Using IceCube Showers.

Cosmogenic neutrinos reach the Earth with energies around ${10}^{9}\text{ }\text{ }\mathrm{GeV}$, and their interactions with matter will be measured in upcoming experiments (Auger, IceCube). Models with extra dimensions and the fundamental scale at the TeV could imply signals in these experiments. In particular, the production of microscopic black holes by cosmogenic neutrinos has been extensively studied in the literature. Here we make a complete analysis of gravity-mediated interactions at larger distances, where they can be calculated in the eikonal approximation. In these processes a neutrino of energy ${E}_{\ensuremath{\nu}}$ interacts elastically with a parton inside a nucleon, loses a small fraction $y$ of its energy, and starts a hadronic shower of energy $y{E}_{\ensuremath{\nu}}\ensuremath{\ll}{E}_{\ensuremath{\nu}}$. We analyze the ultraviolet dependence and the relevance of graviton emission in these processes, and show that they are negligible. We also study the energy distribution of cosmogenic events in AMANDA and IceCube and the possibility of multiple-bang events. For any neutrino flux, the observation of an enhanced rate of neutral current events above 100 TeV in neutrino telescopes could be explained by TeV-gravity interactions. The values of the fundamental scale of gravity that IceCube could reach are comparable to those to be explored at the LHC.

TeV gravity at neutrino telescopes

The observation of lepton flavor violating processes at colliders could be a clear signal of a nonminimal neutrino sector. We define a 5-parameter model with a pair of TeV fermion singlets and arbitrary mixings with the three active neutrino flavors. Then we analyze several flavor violating transitions (l→l′γ, l′l′′l¯′′′, or μ-e conversions in nuclei) and Z→l¯l′ decays induced by the presence of heavy neutrinos. In particular, we calculate all the one-loop contributions to these processes and present their analytic expressions. We focus on the genuine effects of the heavy Majorana masses, comparing the results in that case with the ones obtained when the two heavy neutrinos define a Dirac field. Finally, we use our results to update the bounds on the heavy-light mixings in the neutrino sector.

Effects of heavy Majorana neutrinos on lepton flavor violating processes

The existence of extra dimensions allows the possibility that the fundamental scale of gravity is at the TeV. If that is the case, gravity could dominate the interactions of ultrahigh energy cosmic rays. In particular, the production of microscopic black holes by cosmogenic neutrinos has been estimated in a number of papers. We consider here gravity-mediated interactions at larger distances, where they can be calculated in the eikonal approximation. We show that for the expected flux of cosmogenic neutrinos these elastic processes give a stronger signal than black hole production in neutrino telescopes. Taking the bounds on the higher-dimensional Planck mass M(D) (D=4 + n) from current air shower experiments, for n=2(6) elastic collisions could produce up to 118 (34) events per year at IceCube. On the other hand, the absence of any signal would imply a bound of M(D) > or approximately 5 TeV.

Cosmogenic neutrinos and signals of TeV gravity in air showers and neutrino telescopes.

Cosmic rays from outer space enter the atmosphere with energies of up to ${10}^{11}\text{ }\text{ }\mathrm{GeV}$. The initial particle or a secondary hadron inside the shower may then interact with an air nucleon to produce nonstandard particles. In this article we study the production of new physics by high energy cosmic rays, focusing on the long-lived gluino of split-SUSY models and a weakly interacting massive particle (WIMP) working as dark matter. We first deduce the total flux of hadron events at any depth in the atmosphere, showing that secondary hadrons can not be neglected. Then we use these results to find the flux of gluinos and WIMPs that reach the ground after being produced inside air showers. We also evaluate the probability of producing these exotic particles in a single proton shower of ultrahigh energy. Finally we discuss the possible signal in current and projected experiments. While the tiny flux of WIMPs does not seem to have any phenomenological consequences, we show that the gluinos could modify substantially the profile of a small fraction of extensive air showers. In particular, they could produce a distinct signal observable at AUGER in showers of large zenith angle.

New physics from ultrahigh energy cosmic rays

The presence of massive sterile neutrinos $N$ mixed with the active ones induces flavor violating processes in the charged lepton sector at the loop level. In particular, the amplitude of ${H}^{0}\ensuremath{\rightarrow}{\overline{\ensuremath{\ell}}}_{i}{\ensuremath{\ell}}_{j}$ is expected to be proportional to the product of heavy-light Yukawa couplings ${y}_{i}{y}_{j}=2{s}_{{\ensuremath{\nu}}_{i}}{s}_{{\ensuremath{\nu}}_{j}}{m}_{N}^{2}/{v}^{2}$, where ${s}_{{\ensuremath{\nu}}_{i,j}}$ express the heavy-light neutrino mixings. Here, we revisit these Higgs decays in the most generic extension of the neutrino sector, focusing on large values of ${y}_{i}$. We show that decoupling effects and a cancellation between the two dominant contributions to these processes makes the amplitude about 100 times smaller than anticipated. We find that perturbative values of ${y}_{i}$ giving an acceptable contribution to the $\ensuremath{\rho}$ parameter imply $\mathcal{B}({H}^{0}\ensuremath{\rightarrow}{\overline{\ensuremath{\ell}}}_{i}{\ensuremath{\ell}}_{j})l{10}^{\ensuremath{-}8}$ for any lepton flavors, a rate that is not accessible at current colliders.

J. I. Illana

Papers

TeV gravity at neutrino telescopes

Effects of heavy Majorana neutrinos on lepton flavor violating processes

Cosmogenic neutrinos and signals of TeV gravity in air showers and neutrino telescopes.

New physics from ultrahigh energy cosmic rays

ρ parameter and H 0 → ℓ i ℓ j in models with TeV sterile neutrinos