by Domenik Ehlert
About the Author: Domenik Ehlert is a fourth-year PhD student in the Institute of Physics at the Norwegian University of Science and Technology. He is interested in all things astroparticle physics with a particular focus on ultra-high-energy cosmic rays. In his doctoral research, he uses numerical and semi-analytical models to understand the sources of these cosmic rays, also considering multimessenger information from neutrinos and gamma rays. In his free time, Domenik likes to enjoy the outdoors, which includes hiking in the summer and skiing in the winter.
Title: On the potential cosmogenic origin of the ultra-high-energy event KM3-230213A
Authors: Antonio Condorelli, for the KM3NeT Collaboration
First Author’s Institution: Université Paris Cité, CNRS, Astroparticule et Cosmologie, F-75013 Paris, France
Status: Published in The Astrophysical Journal Letters [open access]
The KM3NeT collaboration has created a lot of excitement recently by announcing the observation of a single muon – the heavy cousin of electrons – with a reconstructed energy of around 120 PeV. This is about the same as the kinetic energy of a big raindrop in free fall, but contained in just a single elementary particle. There are good reasons to be excited about this observation! Because of the arrival direction close to the horizon and the high energy, it is very likely that this muon was produced near the detector by an astrophysical neutrino with an even higher energy of 200 PeV or more – making it the highest-energy neutrino ever detected. If you’re wondering what exactly KM3NeT even is and how it works, have a look at the infographic below.
The main paper published by the KM3NeT collaboration was already covered in a recent bite – check that out for more details! Today, we will look in more detail at one possible origin scenario of this neutrino – the cosmogenic case.
Photons, photons everywhere!
To understand the cosmogenic model (cosmos + genesis = production in the cosmos), we first have to realize that there are photon fields (a collection of photons of similar origin) absolutely everywhere in the Universe. Even the space between galaxies is not empty at all but filled by these fields, in particular the cosmic microwave background CMB and the extragalactic background light EBL, which is the sum of all the radiation in the Universe produced by stars, active galactic nuclei, etc. The CMB and EBL exist everywhere in space, but because they are relatively dim, these fields are only relevant targets for interaction when particles travel over truly astronomical distances – such as between galaxies.
The (possible) origin story of KM3-230213A
In the cosmogenic scenario, the origin of the neutrinos is tied closely to the escapades of high- and ultra-high-energy cosmic rays UHECRs during their long journey through the galactic and extragalactic environments. Cosmic rays are charged particles, mainly protons and nuclei with a mass between hydrogen and iron, that are accelerated to kinetic energies way beyond their rest-mass energy by electromagnetic processes in some yet-to-be-discovered sources in the Universe.
Every now and then, when a cosmic ray gets unlucky (or lucky, depending on your view), it encounters a photon from the CMB or EBL on its path. When the combined energy of both particles is high enough, this chance meeting can lead to an interaction in which the cosmic ray loses part of its energy and new particles are produced. Sometimes this happens to be a charged pion which, after some time, decays into more fundamental particles — among them neutrinos! As a result, the expected fluxes of high-energy neutrinos and UHECRs are inextricably linked. Since we know that cosmic rays are out there because we observe them here on Earth, there should be a guaranteed flux of cosmogenic neutrinos. The problem is that the exact amount depends a lot on the details of the cosmic-ray flux, which have huge uncertainties. In general, there will be more cosmogenic neutrinos if most UHECRs are protons, because these produce more charged pions during their propagation and if cosmic- ray sources were more common or powerful in the Universe at earlier times (i.e., at higher redshifts).
These days, observations of UHECRs suggest that their composition must be a mix of different nuclei, getting progressively heavier toward the highest energies. This was discovered through observations of UHECRs interacting in the atmosphere, which lead to the production of “showers” of secondary particles. These showers start developing at higher altitudes the heavier the initial cosmic ray. If UHECRs are mostly heavy nucleiIn this case, the expected flux of cosmogenic neutrinos is relatively small. While this is compatible with current upper limits on the UHE neutrino flux by IceCube, it is difficult to use the cosmogenic model to explain the high neutrino flux derived from the KM3NeT event.
Boosting the neutrino flux
As pointed out in today’s paper, the observed UHECRs should be produced mostly by nearby sources because their interactions when travelling through space would cause them to decay into lower energy particles. That is why those observations cannot really be used to constrain more distant (z>1) sources. It just may be that there really are way more (or more powerful) cosmic-ray accelerators hiding in the distant reaches of the Universe. Indeed, for some astrophysical objects, like active galactic nuclei or gamma-ray bursts, this has been observed to be the case, although above a certain redshift, this trend tends to stop or reverse.
As it turns out, if you consider possible cosmic-ray sources up to z = 6, and assume that sources were much more abundant or powerful back then, the expected flux of cosmogenic neutrinos is enhanced by about a factor of ten (see Fig. 2). Although the expected flux is not quite high enough to comfortably explain the observed KM3NeT neutrino, even when assuming very strong evolution of the emissivity of the sources as function of redshift, the remaining tension is now well below 3 sigma. However, this assumes a steep, continuous increase of the emissivity as a function of redshift. For more reasonable evolutions, perhaps similar to the star formation rate evolution, the increase flattens out somewhere between z = 1-2. In this case, the expected flux of cosmogenic neutrinos will be much lower. In short, the redshift evolution of cosmic-ray sources can have a huge impact on the expected flux of cosmogenic neutrinos.
Building on this, the authors also show that there can be a ton of extra neutrinos when adding a second population of cosmic-ray sources, which accelerate protons up to the highest observed energies; see Fig 3. This especially boosts the neutrino flux at the highest energies (above 100 PeV) since the cosmogenic neutrinos receive about 5% of the original cosmic-ray energy when they are produced by the decay of secondary pions originating from the interactions of UHE protons. At the highest energies those 5% can amount to many tens to hundreds of exa-electronvolt.
Curiously, both IceCube and the Pierre Auger Observatory, which have been taking data for much longer than KM3NeT, haven’t really found any significant evidence for such UHE neutrinos. Given the known exposure of those two observatories, the absence of detections allows them to place upper limits on the time-averaged flux of UHE neutrinos (see Figs. 2 & 3). Crucially, the neutrino flux predicted in today’s paper is still below the existing limits, even for the most extreme scenarios… This is important, since a model should always be consistent with all the available data and limits. In that sense, if KM3-230213A is interpreted to be of cosmogenic origin it likely represents a fortunate upward fluctuation of a neutrino flux that is much lower when averaged over a longer time.
Astrobite edited by Samantha Wong
Featured image credit: KM3NeT
