Ultrahigh-Energy Event KM3-230213A within the Global Neutrino Landscape

On February 13th, 2023, the KM3NeT/ARCA telescope detected a neutrino candidate with an estimated energy in the hundreds of PeV. In this article, the observation of this ultrahigh-energy neutrino is discussed in light of null observations above tens of PeV from the IceCube and Pierre Auger observatories. Performing a joint fit of all experiments under the assumption of an isotropic <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:msup><a:mi>E</a:mi><a:mrow><a:mo>−</a:mo><a:mn>2</a:mn></a:mrow></a:msup></a:math> flux, the best-fit single-flavor flux normalization is <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"><c:mrow><c:msup><c:mrow><c:mi>E</c:mi></c:mrow><c:mrow><c:mn>2</c:mn></c:mrow></c:msup><c:msubsup><c:mrow><c:mi mathvariant="normal">Φ</c:mi></c:mrow><c:mrow><c:mi>ν</c:mi><c:mo>+</c:mo><c:mover accent="true"><c:mrow><c:mi>ν</c:mi></c:mrow><c:mrow><c:mo stretchy="false">¯</c:mo></c:mrow></c:mover></c:mrow><c:mrow><c:mn>1</c:mn><c:mi mathvariant="normal">f</c:mi></c:mrow></c:msubsup><c:mo>=</c:mo><c:mn>7.5</c:mn><c:mo>×</c:mo><c:msup><c:mrow><c:mn>10</c:mn></c:mrow><c:mrow><c:mo>−</c:mo><c:mn>10</c:mn></c:mrow></c:msup><c:mtext> </c:mtext><c:mtext> </c:mtext><c:mi>GeV</c:mi><c:mtext> </c:mtext><c:msup><c:mrow><c:mi>cm</c:mi></c:mrow><c:mrow><c:mo>−</c:mo><c:mn>2</c:mn></c:mrow></c:msup><c:mtext> </c:mtext><c:msup><c:mrow><c:mi mathvariant="normal">s</c:mi></c:mrow><c:mrow><c:mo>−</c:mo><c:mn>1</c:mn></c:mrow></c:msup><c:mtext> </c:mtext><c:msup><c:mrow><c:mi>sr</c:mi></c:mrow><c:mrow><c:mo>−</c:mo><c:mn>1</c:mn></c:mrow></c:msup></c:mrow></c:math> in the 90% energy range of the KM3NeT event. Furthermore, the ultrahigh-energy data are then fit together with the IceCube measurements at lower energies, either with a single power law or with a broken power law, allowing for the presence of a new component in the spectrum. A slight preference for a break in the PeV regime is found for one of the three investigated IceCube samples and no such preference for the other two. In all cases, the observed tension between KM3NeT and other datasets is mild to moderate (<j:math xmlns:j="http://www.w3.org/1998/Math/MathML" display="inline"><j:mrow><j:mn>1.6</j:mn><j:mi>σ</j:mi><j:mi>–</j:mi><j:mn>2.9</j:mn><j:mi>σ</j:mi></j:mrow></j:math>), and increased statistics are required to resolve this apparent tension and better characterize the neutrino landscape at ultrahigh energies.