Highlights from the 7 year High Energy Starting Event sample in Icecube

TL;DR

This paper summarizes seven years of IceCube's High Energy Starting Events, emphasizing new physics searches with astrophysical neutrino flavor data that probe energy scales up to the Planck level.

Contribution

It presents the longest IceCube HESE dataset and introduces a novel physics search reaching the Planck scale using neutrino flavor data.

Findings

Seven years of HESE data collected

First physics search reaching the Planck scale

Insights into astrophysical neutrino flavor composition

Abstract

Here we outline the main highlights from the 7 year High Energy Starting Events (HESE) event sample. The next new physics search using astrophysical neutrino flavor data is described, where we reach the Planck scale for the first time.

Full text

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1 High Evergy Starting Events (HESE) in Icecube

The HESE data sample in IceCube [1] is selected to extract a sample of astrophysical neutrinos with high purity [2, 3]. Events are accepted into the sample if the interaction vertex is contained in a subvolume of IceCube, defined by the inner part of the detector as the outer most layers are used as a veto region to reject atmospheric backgrounds [4]. We find 102 events observed over 2635 days with 60 events above 60 TeV in deposited energy. HESE is a low atmospheric background event selection used in IceCube to study astrophysical neutrinos, including dark matter (DM) searches and anomalous spacetime effects through the astrophysical neutrino flavour composition.

Key changes in the event reconstruction include global changes to the ice model. This includes both ice anistropy and tilt effects. HESE 7 is consistent with a single power law fit with spectral index of $\gamma\sim 2.9$. Systematic uncertainties include contributions from atmospheric neutrino fluxes as well as detector systematics. We show the energy and angular distributions in Fig. 2.

The preliminary best fit for the flavour composition of diffuse neutrinos is $(0.29:0.50:0.21)$, where the current flavor contour is consistent with $(1:1:1)$. Note zero tau events cannot be ruled out with best fit $E^{-2.9}$ spectrum. Contours are computed with Wilk’s theorem, and work in this avenue is ongoing. A BSM search using the Standard Model Extension [5] was performed, attempting to detect anomalous flavor ratios. Such ratios could arise due to the presence of effective operators at high energy scales. Three source flavour compositions of the form ($f_{e}:f_{\mu}:f_{\tau}$) are believed to dominate, namely $(1:2:0)$, $(1:0:0)$ and $(0:1:0)$. We expect to place the most stringent limits on higher order new physics operators. Furthermore, two tau candidate neutrinos have been observed in HESE, with double cascade energies $E\sim 100~{}\textup{TeV}$ and $1.8~{}\textup{PeV}$. This corresponds to the first astrophysical tau neutrino candidates in IceCube. Current work is ongoing to quantify their significance. In conclusion, our first ever search of Lorentz violation (LV) was done using astrophysical neutrinos. Most notably, our limit resides several orders of magnitude below the Planck scale. More work is needed to constrain all source flavor paradigms. Our technique reaches the quantum gravity regime for the first time. Future searches will be able to probe the region of flavor space currently inaccessible with 7 years of HESE data.