IceCube: Light at the end of the tunnel?
26 September 2013
Ernie, high-energy neutrino observed with IceCube (picture: IceCube Coll.)
In one of the episodes of the “Sesame Street”, the protagonists Ernie and Bert debate on Ernie’s frozen ice cubes which miraculously had disappeared. The story makers could not have guessed that some forty years later, members of a 270M dollar endeavour called IceCube would debate on the miraculous appearance of two events, and that these events would be nicknamed “Ernie” and “Bert” by some imaginative PhD students. Moreover, it would have gone beyond the imagination of the Sesame Street people that these two events would be interpreted as a first hint to something what scientists call “high-energy extraterrestrial neutrinos”.
IceCube is a cubic kilometre neutrino telescope installed in the deep Antarctic ice at the South Pole. The detector consists of 86 strings each equipped with 60 light sensors. The sensors record the faint light emitted by relativistic charged particles – including those which have been generated in neutrino interactions. IceCube construction was started in December 2004 and completed in December 2010, but data have been taken already before completion, year by year, with the strings already installed.
The ultimate goal of IceCube is to identify individual sources of extraterrestrial energetic neutrinos. These are neutrinos which have not been generated in cosmic-ray collisions in the Earth’s atmosphere (IceCube has recorded more than 105 of such “atmospheric neutrinos”), but which have reached us directly from distant cosmic objects like supernova remnants or active galactic nuclei. A neutrino signal would provide a watertight proof that the corresponding object is also a source of charged cosmic rays. Although IceCube has improved the sensitivity to energetic neutrinos by a factor 1000 over the last twelve years, it could not yet pinpoint any individual source. Neutrinos from such “point-like” sources would appear as an accumulation of events from a certain direction of the sky. Instead, the IceCube scientists collected growing evidence, that there is a tiny excess of events at the very highest energies which seem to arrive as a diffuse flux from many sources, or even from all directions.
High-energy tails in the energy spectrum had been observed repeatedly in analyses of data taken with the 40-string and 59-string configurations of IceCube. These deviations exceeded the predicted spectrum for atmospheric neutrinos by roughly two standard deviations (~2s). The first step clearly beyond 2s was made with an analysis of data taken in 2010 and 2011 with the 79-string and 86-string configurations. This analysis focused to energies larger than ~500 TeV and provided two events with reconstructed energies of 1.04 and 1.14 PeV (1 PeV = 103 TeV = 106 GeV): “Ernie” and “Bert” (see the figure). Ernie and Bert represent a 2.8s excess over the expectation for atmospheric neutrinos 1.
Motivated by this result, an alternative analysis of the same data was performed. It constrains the event to start in the inner volume of IceCube (using the outer part as veto layer), and at the same time considerably lowers the threshold compared to the first analysis (down to some tens of TeV).
Bert, high-energy neutrino observed with IceCube (picture: IceCube Coll.)
Results of this analysis have been firstly presented at May 14, 2013 at a conference in Madison/USA. It provides 28 events with energies deposited in the detector ranging from ~30 TeV to 1.14 PeV. “Ernie” and “Bert” stoically defend their top position in energy. Notably also the events at somewhat lower energies (~30 TeV – ~250 TeV) can hardly be explained alone by atmospheric neutrinos or by muons sneaking unrecognized from above into the detector. The calculated contribution of such “trivial” sources to the total of 28 events is estimated to only 12 events 2.
Does that mean that extraterrestrial high-energy neutrinos have incontrovertibly been detected? Certainly not! With a statistical significance of 4.1s, the excess has not yet reached the magical benchmark of 5s. Moreover one cannot exclude that estimates of the background from atmospheric neutrinos are still somewhat too low. This particularly applies to the so-called prompt neutrinos which emerge from the decay of short-lived charm particles high in the atmosphere. Such an underestimate would lead to an overestimation of the extraterrestrial contribution. However, the IceCube collaboration will have analyzed about twice the amount of data in autumn. The doubling of the statistics will go hand in hand with steadily improving understanding of systematic effects.
Summing up: we see light at the end of the tunnel – 40 years after the first detector of this kind was conceived3)! We are reluctant to open the bottles right now, but the Champagne is already in the fridge. With some luck we may pass the 5s benchmark within a few months…
1) M. Aartsen et al. (IceCube Coll.), First observation of PeV-energy neutrinos with IceCube, accepted for publication in PRL, arXiv:1304.5356
2) M. Aartsen et al. (IceCube Coll.), Evidence for High-Energy Extraterrestrial Neutrinos at the IceCube Detector, paper submitted for publication.
3) C. Spiering: Towards High-Energy Neutrino Astronomy. A Historical Review, EPJ-H 37 (2012) 515, arXiv:1207.4952