This weeks issue of Science has a the IceCube paper
that we’ve all been waiting for: Evidence for high-energy extra-terrestrial neutrinos. The paper describes a follow-on analysis to
Bert and Ernie (our two 1-PeV neutrinos. The analysis was designed to find more
events like Bert and Ernie. It did
not. It did, however, find 28 events
that appeared to come from interactions within the detector, with no evidence
of an incoming muon track as expected from downward-going cosmic ray muons. One
of the events even made the cover of Science. Unfortunately, Science requires a subscription, but we will release freely-available version of the paper this afternoon; I'll post the URL when it comes out.
The
thing that makes this analysis so successful is that it brought together
multiple techniques to reject most background and estimate the reset, leading
to a convincing detection of a 4-sigma excess of events above the background
level expected from atmospheric neutrinos.
The first technique has been around since the first IceCube cascade
analysis: using the edges of the detector for a veto, with a smaller fiducial
(active) volume in the center. This
eliminates most background from downward-going muons entering the
detector. These downward-going muons
outnumber the neutrinos by 500,000 to 1, and estimating the fraction that
sneaks through the veto region is tricky, requiring voluminous simulations. The new
analysis uses a data-driven estimate instead.
The estimate uses two independent nested veto regions surrounding a
smaller fiducial volume. It counts events
tagged in the outer veto which miss the inner veto to determine the veto miss
fraction.
The other background is atmospheric neutrinos. These are, on average, less energetic than
the extra-terrestrial events. The new
analysis considers the expected energy spectrum, but it adds a new handle. Energetic downward-going atmospheric neutrinos
should be accompanied by a cosmic-ray muons which may trigger the veto
mentioned above, so they are less likely to pass the final event
selection. The new study is the first
one to search for downward-going cascades.
This atmospheric neutrino ‘self-veto’
probability is included in the background estimates. The background estimates also took advantage
of the latest IceCube measurements of the atmospheric neutrino rates.
In total, our best estimate of the background was 12.1
events (including 1.5 ‘prompt’ atmospheric neutrinos from the decay of charmed
particles), giving a significance as an extra-terrestrial signal ‘at the
4-sigma level.’ Of course, there are
some caveats, but this looks like a fairly robust detection, especially with
Bert and Ernie.
The energy spectrum of the events is shown in the figure above (the points with errors). The blue
histogram shows the atmospheric neutrino background, while the magenta and
green lines include two estimates of the prompt atmospheric neutrinos; the
shading shows the uncertainty. The red
shows the remaining downward-going muon background, while the grey line
includes these backgrounds, plus an assumed astrophysical component. The extra-terrestrial signal is significant
starting at energies above 60 TeV. The
absence of events at energies much above 1 PeV is significant, indicating that
the spectrum is cut off at very high energies (between 2 and 10 PeV); this may
be a clue about the accelerators that produced the neutrinos.
The apparent flux of extra-terrestrial neutrinos is toward the high end of current theoretical estimates, near the Waxman-Bahcall (WB) bound. The WB bound is a calculation based on the measured cosmic-ray flux, assuming that, when these particles are accelerated, they interact with background gas or photons (light) in the accelerator, producing particles (pions) which decay, producing the neutrinos that IceCube observes. Further studies, with more data, should give us clues which will help us located these accelerators.
For comparison, the only other observations of
extra-terrestrial neutrinos have been from our Sun (created by the nuclear
fusion that powers it) and a short burst of neutrinos when supernova 1987a
exploded. These neutrinos were all a
million times lower in energy than the ones that IceCube observed.
Many institutions
have issued press releases and feature stories about the paper, A few of them are
My apologies for the length and technical level of this
post, but this analysis is quite intricate, and I wanted to do it justice.
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