The curious case of the softening spectrum: more on astrophysical neutrinos

IceCube has posted a set of papers on the arXiv, giving new results on starting events: neutrino interactions that occur within IceCube.   These analyses use 102 events observed in 7 1/2 years of data,.  There are many new results, including new measurements of the astrophysical neutrino flux and energy spectrum, evidence for the observation of tau neutrinos and  the  first measurement of the neutrino-nucleon cross-section using starting events.  The papers are available on the arXiv preprint server, and have been submitted for journal publication:

"The IceCube high-energy starting event sample: Description and flux characterization with 7.5 years of data," R. Abbasi et al., available as arXiv:2011.03545.

"Measurement of Astrophysical tau neutrinos in IceCube's high-energy starting events, R. Abbasi et al., available as arXiv:2011.03561.

"Measurement of the high-energy all-flavor neutrino-nucleon cross section with IceCube, R. Abbasi et al., available as arXiv:2011.03560. 

There were a couple of reasons to have three publications.  These are three very different topics, based on rather different analysis techniques.  But, length was also an issue: the first paper comes in at 51 pages, definitely on the long end of the spectrum for physics papers.   This post will focus on the first paper, which also describes the data sample.

The analyses in the first paper are very similar to those in previously published starting event analyses, which I discussed here.   The current analyses benefits from more data, and better detector calibrations and better analysis software, giving better measurements of the energy deposited in the detector, better measurements of the neutrino directions, etc.

That said, the results have changed more than we would have expected.  Most notably, the measured neutrino energy spectrum has gotten softer (i. e. there are fewer very energy astrophysical neutrinos, and more with lower energy).  The figure immediately above shows the energy spectrum (expressed as energy deposited in the detector) and the zenith angle (where cos(theta)=+1 is going vertically downward, and cos(theta)=-1 is going vertically upward), compared to the expectations for atmosphe ric muons, atmospheric neutrinos (labelled as Atmo Conv.) and a fit to the astrophysical spectrum.  The fit found the astrophysical spectrum was consistent with a flux phi=phi_0 (E_neutrino/100 TeV)^-alpha, where alpha=2.87+/-0.20.  Here, phi_0 is a normalization constant.  In comparison, previous contained event analyses found alpha in the 2.3 to 2.6 range, depending on which years of data were studied.    The collaboration spent much time trying to determine what has changed.  Otherwise, this paper would have been out some time ago.  

We looked at every plausible explanation that we could find, and even some that were clearly less plausible.  If we use just the first 4 years of data, the results were similar to those in the previous analysis.  If we swap the old and new software and calibration, very little changes.  There is no evidence for any change in the detector behavior; one expects detectors buried under a mile of ice and held under constant conditions to be very stable, and, as expected, we see no significant changes in atmospheric neutrinos, cosmic-ray muons, or any other measure of detector performance.   The interactions were spread pretty evenly throughout the detector, so it is not a problem in a small part of the detector.   The astrophysical neutrinos come from a large number (very likely >50) source from many directions in the sky, so it is not plausible that this is due to a change in their source.   So, in the end, I am just chalking this up to statistics - once in a while, we expect large (roughly 2 sigma) statistical fluctuations, and this seems to be one of those occasions.

 The neutrino arrival directions have also changed somewhat.  This is better understood, and comes from a combination of improved analysis techniques and a better understanding of how light scatters and is absorbed in the Antarctic ice.  For each neutrino candidate, we estimate the probability of it coming from any given direction in the sky.  The result is a blob (which may be regular or irregular, depending on the reconstruction) centered around the most likely arrival direction.  The graphic at the top shows our revised sky map, which shows the estimated flux coming from different directions, where we add up the probability that each neutrino came from a given direction.   The gray dot shows the center of our galaxy, and the gray curve shows the galactic plane.

The color code gives the "Test statistic," a measure of how likely the measured flux from that direction can be explained by background.   There is a hot spot (every map must have a hottest spot), but it is not statistically significant; this map shows no evidence for any specific neutrino sources.  It should be noted that, because we have only a handful of contained events, this search is less sensitive than studies using through-going muons.