More neutrino interaction physics with IceCube

Once again, IceCube has shown that we can study high-energy neutrinos in their own right, rather than just as astrophysical probes.   This analysis used a sample of starting tracks in 5 years of data, from neutrinos that interacted within the detector, producing a hadronic cascade from the nuclear target recoil, and a muon from the lepton, in a reaction written as neutrino + nucleon (proton or neutron) -> muon + X, where X is the shower of particles produced by the recoiling nucleon.   In these interactions, there are two quantities to measure, the energy of the muon, and the energy of the shower.  The inelasticity is the energy of the cascade divided by the total energy (the sum of the shower and muon energy).  The distribution of inelasticity is well predicted by the Standard Model of Particle Physics, but has not been measured at energies above 500 GeV (5*10^11 electron volts).  With IceCube, we have extended the measurement to energies above 100 TeV (10^14 electron volts) - a factor of 200 upward in energy.  This plot shows the measured average inelasticity, from our new Icecube preprint, available here, or directly as pdf. 

The points show the inelasticity, while the blue and green curves show the standard model predictions for neutrinos and antineutrinos respectively.  The red curve shows the expectation for the mixture expected in IceCube.  For aficionados, this calculation is done at next-to-leading order accuracy, with BFKL evolution to low-x partons.

This measurement is sensitive both to potential beyond-standard-model physics, which would likely have a rather different inelasticity distribution than for the expected interactions.  Even the standard model cross-section is sensitive to the number of low-momentum quarks and antiquarks in the target nucleus.

Inelasticity is interesting in it's own right.  But, the inelasticity can also be used to probe a number of additional physics topics.  Neutrinos and antineutrinos have different inelasticity distributions, so by assuming the standard model values, we can measure the neutrino:antineutrino ratio.  As can be inferred from the plot above, it is exactly as expected.  Unfortunately, at the energies where IceCube is sensitive, we are mostly studying atmospheric neutrinos, not astrophysical.

We can also use inelasticity to probe astrophysical neutrinos.  Although the neutrinos selected here are mostly muon neutrinos, some tau neutrinos make it into the fit, and it is possible to use similar criteria to select a matching set of cascades.  The plot below shows the flavor triangle found from this study.

 Each point on the flavor triangle corresponds to a unique mixture of electron, muon and tau neutrinos.  The upper point is all muon neutrino, with the lower left and lower-right points corresponding to all tau neutrinos and all electron neutrinos respectively.  The colors show the relatively likelihood, with the best-fit point (cross) corresponding to 83% tau neutrino, 17% electron neutrino and no muon neutrino.  Unfortunately, the errors are large, so none of the different standard acceleration scenarios can be ruled out.

This work was done by my student, Gary Binder.  Besides the IceCube paper, he wrote a very nice dissertation, available here.  For this work, he won the GNN (Global Neutrino Network) dissertation prize for 2018. 

Found: one cosmic accelerator - TXS0506+56

Yesterday, in two papers published in Science, IceCube and collaborating experiments announced the observation of high-energy astrophysical neutrinos coming from a source, the blazar TXS0506+56. (the numbers denote its position in the sky).  This was announced at a press conference at National Science Foundation headquarters, and accompanied by press releases from multiple institutions, including from one from Berkeley Lab, and made the cover of Science (above).

Blazars are a type of active galactic nuclei (AGNs), which are themselves galaxies with a supermassive black hole at the center.  If the black hole is surrounded by a dust cloud (or other matter), it will gradually accrete that matter.  In the process, it will eject a fraction of it in a relativistic jet perpendicular to the galaxies axis of rotation.  The jet is turbulent, and thought to be a likely site to accelerate particles to extremely high energies.  In blazars, this jet is pointed nearly directly at Earth, giving us the best chance to see these ultra-energetic particles. 

The story begins on Sept. 22, 2017, when IceCube observed a neutrino with an energy around 300 TeV (about 50 times the energy of the protons accelerated at CERNs Large Hadron Collider).   The event display shows the neutrino; each colored dot shows the photons registered by one IceCube optical module, with color indicating relative time (from red to blue), and the size indicating the number of photons.

IceCube has seen many neutrinos that were more energetic than this, but this was still enough energy that the neutrino was likely of astrophysical origin.  So, computers clacked and whirred, and 43 seconds later we sent out an automatic alert, telling many partner observatories that we had seen an energetic neutrino coming from a specific direction.  Several of these observatories pointed telescopes in the direction of the neutrino, and (to make a long story short) that the location coincided with a minutes to years; this neutrino came during a time when TXS0506+56 was emitting at particularly high levels, from radio waves through (at least) 1 TeV photons.  The  first paper, by IceCube and the other experimental collaborations, discusses this coincidence in space and time.   TXS0506+56 is a relatively energetic, quite nearby (with a redshift of 0.3365), so it is a likely candidate for a first observation. 

Shortly after this observation, IceCube went back and looked at archival data, searching for excess emission from the source.    We found an excess of neutrino events coming from that direction during the period from September 2014 to March 2015.   This is reported in the second paper.

The exact statistical significance of these observations depends on some of the details of the analysis - the first paper gives a range of significances, depending on the preferred assumptions. But, taken together, this is strong evidence that we have seen neutrinos coming from a specific source: we have found at least one cosmic accelerator, far more powerful than CERN's LHC.   Besides the observed neutrinos, there is strong suspicion that AGNs also accelerate the ultra-energetic protons and/or heavier nuclei cosmic-rays that led us to look for neutrinos in the first place.  Unfortunately, since protons and heavier nuclei are bent by interstellar magnetic fields, they do not point back to their sources.

One still-open question is whether blazars are responsible for all of the neutrinos that IceCube sees.  In 2016, IceCube published a paper (freely available arXiv version here) which set limits on the fraction of the astrophysical neutrino that could come from blazars, setting a limit between 27% and 50%, depending on the spectral index.  This paper studied 862 blazars, and had to make some assumptions about the relationship between the observed gamma-ray flux and the expected neutrino flux.   As you can imagine, extensive work is ongoing to revisit this question.