Do sterile neutrinos exist?

Sterile neutrinos are among the most intriguing BSM (beyond-standard-model) ideas around, with a long history of data hinting that something unusual is going on.  The idea that neutrinos might oscillate into  a new type of invisible (or nearly invisible) neutrinos has attracted much theoretical interest, along with a large number of experiments.  Unfortunately, even after 25 years of study, we still don't know if sterile neutrinos exist or not.

The  sterile neutrino story starts in the mid 1990s, when the LSND (liquid Scintillator Neutrino Detector) studied neutrinos produced by the decay-at-rest of pions from LAMPF (the Los Alamos Meson Production Facility).  They observed a significant excess of electron-flavored neutrinos (henceforth electron-neutrinos) over the expectations.   The excess could naturally be explained via neutrino oscillations, but the oscillation parameters required to explain the data were inconsistent with the known neutrino masses and mixing.   It could, however, be explained by positing a fourth neutrino flavor (beyond those connected with the electron, muon and tau leptons).   These models are sometimes called 3+1 models, denoting three conventional neutrinos plus one sterile neutrino.

 The result was immediately controversial, even within the LSND collaboration. The concern was that there could be some type of unmodelled background, such as neutrons sneaking into the detector.    The KARMEN experiment at Rutherford Appleton Laboratory (in England) searched for similar oscillations, but did not find anything anomalous.

LSND was followed by the MiniBoone experiment at Fermilab, which ran from 2002 to about 2008.  MiniBoone was designed to confirm or refuse the LSND excess.  Unfortunately, it also found an anomalous result, but one which was in some tension with the LSND result, at least within a 3+1 model.; their 2013 paper used the phrases 'have some overlap with' and 'marginally compatible.'  This led to a profusion of more complex models, with a 3+2 model, with two sterile flavors gaining some popularity.   Of course, one expects that a model with more free parameters to do a better job of fitting the data.  Models with unstable (decaying) sterile neutrinos were also proposed.  

By now, a good number of experiments were in position to search for sterile neutrinos.  Unfortunately, they found a wide spectrum of results, with some favoring sterile neutrinos, and some not.   IceCube was in the latter category, reporting results consistent with the standard model.

There were also dedicated experiments, most notably MicroBoone at Fermilab.  MicroBoone is a liquid argon time projection chamber, a follow-on to MiniBoone.   The collaboration recently released their results late this year, with four analyses (of the same data) finding that their data was consistent with the standard model.   

Of course, many theorists have pointed out ways that the MicroBoone null results could be compatible with the previous LSND and MiniBoone positive results.  So, we still don't really know if sterile neutrinos exist or not.  However, MicroBoone is a strong experiment, designed to avoid MicroBoone's weak spots (e. g. the ability to distinguish photons and electrons).  It did not, however, cover exactly the same range of parameters that MiniBoone did.   It would be a bit of a coincidence that nature provided sterile neutrinos with the right characteristics to elude MicroBoone.  So, although sterile neutrinos are not impossible, but they seem less likely than they did on New Years Day in 2021.

Seeing antineutrinos in a new way - the Glashow resonance

 Recently, IceCube made its first definitive observation of an antineutrino, as it interacts with an atomic electron.  The result was published in Nature, and is now available publicly on the arXiv preprint server, as arXiv:2110.15051.

The reaction is very different from the usual Deep Inelastic Scattering interactions, where a neutrino or antineutrino interacts with an atomic nucleus.   In this process, known as the Glashow Resonance, an antineutrino and an electron annihilate each other, producing a W boson, as is shown in the diagram to the right     The W boson is heavy (weighing about 85 times the mass of a proton), so it decays essentially immediately, usually into a quark and an antiquark which then fragment producing two jets of particles. In IceCube, this leads to a cascade of particles, which looks like (nearly) a point source of light.   For antineutrinos with the right energy (about 6.3 PeV), the interaction probability is very high - antineutrinos near the peak of the Glashow resonance only have a range in ice of about 100 km, only about 1% of the range for neutrinos of the same energy.

This reaction is of great interest because it only happens with antineutrinos.  Its not a big surprise that there are astrophysical antineutrinos, but it is nice to have clear confirmation.  Later, with enough statistics (this will take a while), we can measure the neutrino:antineutrino ratio, which will tell us something about how the neutrinos are produced.  

 

There were some interesting technical aspects of the event.  The event display (above) shows a large cascade near the edge of the instrumented volume.   In fact, the most likely location of the actual interaction is outside the detector, but close enough that we can reconstruct it well.   However, closer examination shows some interesting features.  

The bottom two parts of the graphic show the signals recorded in two of the optical modules, as a function of time.  The blue curves show the expected light profile from a pure cascade at the reconstructed interaction point.  The red curves show unexpected 'early' light.   We believe that this light came from muons produced in the cascade.  

The muons travelled at nearly the speed of light, while the light moves more slowly.  This may sound surprising but in dense materials like ice, the light interacts with the medium (one way to think about is as if the light bounces around as it moves), and so only moves at about 3/4 of the speed of light.  So, the muons will reach the vicinity of the optical sensors first, emitting early light which will reach the sensors before the light from the rest of the cascade.   This early light signals the presence of muons, which show that the cascade was a hadronic shower, rather than purely electromagnetic.  So, the cascade was not due to an electron-neutrino charged-current interaction.  By eliminating the electron-neutrino hypothesis, we strengthen the case that this is indeed the Glashow resonance.  Which, in turn, strengthens the case that we have observed an antineutrino.

Conferences - the International Cosmic Ray Conference

 

Last month, Berlin 'hosted' the 37th International Cosmic Ray Conference (ICRC) - the major conference for IceCube physics.  It is a chance to meet, present new results and chat informally with colleagues from different experiments around the world - an important opportunity to exchange ideas, plan for future experiments, network, and, for the younger people, formally or informally job hunt.

Unfortunately, Covid forced us into the virtual world.  Although the organizers worked very hard and did a good job, it just isn't the same.  Virtual meeting rooms may be getting better, but they're nowhere near in-person meetings, and the time differences limited the opportunity to interact.

The ICRC program included 693 talks and 687 posters, with 84 presentations from IceCube.  To cope with the time differences, the talks were pre-recorded, viewable at leisure. Posters were also made available, accompanied by short 'flash' talks by the presenters.   The organizers scheduled discussion sessions, clustering talks on similar topics.   I found these were quite valuable, although there was so much to cover that some presentations did not get the attention than they deserved. 

Although no major new results were presented (by IceCube or by other experiments), it was still a good opportunity to assess progress in the field.   There was steady progress in most areas.  IceCube presented a host of new searches for astrophysical neutrino searches, plus progress reports on a number of new studies of the diffuse (aggregate) neutrino flux, two measurements of the neutrino-nucleon cross-sections, and several contributions on neutrino oscillation studies.  IceCube Gen2 also received some attention, with reports on the science case and hardware developments.  The IceCube talks are linked to a master arXiv submission available here.

One subfield with some nice developments is high-energy gamma-ray astronomy.  The Chinese Large High Altitude Air Shower Observatory (LHAASO) is now operational in Tibet.  It features a large surface array consisting of water Cherenkov detectors to detect air shower particles that reach ground level, buried muon detectors to separate gamma-ray and hadronic showers, and Cherenkov telescopes for further gamma/hadron rejection.  The large-area surface coverage and high-altitude site give it good acceptance for gamma-rays with energies down to 500 GeV.   At TeV energies, it is the most sensitive observatory we have.

Although it is still early days, LHAASO has presented observations of a twelve sources, including seeing photons with energies up to 1.4 PeV.   One has to be careful about claims of maximum photon energies, but the events look good, and this is a considerable step up from previous maximum energies.  At least most of the sources are likely to be in our galaxy.  This is expected since photons with energies above about 50 TeV are attenuated in-flight, through interaction with lower-energy photons from the cosmic microwave background radiation.  Even within our galaxy, only about 1/3 of the most energetic photons survive the trip to Earth.

Happy Birthday to IceCube

 IceCube turned 10 years old today!  

Of course, there many way to determine IceCube' birth date.  The one that we are choosing to celebrate is the 10th anniversary of the start of the first production data run using all 86 strings.  We could also have celebrated the end of deployment of the 86 strings, which happened on December 17, 2020.  But, the May date was more convenient; pre-Covid, we had intended to schedule our collaboration meeting around it, and also have a celebratory 'What have we learned' workshop.  Alas, the in-person celebration and workshop will have to wait until we can safely travel again.

For those who are interested, many IceCube institutions issued press releases and features.  The LBNL story is available here, while the UW Madison release is available here.

Here's to another 10 years, including the IceCube Upgrade and Generation 2!   Minus the adolescent angst, of course.

Here today, gone tomorrow: searching for transient sources in astrophysics

Coming from a particle/nuclear physics background, when I started working on IceCube one of the bigger mental adjustments I had to make was to get used to the idea of transient sources.  When an accelerator is running, its particle output is more-or-less constant.  Not so with astrophysical objects. Many (not all) of the most interesting astrophysical objects vary considerably in output (by a factor of 10 or more), over different time scales.  Depending on the source, periods of increased emission may or may not repeat, on either regular or irregular time scales.

In fact, IceCube's most statistically significant signal, from the source TXS0506 was based partly on a search for transients, where we found a transient lasting about 7 months, as I discussed in a previous post. Transients can come over a wide range of length scales, from millisecond long bursts of radio waves called Fast Radio Bursts, up to sources that probably change on time scales longer than we have been observing them.     

In IceCube, time-varying sources add additional complexity to source searches, since searching over a wide range of time scales, degrees of repeatability, etc. can lead to a large increase in the number of trial factors: the more ways you slice and dice the data, the more likely you are to get a statistically significant result.  It is critical to keep track of the number of different observations (positions in the sky, possible pulse start times and lengths etc.) to know if an observation is really statistically significant.  For some sources, we can use radio, optical or X-rays to tell us the best places to look, reducing the number of trials factors

IceCube has recently released a paper on a search for time-varying sources.   The paper included two types of searches.  The first was an all-sky search that looked for emission on different time scales, from about 1/10 second to 100 days.  This suffered from a large trials factor, for the reasons noted above. 

The second search examined one object of particular interest: 3C279, which is a quasi-stellar object.  Despite the 'quasi-stellar' name, it is a distant galaxy containing a massive black hole which powers the emission of powerful particle jets, which were recently imaged  by the Event Horizon Telescope - the image above is from their web page.  3C279 is known to exhibit strong variability in radio, optical and X-ray emission.  These factors made it an attractive place to search for neutrino emission, despite the long distance (5 billion light years).   We used gamma-ray data (using photons with energies above 100 MeV) from the Fermi telescope to select time periods when 3C279 was particularly active.  By focusing on the active periods from a single source, we were able to make a much more sensitive search.

Unfortunately, we did not find anything using either approach.   We are, however, reducing the number of ways that Nature can hide the cosmic-ray accelerators that we know must exist.   We use the non-detection of neutrinos to put limits on how 3C279 could work as an accelerator.

IceCube has won the American Astronomical Society's Bruno Rossi Prize

 

 

The American Astronomical Society has awarded the 2021 Bruno Rossi Prize   to Francis Halzen and the IceCube Collaboration "for the discovery of a high-energy neutrino flux of astrophysical origin." 

We are very proud of this award, which reflects on both the construction of IceCube and on the data analysis (plus help from Mother Nature, for making the flux large enough to be detectable).  The announcement is posted at

https://head.aas.org/rossi/rossi.recip.html#2021_ic

and there is an IceCube press release at

https://icecube.wisc.edu/news/view/799

 

The prize is named after the Italian physicist Bruno Rossi,who was one of the pioneers of cosmic-ray physics.  He won the 1954 Nobel Prize in Physics for the invention of coincidence circuits, which he used to show that large groups of cosmic-ray particles reached the ground simultaneously, i. e. that very high-energy cosmic-rays produce air showers consisting of large numbers of particles.