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.

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.

 

Science in the age of Covid

 Hi,

Apologies because I have not updated this blog in quite a while.   I'm healthy, but Covid has brought many changes to my work life, and almost everything takes longer.    

Covid has had an enormous effect on scientists, and a somewhat lesser but still very sizable effect on science.  I am one of the fairly large fraction of physicists who mostly work on a computer.  So, I am able to work at home, and I do so.  There is a significant hit to productivity because I can no longer walk next door and talk to my colleagues - everything requires an appointment and a zoom call. 

Laboratory science has taken a much larger hit.  Lawrence Berkeley Lab, like most other research institutions, was almost entirely shut down for about six months, so all laboratory work stopped.  Now, we are slowly and carefully ramping up lab work, with a whole host of anti-Covid precautions, about physical separation, etc.   Of course, Covid-related work has become very high priority.  At LBNL, this includes using the Advanced Light Source (it produces intense beams of X-rays) to study the structure of important proteins, using the NERSC supercomputers to study protein-Covid interactions, and, of course, much biological research. 

Polar science has been affected even more.  There is a strong determination to keep Covid out of Antarctica.  To do so, the U. S. polar program has slashed the number of people who are going there this coming Astral summer.   The only activities that are supported are those that keep the U. S. stations running, and prevent damage to scientific infrastructure.   For IceCube, we will be able to swap winter-overs as the Pole, but not much more.   The usual transportation, using Air Force and NY Air National Guard LC-130 transports will not occur; instead there will be a small number of flights on Baslers (shown above, they are much upgraded DC-3s with turboprop engines), with very limited passenger space, and even more limited cargo transport.  

Everyone going to Antarctica will spend time quarantining in both the U. S. and New Zealand.  Even this has been tough - New Zealand is essentially closed to visitors, so special arrangements were required to allow polar program personnel in.   Fortunately, IceCube is running well, so the main effect is to put off some planned software upgrades, plus the surface deployment of new prototype air shower detectors.

For younger scientists without career positions,, the effects of Covid are especially drastic.  Colleges and universities are in dire straits financially, and have consequently cut faculty hiring.  One estimate I saw was that the number of advertised faculty positions is down by 70%.    This is a huge cut, especially for people who were positioning themselves to apply for jobs this year.  The situation in industry is better, but it is still not as good as last year.    Overall, physicists are probably no worse off than most other professions, but young people have a limited time window to apply for faculty positions, and budget limitations will create a squeeze that will likely last for several years.

Here comes the tau?

Simulated tau neutrino event in IceCube.  Each sphere is an optical module that observed light, with the size scaling with the number of photons.  The color indicates the timing of the light, from red (earliest) to blue (latest).

 One of the more interesting/embarrassing holes in IceCube's physics portfolio was the tau neutrino.  Neutrinos come in three flavors: electron, muon and tau, each tied to the charged lepton of the same name.  Over long distances, these neutrinos can oscillate, changing flavors.  So, no matter what flavor ratio a neutrino beam is produced with, over long distances, we expect it to oscillate and reach Earth as a roughly 1:1:1 ratio of electron:muon:tau neutrinos.

Since very few tau neutrinos are produced directly on Earth, the observation of tau neutrinos was considered to be a clear sign of astrophysical neutrinos, and many many papers discussed the signatures and expectations.  A beautiful 1995 paper by John Learned and Sandip Pakvasa   (also available on the arXiv - soon after it was founded) pointed out that sufficiently energetic tau neutrinos could produce a distinctive 'double bang' signature - a large cascade when the neutrino interacted, and a second when the resulting tau lepton decays.  Even though the tau lepton lifetime is very small (3*10^-13 s), when it has energies of a PeV (10¹⁵ eV) or higher, a Lorentz boost extends its lifetime in the Earth frame of reference so that the two bangs can be separated by an average of (Energy/1 PeV) * 50 meters, making for a distinctive signature seen in the simulation shown above, with two distinctive light clusters.  Unfortunately, IceCube has not seen this signature, and we have also not seen enough PeV-energy neutrinos so that we can expect to see it.

A candidate tau neutrino event seen in IceCube. Each sphere is an optical module that observed light, with the size scaling with the number of photons.  The color indicates the timing of the light, from red (earliest) to blue (latest).  The seven plots show the waveforms (light vs. time) for certain optical modules; several show apparent double-pulse signatures.

However, IceCube is developing techniques that will allow us to see tau neutrinos with lower energies, where the two bangs are closer together.  Even if they are so close together (10-30 m) that we cannot separate the overall light clouds,  there may be some optical modules that see pulses from the two cascades at separate times, producing a double-pulse topology in an individual optical module.   The figure above shows one candidate event, along with waveforms from some of the modules, showing the double-pulse signature.   A word of caution is in order - there are some possible background processes that could mimic these signatures - but this is considered by IceCube to be evidence for tau neutrinos.  "Evidence" typically means that the statistical significance is 3 sigma or more, not the 5 sigma required to claim a detection.  Since we expect to see tau neutrinos, this is reasonably convincing, an it seems safe to say that the holes has largely been filled in.  We look forward to more precise measurements, of course, to check in more detail for consistency with the standard acceleration scenarios.

The tau neutrino work has been presented in several recent conference presentations, including ones with writeups by Daan van Eijk and Logan Wille and Juliana Stachurska.