The mystery of the most energetic cosmic rays deepens

 Recently, the Telescope Array (TA) Collaboration reported on the observation of a cosmic ray with enormous energy: 244 exa-electron volts (EeV, 2.44*10²⁰ eV), or roughly 40 Joules.  This is one of a handful of events seen in this energy range - with an energy roughly comparable to a well hit tennis ball.   Although this is not the most energetic particle ever seen, it is pretty close.  

Fig. 1. A skymap, showing the best-estimate directions for the event, after correcting for expected deflection in galactic and extra-galactic magnetic fields.   The labelled circles are the best estimates for different assumptions about nuclear composition, from protons (p) through iron (Fe) for two different models of the galactic magnetic fields - PT2011 and JF2012 which predicts larger deflections .  The active galaxy PKS1717+177 is a flaring source, but it is 600 Mpc away - probably too far to be the source of this event.  Also shown is a broad hotspot previously seen by TA at lower energies, along  with the galactic plane (the solid line), with the galactic center also indicated.

 

 The intriguing thing about this event is its direction.  The arrival direction was measured to better than 1 degree.  Fig. 1 (above) shows the predicted arrival direction.   At these energies, cosmic rays are not expected to bend very much in galactic and intergalactic magnetic fields.   The TA Collaboration calculated the expected bending for different hypotheses about the nuclear species of the incident particle, from protons to iron, and accounted for that bending.  Then, they looked in that general direction, and found nothing that seemed likely to be able to accelerate particles to those energies. Because 250 EeV particles can interact with the cosmic microwave background radiation and lose energy, their range is limited to 10-30 megaparsecs (Mpc), depending on nuclear species.  The active galaxy PKS1717+177 was considered as a source, but, at 600 Mpc distance, is too far to be a likely source.

This is a finite volume for a possible source, and it was devoid of 'interesting' objects.  'Interesting objects' include active galactic nuclei (galaxies with supermassive black holes at their center, with significant accretion which leads to a relativistic jet) and other sites that may contain the ingredients for a powerful particle accelerator.

There are a couple of possible explanations for the lack of an apparent source - all interesting.  It may be that we need to expand our definition of what is 'interesting' here - the accelerating sites are something that we have not thought of.  Or, maybe, the accelerators do not leave obvious other traces, or are distributed in space.  Or, possibly, the galactic and/or intergalactic magnetic fields are significantly larger than we expect.   

As the name implies, the Telescope Array is an array of surface scintillator detectors (to detect charged particles in cosmic ray air showers) and fluorescence detector telescopes to detect the fluorescence from nitrogen in the air as these charged particles propagate through the atmosphere.

The paper was published in Science, but is also freely available on the arXiv. 

Neutrinos from our galaxy

 IceCube has found a new source of neutrinos - our own Milky Way galaxy, with a significance of 4.5 sigma!   This observation was published in a recent paper in Science; a freely available version is available on the arXiv.  This study is both technically and scientifically very different from the two observations of neutrinos from active galactic nuclei (AGN) that IceCube previously observed.

Earth is within the Milky Way, which is, from our observation point, largely a plane in space so the source surrounds us.  High-energy gamma-rays have been observed coming from the Milky Way  The line has a width of a couple of degrees, depending on how you define the width.  This is very different from other galaxies, which are point sources (or close to that); the different geometry calls for a different analysis technique.  Instead of using muons from muon-neutrinos, this study used `cascades,' which come from electron-neutrinos, and neutral-current interactions of all neutrino flavors.  The advantage of using cascades is that the background of atmospheric neutrinos is much lower, so the signal:noise ratio is higher.  The disadvantage is that the angular resolution isn't nearly as good.  However, IceCube used a machine-learning technique, a convolutional neural network (CNN), to determine cascade directions.   A CNN works in a roughly similar manner to our own brains, with neuron-like processing steps that looked at the light deposition in IceCube's sensors.  This approach gives a resolution that is about two times better than previous cascade directional studies, lessening the difference with nu_mu's.  And, it used many more events.  since the Milky Way is not a point source, the angular resolution is less important.   The figure below shows how the analysis was done:

 

The Milky Way, seen in different ways.  The galactic center is in the middle.  The images extend +/- 15 degrees from the galactic plane., and cover the full 360 degree panorama.   The top panel is a composite optical image of the Milky Way.  The second panel down show the Milky Way as seen by the Fermi satellite Large Area Telescope, using photons with energies above 1 GeV.  The third panel shows a template developed from the Fermi data, assuming that the photons come from π⁰ decays. The fourth panel shows a template for what IceCube should see, after accounting for angular resolution and other detector effects.  Finally, the bottom panel shows the neutrino observations.

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The other interesting thing about the neutrinos from the Milky Way is that we are in the galactic plane.  This is very different from the previous observations, of neutrinos from NGC1068 and TXS0506, both of which are AGNs, with considerable high-energy activity.   We are also observing them from relatively close to their axis of rotation, where high-energy emission is more likely. In contrast, we are in the plane of the Milky Way, and it appears that the neutrinos are coming from many directions.  

IceCube found that the neutrino emission was consistent with the pattern of photon emission, assuming that the photons came from pi^0 and neutrinos come from charged pions.  However, the measured neutrino flux was considerably higher than one would expect based on pi^0 extrapolations.  There could be several reasons for this, including photon absorption en-route to Earth.    The neutrinos and pi^0 might be produced in sources, of when high-energy cosmic-rays interact with atoms or dust while moving around the galaxy.    However, a very recent new search for emission from a number of known sources of energetic (TeV) gamma-rays did not find evidence for an excess of neutrinos. In short, this is an important observation that raises many interesting new questions.

Scientists behaving badly? In Antarctica?

 Over the past decade, sexual harassment (and other related bad behavior) has become a hot topic among scientists, and everyone - employers, government agencies, and scientific societies are (properly) under pressure to take action to combat it.  Although almost everyone agrees that something must be done, the details are not always simple.  

This is especially true in Antarctica, where the problem is enormous, and solutions may be hardest.   In September, the NSF released a comprehensive report (available here) which documented the scale of the problem, and gave some thoughts about the solution.   A survey provided hard data, which backed up many quotes from interviews.  It all made for some very disturbing reading.  Briefly, women (and some men) were broadly harassed, and there was often little that was done about it.  One interviewee said "“Every woman I knew down there had an assault or harassment experience that had occurred on ice...”   Another said "It's so self-evident that [it's] barely work speaking out loud. [Sexual assault and sexual harassment] are a fact of life [here], just like the fact that Antarctica is cold and the wind blows."  Another quote describe a male supervisor attempting to break into a women's bedrooms using his master keys, and still another described a violent sexual assault.  

The report also touches on some of the reasons that the problem is both so painful for victims, and also hard to fix.  First, there is no single person in charge.  People at the U. S, Antarctic bases work for many different employers - universities and national labs in multiple countries, one of multiple logistical contractors, the U. S. Air Force and the New York National Guard, to name a few.   It can be difficult to figure out who someone works for, much less how to complain to their employer.  People also come from a variety of backgrounds - scientists are in the minority. 

Second, it is very isolated, both physically and psychologically.   Communications bandwidth (both phone and internet) is very limited, and most workers at McMurdo station have limited access to the outside world.  So, even if there was a clear address to complain to, it would not be easy to do so.  And, it would be almost impossible for any outside agency to investigate happenings in Antarctica.  And, even when there is access, the rest of the world is so distant, and engagement doesn't seem important.

Third, there is a significant gender imbalance.  This is partly (but only partly) a self-perpetuating complication, since the toxic atmosphere in Antarctica discourages many women from applying for jobs there, worsening the imbalance.

The problem of reporting, at least, has a fairly simple solution.  There needs to be offices, in McMurdo, and some of the other large bases, where people can report problems, which will then be investigated by people who are stationed there.  The NSF cannot fire people directly, but they should be able to quickly remove people from Antarctica when needed.  This will not solve every problem - it may not be practical to have relevant offices in small field camps, but there hopefully the camp managers should be on top of things (unless they are unfortunately part of the problem).  The U. S. military may be another issue - I do not have any clear knowledge that they are part of the problem, but they are clearly a law unto themselves, and military personnel were not allowed to participate in the survey until it was too late.  Still, this would provide a clear reporting path for 90% of the problems.  

Such a reporting system would pay significant dividends beyond reducing the amount of sexual harassment and assault.  Improving the atmosphere would increase the applicant pool for Antarctic jobs, and should lead to a better work force, and, hopefully, a happier one.

From what I can see, the NSF took the report seriously.  And, many of the groups that send people to Antarctica (IceCube included) are also making changes.  Unfortunately, so far,  changes have been limited.  A recent Ars Technical article discusses some of the changes - including a confidential advocate that victims can talk to - but clearly more drastic changes are needed.  

Elections as measurements

 

The complete debacle in the recent (Nov. 8^th, 2022) election for the Oakland School Board (OSB) led me to think more about elections.  For those of you who are from outside the San Francisco Bay Area, after tabulating and announcing the election results, the Alameda County registrar found an error in how ballots were counting, and On Dec. 28^th announced a new top vote-getter, less than two weeks before inauguration day.

 

Oakland is a city of about 440,000 people just south of Berkeley (where I live).  The OSB is important, since Oakland schools are facing many problems, including declining enrollment, educational recovery from Covid closures, and financial problems.  The OSB election used rank choice voting, so counting took time; the results were announced by early December.  Nick Resnick won in District 5.  This result was duly certified.  

 

Then, on December 28^th, the registrar dropped a bombshell.  There was a mistake in tabulating ranked choice votes, and a different candidate, Mike Hutchinson, was the actual top vote getter.  Some voters did not select a first-choice candidate, but did select ones for second or third choice.  Those second or third choices should have been tabulated when the ranked choice algorithm got to the second or third choice, but they mistakenly were not counted at all.  This was pointed out by a non-profit that looked over the voting results and spotted a problem.  When this was fixed, the District 5 results changed.

 

Despite this bombshell, the originally-certified winner, Nick Resnick was sworn in to office on Monday (Jan. 9^th).  The ultimate disposition is in the hands of the courts, since Hutchinson has, not surprisingly, sued.   The first hearing will be in May, after many OSB meetings and votes.

 

What does this have to do with physics?  Elections can be considered measurements of the will of the people.   Some measurements give very clear results, while others are ambiguous.  Like measurements, elections have statistical and systematic uncertainties.   

 

The statistical uncertainties are from random fluctuations in who did or did not vote. If the election were rerun, some different people would vote, because they were sick that day, or out of town, or just forgot.  Further fluctuations come from the lag in voting when people move into or out of the district, turn 18, or pass away. Mail-in ballots change the details, but not the overall picture – fluctuations remain.

 

From the measurement analogy, the statistical fluctuations are roughly the square root of the number of voters.  If 100,000 people cast votes then the uncertainty is the square root of 100,000, or 316 votes.   There are other, more sophisticated (binomial) formulae, but this is a reasonable estimate.   Elections with a smaller vote difference could easily have gone the other way. 

 

Systematic errors may be larger, and stem from systemic issues, such as the OSB debacle.  One difficulty with the analogy is that not everybody agrees on what ‘features’ of our election system are systematic uncertainties and which are ‘the rules.’     Funding inequity and unwarranted voter suppression are both major issues which can lead to unfairness, and incumbents certainly have advantages.  It is a matter of defining the question we ask.  Are we measuring ‘the peoples will, according to rules, or ‘the peoples will, as would be measured in a perfect, unbiased system?  The problem with the latter choice is that people disagree about the biases.  ‘According to the rules’ is a clearer baseline. Methodologies exist to try to estimate biases for the latter case.

 

Either way, though, when elections are within the combined error, the vote counts are statistically indistinguishable.  This is not to say that the different candidates are similar, or that partisans on both sides will not feel strongly about the result.  But, there is no discernible difference in the people’s preferences.

 

Treating elections as experimental measurements can help put their results in perspective.   a 51%:49% split is not a mandate, but a small, and perhaps statistically insignificant difference.    Successful candidates will govern better if they keep this in mind.

 

The opinions expressed here are my own, and not necessarily those of my employer, colleagues, family, friends, or anyone else (although they should be). 

Cool visualization

 Some members of IceCube (led by Lu Lu, at UW Madison) have created a cool new visualization for IceCube events.  It shows where the neutrinos that reach IceCube entered the Earth (only upward-going muon neutrinos are shown).  The Earth can be rotated, and it is possible to set the minimum energy.  These are mostly atmospheric neutrinos, so are distributed over the Earth, but it is still possible to see some interesting things.  

In the screen-capture above, with an energy threshold of 100 TeV, it is clear that the neutrinos are predominantly in the Southern hemisphere.   This is because, at these energies, most neutrinos are absorbed in the Earth, so, with their longer trajectories through the Earth, most Northern hemisphere neutrinos are absorbed before reaching IceCube.

More on Science in the age of Ukraine

 I wanted to give you an update on how international science (CERN, in particular) has been reacting to the continued Russian invasion of Ukraine.  The invasion is now in its 8th month, and the four large LHC experiments still have not decided how to publish papers with author lists, affiliations and acknowledgements of funding agencies that reflect that fact.  Papers from the LHC experiments are being sent to the Cornell arXiv with the authors listed as "The ALICE Collaboration," without lists of individual authors, institutional affiliations, or acknowledgements to the funding agencies. 

The papers are also being submitted to diverse scientific journals for review, with the understanding that versions with author lists, affiliations and acknowledgements will be forthcoming, hopefully allowing for timely publication.  Unfortunately, after 8 months, it is clear that the 'timely' part of this is not happening, and it is likely that some journals are becoming less happy about the situation.   They do best on a steady diet of publications.  It will not be easy for them to deal with a brief flood of manuscripts that are ready for publication once the author lists, etc. arrive.  

This will also not make things easier for some of our younger colleages who may be in the job market.  Everybody involved in CERN is well aware of the situation.  However, although the collaborations are making every effort to document their contributions, not having actual published papers may not make things easier farther afield, either in smaller institutions where one interviews with people in other subfields of physics, or in industry.

CERN, for its part, has not taken further visible action, with the last pronouncement being their March 8th announcement. 

A new source of astrophysical neutrinos

 

A likely new source of astrophysical neutrinos has been identified!  In a paper published in Science, the IceCube Collaboration reported on the observation of neutrinos coming from an object called NGC1068, with 4.1 sigma significance.  The probability that this is a random statistical fluctuation is about 1.1 in 100,000.  This calculated significance accounts for the fact that this was part of a search for neutrinos from 110 likely sources that were studied - this is known as correcting for trial factors - essentially the number of experiments that were tried at once.  A version of the paper is also available here, on the Cornell arXiv preprint server).

NGC1068 (also sometimes known as M77) is an Active Galactic Nucleus (AGN) about 47 million light years from Earth.  AGNs are spiral galaxies with supermassive black holes at their center.  These black holes accrete (pull in) matter from the surrounding galaxy; in the process, some of the matter is ejected at relativistic velocities (speeds a fair fraction of the speed of light) in a collimated jet that follows the axis of rotation of the galaxy.  NGC1068 is one of the closer AGNs, so it is not too surprising that it should be one of the first (after TXS0605+56) high-energy astrophysical neutrino sources to be observed.  The signal was composed of about 80 neutrinos over the background, which was mostly  atmospheric neutrinos. The figure above shows the position of the hotspot (including contours corresponding to 68% and 90% positional uncertainty) compared with that of NGC1068, denoted by the red star.   The agreement is good, and there are no other likely sources within the error contours.

NGC1068 was first seen in an earlier IceCube paper, published in 2020 in Physical Review Letters  (also freely available here, from the Cornell arXiv).   That paper used 10 years of data, and observed NGC1068 with a significance of 2.9 sigma.  The higher significance came from a couple of factors: more new data (and some old partial-detector data was dropped), improved calibrations, and an improved analysis method.  Going from 2.9 sigma to  4.1 sigma is a pretty significant improvement for a re-analysis like this; it seems that the largest single factor was the improved calibrations.

Assuming that this holds up (it is not over the 5 sigma threshold ), this will be the second neutrino source seen by IceCube.  Both are AGNs, so those of us who are impatient can see a pattern developing.  That said, there are differences.  TXS0506 is much further away  - 5.6 billion light years.   It is a known source of gamma-rays, while NGC1068 is not - it's jet is surrounded by a dust cloud that would obscure X-rays and gamma-rays.  And, neutrino emission from TXS0506 was associated with a period exhibiting an increase in gamma-rays, while the NGC1068 source is steady state (at least we have not seen evidence for variability).