Stopping a neutrino beam; measuring their interaction cross-section

Neutrinos are popularly known as the particles that go through anything and everything.  Neutrinos from beta decay can escape from the best shielded nuclear reactor, and neutrinos from nuclear fusion escape from the center of the sun.  Neutrinos interact only via the weak interaction, which is indeed weak.  But, that doesn't mean that they can go through anything - the IceCube Neutrino Observatory recently demonstrated experimentally that it is possible to stop a beam of neutrinos, in a paper published in Nature (also freely available on the arXiv).

To do this, IceCube used two tricks. 

First, it use extremely energetic neutrinos, with energies above 1 TeV (1 tera-electron volt, or 10¹² electron Volts), extending up to 1 PeV (1 peta-electron volt, or 10¹⁵ eV), millions of times more energetic than neutrinos from nuclear fusion or radioactive ion decay.  The cross-section (probability) for neutrinos to interact rises with energy (linearly at first, then moderated to scale roughly as Energy^0.3.  So, at an energy of 30 TeV (the rough mid-point of the measurement) the cross-section is several million times higher than it is for neutrinos from radioactive decay.  Of course, there aren't that many neutrinos this energetic, but, at 1 cubic kilometer in volume,  IceCube is big enough to collect a good sample.  The analysis used 10, 784 energetic muons from neutrinos that passed through at least some of the Earth.

Second, it used a very thick absorber - the Earth.  With this, the measurement was quite simple.   It Compared to a baseline of near-horizontal neutrinos that traversed only a relatively small amount of matter, energetic near-vertical neutrinos were absorbed going through the Earth.  The figure above shows the predicted transmission probability (= 1 - absorption probability), as a function of neutrino energy and zenith angle; the latter shows how much Earth matter was traversed.   

There are of course many complications - experimental uncertainties on the neutrino energy, neutral current interactions, where a neutrino may emerge from the Earth with a lower energy than it entered, modelling the material within the Earth, etc., but the result clearly showed that neutrinos are absorbed at about the expected rate.  More precisely, the best-fit cross-section was. 1.3 +/- 0.5 times the predictions of the Standard model where I have combined the statistical and systematic uncertainty.  It was not trivial to find a good definition for the neutrino energy range for which this measurement applies, because different methods give somewhat different energy ranges, but we settled on a method that returned a range from 6.3 TeV to 980 TeV.  For comparison, the highest energy measurements at an accelerator laboratory only reached 0.37 TeV - our measurement reaches order of magnitude higher energies than than.  The figure below puts this in perspective, comparing our measurement with the previous accelerator work.  The cross-sections (y axis) are divided by the neutrino energy so that everything fits on the graph better; otherwise, it would span many orders of magnitude.

I have to mention that this was the dissertation work of my (now graduated) graduate student, Sandra Miarecki.  Sandy had a very interesting preparation for graduate school - she was a career US Air Force Pilot, serving many roles, including as a test pilot, before retiring from the Air Force and coming to graduate school in Berkeley.   After graduate school, she became an Assistant professor at the US Air Force Academy.   The LBNL news center has a very nice article about her.

The Nature article also recieved a fair amount of press coverage.  I will just mention one article,  in Symmetry magazine, which goes into more detail about the analysis than other press writeups.

Gravity waves, Gamma-rays and gold jewelery

It has been a bumper month for astrophysicists.

On October 16th, the combined LIGO/VIRGO collaborations announced the observation of gravitational waves from an even that occurred on August 17th.  Unlike the previous observations, these waves came from relatively 'light' objects, reflecting the collisions of two presumed neutron stars, with masses around 1.1 to 1.6 times the mass  of the sun, forming a black hole with a mass around 2.74 times the mass of the sun.   Previous gravitational wave events had come from the collisions of much heavier objects.

But, that's not all.  Two seconds later, the FERMI observatory, a satellite containing a large gamma-ray detector, and the INTEGRAL satellite both observed pulses of gamma-rays coming from the same direction.   This is the classical signature of a 'gamma-ray burst' (GRB).  GRBs were first observed in the 1960's by the VELA satellites, built to monitor gamma-rays from possible atmospheric or space-based nuclear weapons tests.   VELA did not observe these, but it did find mysterious bursts of gamma-rays coming from space.     These bursts have been the subject of scientific speculation for decades, and the conventional wisdom was that some GRBs came from the merger of neutron stars or black-hole on neutron star mergers.  That theory has now been amply confirmed by the LIGO/VIRGO/FERMI/INTEGRAL observation.   The graphic above, from the LIGO collaboration, shows the process.

Of course, this collision site was studied by many many other astronomical instruments.  IceCube looked, but we didn't see anything.   However, the optical studies were very fruitful.  Multiple telescopes observed an optical signal that lasted for a few days, plus an infrared signal that lasted for nearly two weeks.  These signals were consistent with some predictions made by my LBNL colleague Dan Kasen and his collaborators.  Kasen made a detailed model of the graviational, nuclear and atomic processes that would occur in a collision of two neutron stars, and, from that, predicted the optical and infrared light emission.  His model predicts considerable production of heavy elements (heavier than iron) via rapid neutron capture (the 'r-process').   The shorter-lived broadband optical emission comes from an initial ejection of lighter nuclei. The long-lived infrared component comes from a secondary emission which is powered by the radioactive decay of heavy elements which heat the plasma that surrounds the newly formed black hole.  Heavy elements (Z between 58 and 90) scatter the light strongly, so it takes longer to escape from the plasma.

This agreement is of great interest to nuclear physicists, since it may provide a new answer to the question: where do the heavy elements in the universe come from?  Previously, it was thought that they were mostly produced in supernovae, explosions that occur when heavy stars reach the end of their livetime and collapse.  However, Dan's simulations  shows that GRBs produce heavy elements, and could account for much or all of the gold used in our jewelry, along with all of the other heavy elements.

Where have all the sunspots gone?

One use for IceCube is to study solar flares, or, more generally, solar weather.  Solar weather is important; solar flares are often accompanied by coronal mass ejections (CME), the ejection of plasma which sometimes hit the Earth.  This plasma can disrupt radio communications, disable satellites, and endanger astronauts.   In 1859, a large CME, the Carrington event, disrupted telegraph communications in the U. S. and Europe, and produced enormous auroras which were visible over much of the globe.  Modern electronics is far more susceptible to damage than simple telegraph systems.  A similar event occurring today would cause incredible ($1 trillion?) damage.   With appropriate warning, we could reduce the damage significantly by turning off and/or shielding as much electronics as possible, grounding airplanes, etc.  For this very practical reason, it is important to understand solar weather in more detail.

In addition,  sunspots and solar flares contribute to the variation in the Sun's total output. Sunspots and solar flares reduce and increase the solar irradiance directly.  So, the lack of sunspots in the current 11-year solar cycle (cycle 24) might be thought to reduce solar irradiance, and hence help combat global warming.  However, the story is a bit more complicated than that.  Both sunspots and solar flares are governed by the sun's magnetic fields; there is a more direct correlation between the Sun's magnetic activity and irradiance, as can be seen from this plot from the Bartol Institute at the University of Delaware, which compares solar magnetic activity and the rate of neutrons from cosmic-rays reaching Earth.  Before ~1950 (i. e. before global warming became significant), these magnetic field variations likely accounted for much of the observed climate variation.  The evidence for this comes from comparing carbon-14 dating curves (carbon-14 measures cosmic-ray activity) with climatic data from tree rings and ice cores.

Most of the particles emitted in a solar flare or CME are, by IceCube standards, low energy, protons, neutrons (which mostly decay before reaching Earth) and photons with energies of at most a few billion electron volts (GeV).   When the individual particles reach Earth, they leave relatively few direct traces at ground level; only a small fraction of them lead to (via a small air shower) particles reaching the Earths surface.   However, a CME contains a very large number of particles, an, if the density is high enough, it can raise the counting rate of terrestrial particle detectors  This is known as a ground level event (GLE).  Space scientists have deployed neutron detectors at many sites around the globe to monitor these signals.  Because of the total sensitive area and low background rates of the 162 IceTop surface array tanks, it is a very sensitive detector for GLE's, and we had expected to detect of order 1 GLE per year. 

Unfortunately, Nature has not cooperated.  At this week's International Cosmic-Ray Conference, IceCube presented an analysis of GLE's from 2011 to 2016.  Only three GLEs were observed, and they were 'quite small by historical standards.'  The reason for the low rate is unknown, but it may be connected with the paucity of sunspots during the current solar cycle. This might be expected to lead to a reduction in solar irradiance.  Data from the SORCE satellite presents a mixed picture, showing a small (~0.07%) increase in solar irradiance from 2009 and 2015, with a similar sized decrease over the past two years.  

We do not understand what is causing these changes; clearly the Sun still has many secrets.

Many thanks to Paul Evenson (Bartol Institute, Delaware) for useful discussions on the relationship between sunspots, flares, CME and magnetic fields.  Of course, any errors here are my own.