Francis Halzen was awarded the 2026 Nobel Prize in Physics, for IceCube!

 IceCube PI Francis Halzen, professor of physics at the University of Wisconsin, Madison has been awarded the 2026 Nobel Prize in Physics, for "“for decisive contributions to the IceCube Neutrino Observatory and the discovery of high-energy neutrinos of astrophysical origin.”  Francis has been a tireless advocate for IceCube, starting in the 1990's, when the idea of a large ice-based neutrino detector was really a laughing matter; few people took it seriously.   This was a simple matter: convince his fellow physicists, numerous committee and funding agencies that the idea was practical, gather one collaboration to produce a proof-of-principle: AMANDA, and a larger one that could be an industrial-scale experiment like IceCube.  Although Francis was not an experimentalist, he was able to bring together a diverse group of scientist, engineers and computer scientists with the necessary skills. IceCube required two key technical development.

The first was drilling deep holes in the ice.  This was most efficiently done using hot water to melt a hole down through the ice, but IceCube required far higher drilling capacity than any previous experiment.  This required a new drill, the development of which often seemed more akin to oil prospecting than astrophysics.   UW Madison and its Physical Sciences Laboratory were responsible for the drilling.  

 

 The second problem was a data readout that would preserve the details (time structure and amplitude) of the light pulses.  Most of this problem fell to Lawrence Berkeley National Laboratory.   The solution was to put the analog and digitizing electronics in the ice, attached to the photo-multiplier tubes, and send slow digital data to the surface.  The challenges here involved reliability (since the modules would be inaccessible after deployment), low-power operation, and a very wide temperature range, down to -55 degrees C.  In essence, we needed to build 5,160 satellites, but with a fraction of the budget that NASA had.  The required tradeoffs made for many interesting discussions, but the result was outstanding; over 98% of the optical modules are still working, with many having spent 20 years in the ice. 

 The final (and perhaps largest) challenge was funding.  Even after construction was approved, there were many people who were skeptical of (a) whether the IceCube detector would work, or (b) whether we would see anything.  Francis deserves enormous credit for convincing the NSF to fund IceCube, and the NSF deserves enormous credit for listening and taking what must surely have been seen as a risk.  We are all better off for it. 

 

 

3I/ATLAS - not just the fastest object in our solar system, but the oldest

I've previously written about 3I/ATLAS, a visitor from outside that arrived in our solar system last year - here and here.  Although it is gone now - from the inner solar system (it is now beyond Jupiter, and getting farther every day) and - mostly - from our consciousness, but new information is still coming out. 

A new study finds that 3I/ATLAS may be 10 to 12 billion years old.  In comparison, our solar system is only 4.6 billion years old - a relative newcomer. 

The study used infrared measurements from the James Webb Space Telescope (such as the picture above) to measure the ratio of deuterium (heavy hydrogen, containing one neutron and one proton) to regular hydrogen (just one proton).  The researchers found that it was almost 1%  - more than ten times higher than in comets that are confined to our solar system.   It also contained a higher ratio of carbon-12 (6 protons + 6 neutrons) to carbon-13 (6 protons + 7 neutrons) than is typically found in objects in our solar system. This is evidence that it formed in a 'metal-poor' environment.  n. b.  Astronomers define any object heavier than helium to be a 'metal.'  They also concluded that 3I/ATLAS formed at very low temperatures, less than 30 Kelvin (-405 Fahrenheit).

The measured carbon ratio is consistent with formation in the early universe, when there were less heavy elements around than there are today, while the high level of deuterium points to formation in a cold environment.  Both of these connections are not trivial, relying on more complex comparisons; for more details, please read the paper, which is open access. 

What do subatomic particles look like?

"What do subatomic particles look like?" sounds like a pretty basic question.  But, in fact, it is not.  Many physicists would considered it ill-posed, since 'look' presupposes observation with visible light, something that is impossible, since, by any measure, these particles are far smaller than the wavelength of light.  Instead, observing subatomic particles requires far more energetic probes.  And, quantum fluctuations mean that they are ever-changing; instead of an instantaneous shape, we can only hope to observe, at best, the average shape, plus some measure of the fluctuations.   Like many larger objects, the way that they look depends on how one looks at them.

Consider the proton, the central nucleus of a hydrogen atom.   When we ask what it looks like, we can expect to learn about its size and hopefully shape.  Is it spherical?  How does its density change in going from the center to its periphery?   This begets an important question: density of what?  There are many different ways to characterize a proton.  One could measure the charge distribution, mass density, or at internal pressure, or look at where the quarks and/or gluons are within it.  One could also probe quarks or gluons within a specific momentum (Bjorken-x) range. 

Or, one could ask where its 'protonness' is.  More generally, one can ask where its baryon number is.  Baryon number is what makes protons and neutrons what they are, and distinguishes them from mesons.  In the valence quark model, protons have three quarks (as in the left drawing above). When one adds in gluons and evanescent quark-antiquark pairs, it looks more like the drawing on the right.  Then one can define baryons as having three more quarks than antiquarks.  But, this is not the only possibility.  However, sophisticated mathematics - the requirement of local gauge invariance of the proton  - points toward the existence of a baryon junction, comprising a nonperturbative localized configuration of gluons which is connected to these three quarks.  This model makes some predictions that seem to be borne out experimentally.   Another consequence is that the protonness is likely to be concentrated in the center of the nucleus.

We can probe this using a reaction that is sensitive to baryon number.  Backward photoproduction (also known as u-channel photoproduction) is one such reaction.  In it, a photon interacts with a proton, producing a meson, plus the original baryon.  Normally, as in diagram (a) above, the produced meson carries most of the momentum of the photon; the momentum transfer from the proton target is small.  But, sometimes, the proton acquires most of the photon momentum, leaving the meson nearly at rest.  This can happen if there is a large momentum transfer from the proton (diagram (c)) or if there is a small momentum transfer, but transfer also exchanges baryon number (diagram (b)).   Either way, the photon is rather directly probing the baryon number in the target.  

We use this reaction to 'image' the proton by looking at the momentum transfer in these backward production reactions.   The higher the momentum transfer, the smaller the object that is struck.  In more technical terms,  a two-dimensional Fourier transform of the transverse momentum of the outgoing meson gives us a density distribution of  baryon number within the proton.  In a preprint (arXiv:2603.03730) we (me, plus colleagues from U Washington, Livermore and UC Davis) did this for a number of different backward production reactions, along with, as a benchmark, some otherwise-matching conventional forward photoproduction reactions.  The plot below shows our results:

And there you have it - pictures of the essence of the proton - the seat of its baryon number. Here, the x axis is the radius (distance from the center of mass of the proton), in fermi; 1 fermi is 10^-15 m.   The y axis is the baryon number density.   For simplicity, we characterized these distributions by their average (rms) radius.  We found that the average transverse (after squashing the proton down to two dimensions) radius of the baryon number was 0.33 to 0.53 fm.  In comparison, a number of other measures of the proton size, based on its charge density, or parton densities, all found larger radii, at least 0.67 fm.  So, a protons protonness is concentrated near the center of the proton.  

There are caveats on this result.  If you are interested, please read the paper (click on the PDF link on the web page above), but overall it seems fairly (pardon the pun) solid.