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.

 

 

 

 

3I/alpha re-emerges from behind the Sun. But, is it an alien artifact?

 The interstellar object (visitor from another sun) 3I/ATLAS has reemerged into view, after being hidden behind the sun.  It continues to attract considerable attention, from both telescopes and humans.  

 The images above, taken by the James Webb telescope are typical of recent images.  In this and other images, it appears to be a comet, with a small central nucleus, with a large coma (ice/dust cloud) on one side.  This cloud is expected; as a comet approaches the sun, surface ice is melted and ejected, taking some of the comet's rock/dust with it.  The coma is quite visible since the dust and ice is effective at reflecting sunlight.  Most of the other pictures are less detailed; there just wasn't much light to work with, so the images are fuzzy.  

3I/ATLAS has attracted much attention on two fronts.

First, it is only the third extraterrestrial object seen.  It appears different from both  1I/ʻOumuamua, which is a bit of an oddball by both accounts, and 2I/Borisov, which also looks a lot like a comet, but is  considerably smaller than 3I/ATLAS.   But, it looks enough like a comet so that NASA has officially called it one (rather than an asteroid).  In fact, if anything, the surprise is that it looks so much like the garden variety comes from our solar system. 

Second, there has been considerable speculation that it be an artifact produced by an alien civilization.  The reflected light has been postulated to be light emitted by the object.  There were also suggestions that it would alter course and head for Earth, or make other dramatic maneuvers.  However, the acceleration that we see is all consistent with a combination of gravity, plus the expected velocity changes due to the out-gassing mentioned above.

The alien-object hypothesis has some attraction.  It would be very very cool, and would radically change our view of the galaxy.  But, extraordinary claims require extraordinary proof.  That is lacking here.  There is no evidence of active maneuvering.  The observed light looks much like we expect from a comet.  

So, scientists have, very reasonably, fallen back on more mundane studies.  An interstellar visitor is plenty cool, even if it 'just' a comet from another solar system.

Antarctica - redux

The  Lawrence Berkeley National Laboratory Engineering Division communications people have put out a nice story about  Thorsten Stezelberger and my work in Antarctica - both ARIANNA and IceCube.  It is available at https://physicalsciences.lbl.gov/2025/10/14/visiting-antarctica-in-service-of-science/

This is somewhat timely, as the 2025/2026 Antarctic season ramps up.  For neutrino astronomy enthusiasts, this is a big one, featuring the deployment of the IceCube Upgrade.   The upgrade will consist of seven additional strings.  The strings will be densely packed (close together), with an energy threshold of a few GeV, giving IceCube a huge boost at low energies.  This should enable us to make a definitive measurement of tau-neutrino appearance through oscillations, and help us pin down the parameters for neutrino oscillation more precisely than previously possible.  The upgrade will also be filled with calibration devices that should allow us to measure the optical characteristics (scattering and absorption)  of the ice far more precisely.  this ice-property data should improve the accuracy of all IceCube measurements, by reducing our systematic uncertainties from ice property uncertainty.  

Alien visitors, redux: Why are we seeing them now?

 After my last post on 3I/ATLAS, an interested reader (full disclosure - my wife, Ruth Ehrenkrantz) asked an interesting question: Why are we only seeing the interstellar visitors now?  We have now seen three in eight years, after centuries of nothing?  

 The answer to this question is: technology.  Finding small (asteroids and comets both qualify) objects in our solar system is a matter of taking multiple photographs of the sky, spaced days or weeks apart, and looking for points of light that are moving against the fixed background of stars.    One needs to look at a lot of images to find a few moving objects.   Finding the even rarer (cf. three in eight years) objects that are moving too fast to originate in the solar system is even harder, and requires fast searches of large regions of the sky.

High-speed searching requires automation, to methodically take photos at appropriate intervals and automatically compare the images.  Correcting for different seeing conditions is not trivial - if the atmosphere is a bit more turbulent or hazy, images will look different.      This is technology that has been developed over the past few decade, driven by "Planetary Defense"which even has its own office at NASA.  Despite a name evocative of "Space 1999," this office is focused on a more prosaic concern, the possibility that an errant asteroid or comet might hit the Earth, ala the Tunguska meteorite or, worse, the asteroid that wiped out the dinosaurs (as shown in the visualization below, from New Scientist magazine). 

One well known search program is Asteroid Terrestrial-impact Last Alert System (ATLAS), which found 3I/ATLAS.  It uses five robotic telescopes, like the one shown above to search around the ecliptic plane (the plane containing the orbits of the planets around the Sun), looking for objects that might come close enough to Earth to pose a danger.  When it finds a moving object, it will follow it to determine its trajectory, and project its movement into the future, to see if it will come close to Earth, or even might hit it.  Of course, the further one projects into the future, the larger the uncertainty.  

Sometime - three times so far - these automated searches will find objects that are moving too fast to be gravitationally bound to the Sun.  It is not at all surprising that our observations of these extra-solar objects has started in the last decade, as searches like ATLAS have ramped up.   

Incidentally, although 3I/ATLAS looks like a typical comet, that has not stopped some astronomers - notably Harvard's Avi Loeb - from speculating that it may be an extra-terrestrial probe, on the basis that it is coming surprisingly close to several planets (not Earth, though).   Other astronomers continue to study it, and nothing too surprising has been observed.  Recently, it has been found to contain water, for example. 

Another alien visitor enters the Solar System

 The Solar System recently welcomed another visitor from another star:  3I/ATLAS.  We don't know much about it yet, but it seems considerably larger than ‘Oumuamua (which I wrote about previously) and 2I/Borisov - maybe 5 kilometers 3 miles) across, although considerably larger diameters - up to 12 km - have also been suggested, based on the hypothesis that it reflects only a small fraction of the incident light.  It is thought to be comet-like, with the outside made up of grains of water ice and silicates.  As the surface is heated by the sun, it is likely to form a coma - a long visible tail of gas and dust released as the ice melts.  Watching this evolution will tell us much about the nature of 3I/ATLAS. 

When it was spotted, it was roughly as far away as Jupiter. It is approaching rapidly and should reach 1.3-1.4 astronomical units (AU) from the Sun (1.3-1.4  times as far away from the Sun as the Earth is) in late November.  Unfortunately its closest approach to the Earth will only be at 1.8 AU.  Worse, when that happens, it will be on the far side of the Sun from us.   Depending on how much it brightens, it should be visible with relatively small telescopes, though.  So far, it appears to have a reddish color, perhaps a sign that it contains methane.   Interestingly, its trajectory is near the ecliptic plane of the Solar System - the plane that contains all of the planets orbits. 

It is also moving fast - faster than ‘Oumuamua -  and will reach 150,000 miles/hour during its closest approach to the Sun.  This simplified the detection, as can be seen from the gif below, combining images taken with the Asteroid Terrestrial-Impact Last Alert System:

 The moving dot shows how 3I/ATLAS stands out from the stationary background stars.  It is coming from the general direction of the center of our galaxy, which is also the direction of the highest density of nearby stars.   It was likely ejected from some other solar system by a close encounter with a planet which left it with a velocity high enough to escape from that sun.  

The three alien visitors that we have discovered are all quite different.  ‘Oumuamua is an oddball, while 2I/Borisov and 3I/ATLAS seem, except for their much higher velocities, comet-like, not too different from  the other comets that we see in the solar system.  More data is needed to draw any conclusions, but it does seem that these interstellar objects are relatively common, with three discovered in 8 years.  

Because of their large velocities and short notice of the arrival times, we probably won't be able to get a sample from one of these objects any time soon.  But, if we are patient, it may be possible to do a fly-by and get some high-resolution photos.  

Two slit interferometry – the fully quantum edition

 

Today, I want to write about something a bit different – a unique 2-slit interferometer (aka the double-slit experiment).  In the standard 2-slit interferometer, a beam of photons, electrons, ions or whatever spreads out and passes through two slits.  The beams passing through the two slits recombine, producing an interference pattern.  You might have seen this demonstrated in high school or college, but the basic idea is pretty straightforward.

Some standard elements of this are that the source is a single beam, and then that the beams recombine after passing through the two slits.  Today, I want to discuss a unique version of this experiment that lacks both of these elements.    

This interferometer consists of two relativistic heavy ions that encounter each other at a collider.   One ion can emit a photon, which can then scatter off of the other ion, and emerge as a particle – this can be a rho⁰, phi , J/psi or other vector meson –particles with the same quantum numbers as the photon.   At low transverse momentum), we cannot tell which nucleus emitted the photon, and which is the target.  Quantum mechanics (QM) tells us that these two possibilities can interfere with each other.  Because vector mesons have negative parity, this interference is destructive, and the production rate goes to zero as the transverse momentum approaches zero.  This suppression was experimentally observed by the STAR Collaboration back in 2009.

One important point is that the two ions come from completely different directions and share no common history, so there is no single source!  Instead, the two sources are synchronized in-phase by the symmetry of the problem. 

Another unique feature is that vector mesons are very short-lived, with the rho⁰ lifetime about  10^-23 seconds, so that, even moving at the speed of light, a rho⁰ travels about 1 fermi (1 fermi = 10^-15 m) before decaying.  This is far less than the minimum ion-ion separation, which is about 15 fermi.   When the rho⁰ decays, it produces two charged pions (pi⁺pi^-).   Because of the near-immediate decays, the two ions are essentially sources of pion pairs.

So, any interference pattern can only appear much later, when the pi⁺pi^- amplitudes overlap.  However, the p⁺ and p^- go in nearly opposite directions, and are well separated by the time pions from the two sources can overlap.  In short, the only way for this interference to happen is if the amplitudes summing and interference appear long after the rho⁰  decays, and the system has spread out.   Strange as this sounds, this well explained by quantum mechanics where the wave function collapse occurs when the interference is observed.   The question of when this observation actually occurs is the subject of debate, but one can think of it happening when the pions interact in the detector.  The figure below nicely illustrates these points of the interference.  

 Why am I writing this now?  Four colleagues and I have written a long review paper discussing this interference, which has just been published in Progress inNuclear and Particle Physics (with open access); it is also available here onthe arXiv.  The article also recaps the data that supports the existence of this interference - both the suppression above, and some interesting angular effects.  

Many aspects of this interference may be somewhat counterintuitive.  However,  they are in accord with the ideas of quantum mechanics, and, more importantly, are well supported by data.  So, they are useful descriptions of the way that Nature works.

Enjoy.  

Neutrinos are lighter

 One of the biggest questions in neutrino physics is: how heavy are they?  We know that there are three types of neutrinos, associated with the electron, muon and tau leptons.  By observing their oscillations, we can observe the mass differences (actually, the square of the mass differences, so we're not sure which is heaviest) between these three types.  However, their actual masses are still unknown.  

Over the past few decades, we have worked to measure their absolute masses, but so far have only determined a succession of ever-more-stringent upper limits.  Most attention is focused on the electron neutrino, because it allows for the most stringent limits.  The technology of choice is the beta decay of tritium - an isotope of hydrogen consisting of one proton and two neutrons.  Tritium decays into helium three, plus an electron and an electron-neutrino, with a half life of 12.3 years.  This is a low-energy decay, releasing about 18,590 electron volts of energy.  For comparison, other, more typical beta decay reactions release ten to thirty times as much energy.  Another point of comparison is an old-fashioned cathode-ray tube in a television receiver, which uses electrons accelerated to up to 25,000 volts - more than from tritium beta decay.

In these beta decay reactions, some of the energy is carried off by the neutrino, with most of the rest going into the electron; the recoiling helium-3 also carries off a bit of energy.   Because beta decay is well understood, we can very accurately predict the distribution of energies carried off by the electron; this predicted spectrum has a small dependence on the neutrino mass;.  The number of events where the electron carries off almost all of the energy decreases as the neutrino mass increases.  We can fit the electron energy spectrum in this endpoint region to determine the neutrino mass. 

The KATRIN collaboration has done this, and just released a new upper limit on the electron-neutrino mass; it is less than 0.45 electron volts, at a 90% confidence level.  For comparison, this is less than one one-millionth of the mass of the electron.  It is about a factor of 2 below previous measurements.  The result is an enormous tour-de-force, since the change in the energy spectrum is tiny.  The collaboration used an enormous spectrometer (pictured above) and studied 36 million electrons, which were already selected for being of particularly high energy.  

Although KATRIN will release results with more data, and, likely, a tighter upper limit, this is near the end of the line for the spectrometric approach that we have used for the past few decades.  A better spectrometer would also have to be a bigger spectrometer, and it is just not possible (economically, and probably technically) to build a much larger instrument.  Instead, we are starting to investigate new approaches that involve trapping electrons in a uniform magnetic field, and observing the synchrotron radiation that they produce, using very sophisticated radio detectors.