Physicists in Europe have reported that it’s possible to transport charged particles on a truck for four hours without disturbing them in any way. This seemingly run-of-the-mill announcement, reported in Nature on May 14, actually contains within its details the possibility of “a new era of precision antimatter spectroscopy”, in the team’s words.
This is because of how the world currently studies antimatter, an elusive form of matter with some properties switched. For example, the electron’s antiparticle is the positron: it has the same mass but behaves like the electron’s mirror image. When a particle touches its antiparticle, they annihilate each other in a flash of energy.
Physicists study antiparticles for what they can reveal about the still-mysterious things about our universe, such as dark matter. They’re also keen to crack the baryon asymmetry problem, which had a breakthrough reported on July 16.
The problem is that antimatter is hard to produce in a machine made entirely of matter. What little scientists already know is based on studying antiprotons and atoms of hydrogen and helium made of antiprotons and positrons at the Antimatter Factory (AMF) at CERN, the European nuclear physics research facility more famous for hosting the Large Hadron Collider. Specifically, the problem is that AMF has instruments to study antiparticles but have limited sensitivity. Antimatter particles are also very sensitive to magnetic fields and the AMF hall has other instruments that emit such energy.
In an ideal world, physicists should be able to produce antiparticles at AMF and transport it to a lab that has very good instruments to study them. The new study delivers a proof of concept showing this is now possible.
At the heart of their effort is a Penning trap, a device that uses a combination of electric and magnetic fields to confine charged particles in a cylindrical tube. The magnetic field is uniform and flows through the cylinder’s central axis, holding the particles together like a string of beads. The electric field is quadrupole, like two positive and two negative charges forming a square shape with alternating charges at the vertices. This field keeps the particles from drifting away.
In their truck, the physicists used a BASE-STEP Penning trap, a special kind of trap developed by the BASE collaboration at CERN. It’s a “Penning-trap system inside the bore of a superconducting magnet that can withstand transport-related forces.” Among other features, it uses cold beryllium ions to cool the stored particles and includes precision measurement techniques to study them.
The team first moved a trap containing a cloud of around 100 protons out of the AMF hall using a pair of overhead cranes, then loaded it on a truck and drove 3.7 km through CERN’s Meyrin campus. They hit a maximum speed of 42.2 km/hr in this run.
The setup included magnetic shielding and a “transport frame to handle acceleration forces apart from gravity of up to 1 g in all directions”, per the paper. On the truck, the apparatus was cooled by an “internal 30-litre liquid helium tank”. The precision voltage supply, frequency generators, and a spectrum analyser were run by a “UPS with two battery units”.
In all, the device — 2 m long, 1.6 m tall, and 0.85 m wide — weighed about 900 kg.
“In our future planned antiproton transport experiments,” the team wrote in its paper, “the last step in the transport campaign would be the extraction of a fraction of particles from the trapped antiproton reservoir, followed by the injection of the extracted fraction into a receiver experiment. Although we do not have such a receiver available yet, we have demonstrated particle separation and extraction after returning to the experiment zone.”
The reason for going to all this trouble with a truck is an idea in physics called CPT symmetry. The material world is made of matter and all matter is made up of subatomic particles. While physicists know a lot about the material universe, there are still many unknowns and lots of room left for physicists to explore and learn. CPT symmetry is one corner of the room.
Each letter stands for a kind of transformation. C (charge conjugation) means switching a particle with an antiparticle. P (parity) means switching left and right, like looking in a mirror. And T (time reversal) means reversing the flow of time. In quantum field theory (the tool physicists use to understand the physical properties of subatomic particles), CPT theorem states that if you applied all three operations to a particle, physics should be the same.
That is to say, if you studied for a physics exam and then someone applied CPT to all particles in the universe, the answers to your question paper wouldn’t change.
CPT symmetry has a sobering history. At the dawn of quantum field theory, scientists assumed all subatomic particles conserve C, P, and T symmetries separately. Then they found that wasn’t true, so they moved the goalpost and said all particles conserve CP symmetry. An experiment in 1964 challenged this as well when physicists found particles called kaons violated CP symmetry. Finally physicists moved the goalpost even further, saying that all subatomic particles should be expected to conserve CPT symmetry.
Since the C part of the symmetry requires swapping a particle with its antiparticle, physicists need both matter and antimatter to check if particles obey or violate CPT symmetry. The more the merrier, too: larger quantities of antimatter will make it easier to tease out any subtle effects that might point to a violation. If all such subtlety can be ruled out, CPT symmetry will hold and physicists might breathe easier.
In the truck study, of course, the physicists only used ~100 protons. “Although the number of stored particles was not pushed to the limit,” they explained, “the transported number would already be enough for our high-precision experiments to operate for several years. As an example, the non-destructive high-precision measurements performed in BASE … typically consume around six antiprotons per year.”
So far, researchers have verified CPT symmetry conservation in anti-hydrogen (hydrogen atoms made of an antiproton and a positron) and anti-protonic helium (helium atoms with antiprotons). These are of course anti-atoms and are considered a type of particle called baryons.
Baryons are the most well-known matter particles: they include protons and neutrons as well as all atoms. Your body, for example, is baryonic matter. Physicists are keen to crack the universe’s baryon asymmetry mystery, too — and the answer is expected to have to do with some particles violating CPT symmetry in a hitherto unknown way.
The physicists wrote in their paper that their findings indicate the “next generation” of antiproton studies could reduce the uncertainty in measurements by a factor of 10 from the “present state of the art”.
Featured image: Left: The route for the first transport demonstration through the AMF hall. Point 1 is the experiment zone from which an overhead crane moved the transport frame to point 2. At point 2, the transport frame was loaded onto a trailer and moved to point 3, where it then got picked up by the second overhead crane. Point 4 is the loading bay with the truck. Right: Road map of the Meyrin site of CERN and the GPS position data recorded during transportation. Credit: Nature 641, 871–875 (2025).
