The BASE collaboration at CERN has reported details of an experiment in which antiprotons were transported by truck and stored for more than a month in a portable apparatus, in a paper published in Nature on September 16. In the transport carried out on March 24, 92 antiprotons were moved 7.5 km around the CERN site with no confirmed particle loss, and during 33 days of storage monitoring, no annihilation from collisions with residual gas was observed.
Because antiprotons can annihilate on contact with matter, carrying them requires maintaining an extremely high vacuum and stable electromagnetic fields despite vibration and changes in the external environment during travel. The peer-reviewed paper shows what measurements were used to verify those conditions, and what is needed to bring antiprotons to quieter environments for precision measurements.
Confirming that 92 antiprotons remained, without losing any particles
An antiproton is an antiparticle with the same mass as a proton but a negative charge. BASE-STEP, developed by the research team, confines antiprotons with a "Penning trap," which combines electric and magnetic fields.
The magnetic field restricts lateral motion, while the electric field suppresses motion along the magnetic field, keeping antiprotons from touching the walls of the apparatus.
However, preventing wall contact alone does not allow long-term storage. Collisions with gas molecules remaining inside the apparatus also cause annihilation, so the trap requires cryogenic temperatures and a very high vacuum.
BASE-STEP has a cooling system operating at about 4.3 K and a superconducting magnet. While disconnected from external facilities, the electrodes and other components are powered by batteries, and cooling is maintained with liquid helium. According to the paper, the whole apparatus weighs about 850 kg. Moving so large a machine just to hold a few dozen antiprotons is a significant undertaking.
The drive in March lasted 24 minutes. From disconnection from external power and cooling to reconnection, the apparatus operated autonomously for 2.72 hours.
So how did the researchers confirm that 92 antiprotons remained after transport?
They used the faint "image current" induced in the electrodes by the antiprotons' motion. In a detector based on a superconducting circuit, interaction with the trapped particles produces a small dip in the noise spectrum. Under matched conditions, the width of this dip corresponds to the number of particles.
By isolating antiprotons and calibrating the signal down to a single particle, the team tracked the particle number without annihilating any antiprotons.
Measurements taken for 24 hours before and after transport gave dip widths of 334.08(8) Hz and 333.92(6) Hz. The numbers in parentheses indicate the uncertainty in the final digits.
The difference between the two is 0.16 Hz. By contrast, the loss of a single antiproton is estimated to change the width by 3.633(16) Hz. The observed difference is less than one-tenth of that, and from this result the team concluded that no particles were lost in road transport.
What matters is that the team did not merely confirm that some signal remained after transport, but compared before and after with enough precision to detect a change in particle number.
That said, a stable signal was not available throughout the drive. Transport shifted the resonance frequency of the detection circuit, and the periods in which the dip width could be calculated from the acquired data were limited. The detection system was also readjusted after the return.
For that reason, the absence of particle loss was confirmed by combining long measurements before and after transport, rather than relying only on monitoring during the drive.
Evaluating vacuum performance from 33 days of storage
The number of antiprotons in BASE-STEP was tracked for 33 days. This period included four tests in which the apparatus was disconnected from external facilities and operated autonomously, totaling 12.3 hours. Even so, no annihilation of antiprotons from collisions with residual gas was observed.
One point needs to be distinguished here: one antiproton was lost during the storage period.
The paper records that this one was lost during an operation that changed the electrode voltages to isolate particles. Thus, while there was zero loss during road transport, it is not the case that no particle was ever lost over the 33 days.
Meanwhile, the fact that no annihilation from residual-gas collisions was observed offers a clue for evaluating the vacuum inside the trap.
The higher the density of residual gas, the higher the probability that an antiproton collides with a gas molecule and annihilates. Conversely, if no annihilation occurs even when many antiprotons are observed for a long time, an upper limit can be placed on the residual gas density consistent with that result.
The quantity observed here is larger than the simple calendar time of 33 days.
Summing the storage time of each antiproton, the team obtained a cumulative observation time equivalent to "observing one antiproton for 8.28 years." This does not, of course, mean that a single antiproton was actually stored for more than eight years. It is the sum of the time over which many antiprotons were observed in parallel.
Treating the occurrence of rare annihilation events as a Poisson process, the team estimated from these observations a lower limit on the storage lifetime of 7.27 years at 68% confidence.
The storage lifetime here indicates the timescale over which antiprotons can be held in this apparatus. The team did not measure the antiproton's natural decay lifetime, nor did it operate the apparatus continuously for 7.27 years.
Using known annihilation cross sections, that is, the probability that an antiproton collides with residual gas and annihilates, the team also derived pressure upper limits for each component of the residual gas.
The estimates were below 9.6×10⁻¹⁹ mbar for hydrogen and below 2.2×10⁻¹⁸ mbar for helium. These are not values measured directly with a pressure gauge, but upper limits derived by applying the observation that the antiprotons remained without annihilating to a physical model.
The point is not to prove a perfect vacuum. It is enough to show that losses from residual gas are small enough for long-term storage of antiprotons.
That a portable apparatus equipped with paths for moving particles in and out maintained this level of vacuum performance even after transport is an important result for future precision measurements.
Separating measured values, statistical estimates, and future plans
The figures presented, "zero loss in transport," "33 days of storage," "a lower limit of 7.27 years on storage lifetime," and "an improvement in precision of more than 100 times," each mean something different.
The statements in the BASE group's September 16 paper on transport, storage lifetime, and future prospects can be organized as follows.
| Item | What the paper reports | What can be read from it, and limits |
|---|---|---|
| Road transport | 92 antiprotons transported 7.5 km with no particle loss | Demonstrated under the transport conditions of this trip on the CERN site |
| Storage observation | 33 days, no annihilation from residual gas observed | One particle was lost during a particle-isolation operation |
| Storage lifetime | Lower limit of 7.27 years at 68% confidence | An estimate from cumulative observation time and a statistical model, not 7.27 years of actual operation |
| Precision measurement | Improvement in precision of more than 100 times anticipated in the future | A future goal premised on moving antiprotons to a laboratory with a quiet magnetic environment |
Source: Nature paper by Leonhardt et al.. This table is not meant for direct numerical comparison, but to sort out what kind of evidence each claim rests on.
Seen this way, the 33-day storage test is more than a supplement to the transport experiment.
To continue experiments after carrying antiprotons to a destination, it is not enough for particles to remain on arrival; they must also be storable afterward while maintaining a sufficient vacuum.
Short road transport and long-term storage observation each verify different conditions.
Separating where antiprotons are made from where they are measured
CERN's Antimatter Factory is a hub that supplies low-energy antiprotons to precision experiments. At the same time, the operation of large facilities such as accelerators causes slight fluctuations in the surrounding magnetic field.
In experiments that measure the frequencies of antiproton motion and spin with high precision, fluctuations in the reference magnetic field become a factor limiting measurement accuracy. As a result, the place suited to producing antiprotons does not necessarily coincide with the place suited to extremely high-precision measurements.
BASE investigates quantities such as the charge-to-mass ratio of the proton and antiproton and the magnetic moment, which describes a particle's magnetic properties.
Such comparisons serve as a way to test "CPT symmetry," the principle that the laws of physics do not change when particle-antiparticle exchange, spatial inversion, and time reversal are performed together.
If differences between the properties of protons and antiprotons beyond the Standard Model's predictions were found, they could offer clues to new physics.
Understanding why matter, rather than antimatter, remained in large quantities in the universe is also one of the major motivations for this research.
However, this transport experiment did not discover any new difference between protons and antiprotons. Its purpose is to raise future measurement sensitivity by making it possible to carry antiprotons to laboratories with low magnetic noise.
Technical challenges remain before longer-distance transport can be realized.
The current apparatus has been tested for up to four hours of autonomous operation, but the trip the team aims for in the future, from Geneva to Heinrich Heine University Düsseldorf in Germany, would take about 10 hours.
In addition to power and cooling systems suited to long-distance transport, technology will be needed to transfer antiprotons safely and in a controlled state from the portable trap to the precision measurement apparatus at the destination.
Only when this whole sequence is established, with antiprotons carried to the destination without loss and still usable for precision measurement after transfer to the local apparatus, will researchers be able to compare matter and antimatter in a quieter environment away from the antiproton supply facility.