Antimatter: Experts Trap Elusive Matter by Blasting It With Lasers

Scientists believed that antimatter plays a significant part in the story of our universe. It’s the equivalent to matter, and with protons, neutrons, and electrons it is similar in every way – but with an opposite electric charge. 

According to the laws of physics, the universe of today should be uniformly inhibited by both matter and antimatter.

However, it’s not. Antimatter is tricky, and one of the major problems in modern physics is how scientists can define a “symmetrical” universe of even parts matter and antimatter when, after years of searching, the universe seems to be almost entirely void of antimatter.

In an attempt to resolve this cosmic riddle, physicists are examining multiple characteristics of antimatter. In particular, scientists were curious about minute discrepancies between matter and antimatter that could explain the asymmetry – in turn verifying existing laws of physics.

But examining antimatter is incredibly challenging. It takes large amounts of energy to generate it, and even then it’s liable to disappear: obliterating itself when it comes into contact with the matter that surrounds us.

Scientists at CERN discovered a way to produce, trap, and laser-cool antimatter for long enough to target an entirely new set of more precise analyses. This experiment could be a vital step in solving the riddle of the missing antimatter in our universe.

Creating Antimatter

Just as matter is composed of atoms, antimatter is composed of antiatoms. The most elementary antiatom to make is antihydrogen, first produced by CERN in 1995 and first measured in 2012. It only had just one antielectron (called a positron) orbiting around one antiproton nucleus, antihydrogen (and hydrogen, its equivalent in the matter) has the most simplistic atomic structure in the universe.

But producing antihydrogen isn’t simple. The traditional high-energy physics strategy to the problem uses a particle collider – like the LHC at CERN – to transform enormous amounts of kinetic energy into a plethora of sub-atomic shrapnel for research.

Antimatter trap
This demonstration of our electromagnetic trap reveals how the forces it generates can hold charged particles in space. Source: Niels Madsen

Particle accelerators can be utilized to produce antiprotons. To create a single usable antiproton, though, scientists require 1 million protons and at least 26 million times the energy that’s ultimately “stored” in an antiproton. This causes each antiproton incredibly valuable.

Once scientists produced enough antiprotons, then antielectrons (positrons) build antiatoms. Happily, positrons can be quite simply collected from a radioactive source. With core elements collected, scientists combine them.

This is accomplished by pushing the antiprotons and positrons into contact within an electromagnetic trap. Crucially, this had to occur in a vacuum, because if the antiparticles were to make contact with any components of the apparatus – which was of course made of matter – they’d simply obliterate on contact, vanishing altogether. 

Measuring Antihydrogen

In this state, it’s feasible to do analyses on the antihydrogen. What scientists are looking to measure here is a fundamental atomic change between two energy states of the antihydrogen atom. This transition is especially suitable for accurate measurements, and the equivalent one in hydrogen has been measured with a staggering 15 decimal places of exactness.

This is more acute than the accurate measurement of ordinary hydrogen by a factor of 1,000, but it’s currently the most reliable measure of antihydrogen anyone has done.

But one important limitation to this measurement is the movement of the antiatoms in the trap itself, due to their kinetic energy. By lessening this movement further, the measurements would be far more accurate. However, for the first time, researchers at CERN achieved this, by blasting the antiatoms with laser light.

Laser Cooling

In its simplest form, quantum theory describes light in laser as consisting of discrete packets of energy, called photons which carry the momentum of their own. When an atom occupies a photon, the atom’s velocity slightly varies. By following this basic principle, scientists identified they could use the momentum contained in the laser beam to decrease the kinetic energy of the trapped antiatoms – cooling them closer to absolute zero.

Scientists only needed to hit the antiatoms with photons when they were impelling towards the laser, as this would help in reducing some of the velocity of the antiatom: a bit like how you’d apply force to slow a child on a swing.

Antimatter in liquid helium
Liquid helium maintains cool antihydrogen in the trap – but lasers reduce the temperature further. Source: Niels Madsen

By using this targeted laser-cooling, scientists succeeded in reducing the temperature of stored antihydrogen by a factor of ten, which has the potential to enhance future measurement accuracy by a factor of four.

Scientists have not yet made enough measurements to publish new, more accurate data on antihydrogen – but that’s coming very soon. Beyond that, the laser-cooling technique has put scientists on a firm path towards higher accuracy in many measurements of both matter and antimatter. It opens up exciting possibilities for measuring antihydrogen.

Journal Reference:

 “Laser cooling of antihydrogen atoms” by A. Capra, C. Carruth, C. L. Cesar, M. Charlton, C. J. Baker, J. M. Michan, T. Momose, P. S. Mullan, W. Bertsche, A. Christensen, R. Collister, N. Evetts, J. Fajans, T. Friesen, M. C. Fujiwara, D. R. Gill, P. Grandemange, P. Granum, A. Cridland Mathad, D. M. Starko, C. So, G. Stutter, T. D. Tharp, A. Thibeault, R. I. Thompson, D. P. van der Werf S. Eriksson, A. Evans, J. S. Hangst, E. Hunter, C. A. Isaac, M. A. Johnson, J. M. Jones, S. A. Jones, S. Jonsell, A. Khramov, W. N. Hardy, M. E. Hayden, D. Hodgkinson, P. Knapp, L. Kurchaninov, N. Madsen, D. Maxwell, J. T. K. McKenna, S. Menary, J. J. Munich, K. Olchanski, A. Olin, J. Peszka, A. Powell, P. Pusa, C. Ø. Rasmussen, F. Robicheaux, R. L. Sacramento, M. Sameed, E. Sarid, D. M. Silveira, and J. S. Wurtele, 31 March 2021, Nature.

DOI: 10.1038/s41586-021-03289-6

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