Antihydrogen

Scientists in Switzerland are making antimatter — an exotic type of matter that explodes on contact with ordinary matter. These experiments may help explain what happened just after the Big Bang.

For some decades, it’s been accepted in physics that the Big Bang that created our universe also created equal amounts of matter and antimatter.

Particles of antimatter would look like matter — but they’d have an opposite electric charge. Physicists also believe that, when antimatter and matter meet, they annihilate each other. Antimatter is rare in our universe, but ordinary matter is everywhere. And physicists wonder why that’s so, if matter and antimatter were created equally.

Also, why is there anything left? Why didn’t matter and antimatter completely destroy each other long ago? Rolf Landua is a particle physicist at the CERN laboratory in Switzerland. His team uses a particle accelerator to create antiparticles and manipulate them — to test the theories of physicists. A basic theory is that most of the matter and antimatter did mutually anihilate each other in the early universe. But due to a slight asymmetry, a tiny amount of matter was left — and this leftover matter is what now makes up you and me and the rest of the universe.

Rolf Landua: So you understand it’s a pretty big question, because if there was no asymmetry between matter and antimatter, we would not exist. And so, we are trying to elucidate that problem.

Everything we see and touch is made out of atoms, which in turn are made out of particles, like protons and electrons. In the 1930s, physicists theorized that in addition to these particles of matter, there are also particles of antimatter. These anti-particles should be just like ordinary particles, except that they have opposite electrical charges. For example, antiprotons — which were first discovered in the 1950s — are the opposite of protons. Positrons are the opposite of electrons.

A normal hydrogen atom is made of one proton and one electron. So an anti-hydrogen atom would contain one anti-proton and one positron. The first human-made anti-hydrogen atom was created several years ago. Now Landua and others are trying to create enough of it to test how it interacts with normal matter and how it interacts with light and gravity.

More Notes:

As exotic as it sounds, antimatter has become a fairly routine topic among physicists.

“When I talk about antimatter to my colleagues, they are not very excited about it. They say, ‘OK, so what’s new? What are you doing with it?'” Dr. Landua says. “When I talk to non-physicists about it, they look at me with great eyes and say, ‘God, it sounds so exotic.'”

In fact, it’s well established that turning energy into mass (a process described by Einstein’s famous equation: E = mc2) produces equal amounts of particles and antiparticles. Particle accelerators routinely use antiparticles as carriers of energy that smash into matter to make new states of matter.

The positron — or anti-electron — was discovered by Carl Anderson in 1932. In 1955, the first antiproton was discovered by Emilio Segre and Owen Chamberlain. The antineutron was discovered shortly thereafter. Physicists believe that most elementary particles have an anti-particle that has the same mass and spin, but has the opposite charge.

Antiparticles normally move around incredibly fast. The equipment at CERN is the only machine in the world that can slow antiparticles down to a tenth of the speed of light, making it possible to trap them and slow them down even more.

The machine works with a combination of electric and magnetic fields. Antiparticles zoom around in a circle through alternating magnetic fields at a rate of about a million times per second. On each lap, the antiparticles lose energy and slow down until they finally come to rest. Once the antiparticles have stopped moving, scientists can put them together to make atoms of antihydrogen.

The biggest obstacle to working with antimatter is that it disappears as soon as it comes into contact with matter. So the next goal of the ATHENA project is to find a good way to trap atoms of antimatter for further study and use. With current technology, the researchers can make 100 particles of antihydrogen every second. At that rate, it would take 10 to the 21 seconds to make a gram of it. “That is,” Dr. Landua says, “a little longer than the age of the universe.”

ATHENA is named after the goddess of wisdom. The researchers at CERN are seeking to illuminate some of their own.

From the ATHENA project web site:

“The laws of physics that govern the interactions of fundamental particles are often collectively referred to as The Standard Model. The Standard Model places some restrictive conditions on the relationship between matter and antimatter. Thus, comparing the characteristics of matter and antimatter serves to test the underlying theory of the Standard Model. Essential to the Standard Model is the so-called CPT theorem, which involves discrete symmetries. The CPT theorem requires that the laws of physics be invariant under the following operation: all particles are replaced by their antiparticle counterparts (Charge conjugation), all spatial coordinates are reflected about the origin (Parity), and the flow of time is reversed (Time reversal). The CPT theorem has important implications for antimatter, including the above-mentioned mass equivalence of particle and antiparticle.

“The CPT theorem also requires that atoms and their anti-atom equivalents behave in the same way. For example, hydrogen and antihydrogen should have the same spectrum Ð the frequencies or colors of light that they emit and absorb. It has long been a goal of physicists to be able to produce atoms of anti-hydrogen, in order to compare their spectra with that of hydrogen. An antihydrogen atom consists of an antiproton (negatively charged) and a positron (the antimatter counterpart to the electron). Antihydrogen atoms were first reported to be observed at CERN in 1996 and at Fermilab (near Chicago in the USA) in 1998, but these experiments produced very few antihydrogen atoms, and these at velocities close to the speed of light. The antihydrogen lived for a very short time before colliding with normal matter and annihilating. There was no possibility for making precision comparison measurements of hydrogen and antihydrogen in these experiments, which only demonstrated the existence of antihydrogen.

Cold Antihydrogen

“The ATHENA experiment takes a completely different approach to producing antihydrogen. The idea is to produce antihydrogen atoms at low energy Ð essentially at rest Ð in order to be able to study their properties. The AD machine at CERN was built to take antiprotons, which are produced in high-energy collisions, and decelerate them to more manageable energies.

“The ATHENA apparatus slows, cools, and traps antiprotons from the AD. The antiprotons are trapped in high vacuum in an electromagnetic ‘bottle’ known as a Penning trap. At the same time, positrons from a radioactive source are accumulated in another trap. The two clouds of charged particles (about 10000 antiprotons and 70 million positrons) are mixed together to produce antihydrogen. All of this takes place in a cryogenic environment at about 15 degrees above absolute zero.

“Antihydrogen atoms that are formed escape the electromagnetic trap because they have no net charge. They then annihilate and are detected by a specially built detector, unique to the ATHENA experiment. Antihydrogen produces a very characteristic annihilation signal in this detector, allowing researchers to confirm its production.

“To date, ATHENA has directly detected 131±22 atoms of antihydrogen. This implies that about 50.000 anti-atoms were actually produced in the apparatus, since most of them escape detection.

“The next step for ATHENA is to try to make measurements of the spectrum of antihydrogen and try to compare these to hydrogen. Any difference in these two spectra would require fundamental changes to our current model of matter and antimatter. These experiments could begin as early as next year, when the AD physics program resumes in May.

“The ATHENA result is a significant milestone in antimatter science, and it opens the door to the anticipated application of modern techniques of atom trapping, cooling, and manipulation to the realm of atomic antimatter. Tests of the behavior of antimatter under the influence of gravity are also an interesting future perspective.”

More excerpts from an interview with Dr. Landua:

CERN has a unique machine which makes antiprotons, which is where you need an accelerator for and then it’s able to not only produce these antiprotons but also to store them and slow them down to a leisurely speed for particles like a tenth of the speed of light, which is a unique machine in the whole world. There is no other machine like that.

There are neither antiparticles nor antimatter in this world except for those we are producing ourselves and except those produced by the cosmic radiation in our neighborhood.

That is our goal to make these comparisons between matter and antimatter so precise that we can make a statement about whether there is any intrinsic asymmetry or difference between matter and antimatter or not. That’s our final goal. It’s not something we expect will shake the world in the next few years. But if we did find something it would be very interesting because it would give another lead to why antimatter has disappeared from our universe.