How is antimatter produced?
In this section you will find answers to the following questions:
The first form of antimatter observed in nature was the antielectron (or positron). This observation was possible because there are many natural processes which produce these particles and the chance of observing them (once you know how and what to look for) is quite high. However, to observe other antimatter particles which are not so frequently produced in nature (such as the antiproton or antineutron), scientists built large, specialized machines that aimed to produce and detect these particles.
However, creating antimatter is not easy. Big quantities of energy are needed. This is what Albert Einstein's famous formula E = mc2 tell us: mass is nothing by energy in a very condensed form. This means that matter can be transformed into energy and energy can be transformed into matter. The process can be observed in nature regularly, for instance, in the combustion of stars. In the sun, 600 million tons of hydrogen turn every second into 595 million tons oh helium and 5 million tons of energy.
E = mc2
We have also learned to produce quantities of energy such that new matter particles are spontaneously created in the process, and so are their equivalent antimatter pairs. The technique entails accelerating particles smaller than the atom (i.e. subatomic particles) to high speeds and smashing them against a metal block or against one another. Some of the energy released in the crash transforms into matter and antimatter. But, in order to achieve this effect, the subatomic particles (normally protons) have to move at speeds infinitesimally close to the speed of light. This is achieved in so-called Particle Accelerators, enormous devices that accelerate particles to high speeds by means of electric fields. CERN, the European Organization for Nuclear Physics, hosts the largest and most powerful accelerator in the world. It is the Large Hadron Collider (LHC), an underground ring, 10 km in diameter and 27 km in length at a depth of 100 meters below ground near Geneva, Switzerland (you can see its dimensions in this map of the zone). Together with CERN, other laboratories in the world produce antimatter nowadays. Fermilab, in Chicago, is one of the most important.
Impact of electrons (e-)on the metal and production of antiparticles (e+) and gamma rays (g)
The first production of antiprotons was accomplished by the physicists Emilio Gino Segré and Owen Chamberlain in 1955 at the University of California (Berkeley) bombarding a copper shield with high energy protons. The experiment was carried out in the particle accelerator built by Ernest Lawrence (Nobel Prize in 1939) at the Lawrence Berkeley National Laboratory. It was the Bevatron, a ring capable of accelerating protons to an energy of 6.2 GeV (gigaelectrons-volt). Segré and Owen's discovery earned them the Nobel Prize in 1959. Nowadays, the largest particle accelerator in the world - the Tevatron at the Fermilab laboratories in Chicago - is capable of generating energies of up to 1.8 TeV (1800 GeV) by colliding protons and antiprotons.
Only a year later, in 1956, B. Cork, O. Piccione, W. Wenzel and G. Lambertson, a second group of Berkeley researchers, announced the discovery of a third antiparticle: the antineutron.
Once the existence of these three antiparticles (antielectron, antiproton and antineutron) had been proven, the efforts shifted to the production of the first antimatter atomic nuclei. The goal was fulfilled in 1965 with the observation of the antideuteron, the nucleus of the atom of antideuterium, consisting of an antiproton and an antineutron. Two teams of researchers succeeded in producing it at the same time: Leon Lederman's team at the Brookhaven National Lab (New York) and Antonino Zichichi's team at CERN.
The following big goal was the production of whole atoms of antimatter, instead of just nuclei. The chosen atom was hydrogen, because it was the simplest element (with just one electron and one proton in its structure). Hydrogen is also one of the best studied elements, and constitutes three-fourths of our universe and the basis for our scientific knowledge of it. In 1995, 30 years after the production of the first nuclei, a team of researchers from Germany and Italy finally announced the production of the first antihydrogen atoms. It was experiment PS210 and it was carried out at CERN. The experiment was possible thanks to a new machine: the Low Energy Antiproton Ring (LEAR). This device decelerated the antiprotons produced in the particle accelerator and stored them in a ring, so that scientists could study them at their leisure and, for instance, combine them with positrons to produce stable atoms. A year later the laboratory Fermilab announced a similar production of antihydrogen atoms as well.
In spite of the importance of these events, these experiments allowed neither direct investigation of the characteristics of antimatter, nor direct comparison with conventional matter. There were three main reasons for this. First, only a few atoms of antihydrogen had been produced. Second, those that had been observed were traveling at a speed close to the speed of light (too fast to analyze their properties). Finally, the atoms annihilated near instantly and disappeared. In order to conduct future detailed studies of the anti-atoms, large quantities of "cold" hydrogen would need to be produced and also stored.
Some of these challenges have already been met. With current technology, particle accelerators at CERN can produce bunches of about 50 million antiprotons every minute. The antiprotons are slowed down (cooled) in the so-called Antiproton Decelerator (AD), which is capable of reducing the temperature of the antiprotons from billions of degrees to temperatures usable in our experiments. These bunches of cold antiprotons allow for the creation of thousands of atoms of antihydrogen. Using the AD, the ATHENA Project (predecessor of ALPHA) and, shortly afterwards, the ATRAP group (from Harvard University) were the first to produce large amounts of cold antihydrogen in 2002.
Producing antihydrogen requires, together with the antiprotons provided by CERN, the participation of positrons (positively charged electrons). Positrons in the ALPHA experiment are produced from special radioactive sodium. Given that these particles possess electric charge, they can be easily trapped and stored using magnetic and electric fields. Next, positrons and antiprotons are combined to form antihydrogen. However, antihydrogen is a neutral atom and cannot be held by the same kinds of electric and magnetic fields that retained its constituent particles. This is the reason why, once produced, the antihydrogen drifts into the wall of the experiment and annihilates (in fact, this is the effect that ATHENA used to detect the first production of antihydrogen). ALPHA is currently developing a trap to contain the antihydrogen for a time long enough to allow the study of its properties. However, what kind of container can we use, given that antimatter annihilates in contact with any form of matter?