ANTIMATTER (part 2/5)

History of Antimatter

In 1928 Paul Dirac developed a relativistic equation for the electron, now known as the Dirac equation. Curiously, the equation was found to have negative energy solutions in addition to the normal positive ones. This presented a problem, as electrons tend toward the lowest possible energy level; energies of negative infinity are nonsensical. As a way of getting around this, Dirac proposed that the vacuum be considered a "sea" of negative energy, the Dirac sea. Any electrons would therefore have to sit on top of the sea.

Thinking further, Dirac found that a "hole" in the sea would have a positive charge. At first he thought that this was the proton, but Hermann Weyl pointed out the hole should have the same mass as the electron. The existence of this particle, the positron, was confirmed experimentally in 1932 by Carl D. Anderson.

Today's standard model shows that every particle has an antiparticle, for which each additive quantum number has the negative of the value it has for the normal matter particle. The sign reversal applies only to quantum numbers (properties) which are additive, such as charge, but not to mass, for example. The positron has the opposite charge but the same mass as the electron. An atom of antihydrogen is composed of a negatively-charged antiproton being orbited by a positively-charged positron.

   

From 1928 to 1995

    The history of antimatter begins in 1928 with a young physicist named Paul Dirac and a strange mathematical equation...

    The equation, in some way, predicted the existence of an antiworld identical to ours but made out of antimatter. Was this possible? if so, where and how could we search for antimatter?

    From 1930, the search for the possible constituents of antimatter, antiparticles, began, and it has been the main influence behind a major scientific and technical evolution over the last 70 years.

    The acceleration era

    Pioneer machines-

    By the 1930s, after the invention of the cyclotron by Ernest Lawrence, it was clear that accelerators were the way forward if physicists wanted to go deeper in to the structure of matter.

    Initially the USA led the way: such machines were just too big and expensive for any European country to build alone. But in 1954, European physicists decided to create in Geneva, Switzerland, a central, European laboratory, that they called CERN (Conseil Européen pour la Recherche Nucléaire). Since then, CERN has played a major role in most technical and scientific developments in High Energy Physics.

    From the early one-magnet cyclotrons and betatrons which could accelerate protons and electrons, respectively, up to some tens of MeV in energy, the new "doughnut-shaped" synchrotron evolved, able to accelerate both types of particle up into the GeV energy range. By the 1950s, using a new focusing technique, machines were pushing up to around 30 GeV.

    From then on, through into the early 1970s, several major steps forward in the search for the basic structure of matter were achieved. An avalanche of new particles was found, thanks to the successful integration of powerful accelerators (like the 28 GeV Proton Synchrotron (PS) at CERN and the 33 GeV Alternating Gradient Synchrotron (AGS) at Brookhaven) and a new, effective particle detector, the bubble chamber.

    Colliders-

    Soon after starting out on the great accelerator adventure, physicists realised that when an accelerated beam of particles hits a stationary target, most of the energy is wasted in the recoil of the target and only a small fraction is left for the real purpose of studying particles and their interactions. If, instead, two particle beams collided head-on with each other, no recoil energy would be wasted, and all the energy would be available for the experiment - think how much more devastating is the head-on collision between two speeding cars than the collision when one of the cars is stationary.

    While other laboratories concentrated on colliding beams of electrons, CERN worked on protons. The idea was to take proton beams from the PS, feed them into the two interconnected rings of a new machine and then force them to collide. The 31+31 GeV Intersecting Storage Rings (ISR), after overcoming many technological challenges, produced the first proton-proton collision in 1971.

    Meanwhile, also particle detectors had to undergo major developments, and the old bubble chamber was replaced by faster and more modern techniques, able to record the increasing number and size of interactions. But a major step had still to come: in the 1980s, made possible by the cooling techniques, available antimatter entered the game, gaining soon a predominant position.

    Two parallel ways opened up to further developments of accelerators; in one, physicists would keep using antiparticles as a tool for further inquiring on the fundamental constituents of matter, going towards the high energy frontier of our knowledge. In the other, antiparticles would became the main subject of study, decelerated to low energy and isolated to explore the properties of antimatter.

        High energy frontier:

        First, in the 1960s, came the electron-positron collider. After Anderson's discovery of the positron, physicists soon learned how to create large quantities of positrons (the interaction of radiation with matter can produce an electron and positron simultaneously). Several colliders were built in Europe and the USA, and with them came many important discoveries about the fundamental nature of matter and our universe.

        The first electron-positron collider was the "Anello d'Accumulazione" (AdA), built by Bruno Touschek in Frascati (Rome) in 1960. The biggest of all is CERN's Large Electron Positron (LEP) collider, which began operation in the summer of 1989 with a collision energy of 91.2 GeV. The year 2000 will be LEP's last year of running, having now reached a massive collision energy of 204 GeV. The detectors around the LEP ring have been able to perform extremely precise experiments, testing and extending our knowledge of particles and their interactions.

        In fact LEP will remain the largest circular electron-positron collider ever built: a property of electrons called "synchrotron radiation" makes it impossible to accelerate electrons to higher energies in a bigger circular collider. But plans are already afoot for the next generation of electron-positron collider - this time as linear colliders, where the electrons and positrons will collide head-on after being accelerated in a straight line over many, many kilometres...

        The proton - antiproton collider, complementary to the studies and discoveries made with electron-positron colliders, unfortunately presented a much greater challenge. Since an antiproton (or proton) is almost 2000 times heavier than an antielectron (or electron), it takes a lot more energy to create them. It was also more difficult to collect antiprotons and store them long enough to make an antiproton beam circulate in a collider.

        However, in the early 1980s, Simon van der Meer at CERN invented "stochastic cooling" - a technique that now made it possible to accumulate, concentrate and control antiproton beams. CERN's Super Proton Synchrotron (SPS) became a 300 GeV proton - antiproton collider, and in 1983 the UA1 experimental team, led by Carlo Rubbia, saw two new particles, the W boson and Z boson, being produced in the SPS collisions. Physicists had suspected for many years that these two bosons existed, and this exciting confirmation brought Rubbia and van der Meer the 1984 Nobel Prize for physics.

        Today the most powerful proton - antiproton collider is at Fermilab, Chicago. With a collision energy of up to 1.8 TeV (that's 1800 GeV!), the Tevatron made news in 1995 with the discovery of the top quark.

        And there's more... Since the early 1990s, CERN has been preparing for its next big collider: the Large Hadron Collider (LHC) will replace LEP in its underground tunnel and collide two proton beams at a record 14 TeV (14,000 GeV!).

        But why proton-proton collisions, and not proton-antiprotons? Well, at such high energies as 14 TeV, proton-proton collisions and proton - antiproton collisions start to look pretty much the same. And as it's still so much easier to produce proton beams than antiproton beams, physicists have chosen to maximise the rate of collisions by just using two very dense proton beams - and thus to maximise the chances of a new discovery.

        The LHC is now under construction at CERN, and four experiments ATLAS, CMS, LHCb and ALICE will be arranged around the collider ring.

        Low energy frontier:

        With the discovery of the cooling technique, available antimatter became a very important tool in particle physics. Dedicated machines were built to handle the different stages of antimatter production, accumulation and acceleration. Since the early days though, they were mainly used to fill the specific needs of high-energy experiments: beams of increasing energy were the goal of most laboratories.

        But there are a lot of interesting things one can do with low energy antiprotons, and low energy (low speed) is definitely the only way to directly test the presumed symmetry between matter and antimatter. Slow antiprotons can be captured in real "traps", and their properties (mass, behavior in a magnetic field,etc.) compared to those of protons. One can make full pieces of antimatter, antiatoms made out of positrons and antiprotons.

        CERN was the only laboratory which chose to specifically invest on this research line. In 1980 it decided to build a new machine, able to "decelerate" the antiprotons produced and stored by its existing rings. In 1982 the Low Energy Antiproton Ring (LEAR) appeared: it could decelerate the antiprotons coming from the PS to different intermediate energies, down to a few MeV.

        Several important scientific achievements were possible thanks to LEAR, one of them being the assembly of the first pieces of antimatter. In 1995 a team of German and Italian physicists (experiment PS210) succeeded for the first time in building up nine atoms of "antihydrogen": while in normal hydrogen an electron orbitates around a proton, in such antiatoms a positron was made to orbit around an antiproton. The result was confirmed, by the end of 1996, by a team at Fermilab. Experiment E862, using antiprotons extracted directly from the Tevatron Antiproton Accumulator, detected several antihydrogen atoms.

        The discovery was exciting: hydrogen atoms has been one of the crucial physical systems for different and fundamental measurements related to the behavior of ordinary matter. The production of antihydrogen was opening the door to a systematic exploration of the proprieties of antimatter and to the test of fundamental physical principles.

        LEAR was formally closed at the end of 1996, but CERN had already foreseen an alternative and powerful way to continue on this research topic: the Antiproton Decelerator (AD).

Antimatter in Cosmology

    Of course, acceleration (and deceleration) of particles are not the only way to study antimatter. Antimatter could exist somewhere in the outer space. Dirac himself was the first to consider the existence of antimatter in an astronomical scale. But it was only after the confirmation of his theory, with the discovery of the positron, antiproton and antineutron, that real speculation began on the possible existence of antiplanets, antistars, antigalaxies and even an antiuniverse.

    In the late 1950s, the amount of antimatter in our galaxy was calculated to be less then one part in a hundred million. If there were an isolated system of antimatter in the universe, free from interaction with ordinary matter, no earthbound observation could distinguish its true content.

    So, even if nothing was visible, the possibility of extragalactic antimatter was wide open. And in the following years, motivated by basic symmetry principles, it was believed that the universe must consist of both matter and antimatter in equal amounts.

    However, it is nowadays strongly believed that there is merely one universe, composed primarily of matter. One could speculate, though, that if any natural antimatter, let say antinuclei from an antimatter galaxy, would try to reach us, it would annihilate with nuclei in the earth's atmosphere, and never be detected.

    Over the past twenty years, scientists have tried to take their instruments as high as possible in the atmosphere (originally with balloons, later with satellites) on the attempt to overcome this annihilation problem, but such an effort is costly and difficult. Nowadays, experiments are planned to be implemented on satellites. In 1998, for instance, a high-energy particle detector, the Alpha Magnetic Spectrometer (AMS), was flown on the Space Shuttle Discovery for a ten-day mission, and it is now being redesigned and upgraded to be installed on the International Space Station for a few years. Orbiting around the Earth above the atmosphere, one of its goals is to measure the fluxes of charged antiparticles and antinuclei to search for any form of cosmic antimatter.