On 30 June 1905 the German physics journal Annalen der Physik published a paper by a young patent clerk called Albert Einstein. The paper, Zur Elektrodynamik bewegter Körper, (On the Electrodynamics of Moving Bodies) set out Einstein's theory of Special Relativity, which explains the relationship between space and time – and between energy and mass – in the famous equation E=mc2.  The paper used Planck's concept of energy quanta to describe how light travels through space.

In 1911 and 1912 Austrian physicist Victor Hess made a series of ascents in a balloon to take measurements of radiation in the atmosphere. He was looking for the source of an ionizing radiation that registered on an electroscope – the prevailing theory was that the radiation came from the rocks of the Earth. To test the theory, in 1909 German scientist Theodor Wulf measured the rate of ionization near the top of the Eiffel tower (at a height of about 300 metres) using a portable electroscope. Though he expected the ionization rate to decrease with height, Wulf noted that the ionization rate at the top was just under half that at ground level – a much less significant decrease than anticipated. Victor Hess's balloon flights took such measurements further. In 1911 his balloon reached an altitude of around 1100 metres, but Hess found "no essential change" in the amount of radiation compared with ground level. Then, on 17 April 1912, Hess made an ascent to 5300 metres during a near-total eclipse of the Sun. Since ionization of the atmosphere did not decrease during the eclipse, he reasoned that the source of the radiation could not be the Sun – it had to be coming from further out in space. High in the atmosphere, Hess had discovered a natural source of high-energy particles: cosmic rays. Hess shared the 1936 Nobel prize in physics for his discovery, and cosmic rays have proved useful in physics experiments – including several at CERN – since. Find out more: About cosmic rays (from the CERN courier) - A discovery of cosmic proportions - Domenico Pacini and the origin of cosmic rays - LHCf: bringing cosmic collisions down to Earth Cosmic rays at CERN - The Large Hadron Collider forward experiment - The CLOUD experiment

In the 1920s, physicists were trying to apply Planck's concept of energy quanta to the atom and its constituents. By the end of the decade Erwin Schrödinger and Werner Heisenberg had invented the new quantum theory of physics. The Physical Institute of the University of Zürich published Schrödinger's lectures on Wave Mechanics (the first from 27 January 1926) and in 1930 Heisenberg's book The physical principles of the quantum theory appeared. The problem now was that quantum theory was not relativistic; the quantum description worked for particles moving slowly, but not for those at high or "relativistic" velocities, close to the speed of light.

In 1928, British physicist Paul Dirac wrote down an equation that combined quantum theory and special relativity to describe the behaviour of an electron moving at a relativistic speed. The equation would allow whole atoms to be treated in a manner consistent with Einstein's relativity theory. Dirac's equation appeared in his paper The quantum theory of the electron, received by the journal Proceedings of the Royal Society A on 2 January 1928. It won Dirac the Nobel prize in physics in 1933. But the equation posed a problem: just as the equation x2=4 can have two possible solutions (x=2 or x=-2), so Dirac's equation could have two solutions, one for an electron with positive energy, and one for an electron with negative energy. But classical physics (and common sense) dictated that the energy of a particle must always be a positive number. Dirac interpreted the equation to mean that for every particle there exists a corresponding antiparticle, exactly matching the particle but with opposite charge. For the electron there should be an "antielectron" identical in every way but with a positive electric charge. In his 1933 Nobel lecture, Dirac explained how he arrived at this conclusion and speculated on the existence of a completely new universe made out of antimatter: If we accept the view of complete symmetry between positive and negative electric charge so far as concerns the fundamental laws of Nature, we must regard it rather as an accident that the Earth (and presumably the whole solar system), contains a preponderance of negative electrons and positive protons. It is quite possible that for some of the stars it is the other way about, these stars being built up mainly of positrons and negative protons. In fact, there may be half the stars of each kind. The two kinds of stars would both show exactly the same spectra, and there would be no way of distinguishing them by present astronomical methods.

In 1932 Carl Anderson, a young professor at the California Institute of Technology in the US, was studying showers of cosmic particles in a cloud chamber and saw a track left by "something positively charged, and with the same mass as an electron". After nearly a year of effort and observation, he decided the tracks were actually antielectrons, each produced alongside an electron from the impact of cosmic rays in the cloud chamber. He called the antielectron a "positron", for its positive charge and published his results in the journal Science, in a paper entitled The apparent existence of easily deflectable positives (1932). The discovery was confirmed soon after by Occhialini and Blacket, who in 1934 published Some photographs of the tracks of penetrating radiation in the journal Proceedings of the Royal Society A. Anderson's observations proved the existence of the antiparticles predicted by Dirac. For discovering the positron, Anderson shared the 1936 Nobel prize in physics with Victor Hess. For years to come, cosmic rays remained the only source of high-energy particles. The next antiparticle physicists were looking for was the antiproton. Much heavier than the positron, the antiproton is the antipartner of the proton. It would not be confirmed experimentally for another 22 years.

In 1929 Ernest Lawrence – then associate professor of physics at the University of California, Berkeley, in the US – invented the cyclotron, a device for accelerating nuclear particles to high velocities without the use of high voltages. Lawrence was granted US patent 1948384 for the cyclotron on 20 February 1934. The machine was used in the following years to bombard atoms of various elements with swiftly moving particles. Such high-energy particles could disintegrate atoms, in some cases forming completely new elements. Hundreds of artificial radioactive elements were formed in this manner. Eventually, the cyclotron was able to accelerate particles such as protons to the energy of a few tens of megaelectronvolts (symbol: MeV. One MeV equals one million electronvolts). Initially driven by the effort to discover the antiproton, the accelerator era had begun, and with it the science of high-energy physics was born. In 1939 Lawrence won the Nobel prize in physics, "for the invention and development of the cyclotron and for results obtained with it, especially with regard to artificial radioactive elements".

The Bevatron in 1958 (Image: Lawrence Berkeley National Laboratory) In 1954, Ernest Lawrence oversaw the building of a proton accelerator called the Bevatron at the radiation laboratory in Berkeley, California. The machine's name comes from BeV, the symbol used at the time for "billion electronvolt", or 109 electronvolts. We now call this unit the gigaelectronvolt, symbol GeV – BeV is no longer used. The Bevatron was designed to collide protons at 6.2 GeV, the expected optimum energy for creating antiprotons. The following is from Experiences with the Bevatron by then Berkeley physicist Edward Lofgren, who was present for the start-up of the machine: Finally, on April 1, 1954, a feeble pulse was obtained at a magnetic field corresponding to 6 BeV. The intensity was measured by counting the tracks in nuclear emulsion that had been inserted into the beam. The intensity was in the range of 104 to 106 protons per pulse. A team of physicists headed by Italian-American physicist Emilio Segrè designed and built a detector specialized to look for antiprotons. The Bevatron was up and running.

A paper titled "Observation of antiprotons," by Owen Chamberlain, Emilio Segrè, Clyde Wiegand, and Thomas Ypsilantis, members of what was then the Radiation Laboratory of the University of California at Berkeley in the US, appeared in the 1 November 1955 issue of Physical Review Letters. It announced the discovery of a new subatomic particle, identical in every way to the proton – except its electrical charge was negative instead of positive. A month before the paper appeared, The New York Times had put the news on the front page: New Atom Particle Found; Termed a Negative Proton With the discovery of the antiproton, Segrè and colleagues had further proof of the essential symmetry of nature, between matter and antimatter. Segrè and Chamberlain were awarded the Nobel prize in physics in 1959 "for their discovery of the antiproton".

The journal Physical Review receives the paper Antineutrons Produced from Antiprotons in Charge-Exchange Collisions by a second team working at the Bevatron – Bruce Cork, Glen Lambertson, Oreste Piccioni and William Wenzel. The paper – which announces the discovery of the antineutron – is published in the issue dated November 1956.

In 1964, James Cronin and Val Fitch at Brookhaven National Laboratory in the US did an experiment with particles called neutral K-mesons, or "kaons". The types of kaons they chose to study can be regarded to consist of one half ordinary matter and the other half antimatter. They started with two types of kaon that had seemingly identical masses but different lifetimes. Kaons of the long-lived type exist for 5.2 × 10-8 seconds before each decays into 3 pions. Kaons of the short-lived type exist for only 0.89 × 10-10 seconds before each decays into 2 pions. Cronin and Fitch shot the two types of kaon down a 17-metre beamline and detected the resulting pion-decays at the other end. Given the different lifetimes of the kaon types and the length of the beamline, you would expect only to see decays from the long-lived kaon type at the detector. Cronin and Fitch expected the short-lived kaon type to decay long before it reached the end of the beamline, and so its decay products would not be detected. In other words you would expect to detect only 3-pion decays and no 2-pion decays at all. But in their experiment, Cronin and Fitch did detect 2-pion decays: 45 of them, out of a total of 22,700 decay events – a ratio of about 1 in 500. The result violated a fundamental principle of physics – the symmetry between matter and antimatter. The pair announced their result in the paper "Evidence for the 2-pion Decay of the K Meson", published in the journal Physical Review Letters on 27 July 1964. They shared the 1980 Nobel prize in physics "for the discovery of violations of fundamental symmetry principles in the decay of neutral K-mesons."

By 1965, all three particles that make up atoms (electrons, protons and neutrons) were known to each have an antiparticle. So if particles, bound together in atoms, are the basic units of matter, it is natural to think that antiparticles, bound together in antiatoms, are the basic units of antimatter. But are matter and antimatter exactly equal and opposite, or symmetric, as Dirac had implied? The next important step was to test this symmetry. Physicists wanted to know how subatomic antiparticles behave when they come together. Would an antiproton and an antineutron stick together to form an antinucleus, just as protons and neutrons stick together to form the nucleus of an atom? The answer to the antinuclei question was found in 1965 with the observation of the antideuteron, a nucleus of antimatter made out of an antiproton plus an antineutron (while a deuteron – the nucleus of the deuterium atom – is made of a proton plus a neutron). The goal was simultaneously achieved by two teams of physicists, one led by Antonino Zichichi using the Proton Synchrotron at CERN, and the other led by Leon Lederman, using the Alternating Gradient Synchrotron (AGS) accelerator at the Brookhaven National Laboratory, New York. The CERN paper, Experimental Observation of Antideuteron Production was published in the Italian particle-physics journal Il nuovo cimento on 1 September 1965 (the journal ended when it was merged into the European Physical Journal in 1999.

CERN issues a press release announcing the first storage of antiprotons. It reads:  Antimatter, in the form of antiprotons, has been stored for the first time in history.  This scientific first occurred at CERN, the European Organisation for Nuclear Research, at the end of July during tests conducted in view of using the SPS European accelerator as a colliding device between protons and antiprotons.  Several hundred antiprotons of 2.1 GeV/c were produced by protons from the PS accelerator and were kept circulating in a machine called ICE (Intial Cooling Experiment) for a period of 85 hours i.e. about 300, 000 seconds (3 × 105). The previous best experimental measurement of antiproton lifetime, acquired during bubble chamber experiments, was about 10-4 second, i.e. a ten-thousandth of a second. 

The Intersecting Storage Rings produced the world’s first proton-antiproton collisions on 4 April 1981, paving the way for proton-antiproton collisions in the Super Proton Synchrotron (SPS), and the Nobel prize for Simon van der Meer and Carlo Rubbia. The ISR proved to be an excellent instrument for particle physics. By the time the machine closed down in 1984, it had produced many important results, including indications that protons contain smaller constituents, ultimately identified as quarks and gluons.

A team led by Walter Oelert created atoms of antihydrogen for the first time at CERN’s Low Energy Antiproton Ring (LEAR) facility. Nine of these atoms were produced in collisions between antiprotons and xenon atoms over a period of 3 weeks. Each one remained in existence for about 40 billionths of a second, travelled at nearly the speed of light over a path of 10 metres and then annihilated with ordinary matter. The annihilation produced the signal that showed that the anti-atoms had been created. This was the first time that antimatter particles had been brought together to make complete atoms, and the first step in a programme to make detailed measurements of antihydrogen. The hydrogen atom is the simplest atom of all, made of a single proton orbited by an electron. Some three quarters of all the ordinary matter in the universe is hydrogen, and the hydrogen atom is one of the best understood systems in physics. Comparison with antihydrogen offers a route to understanding the matter–antimatter asymmetry in the universe.

In 1996 CERN's antiproton machines – the Antiproton Accumulator (AC), the Antiproton Collector and the Low Energy Antiproton Ring (LEAR) – were closed down to free resources for the Large Hadron Collider. But a community of antimatter scientists wanted to continue their LEAR experiments with slow antiprotons. Council asked the Proton Synchrotron division to investigate a low-cost way to provide the necessary low-energy beams. The resulting design report for the Antiproton Decelerator concluded: The use of the Antiproton Collector as an antiproton decelerator holds the promise of delivering dense beams of 107 protons per minutes and low energy (100 MeV/c) with bunch lengths down to 200 nanoseconds. The Antiproton Declerator project was approved on 7 February 1997.

Two CERN experiments, ATHENA and ATRAP, created thousands of atoms of antimatter in a “cold” state in 2002. Cold means that the atoms are slow moving, which makes it possible to study them before they meet ordinary matter and annihilate. Antihydrogen formed in the experiments when cold positrons and antiprotons were brought together and held in a specially designed “trap”. Once formed, the electrically neutral antihydrogen atoms drifted out of the trap and annihilated. Find out more

The ALPHA experiment at CERN reported today that it succeeded in trapping antimatter atoms for over 16 minutes: long enough to begin to study their properties in detail. ALPHA is part of a broad programme at CERN’s antiproton decelerator investigating the mysteries of one of nature’s most elusive substances. ALPHA studied 300 trapped antiatoms. Trapping antiatoms will allow antihydrogen to be mapped precisely using laser or microwave spectroscopy so that it can be compared to the hydrogen atom, which is among the best-known systems in physics. Any difference between matter and antimatter should become apparent under careful scrutiny.

In a paper published today in the journal Nature, the Japanese-European ASACUSA experiment at CERN reported a new measurement of the antiproton’s mass accurate to about one part in a billion. Precision measurements of the antiproton mass provide an important way to investigate nature’s apparent preference for matter over antimatter. To make these measurements antiprotons are first trapped inside helium atoms, where they can be ‘tickled’ with a laser beam. The laser frequency is then tuned until it causes the antiprotons to make a quantum jump within the atoms, and from this frequency the antiproton mass can be calculated. However, an important source of imprecision comes from the fact that the atoms jiggle around, so that those moving towards and away from the beam experience slightly different frequencies. A similar effect is what causes the siren of an approaching ambulance to apparently change pitch as it passes you in the street. In their previous measurement in 2006, the ASACUSA team used just one laser beam, and the achievable accuracy was dominated by this effect. This time they used two beams moving in opposite directions, with the result that the jiggle for the two beams partly cancelled out, resulting in a four times better accuracy.

The kick-off meeting for ELENA, the Extra Low Energy Antiproton Ring, starts today at CERN, bringing together scientists from Canada, Denmark, France, Germany, Japan, Sweden, the UK and the USA.    ELENA will consist of a small new decelerator ring that will be installed in same building that houses CERN’s existing Antiproton Decelerator (AD). It will slow antiprotons down to under a fiftieth of the current AD energy, bringing an improvement of a factor of 10-100 in antiproton trapping efficiency. At the AD, antiprotons have to be slowed down by passing them through a series of foils, a process that results in the loss of some 99.9% of the antiprotons extracted from the AD before they reach the experiments.

In Physical Review Letters, the Antihydrogen TRAP (ATRAP) experiment at CERN's Antiproton Decelerator (AD) reports a new measurement of the antiproton magnetic moment made with an unprecedented uncertainty of 4.4 parts per million (ppm). This result is 680 times more precise than previous measurements. The unusual increase in precision is due to the experiment’s ability to trap individual protons and antiprotons, and to use a huge magnetic gradient to gain sensitivity to the tiny magnetic moment. ATRAP’s new result is partly an attempt to understand the matter-antimatter imbalance of the universe, one of the great mysteries of modern physics. Using a device called a Penning trap, a sort of electromagnetic cage, the antiproton is suspended at the centre of an iron ring electrode sandwiched between copper electrodes. Thermal contact with liquid helium keeps the electrodes at 4.2 K, providing a nearly perfect vacuum that eliminates the stray matter atoms that could otherwise annihilate the antiproton. Static and oscillating voltages applied to the electrodes allow the antiproton to be manipulated and its properties to be measured. The ATRAP team found that the magnetic moments of the antiproton and proton are "exactly opposite": equal in strength but opposite in direction with respect to the particle spins, consistent with the prediction of the Standard Model and its CPT theorem to 5 parts per million. However, the potential for much greater measurement precision puts ATRAP in position to eventually test the Standard Model prediction much more stringently. Explore resources for the media.

The LHCb collaboration at CERN publishes a paper in Physical Review Letters on the first observation of matter-antimatter asymmetry in the decays of the particle known as the B0s. It has a significance of more than 5 sigma, and is only the fourth subatomic particle known to exhibit such behaviour.  By studying subtle differences in the behaviour of particle and antiparticles, experiments at the LHC are seeking to cast light on this dominance of matter over antimatter. Explore resource for the media.

The ALPHA collaboration at CERN1 has published a paper in Nature Communications describing the first direct analysis of how antimatter is affected by gravity. ALPHA was the first experiment to trap atoms of antihydrogen — neutral antimatter atoms held in place with a strong magnetic field for up to 1000 seconds. The original goal of the experiment was not to study gravity, but the researchers realised that the data they had already collected might be sensitive to gravitational effects. Current theoretical arguments predict that hydrogen and antihydrogen atoms have the same mass and should interact with gravity in the same way. If an atom is released, it should experience a downward force whether it’s made of matter or antimatter. The ALPHA scientists have retroactively analysed how their antihydrogen atoms moved when released; this has allowed them to put a limit on anomalous gravitational effects.

In a paper published in the journal Nature Communications today, the ALPHA experiment at CERN's Antiproton Decelerator (AD) reports a measurement of the electric charge of antihydrogen atoms, finding it to be compatible with zero to eight decimal places. Although this result comes as no surprise, since hydrogen atoms are electrically neutral, it is the first time that the charge of an antiatom has been measured to high precision. Antiparticles should be identical to matter particles except for the sign of their electric charge. So while the hydrogen atom is made up of a proton with charge +1 and an electron with charge -1, the antihydrogen atom consists of a charge -1 antiproton and a charge +1 positron. We know, however, that matter and antimatter are not exact opposites – nature seems to have a one-part in 10 billion preference for matter over antimatter, so it is important to measure the properties of antimatter to great precision: the principal goal of CERN’s AD experiments. ALPHA achieves this by using a complex system of particle traps that allow antihydrogen atoms to be produced and stored for long enough periods to study in detail. Understanding matter antimatter asymmetry is one of the greatest challenges in physics today. Any detectable difference between matter and antimatter could help solve the mystery and open a window to new physics. To measure the charge of antihydrogen, the ALPHA experiment studied the trajectories of antihydrogen atoms released from the trap in the presence of an electric field. If the antihydrogen atoms had an electric charge, the field would deflect them, whereas neutral atoms would be undeflected. The result, based on 386 recorded events, gives a value of the antihydrogen electric charge as (-1.3±1.1±0.4) × 10-8, the plus or minus numbers representing statistical and systematic uncertainties on the measurement. Explore resources for the media.

Geneva, 12 August 2015. In a paper published today in Nature, the Baryon Antibaryon Symmetry Experiment (BASE1) at CERN2's Antiproton Decelerator (AD), reports the most precise comparison of the charge-to-mass ratio of the proton to that of its antimatter equivalent, the antiproton.  The charge-to-mass ratio — an important property of particles — can be measured by observing the oscillation of a particle in a magnetic field. The new result shows no difference between the proton and the antiproton, with a four-fold improvement in the energy resolution compared with previous measurements. To perform the experiment, the BASE collaboration used a Penning-trap system comparable to that developed by the TRAP collaboration in the late 1990s at CERN. However, the method used is faster than in previous experiments. This has allowed BASE to carry out about 13 000 measurements over a 35-day campaign, in which they compare a single antiproton to a negatively-charged hydrogen ion (H-). Consisting of a hydrogen atom with a single proton in its nucleus, together with an additional electron, the H- acts as a proxy for the proton. “We found that the charge-to-mass ratio is identical to within 69 parts per thousand billion, supporting a fundamental symmetry between matter and antimatter,” said BASE spokesperson Stefan Ulmer. While matter and antimatter particles can differ, for example, in the way they decay (a difference often referred to as violation of CP symmetry), other fundamental properties, such as the absolute value of their electric charges and masses, are predicted to be exactly equal. Any difference – however small — between the charge-to-mass ratio of protons and antiprotons would break a fundamental law known as CPT symmetry. This symmetry reflects well-established properties of space and time and of quantum mechanics, so such a difference would constitute a dramatic challenge not only to the Standard Model, but also to the basic theoretical framework of particle physics. Explore resources for the media.

The ASACUSA experiment at CERN reports in Science a new precision measurement of the mass of the antiproton relative to that of the electron. Such measurements provide a unique tool for comparing with high precision the mass of an antimatter particle with its matter counterpart.  This result is based on spectroscopic measurements with about 2 billion antiprotonic helium atoms cooled to extremely cold temperatures of 1.5 to 1.7 degrees above absolute zero. In antiprotonic helium atoms an antiproton takes the place of one of the electrons that would normally be orbiting the nucleus.  The measurement of the antiproton’s mass is done by spectroscopy, by shining a laser beam onto the antiprotonic helium. ASACUSA has now managed to cool down the antiprotonic helium atoms to temperatures close to absolute zero by suspending them in a very cold helium buffer-gas. In this way, the microscopic motion of the atoms is reduced, enhancing the precision of the frequency measurement. The measurement of the transition frequency has been improved by a factor of 1.4 to 10 compared with previous experiments. Experiments were conducted from 2010 to 2014, with about 2 billion atoms, corresponding to roughly 17 femtograms of antiprotonic helium.  According to standard theories, protons and antiprotons are expected to have exactly the same mass. The observation of even a minute breaking of CPT would call for a review of our assumptions about the nature and properties of space-time.  Explore resources for the media.

The ALPHA collaboration reports in Nature the first ever measurement on the optical spectrum of an antimatter atom.  When the electrons move from one orbit to another they absorb or emit light at specific wavelengths, forming the atom's spectrum. ALPHA result is the first observation of a spectral line in an antihydrogen atom, allowing the light spectrum of matter and antimatter to be compared for the first time.  Within experimental limits, the result shows no difference compared to the equivalent spectral line in hydrogen. This is consistent with the Standard Model of particle physics, the theory that best describes particles and the forces at work between them, which predicts that hydrogen and antihydrogen should have identical spectroscopic characteristics. Measuring the antihydrogen spectrum with high-precision offers an extraordinary new tool to test whether matter behaves differently from antimatter and thus to further test the robustness of the Standard Model.  The measurement was done by observing the so-called 1S-2S transition. The 2S state in atomic hydrogen is long-lived, leading to a narrow natural line width, so it is particularly suitable for precision measurement.  Explore resources for the media.

The BASE collaboration, publishes today in Nature a new measurement of the magnetic moment of the antiproton, with a precision exceeding that of the proton. Thanks to a new method involving simultaneous measurements made on two separately-trapped antiprotons in two Penning traps, BASE succeeded in breaking its own record presented last January.  The magnetic moment of the antiproton is found to be 2.792 847 344 1(42), to be compared to the figure of 2.792 847 350(9) that the same collaboration of researchers found for the proton in 2014, at the BASE companion experiment at Mainz, in Germany. Explore resources for media.

The LHCb collaboration at CERN has seen, for the first time, the matter–antimatter asymmetry known as CP violation in a particle dubbed the D0 meson. The finding, presented at the annual Rencontres de Moriond conference in 2019 and in a dedicated CERN seminar, is sure to make it into the textbooks of particle physics. CP violation is an essential feature of our universe, necessary to induce the processes that, following the Big Bang, established the abundance of matter over antimatter that we observe in the present-day universe. Explore resources for the media. 

The ALPHA collaboration at CERN has succeeded in cooling down antihydrogen atoms – the simplest form of atomic antimatter – using laser light. The technique, known as laser cooling, was first demonstrated 40 years ago on normal matter and is a mainstay of many research fields. Its first application to antihydrogen by ALPHA, described in a paper published in Nature, opens the door to considerably more precise measurements of the internal structure of antihydrogen and of how it behaves under the influence of gravity. Comparing such measurements with those of the well-studied hydrogen atom could reveal differences between matter and antimatter atoms. Such differences, if present, could shed light on why the universe is made up of matter only, an imbalance known as matter–antimatter asymmetry. Explore resources for the media.