Article curated by Ed Trollope
Everything around us is made of matter - some combination of building blocks called particles.
Antimatter is the mirror image of matter, with a corresponding antiparticle for each particle of matter. Any particle has a set of properties, defined in the standard model of physics, such as charge, mass, spin and many other things. With the exception of mass, as it’s really a form of energy on that scale, all of the properties of a particle can be reversed to make its antiparticle.
When particles of matter meet their corresponding anti-particles, they annihilate, releasing pairs of high energy electrons. The famous equation E = mc2 describes the relation between mass and energy - we know today that even a tiny bit of mass, like that of the electron or positron, can release huge amounts of energy upon annihilation.
The existence of antimatter has been known for a number of decades, having been proposed in the presently understood form in a 1928 paper by Paul Dirac when his equation to describe motion of particles gave two answers - one positive and one negative. These antiparticles have always been thought to possess almost all of the same basic properties as ordinary matter particles, (other than a few things like charge) however some properties, including the behaviour under gravity, of antimatter have never been measured directly. This area of study could provide interesting clues to questions regarding the expansion of the universe and the difference between the quantities of matter and antimatter observed in the universe today. Antielectrons were seen a few years later by Carl Anderson, who renamed them positrons. Researchers are producing large quantities of Positronium. Positronium is a bound state between an electron and a positron (the electron's antimatter equivalent) which decays rapidly in normal circumstances when the components annihilate. The short lifetime of these atoms makes their properties particularly difficult to study. Scientists are now studying individual properties of antimatter in laboratory settings.2
Where has all the antimatter gone?
Since the discovery that matter and antimatter are created in equal amounts, physicists have asked themselves why is the universe we see only made up of matter? Just after the big bang, while the universe was very hot and dense, lots of particles were so close to their corresponding antiparticles in this very small universe that they collided and annihilated each other. This released energy that could be converted into matter/antimatter pairs again. As the universe expanded, the chance of the particles annihilating decreased, but there should still be equal amounts of matter and antimatter.
But we know from our everyday experiences that the universe around us is made up of matter. Even astronomers tell us that as far out into the universe as we can see, everything is made from matter. This abundance of matter is great for making planets and stars, however, it defies physical explanation. Everything that we currently know about particle physics is described by the standard model. This model is well established and has been rigorously tested. However, it cannot explain the abundance of matter we see in the universe. This means physicists need to expand this model in new ways to help explain the phenomena.
There are several possibilities that may account for the missing antimatter. It’s possible that by chance, all of the antimatter is somewhere else in the universe and entire areas of space are filled with antimatter, but we would expect to see the energy release coming from annihilation at the boundaries of these places. Antimatter regions in distant clusters of galaxies, if they exist, may be detectable via the release of certain antinuclei (the cores of atoms of antimatter). Space-based missions such as PAMELA are searching for this evidence, but these have not found large antimatter regions yet.
One possibility is that more matter than antimatter was produced in the first place, say 51% matter and 49% antimatter, and annihilation occurred until all that was left in the universe was that 2% matter, and a lot of energy. But as far as scientists can tell, the laws of physics are exactly the same for matter and antimatter. If there were galaxies, solar systems, planets and life forms all made of antimatter, an anti-football would bounce off an anti-wall through anti-air in just the same way as we would expect for normal matter.
The precise mechanism by which this asymmetry appears is unclear, however, it is postulated that this arises from violation of charge-parity symmetry. This symmetry suggests that the laws of physics are unchanged in a situation where a particle is replaced with it's antiparticle (charge symmetry), and left and right are flipped as by a mirror (parity symmetry). This symmetry is known as CP-symmetry for short, and a violation of this symmetry is known as a CP-violation.
While this holds true for most particles, particularly at the low energies we encounter on a daily basis, studies of particles called kaons in the 1960s showed a certain asymmetry. Neutral kaons are able to transform into their antiparticles, and vice versa, but experimental data have shown that the process is not equally likely to occur in both directions. More recent experiments have shown that other neutral mesons (particles containing one quark and one anti-quark) display similar asymmetry.
This asymmetry goes some way to explaining the prevalence of matter over antimatter in our observed universe. However, quantitative simulations of the period after the big bang that take into account the observed asymmetries show that they cannot account for the size of the imbalance. As a result, scientists are continuing to search for further examples of CP-violation. Further examples of CP-violation in quarks are likely to be hard to detect, in part due to the colossal energy levels required to produce the mesons to be studied, but also because extremely sensitive measurements are required to discern a disparity in the rates of decay.
So far, CP-violation has only been detected for interactions involving the weak nuclear force (one of the four fundamental forces in nature, along with the strong nuclear force, the electromagnetic force, and gravity). Interactions involving the strong nuclear force (a class of interactions known as Quantum Chromodynamics, or QCD for short) have thus far not exhibited any evidence of CP-violation, and the reason for this is unknown. This mystery is known as the strong CP problem.
There exist more particles in the universe with mass than quarks however, so a natural question for physicists to ask is whether CP-violation can be found in other particles. Known as leptons, the other particles with mass interact via the weak nuclear force, but not the strong nuclear force, so any CP-violation found here would not contradict the strong CP problem. These leptons fall into two groups,: charged leptons, which include the electron, muon and tau particles, and their corresponding neutrinos. Searches for CP-violation in these areas tend to focus on decays of the Tau particle because it is the heaviest particle and the likelihood of it decaying in a particular way is well theoretically grounded, hence any deviation from this predicted rate would indicate new physics.
Neutrinos are another a promising particle with which to look for CP-violation. These chargeless particles have incredibly small masses and can oscillate between their three different types. Experiments designed to test the probability of these changes in type are also aiming to see whether this probability is the same for neutrinos and antineutrinos. A difference in these probabilities would indicate a violation of CP symmetry, but the minuscule masses and tendency of neutrinos not to interact make them difficult to examine precisely.
Neutrino oscillations and CP violation
In 2013, the Tokai to Kamioka (T2K) experiment in Japan observed that neutrinos are able to oscillate between its three forms: electron, muon, and tau. In 2016, New Scientist reported that their results have showed evidence of
32 muon neutrinos morphing into the electron flavour, compared to just 4 muon antineutrinos becoming the anti-electron variety.
This could be considered evidence of the CP symmetry violation required to explain matter-antimatter asymmetry, as, if they assume CP symmetry holds, this would indicate
more matter and less antimatter than they expected. They have been able
to rule out CP symmetry holding at the 2 sigma level.
NoVA (similar to T2K) are planning on running experiments in the future to try and confirm the T2K results, which will help answer the question of whether neutrino oscillations can explain matter-antimatter asymmetry.
How is Charge-Parity symmetry violated in the quark sector? Charge-Parity symmetry is the idea that the laws of physics are identical for a system of particles as they are for a system featuring the corresponding antiparticles (the charge symmetry) and left and right flipped as by a mirror (the parity symmetry). While this holds true for most particles, particularly at the low energies we encounter on a daily basis, studies of particles called kaons in the 1960s showed a certain asymmetry. Neutral kaons are able to transform into their antiparticles, and vice versa, but experimental data have shown that the process is not equally likely to occur in both directions. More recent experiments have produced results indicating that other neutral mesons (particle containing one quark and one anti-quark) display similar asymmetry. This asymmetry goes some way to explaining the prevalence of matter over anti-matter in our observed universe, since we would expect these to have been created in equal proportions in the Big Bang. However, quantitative simulations of the period after the big bang, taking into account the asymmetries discovered thus far reveal that these cannot begin to account for the size of the imbalance. As a result, scientists are continuing to search for further examples of CP-violation. Further examples of CP-violation in quarks are likely to be hard to detect, in part due to the colossal energy levels required to produce the mesons to be studied, but also because of the extreme sensitivity of the measurements required to discern a disparity in the rates of decay.
While some of the antimatter that was produced in the big bang may be out there somewhere in the universe, the fact that some amount of CP-violation has been measured demonstrates clearly that there exists an imbalance capable of generating some degree of asymmetry in the ratio of matter to antimatter in the universe. The search for further examples of this, enough to explain the abundance of matter we actually observe, will continue in particle accelerators and physics labs around the world.
What particles are their own anti-particles?
According to theory, the particles which make up the matter around us each have a corresponding anti-particle, and many of these anti-particles have been seen in experiments. In 1937, an Italian physicist named Ettore Majorana established that the equations which had predicted the existence of these antimatter particles, could in some circumstances be solved such that a particle was it's own antiparticle. These hypothetical particles are known as Majorana fermions.
Detecting Majorana fermions is not a simple matter. The possibility exists that Neutrinos (particles with very very little mass, which interact very rarely with most matter) are majorana particles, and if this is the case then a particular sub-atomic process (neutrinoless double beta decay) would be allowed. Scientists are searching for the tell-tale signs of this particular event, but it's predicted rarity means it's difficult to say for certain that it's not occurring if we see nothing.
There are also states in which Quasi-particles (phenomena caused by actions within certain excited states of matter) can be observed to behave like Majorana fermions, but these are not fundamental particles.
Do protons decay?
Protons are some of the most stable particles in existence, but some theories predict that, very rarely, one ought to decay. This is at least partially motivated another possible way to explain the apparent lack of antimatter in the observable universe. Attempts to observe these decays have thus far not produced any evidence that they occur, but as with the Majorana fermions this doesn't conclusively show that they don't.
Because these theories predict that on average a single proton ought to take an incredibly long time to decay (about a billion billion times the age of the universe!) to have any chance of spotting a decay, scientists need to watch a huge number of protons at once, and so the detectors which are set up to spot this type of decay are colossal and expensive to build. This equipment also needs to be deep underground, to try and shield it from background radiation which would dominate the observations and make it harder to spot the particular type of event the experiment was looking for.
This article was written by the Things We Don’t Know editorial team, with contributions from Jim Sadler, Jon Cheyne, and Alice Wayne.
why don’t all references have links?
 Zito, M., Kaplan, D., Goodman, M., & Sullivan, Z. (2010, March). SuperBeam experiments: T2K, NOvA and beyond. In AIP Conference Proceedings (Vol. 1222, No. 1, p. 10). DOI: 10.1063/1.3399269