Antimatter

Antimatter is a form of matter that is very similar to ordinary matter, of which it is a kind of mirror. Antimatter particles, called “antiparticles”, are actually identical in mass to the corresponding particles, but have some quantum numbers (including the electrical charge) of the opposite sign. The meeting between a particle and its antiparticle produces a phenomenon known as “annihilation”. This leads to both particles’ complete conversioninto energy, according to the known relativistic equation E = mc2.
The theory of antimatter was initially proposed by the British physicist Paul Dirac in the early 1930s. This was as a result of the equations that describe the behaviour of electronsin quantum mechanics. The first experimental confirmation of the existence of antimatter was actually obtained in 1932 by the physicist Carl Anderson, with the discovery of the positron, antiparticle of the electron.
The near total absence of antimatter in the universe represents one of the most importantopen problems of contemporary physics.

First image of a positron obtained from a cloud chamber, 1932 (public domain)

A transaxial slice of the brain imaged with positron emission tomography (PET). (Public domain)

Currently accepted cosmological theories consider that the Big Bang should have produced equal quantities of matter andantimatter, but, for some still unknown reason, antimatter almost totally disappeared, to the benefit of matter.
The generation of asymmetry between matter and antimatter in the universe requires, among other things, that the fundamental interactions distinguish particles from antiparticles. The symmetry that exchanges particles and antiparticles in the Standard Model, which takes the name of CP-symmetry, cannot, thus, be exact. The CP violation isactually present in the Standard Model and was observed in the oscillations and weakdecays of mesons (quark-antiquark bound states) starting from the 1960s. However, the amount of CP violation is not enough to explain the asymmetry between matter and antimatter in the universe, which thus points to the existence of “new physics”.
Antimatter has also led to the development of very important applications, for example in medicine, where PET (positron emission tomography) imaging, in particular, is used indiagnostics techniques. This exploits the photons emitted by the annihilation between positrons and electrons to create detailed images of some organs of the human body.

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