High-energy particle collisions can lead to the appearance of particle / antiparticle pairs or photons, and the annihilation of particle / antiparticle pairs also leads to the appearance of photons, as shown by these traces in the bubble chamber. But what determines whether a particle belongs to matter or antimatter?Every known particle of matter in the universe has an anti-material counterpart. Antimatter has many properties similar to those of normal matter, including the types of interactions, the mass, the magnitude of the electric charge, and so on. But there are some fundamental differences. However, two things about the interaction of particles of matter and antimatter can be said with certainty: if you push a particle of matter with its counterpart from antimatter, they instantly annihilate, turning into energy, and in any interaction creating a particle of matter, its counterpart of antimatter will also arise. So what makes antimatter special? This is what our reader wants to know, who asks:
What are the differences between matter and antimatter at a fundamental level? Is there any intrinsic property forcing a particle to become matter or antimatter? Is there any intrinsic property (such as a spin) that distinguishes quarks and antiquarks? What gives the prefix "anti" antimatter?
To understand the answer to the question, it is necessary to look at the existing particles (and antiparticles).
Particles and antiparticles of the Standard Model obey all sorts of conservation laws, but there are fundamental differences between fermions and bosonsThis is the Standard Model of elementary particles: a complete set of open particles in the known Universe. They are usually divided into two classes: bosons with whole backs (..., -2, -1, 0, +1, +2, ...) that do not belong to matter or antimatter, and fermions with half-integer spins (..., -3/2, -1/2, +1/2, +3/2, ...), obliged to fall into one of two categories: matter or antimatter. Any particle you want to create will have many of its inherent properties, defined by what we call
quantum numbers . A single, isolated particle will have both familiar properties and several properties that may be unfamiliar to you.
The possible configurations of an electron in a hydrogen atom differ remarkably from each other, and yet they all represent the same particle in slightly different quantum states. Particles and antiparticles also have their own inherent immutable quantum numbers, and they play a major role in determining whether a particle belongs to matter, antimatter, or to any of the categories.From simple you can remember the mass and electric charge. For example, the electron rest mass is 9.11 × 10
-31 kg, and its charge is -1.6 x 10
-19 C. Also, electrons can bind to protons, which gives a hydrogen atom with a set of spectral lines and emission / absorption lines, depending on their electromagnetic interaction. The electron spin is +1/2 or -1/2, the
lepton number is +1, the lepton family number is +1 for the first of three (electron, mu, tau) lepton families (for simplicity, we omit such numbers as
weak isospin or
weak hypercharge ).
Given these properties of an electron, you can ask the question - how should a particle-twin of an electron from antimatter, according to the rules governing elementary particles.
In a simple hydrogen atom, a single electron orbits around a single proton. In an atom of antihydrogen, one positron moves around one antiproton. Positrons and antiprotons are doubles in antimatter for electrons and protons, respectively.Quantities of all quantum numbers should be saved. But antiparticles signs of these numbers must be reversed. For an anti-electron, this means that it must have the following quantum numbers:
- rest mass 9.11 × 10 -31 kg,
- electric charge 1.6 x 10 -19 C,
- spin -1/2 or +1/2,
- lepton number -1,
- lepton family number -1.
When you bind it to an antiproton, it must produce exactly the same series of spectral lines and absorption / emission lines that the electron / proton system exhibits.
The electron transitions in the hydrogen atom and the wavelengths of the resulting photons demonstrate the effect of binding energy and the interaction between the electron and the proton in quantum physics. The identity of the spectral lines in positrons and antiprotons is confirmed.All these facts have been experimentally confirmed. The particle, which exactly corresponds to the description of the anti-electron, is known as a positron. This is necessary if you consider how we create matter and antimatter: we usually create them from nothing. That is, if you collide two particles at sufficiently high energies, you can often get an additional particle / antiparticle pair from the excess energy (from Einstein E = mc
2 ), according to the law of conservation.
By colliding a particle with an antiparticle one can expect them to annihilate, turning into energy. And from this it follows that by pushing two any particles with sufficiently high energy, you can create a particle / antiparticle pair.But not only energy must be conserved; there is still a whole mountain of quantum numbers that also need to be saved! These include:
- electric charge,
- angular momentum (combination of spin and orbital angular momentum; for individual particles, it’s just spin),
- lepton number
- baryon number
- lepton family number
- color charge.
And of all these intrinsic properties, two determine the belonging to matter and antimatter - the baryon number and the lepton number.
In the early universe there were extremely many all particles and their antiparticles, but as it cooled, most of the particles annihilated. All of our usual matter originated from quarks and leptons, with positive baryon and lepton numbers, exceeding in the number of their twins, antiquarks and antileptons.If any of these numbers is positive, then the particle belongs to ordinary matter. Therefore, quarks (with a baryon number +1/3), electrons, muons, tau, neutrinos (with a lepton number +1) belong to matter, and antiquarks, positrons, anti-muons, anti-neutrons, anti-neutrinos - to antimatter. These are all fermions and antifermions, and each fermion is a particle of matter, and antifermion is a particle of antimatter.
Particles of the Standard Model with masses in MeV / s 2 indicated in the upper left corners. The three left columns are fermions, the two right ones are bosons. And although all particles have their own antiparticle, only fermions are related to matter or antimatter.But there are also bosons. There are gluons whose antiparticles are gluons with opposite color combinations; is W
+ with antiparticle W
- (with opposite electric charge); there is Z
0 , the Higgs boson, and the photon, of which they themselves are antiparticles. However, bosons are neither matter nor antimatter. Without the lepton or baryon numbers, these particles may have an electric charge, color charge, spin, etc. - but no one can call them "matter" or "antimatter". In this case, bosons are just bosons, and if they have no charge, then they are themselves antiparticles.
On all scales of the Universe, from our region to the interstellar space, from individual galaxies to clusters and filaments and the great cosmic web, everything that we see is seen as consisting of ordinary matter, but not of antimatter. This mystery remains unsolved.So what gives antimatter the prefix "anti"? If we take a separate particle, then its antiparticle will have the same mass, and all the same quantum numbers with the opposite sign: this is a particle that can annihilate from the first and turn into energy. But in order to be matter, a particle must have either a baryon or a lepton number positive. To be antimatter, you must have a negative or baryonic or lepton number. In addition, in our Universe, no fundamental reasons are known for which matter would somehow surpass antimatter; we still do not know how this symmetry was broken (although we have ideas). If everything went differently, we would probably call everything we are made of, “matter,” and the rest, “antimatter,” but these names are given arbitrarily. As always, the universe is on the side of those who survived.