Quarks and Fundamental Forces: Our building blocks
Quarks are the particles that make up protons, neutrons and many other particles. There are six of them, classified into three generations while the name “quark” is borrowed from the book Finnegan’s Wake (by James Joyce).
These strange names of quarks do not have any physical meaning, but were chosen arbitrarily, for ease of remembering.
Quarks, with these interesting names, are very strange particles. They are never alone, but always appear with other quarks and thus build the world around us.
The matter that surrounds us is made of quarks (and leptons) of the first generation. In nature, everything tends towards the minimum of potential energy, so massive quarks quickly decay into lighter ones.
Because of this, particles built from quarks of the II and III generations live for a very short time. Unlike leptons, which interacted with all but the strong force, quarks interact via all four fundamental forces.
Everyone probably remembers the physics taught in school, and the teachers and professors who repeat that the smallest possible charge is the one carried by the electron, the charge e, but it is not quite so.
The charge of quarks is smaller, it has a value of 1 or 2 thirds of the charge of an electron.
Detecting quarks is quite a difficult task because they do not exist alone, they are always part of some other more massive particle, now with other quarks.
The only way to detect it is by applying some indirect methods and checking whether these results agree with the predictions of the theory. Only five years after theorists predicted the existence of quarks, the first experimental confirmation arrived (1969).
The last quark, the top quark, posed a major challenge to experimental physicists. It was searched for a long time and was detected at Fermilab in 1995.
Like other particles and quarks, they have their antiparticles, which differ in spin and charge.
Quarks are building blocks that make up a large number of particles, called hadrons. Hadrons consisting of three quarks are called baryons.
Baryons include the well-known proton (uud – quarks) and neutron (udd). Mesons are hadrons that are made up of one quark and one antiquark. Hadron antiparticles, e.g. antiproton and antineutron are made up of antiquarks. Mesons are short-lived unstable particles, on the order of 10-20 seconds.
One of the baryons that puzzled physicists and cast some doubt on the Standard Model was the baryon which consists of three up quarks (uuu). The problem that arose was a consequence of the Pauli exclusion principle.
According to the known laws of physics, two quarks had to have the same spin, which the Pauli principle did not allow. What physicists knew until then was that this particle could not exist, but it did exist.
The solution to this mystery was quickly found, it was established that quarks possess, in addition to electric, another type of charge – color.
Color has no real meaning here, quarks do not differ in color (they are too small to have any color at all), but that term was chosen similarly to their names, to make it easier to remember.
Quarks occur in three colors. Just as charge is related to the electromagnetic force, color charge is related to the strong force.
The introduction of color solved the problem of the baryon which consists of three up quarks (uuu) particle very simply – the quarks from which this particle is made have different colors, so the validity of the Pauli principle is not violated.
Forces
We have already seen that the Standard Model, in addition to quarks and leptons, also includes fundamental forces and particles that carry fundamental interactions.
The most famous of all forces is electromagnetic. It acts between all charged bodies, and the particle that is the carrier of this interaction is a photon. A photon is a particle that has no rest mass, no charge, and spin 1.
The next force is the weak force. This force is responsible for some processes at the atomic level, the most famous being beta decay. Just as the photon is the transmitter of the electromagnetic force, so the W and Z bosons are the transmitters of the weak force.
In 1979, Sheldon Glashow, Abdus Salam and Steven Weinberg were awarded the Nobel Prize in Physics for showing that the electromagnetic and weak forces become one force at sufficiently large energies.
That unique force is called electroweak. This was the first confirmation of the cosmological idea that all four forces were once one and that only later, during the evolution of the universe, their separation occurred.
Strong force is, as the name suggests, the strongest, but also the shortest acting. It acts only at distances of the dimensions of the atomic nucleus. This force is responsible for the stability of the atomic nucleus and the particles that make up that nucleus.
The carriers of this force are gluons, which were discovered in 1979 in the PETRA accelerator (DESY, Hamburg, Germany). The characteristic of this force is that with increasing distance its intensity becomes greater (the opposite of other forces).
The action of this force is achieved by quarks exchanging gluons. The strong force, sometimes called the color force, acts between individual quarks (of different colors) and allows protons and neutrons to survive.
A strong force, but of lesser intensity, also acts between quarks belonging to different protons or neutrons. This force, called the nuclear force, is responsible for the stability of atomic nuclei.
There is one more force left, perhaps the most familiar to everyone.
Yes, it is gravity, but sadly it doesn’t fit the Standard Model!
Gravity is described by the general theory of relativity, and one of the biggest challenges for theoretical physics is to find a theory that will “reconcile” relativity and the standard model.
A few decades after the theoretical establishment of the standard model, experiments have confirmed almost all predictions of this theory.
There is only one thing left, the search for the God Particle (as Lion Lederman called it, in the book of the same name). We are talking about the Higgs boson, a particle that should answer the simple question “why there is mass”.
The Standard Model predicts the existence of this particle, but it has not yet been found, but its discovery is expected. Physicists hope that the LHC accelerator, operating in Geneva, will enable the detection of this particle. This would fully confirm the standard model.
So, summing up, about 2500 years have passed since the first ideas about the atom. The idea of the atom has undergone many changes.
From the idea of 4 elements, we reached abstract concepts and 12 particles (plus the same number of antiparticles, and all painted in three colors).
This is currently the best theory that describes the world around us, but physicists know that it certainly does not fully end there.
This is the end of our story about the history of atoms, but this is not the end of particle physics, because no one can even guess what the next years and decades will bring.
