These states of matter or particles are distinct from anything that is ordinary — and therefore observable. Often, the exotic is more difficult to experience and is, at this stage of knowledge, part of the domain of the hypothesis.
You are surrounded by matter. A tangible matter, which you can observe, and which forges your spatial reality. It is made of particles, which form “ordinary” matter. But can there be other forms of matter? Shapes… “exotic”? This is indeed a quest for particle physics.
Ordinary matter, or baryonic
Almost all ordinary matter is called “baryonic” because it is made up of baryons. Baryons are composite particles. “Composites”, because they are themselves composed of elementary particles. We know the composition of composite particles, but not that of elementary particles — perhaps they are in turn composed of even smaller unidentified particles. In any case, they are called “elementary” as the fundamental blocks, to our knowledge, of the other particles.
The composition of baryons is made of quarks – which are part of the famous elementary particles! — and more precisely of three quarks. Protons and neutrons are among the most important baryons in ordinary matter.
Except that ordinary matter is not enough to explain everything. A significant part of the Universe is made of non-baryonic matter. Particle physics calls “exotic” almost anything that, by definition, is not ordinary matter. As soon as matter has bizarre properties, it is exotic.
“Exotic” Particles and States of Matter
The quark-gluon plasma, that is to say the state of matter just after the Big Bang, in an exceptionally dense and hot universe, is “exotic” by definition: it is not usually found, and it is not is not easy to generate (you have to use a particle accelerator).
The term often designates states of matter or particles that are hypothetical — that have not been experienced. For example, the graviton is an elementary particular which has never been observed, and which therefore turns out to be purely theoretical at this stage. Dark matter, which has never been observed directly, but which is postulated to fit the standard model of physics, is postulated to be exotic since it differs from ordinary observable matter.
A particle is designated as exotic when, unlike protons and neutrons, it does not have the predicted number of elementary particles. As we said before, protons and neutrons have three quarks. But physicists discovered a few years ago that there could be variations: particles called tetraquarks, which therefore have four quarks. In 2021, CERN spotted the signal of such an “exotic hadron”.
In 2003, a particle called “particle X” was discovered. The “X” comes from its structure which is unknown to us. It is supposed to have appeared by stealth just after the Big Bang, when the Universe was an extremely dense and hot quark-gluon plasma. But recently, by reproducing an equivalent of this state of matter, physicists were able to identify a hundred signals whose data confirm that it would indeed be this X particle. now, what is its structure. But what is certain is that this is indeed an exotic particle, either with a different number of quarks than usual, or an even more different molecule which would be made of two mesons ( a single meson has two quarks).
The differentiation between ordinary and exotic is in any case an important distinction in physics, in particular to distinguish what we already know and can observe, from more strange states and particles that require digging any further.