If you read Part 1 of this article you may wonder: “so, the bosons are like this, fermions like that, so what? What are the consequences of that?” Actually, these particles differ in something very important: while fermions interact with the forces, the bosons mediate the forces!
Newton introduced the idea of “action at a distance” (force at a distance) to formulate gravity. It was an extremely important concept to understand gravity and later electromagnetism. Then, it was discovered that electromagnetism is mediated by the photons (which are bosons), and that the weak and strong forces also have their own mediators which are also bosons. Interestingly, we are still looking for a boson to mediate gravity (the graviton), which constitutes a very important open problem in particle physics (within the Standard Model). In fact, the Standard Model is not compatible with the graviton, because this particle brings some contractions into the model. The most famous theory to solve these (and other) problems is String Theory (which I will talk about in another article).
(If you prefer to read this article in Portuguese: O Mundo das Partículas II.)
The Standard Model is the theory that have provided the most valid results at the limits that our instruments allow us to make experiments – the high energy physics that are tested in particle colliders like the LHC, the Large Hadron Collider. Within this theory some new particles have been predicted and later on observed. Moreover, an outstanding agreement between measurable parameters from experiments and theory has been found. This theory brings together Quantum Mechanics and Special Relativity and is able in principle to describe the behavior of all particles and forces except gravity.
The reason why the theory doesn’t include gravity is because Quantum Mechanics is incompatible with General Relativity. That is why black holes are so interesting at the theoretical level, for they are the only known objects in our universe that need to be described by both theories simultaneously. An understanding about them may lead to clearer ideas about the problem of how the theories may reconcile with each other.
Why do we not assume that one of them is wrong (Quantum Mechanics or General Relativity)? Simply because both seem to work perfectly well when we do not need to consider the other one. There is no good reason to consider that one is correct and the other theory is incorrect.
However, the Standard Model has other problems. It’s not only incomplete (given that it excludes gravity), but also it should be incomplete for it is not autonomous: there are 19 “arbitrary” parameters, which can only be obtained from experiments (like a calibration). Physicists would prefer a theory that could explain those constants, a theory that could explain nature without needing experiments to fit constants. The experiments should only be necessary to test the theory, nothing else.
The Standard Model has one other problem related with antimatter. As you may know, there is not so much antimatter in our universe. However, the Standard Model predicts equal quantities of matter and antimatter. Of course, the Big Bang Theory demands symmetry breaking of these two entities immediately after the “beginning” of the universe. Thus, the Standard Model is missing this important “detail”: the origin of this asymmetry. Also, the Big Bang Theory includes the Inflationary theory (there are other alternative theories, such as the theory that light could have had a larger speed when the universe was “young”), which is a fundamental theory to explain the first moments of the universe in expansion. Since the Standard Model doesn’t include gravity, it is unable to explain the inflationary universe.
There is another reason to believe that this theory cannot be “The Theory”: its beauty. Do you think it is a naive argument? Well, Copernicus, for instance, believed that the Sun should be in the center of the universe because it was more “beautiful” than the planet Earth (among other more scientific arguments). The truth is that most of our successful theories, which better describe nature, show an inherent simplicity (and “beauty”). A pattern, a rule, may be something that simplifies and makes an explanation more elegant. If we have a number of phenomena to elucidate, it might be possible to explain one by one, or explain one and show how this explanation may be generalized for all of them. Which way do you think is the most effective to find new knowledge?
All this is meant to say that the Standard Model also fails in this aspect. As we will see, this theory predicts a lot of “fundamental” particles. (In contrast, I could refer the String Theory, which has the “beauty”, but I’ll discuss it in another article together with its problems.)
The fundamental particles in the Standard Model, in particular the fermions, are divided in three families (also called generations), according to their mass, in which the first family corresponds to the particles of smaller mass and the third of larger mass. The particles of the first family are predominant in nature, because they are more stable. The proton and the neutron are constituted by particles of this generation.
Each family has two quarks and two leptons. The main difference between these two types of particles is that the quarks “feel” the strong force, whereas the leptons don’t. Also, the quarks are much heavier than the leptons.
So, since there are 2 quarks per family and there are 3 families, then there are 6 quarks. They have the following names: top, bottom, charm, strange, up and down. The last two (up and down) belong to the first family and they constitute the proton and the neutron. The up quark has charge 2/3 and the down quark has -1/3, therefore the proton has two up quarks and one down, whereas the neutron has two down quarks and one up. Of course, if we sum the charge we obtain charge 1 for the proton and 0 for the neutron, as required. These charge values are normalized taking into account the charge of one electron. In other words, if we take into account that the charge is given in units of Coulomb (like grams are the units of mass), then the above values should be multiplied by the electron’s charge value to determine the charge of each quark.
The leptons are the electron, the muon, and the tau particle (all of them with negative electrical charge). There is one corresponding neutrino for each of these particles: electron neutrino, muon neutrino and tau neutrino. The neutrinos have the peculiarity of a null electrical charge and very small mass (people thought that it could be zero), therefore their interaction with other particles is very tiny: since they are leptons, they do not interact with the strong force; no charge means no electromagnetic interaction; very small mass implies a very small interaction by means of the gravitational force; and the weak force, well, it’s weak. Nevertheless, their existence has been detected. How was it possible? Because there are many neutrinos in our universe (since the detection method depends on their interaction, it means that it is indeed necessary to have many neutrinos in order to find just a few of them).
Does the story end here? No. Besides these 12 particles, due to symmetry reasons, 12 other particles were predicted (and found): the corresponding antiparticles. These antiparticles have equal mass, spin (for they are also fermions, I mean: anti-fermions) and parity (something related with wave functions and therefore with probabilities that appear in quantum mechanics) as their respective particles. The distinction is in their electrical charge (which is symmetric) and other quantum properties. When a particle meets its antiparticle there is a reaction in which they annihilate each other resulting in pure energy – 100% reaction yield.
Beyond these 24 fermionic particles, there are also the bosonic particles: the photon (already mentioned in part I) mediates the electromagnetic force; the gluon is responsible for holding quarks together (hadrons, like the proton and the neutron), it mediates the strong force; and W boson and Z boson mediate the weak force (“W” from weak and “Z” because this one has zero electrical charge). These last two bosons were found in 1983 at the CERN, which was one of the biggest successes of the Standard Model. Why two particles and not just one? It is related with the properties of this force (which I’ll not explain here). Finally, we are still trying to find the graviton.
You may be wondering: what about the “big star”? Where is the “god particle”? The Higgs boson seems to have been finally found at the LHC – I said “seems”, because the particle that has been found is a boson (integer spin), but not necessarily the Higgs boson, for further measurements have to be done in order to determine missing properties. The measured energy (which is a very important property) is the one expected (within a certain interval), that’s why everybody is so excited and confident: it really looks like it is the Higgs boson (if it isn’t, then it’s also interesting, for it is a new particle and it demands a new theory). The experiments will still take a while, because it is necessary to perform an astonishing number of collisions in order to find just one Higgs boson (the probability is something like 1 in 10 thousand million collisions). Of course, in order to characterize this boson it will be necessary to get many Higgs bosons.
Taking into account its name, you may deduce that this particle should mediate something. That’s right, it mediates the Higgs field, which is something necessary to explain why the fundamental particles have their specific masses. One might think that it is somehow similar to the graviton, but it’s not. The Higgs boson does not have infinite range, whereas the graviton must have (also, the Higgs is a scalar field and the graviton cannot be). The discovery of this particle was (is) extremely important for the Standard Model, because it is one of the main predictions of the model, and plays a central role in it. The non-existence of the Higgs boson would have meant that the model was incorrect and consequently it would imply that high energy physics had been orbiting around wrong ideas in the past 40 years. It would be very strange, since the model has already proven to be correct in a number of cases. Anyway, as I said, the Standard Model is not a complete theory, therefore we are still far from a complete understanding of the universe. It’s not only the graviton that is missing, we have also other challenges, such as explaining dark matter and dark energy. We estimate that dark matter constitutes 27% of the universe and dark energy 68%. Basically we have an incomplete knowledge of about just 5% of our universe!
If you have any doubt, comment, remark, please do it. I also remind you that I’m not a native speaker, so I’ll appreciate any corrections to the text. Thanks!