There are many kind of subatomic particles. The protons and neutrons, and a number of other exotic kinds of matter, are made out of particles called quarks. The electron is a familiar to most people as another kind of subatomic particle. Less familiar to most people is another kind of particle called a neutrino.
A neutrino is the lightest of the particles called fermions that consist of quarks, electrons and neutrinos (each of which come in ordinary and anti-matter varieties). The vast majority of quarks, electrons and neutrinos, which are also the only stable ones, are what are called "first generation" quarks, electrons and neutrinos. But, particles that are identical to these in all respect except that they are heavier and prone to decay into lighter particles called "second generation" particles. There is also an even heavier and faster decaying "third generation" of these particles.
No one has ever directly observed a fourth generation particle, which, we would expect to be heavier, and decay faster, than a third generation particle of the same type. But, a new study of the universe's cosmic background radiation (source scientific journal article here) (cosmic rays are an important cause of radio static that is found in all directions from early at different intensities and believed to date back to the earliest days after the Big Bang), in addition to confirming a large number of mainstream theories about the make up of the universe, also provides evidence that there are four, or even five, generations of neutrinos.
Ways to use cosmic background radiation observations to determine how many kinds of neutrinos there are were proposed at least ten years ago. But, now the evidence needed to apply that test is in hand.
The answer so far from this indirect evidence isn't definitive. There are experimental error issues and other kinds of random variation in space that prevent indirect observation of how many generations of neutrinos there are from being definitive.
In light of the evidence and the experimental uncertainty involved, the cosmic background radiation observations we have make it appear most likely that there are four generations of neutrinos. The next most likely possibility (within the roughly 68% chance that the true result is within one standard deviation from the expected result, but further from the expected result than four generations of neutrinos) is that there are five generations of neutrinos. The next most likely possibility (within two standard deviations from the expected result, so a considerably smaller than 17% chance given our experimental evidence, since this is a two sided distribution) after that is that there are only three generations of neutrinos.
We know from other evidence that there are at least three generations of neutrinos, and there is not a statistically significant chance (in other words it more than three standard deviations from the expected value, which is a far less than 1% chance in a two sided distribution) that there are six or more generations of neutrinos, given the evidence we have at this point.
In other words, the odds against there being only three generations of neutrinos given current scientific evidence are about 9-1, and the odds against there being six or more generations of neutrinos are worse than 100-1. Four generations of neutrinos are more likely, and five generations aren't ruled out.
The heavier a particle is, the quicker it decays, and thus, the harder it is to observe. Since the neutrino is always the lightest particle in each generation of particles consisting of quarks, electrons and neutrinos, we would expect that it would be easier to find evidence of a fourth generation neutrino, than it would be to find evidence of a fourth generation quark or electron.
But, given the patterns we have seen in the first three generations of quarks, electrons and neutrinos, almost everyone thinks that if there are four generations of neutrinos, that there are probably also four generations of quarks and electrons. And, knowing that it is likely that there is a fourth generation neutrino out there makes it seem much more worthwhile to do the very expensive scientific experiments necessary to observe one more directly and determine its properties. A fourth generation neutrino should be light enough that there is a real chance that we could detect one in the next round of particle accelerator experiments (the more powerful a particle accelerator, the heavier the subatomic particles it can detect).
If we learn the mass of a fourth generation neutrino, we could make quantum mechanics, in general, more accurate, and we could make better guesses concerning the mass of fourth generation quarks and electrons. Simply knowing that fourth generation quarks and electrons exist and being able to make somewhat accurate guesses concerning that mass of those particles would make quantum mechanics calculations a lot more accurate. These facts make quantum mechanics more accurate because quantum mechanical equations, in theory, require considering every possibility in a certain situation (for example, that a high energy photon that hits a third generation particle will cause it to turn into a fourth generation particle of the same type), and fourth generation particles make more possibilities available, even if those possibilities are usually relatively unlikely ones.
Knowing a fourth generation quark exists would also have deep implications for theoretical quantum physics. Each new piece of information on higher generation particles helps us to put together some sort of theory about why each particular particle of a particular generation weighs what it does and behaves the way that it does.
There are many different theories out there that provide a deeper explanation than what we have now about the nature of the universe and the particles and forces at work in it. Many of those theories require that there be precisely three generations of quarks, electrons and neutrinos. If we know that there are four generations of neutrinos, all of those theories can be thrown out as interesting but irrelevant junk, and scientists can focus their attention on theories with four generations of particles.
Since there are differences in kind between theories that expect exactly three generations of particles from those that expect more than three, it might even be possible to make accurate scientific predictions from these differences without actually knowing which particular theory that permits four or more generations of particles is right. After all, the theories scientists have not been able to rule out already have lots of things in common because they must all be able to agree with all the evidence that science has gathered so far, which forces them to all have a great many similarities already. Each new fundamental discovery, like the discovery that there are more than three kinds of neutrinos (if supported by further evidence) culls the group of viable theories. Even small bits of new fundamental information beyond that, such as the mass of a fourth generation neutrinos or the presence or absence of a Higgs boson, both things that the Large Hadron Collider (LHC) might be able to tell us, could dramatically narrow the ranks of theories that can fit the empirical evidence. If the LHC, for example, finds that there is no Higgs boson, confirms that there is a fourth generation neutrino of a particular mass, this would rule out many of the leading proposals of theoretical physics, leaving a narrow group of theories that deserve further examination.
This would also highlight the distinctions between the theories that remain viable, allowing experimenters to focus their attentions on issues that otherwise might not have seemed worth the trouble to resolve.
A 3+1 rule seems to dominate basic elements in complex natural systems (I've seen it from fundamental particles all the way up through the periodic table, the genetic code, and human languages). Simply put, a tetrahedral symmetry starts out that is then broken so that one element in the system is isolated behaviorally from the other three, and the three form their own more coherent subsystem. Examples include neutrinos which have zero charge so have no place in normal atoms, leaving the two quark and one lepton types remaining. Another is the breaking of electroweak symmetry to yield the two W and one Z weak force particles on one side as a system, and the photon on the other.
If there is a fourth generation, it is likely to be the odd-man-out of such 3+1 sets, and will exhibit behavior unexpected from the perspective of the other generations we already have seen. Thus simply extending our existing experiments may not work. We have to broaden the net.
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