Yesterday, we presented a comprehensive guide to the world of subatomic particles, exploring all the known elementary and composite particles. But now it's time to put certainty to one side and explore the wild, mind-bending world of undiscovered particles.
There are three basic types of hypothetical particles. In the first group, we find the particles that most physicists believe exist, and we just don't have sufficiently advanced technology to detect them yet. In the second group, there are the particles that a minority of scientists believe are out there, or particles that once were more widely accepted but have now fallen out of favor. And in the third group, there are any particles that scientists haven't even really conceptualized yet but are out there lurking, waiting to be discovered. There might not actually be any particles in that third group - by definition, we can't know - but it's never a bad idea to leave at least a little room for the unexpected.
In any event, we'll be focusing on just the first two groups. In general, all of the particles we're about to discuss would fundamentally enhance and perhaps alter our understanding of the universe if they were to be discovered. These particles could confirm or refute decades old theory, lay the groundwork for a grand unified theory, and maybe even bring the impossible into the realm of the possible. But let's not get too romantic about these particles - let's first try to understand what they might be, before we speculate about what they might do.
The Two Holy Grails: the Higgs Boson and the Graviton
We mentioned both the Higgs boson and the graviton yesterday, but now we can properly discuss them. Both of these would be elementary particles, specifically bosons like the photon, gluon, and W and Z bosons. In other words, they would mediate fundamental properties of the universe. For the Higgs boson, that property is mass, and for the graviton, that property is (unsurprisingly) the gravitational force.
The Higgs boson is the only elementary particle predicted by the Standard Model that we have yet to discover. Now, whether we find it or not doesn't actually invalidate the Standard Model - it's already proven itself over several decades to be an excellent model for how the universe works in most cases, and whether or not we find the Higgs won't change that fact.
The discovery of the Higgs would explain the existence of mass in the universe, which is one of the many long-simmering mysteries physicists are trying to solve, but it's questionable how much use the Higgs will be in solving larger physics issues, like the search for a grand unified theory. At best, learning more about how it interacts with other particles could help rule out a couple models for a grand unified theory, but that's probably the most we can hope for, and even that might be overly optimistic.
That's part of the reason why the science community has officially discouraged the media from calling the Higgs boson the "god particle", preferring the relatively restrained nickname "the champagne bottle particle." Finding it will be a cause for celebration, yes, but physicists will still have plenty of mysteries left to solve.
The graviton, on the other hand, could have much larger implications. The existence of a gauge boson that mediates gravity would prove that gravity is, in fact, a quantized, fundamental force in the same way as the strong, weak, and electromagnetic forces. That doesn't quite fit into the Standard Model as presently constructed, and would strongly suggest we need to add some sort of quantum gravity theory to resolve the discrepancy.
Of course, quantum gravity disagrees in a much more fundamental way with one of the granddaddies of modern physics: Einstein's theory of general relativity. According to Einstein, gravity isn't so much a fundamental force as it is something created by the curvature of spacetime. That flatly contradicts the existence of a graviton.
But then, that's not the only place where general relativity and quantum mechanics are incompatible. Again, I'm condensing very complex topics into a few sentences, but in essence, general relativity explains objects that are very massive, while quantum mechanics explains objects that are very tiny. It's when you deal with places that are both very massive and very tiny, like the epicenters of black holes or the beginnings of the universe, that you run into problems, because quantum mechanics and general relativity are fundamentally incompatible.
There are a number of theories that attempt to reconcile this discrepancy - string theory is probably the most famous of these - and quantum gravity forms a part of many of these theories. Admittedly, a graviton can't fit within the bounds of general relativity, but perhaps it doesn't have to. They might just coexist, with the graviton explaining certain situations and general relativity explaining other ones.
A somewhat similar example of this is light. In certain scenarios, we deal with individual photon particles, while in others we deal with radio waves, which are fundamentally different things that somehow encompass the same phenomenon. The difference between the graviton and the curved space of general relativity is a much larger gap to bridge, but finding the graviton might help us start to knit these two opposed ideas together.
Entering the Spin Zone
If you've had a chance to check out my cheat sheet for the elementary particles, you'll notice I listed one property that I barely mentioned in yesterday's post: spin. The reason I left spin out of the discussion was a practical, if somewhat cowardly one - spin is a confusing, non-intuitive concept, the sort of thing where quantum weirdness really kicks into high gear. But if we're going to discuss the single largest group of undiscovered particles, we've got to bite the bullet and talk about spin.
At first glance, quantum spin doesn't seem that much different from things we observe in the classical world. If you take a charged object and spin it around, the charge creates a loop of current, and that current in turn creates a magnetic field. That is, more or less, how you make an electromagnet, and that's basically what spin is in a quantum sense as well. Seems simple enough, right?
Here's where things get a bit weird. If we're talking about an electromagnet in the observable, classical world, it's perfectly easy to make the charged object spin a little slower or a little faster, alternately decreasing or increasing the strength of the magnetic field. But an electron doesn't work that way - its spin will always be the same, and there's absolutely nothing in the universe that will change it. The electron's spin is an intrinsic, unchanging property - rather like if our charged object in the physical world was always spinning at the same speed, regardless of any outside interference.
Besides, this analogy only works for particles that have a charge to begin with. Particles with neutral charge like the photon and neutrino also have spin, but since they have no charge there's no related magnetic effect. There's really no completely accurate way to talk about spin without at least a few semesters of college level physics (at the very least), but for our purposes, three things are really important to know: it's an intrinsic, unchanging property of all particles, it represents the angular momentum of the particle, and it creates a magnetic moment. As for the elementary particles, all the leptons and quarks have spin-1/2, and all the bosons have spin-1.
Before we move on to the undiscovered particles, one last rather mind-bending point about spin. Let's imagine you're standing at the center of a rotating platform. Once the platform has rotated 360 degrees, your position will be exactly the same, and you will interact with the universe in precisely the same way as you did before the platform started rotating. That's true of anything in the classical world.
But let's say you took a particle with spin-1/2, like an electron, and put it on that platform. If you rotated the platform 360 degrees, the electron wouldn't be in the same position as it was before the rotation. It would actually be in the opposite quantum phase, and you would have to rotate it another 360 degrees to get it back to its original state. We don't know if there are any spin-2 particles - the graviton might be one, but like I said, we haven't found it yet - but they would only need to be rotated 180 degrees to get back to its original position. And certain mesons, like the kaon, have spin-0, which, in terms of our analogy, means it would actually be impossible to rotate it on the platform at all.
Why does the quantum world work like this? Because, well, it just sort of does. The quantum world is sort of like the fifties motorcycle punk to classical physics's crusty old authority figure. It plays by its own set of rules.
A Question of Symmetry
Now, this is why spin is important. Like I said earlier, the leptons and quarks - collectively, the fermions - all have spin-1/2, and all the gauge bosons have spin-1. More generally, all fermions will have half-integer spin, while all bosons will have integer spin.
There is a theory that all the elementary particles should have a corresponding particle that has the same mass and internal properties, with the only difference being that its spin is 1/2 less. So, let's go back to the most famous boson, the photon. In this theory, the photon, with spin-1, has a particle that's identical apart from the fact that it has spin-1/2. Of course, that means this particle is a fermion, because all fermions have spin-1/2. The counterpart of a fermion like the electron would also have spin-0 - and since 0 is an integer, that means this particle is a boson. And the graviton, if it exists, has a supersymmetric partner with spin-3/2.
This theory is known as supersymmetry. It was independently proposed by a number of physicists in the late 1960s and early 1970s, but the real breakthrough came in 1981, when Howard Georgi and Savas Dimopoulos proposed what they called the Minimal Supersymmetric Standard Model. The MSSM had one crucial innovation - it suggested these supersymmetric particles had the same quantum properties as the particles we're familiar with, but not the same mass.
In fact, the mass of these particles might be in the scale of tera-electronvolts, which is a thousand times heavier than any known elementary particles. Particles in the TeV scale would be so heavy and unstable that they would only last the tiniest fractions of a second - they only would have existed naturally in the shortest of instants after the Big Bang. This broken symmetry of masses between the two types of particles explained why the supersymmetric particles had never - and still haven't - been observed, because they're too big and require too much energy for our current particle accelerators to create them.
These supersymmetric particles, better known as sparticles, have some of the most awesomely silly names of any particles. As a rule, all the supersymmetric partners of the fermions (which are bosons) just take the name of the original fermion and add an "s" to the start. So all the supersymmetric leptons are called sleptons, all the supersymmetric quarks are called squarks, and so on. The new names vary from the reasonable (selectrons) to the ridiculous (smuon sneutrino) to the downright unpronounceable (sbottom squark).
The bosons, on the other hand, change the "on" at the end of their names to an "ino". This means the photon's supersymmetric partner is the photino, the gluon is the gluino, and the W and Z bosons become the wino and zino. There's also a theoretical combination of the supersymmetric partners of the charged and neutral bosons, known as charginos and neutralinos. We'll come back to the neutralino in just a moment.
So will we find the sparticles? That's what the Large Hadron Collider is hoping to accomplish, and with good reason - there's a whole lot of theoretical physics riding on the hopes that these sparticles exist. Supersymmetry is a major part of most variations of string theory, it could help us unify different forces into as single equation, and it could help solve of the most pressing mysteries of the universe: dark matter.
The Search for the Dark Matter Particle
I'd need another few thousand words apiece to fully explain dark matter and dark energy, but thankfully I don't have to. Dr. Dave Goldberg, author of the always excellent "Ask a Physicist" series, has already written a couple great primers on dark matter and dark energy. So I'll just very quickly summarize what these are, then get to the particles.
Both dark matter and dark energy are our current best explanations for certain unexpected properties of the universe. Dark matter was first proposed in the 1920s to account for the fact that both galaxy clusters and galaxies should fly apart if there isn't a lot of extra, unseen mass holding them together. Dark energy is a mysterious force that is causing the expansion of the universe to accelerate, rather than slow down and eventually start to contract, which is what would happen if gravity was left to its own devices.
For the universe to fit all our observations, then visible matter can only account for 5% of the universe's mass, while dark matter is responsible for 25% and dark energy 70%. That's a lot of unknown stuff for us to find, and the search is on for particles that can account for them. It's not actually clear whether any particles are involved in dark energy, and one of the better current theories is that dark energy is essentially a byproduct of the cosmological constant. But we do have a bunch of candidates for dark matter, and the most famous is probably the WIMP.
The WIMP, or weakly interacting massive particle, has a lot of its basic properties in its name. This particle would only interact through the weak nuclear force and gravity, but not the much more powerful strong nuclear force or electromagnetism. That fact immediately explains why dark matter can't be detected through conventional means: if something doesn't interact through electromagnetism, we can't see it, and if it doesn't interact through the strong force, then it won't affect atomic nuclei in any easily noticeable way.
The other big problem is that it's massive, meaning it's a lot bigger than the elementary particles we're familiar with. As we discussed with the sparticles, it requires a lot more energy to create or detect massive particles under laboratory conditions, which greatly complicates the search for these WIMPs. It should be pointed out that WIMP is more a description of a particle than an actual name, and that one of the other theorized particles is likely the secret identity of these WIMPs.
And with that, we return to neutralino, a particle formed by mixing together the supersymmetric partners of the W and Z bosons, photon, and neutral version of the Higgs boson. Such particles would generally have masses between 100 GeV to 1 TeV and behave very similarly to the smaller, weakly interacting neutrinos. Like all the sparticles, that makes them very difficult to detect in particle accelerators, but it also perfectly describes the WIMP. The lightest neutralino, which could have mass anywhere between 10 GeV and 10 TeV, is considered the leading candidate for the dark matter particle.
Another dark matter candidate is the axion. This particle was theorized in 1977 by Roberto Peccei and Helen Quinn as a solution to what's known as the strong CP problem. Basically, CP symmetry states that the laws of physics should remain the same if a particle is swapped with its antiparticle (C symmetry) and if left and right were switched (P symmetry). That holds true for almost all particles, but the weak force decay of the neutral form of the K-meson, or Kaon, was found to violate CP symmetry. This finding won its discovers a Nobel Prize, and prompted a mild rewriting of the laws of physics to account for the violation.
None of this would be a problem, except for the fact that CP violation only occurs in reactions involving the weak force, with no obvious sign of similar violation in those involving the strong force. Now, it makes sense mathematically for there to be total CP symmetry or for there to be the same CP violations with the weak and strong forces, but it doesn't make sense for the violation to occur with one force but not the other. That's left physicists with a puzzle to solve, and that's the strong CP problem in a nutshell.
There are a few ways out of this predicament. One is that there is strong CP violation, but the amount of violation is many orders of magnitude smaller than its weak equivalent and thus beyond our current ability to detect. The other major theory is that an undiscovered particle, the axion, affects CP symmetry in strong force interactions, externally reducing the amount of CP violation to zero and explaining why we don't see any violation.
Axions figure into the dark matter question in a couple ways. If they exist, they have no electric charge, have a tiny mass that's anywhere from 10^-6 to 1 eV, and have very limited interactions through the strong and weak forces. That could explain why we've had such trouble detecting them, but that incredibly low mass means they likely aren't themselves the dark matter particles.
Instead, their supersymmetric partner, the axino, could be the lightest supersymmetric particle, and therefore one of the best candidates for dark matter. It's also possible that axions created in the first moments of the Big Bang have since decayed and clumped together into a strange, very cold form of matter known as a Bose-Einstein condensate. The universe could be filled with these axion clumps, and that might account for the dark matter.
Of course, it doesn't have to be an all-or-nothing solution, and a combination of any of these particles or other phenomena could account for the missing dark matter. There are also sterile neutrinos, which are nearly identical to neutrinos except they only interact through gravity, making them fiendishly difficult to detect. The chameleon particle is a truly bizarre theoretical particle whose mass varies depending on its location: in deep space, it has a very small mass, but as it approaches a dense body like Earth it gains a huge amount of mass, which then makes it difficult to detect in terrestrial particle accelerators.
These various possibilities enjoy different levels of scientific support, and none have been definitively ruled out as candidates for dark matter. Indeed, many of them have at least some possible evidence for their existence, although these findings are almost all disputed and would require another article entirely to sort through. Suffice it to say that we haven't found the dark matter particle yet, but we've got a bunch of good leads and plenty of reason to think we will find the right WIMP eventually. And we haven't even touched on the equally wonderfully named RAMBOs and MACHOs, which are good dark matter candidate but don't involve undiscovered particles.
Where We Stand
Even after several thousand words spent discussing particle physics, I still feel like we've only just scratched the surface of this topic. I've provided a fairly comprehensive overview, but there are a ton of subtleties to shade in, and there were a few topics that just couldn't be done justice in these posts - topics like string theory, which is dancing around the outer edges of most of these topics, or even things like tachyons, the theoretical faster-than-light particles that are just maybe clawing their way back into scientific respectability. And I didn't even get into ghosts or quasiparticles, which we actually know exist. Clearly there's a lot more to talk about, and this isn't the end of our discussion. It's just the beginning.
Be sure to check out yesterday's post on the known particles and my cheat sheet to the elementary particles. If you're still confused about any of the topics I've discussed, feel free to leave questions in the comments or send an email to firstname.lastname@example.org.
Huge thanks go to Dr. Dave Goldberg for his help with spin and the Higgs Boson; you can read more of his stuff at A User's Guide to the Universe and in his "Ask a Physicist" series here at io9. I'd also like to thank my friend Tova Holmes, currently a senior physics concentrator at Harvard, for patiently explaining the strong CP problem to me with no advance warnings. Any errors or mistakes in the explanations are of course my own, not theirs.