B How will the sparticles work?

This will be difficult. Think about the hardest thing you learned in school in the last month. How would you explain that to someone much younger than you - say a first grader?

My physics teacher used to say "Beyond the scope of this course. And the next one. And the one after that." When I was your age, to get to supersymmetry, I needed to finish grade school. Then four years of high school. Then college. Then two years of graduate school. By then I didn't understand supersymmetry, but at least I was prepared to learn it.

 

btw, the word "sparticle" can generate an extra of confusion, because the s in "squark" (say) means "scalar", not super.

 

In fairness, there is really no harm in thinking of the "s-" in a sparticle name as referring to a "superpartner" instead of a "scalar" even though it is both. In the same way, a "b" quark is sometimes called a "bottom quark" but sometimes called a "beauty quark" although the former is now more fashionable as it corresponds to calling a "t" quark a "top quark" which has not "beauty" counterpart alternative name.

 

To a bit less dismissive without getting into all the technical details, supersymmetry (also known as SUSY) is basically about the idea that there are technical reasons that makes it desirable for there to be fundamental balance between fundamental fermions that make up what we crudely in layman's language think about as "matter" (i.e. the quarks that protons, neutrons and more exotic hadrons are made of, electrons, muons (heavy electrons), taus (really heavy electrons), and neutrinos) and fundamental bosons that we crudely think of in layman's language as the particles that make up force fields (gluons for the strong force, W and Z bosons for the weak force, photons for the electromagnetic force, and gravitons (hypothetically) for gravity, and the Higgs boson which gives rise to the inertial mass of fundamental particles).

The theoretically easiest way to get that balance is to imagine that every fundamental fermion has a boson counterpart (squark and sleptons), and that every fundamental boson has a fermion counterpart.* This then gets jumbled a bit because some of these counterparts have very similar physical properties that cause them to blend into each other and look like different particles (something that happens in the Standard Model as well in the way that the electromagnetic force and weak force are related to each other in very deep ways), and the theory also requires at least four extra Higgs bosons to work out (a positively charged one, a negatively charged one, an extra heavy one, and one with a different parity - i.e. left handedness v. right handedness than a usual Higgs boson).

We don't notice any of this in everyday life, or even in high energy physics experiments (if the theory is true) because all of the particles created by supersymmetry except one (which explains dark matter) are unstable and decay into ordinary matter before we have time to see it, and also because they only form at all in very high energy situations.

(If this sounds familiar, it should. Most of the particles we do know exist decay extremely rapidly into ordinary matter and only form at all in very unusual high energy situations.)

Aside from the obvious problem that nobody has ever seen any of the supersymmetric particles the benefit of assuming that this is all real and that the new particles are simply too heavy to see in colliders or otherwise not visible due to something called "R-parity" (a property that basically keeps superpartners and regular matter separated), is that:

(1) it provides natural candidates for dark matter particles of a variety called "WIMPS",
(2) it makes the constants of the Standard Model such as the Higgs boson mass seem more "natural",
(3) it makes it much easier to do math that sheds light on how particles interact at very high energy
(4) it unified the three fundamental forces into one master force at high energies called the GUT scale,
(5) it provides a way to explain where the matter in the universe came from that are unexplained in the Standard Model, and
(6) it sheds some light on the kind of reasons that Standard Model constants might have the values they do although not particular clear guidance.

Another of the big reasons to explore supersymmetry before we knew the mass of the Higgs boson was that lots of Standard Model predictions in high energy situations were nonsense answers where the likelihood of all possible events didn't add up to 100% before the Higgs boson mass was discovered to be just right, while this doesn't happen in supersymmetry. But, this is less of a big deal than it used to be because the mass of the recently discovered Higgs boson prevents the Standard Model from becoming pathological mathematically at high energies in the way that it would if the Higgs boson where much heavier or much lighter than it is in reality.

Supersymmetry is also a very natural low energy approximation of string theory (many versions of string theory require, for mathematical reasons, that fundamental fermions and fundamental bosons have counterparts for each other for reasons related to the way a fundamental superstring in that theory can vibrate).

Supersymmetry itself does not unify quantum mechanics and relativity. Instead, it unifies the three forces of the Standard Model (electromagnetism, strong force, weak force) into forms of the same underlying force that is unified at high energies, making it what is known as a "GUT" (grand unified theory).

If you add quantum gravity to the supersymmetry mix by adding the graviton (a fundamental boson) and a superpartner called a gravitino (a fundamental fermion), you get supergravity also known as SUGRA which is a low energy approximation of a theory of everything, and supergravity, in turn is usually a foundation of string theory.

A potential connection to string theory is attractive because string theory offers a reasonable hope that it could provide a mathematically consistent way to create a theory of quantum gravity that could be consistent with the rest of quantum mechanics which is called the Standard Model. String theory is pretty much the only game in town that creates a potential theory of quantum gravity with particle based force fields like those used in the rest of quantum mechanics so it is very tempting to find a way to connect what we know to it. Scientists from Einstein onward have been trying very hard to unify gravity and other forces of nature ever since general relativity and quantum mechanics were conceived in the early 1900s. So far, no one has even come close to succeeding.

(There is another approach to quantum gravity that involves applying quantum mechanical concepts to the nature of space-time itself, which includes approaches known as Loop Quantum Gravity (LQG), rather than using the force field carried by particles approach of string theory, but that is a story for another day that doesn't involve supersymmetry.)

The problems with supersymmetry are that (1) if superpartners exist, the LHC has determined that they are much heavier than they were expected to be, (2) "naturalness" is being questioned as a useful theoretical concept, (3) that evidence of force unification that should have showed up by now has not appeared, and (4) the LUX experiment has pretty much ruled out the kinds of WIMP dark matter particles that supersymmetry predicted.

But, because supersymmetry is so mathematically similar to the Standard Model of particle physics, it is easy to tweak properties of particular versions of supersymmetry like particle masses in such a way that it predicts essentially the same things as the Standard Model down to the limits of experimental error. So it is hard to reject outright. And theoretical physicists are very reluctant to abandon supersymmetry because that would mean giving up hope that their best shot at a good theory of quantum gravity. So they'd have to start over from scratch trying to merge quantum mechanics and general relativity (the reasons that those two are incompatible are quite mathematical and technical but include, for example, the fact that point particles which are assumed in quantum mechanics would instantly turn into black holes in general relativity).

String theory has all of the problems of SUSY and SUGRA and also lots of problems of its own. Basically, there are thousands or millions or more versions of string theory (called "vacua") and nobody knows which version is remotely close to our reality.

The Standard Model, in contrast, has no obvious generalizations that could for example be used as a basis for a version of string theory. So figuring out how to meld it with quantum gravity is even more difficult.

* It could be that a balance between fundamental fermions and fundamental bosons already exists in the Standard Model in a much more subtle way than the crude and obvious balancing present in supersymmetry theories, which would explain how seemingly "unnatural" aspects of the Standard Model "miraculously" balance out, but so far only the vaguest hints that this might be the case have been worked out by theoretical physicists and only as conjectures and hypotheses, not as proven theories.

 

Think a 7th grader got all that?

 

My friends and I had discussions about supersymmetry and string theory with not much less detail when we were in the 7th grade. Those discussions at one of my birthday parties became one of my mother's favorite stories of my childhood that she keep telling until she passed.

Anyway, if even 50% is understandable and that leads to the right kind of questions, that counts as success.