# 13. SUPERSYMMETRY

I think the discovery of supersymmetric partners for the known particles would revolutionize our understanding of the universe.— Stephen Hawking

A popular idea that extends the Standard Model is the Supersymmetric Model (or SUSY).

Supersymmetry is actually a specific mathematical relationship between certain elements in the Standard Model equations for fermions and bosons. The benefit is that it vastly simplifies the equations by allowing certain terms to cancel out. Without supersymmetry, certain physical inconsistencies such as infinite values and imaginary energy levels creep into the equations.

Is supersymmetry merely a mathematical sleight of hand? Could be. But if supersymmetry is true, there are wide-ranging consequences. Supersymmetry can explain why gravity is so much weaker than the other forces of nature, why the Higgs boson is so light. It could even account for dark matter (that makes up about 27% of that invisible 95% of the universe). A very popular idea is that dark matter is made of neutral—but still undiscovered—supersymmetric particles.

How do we know that supersymmetry is actually observed in nature?

Supersymmetric theories predict that for every particle that we observe, there is a super-partner (SUSY particles) whose spin differs by ½, implying the existence of a bosonic species for each fermionic species and vice versa. The two different classes of particles are thereby linked, although they are as different as could be, yet supersymmetry brings the two types together.

Supersymmetric particles have strange names: an electron’s supersymmetric partner is called a “selectron” and a quark’s partner is “squark”, and so on. Bosonic ordinary particles have fermonic super partners with the same name except with the added suffix ‘ino’, while fermonic ordinary particles have bosonic (scalar) super partner names with the prefix ‘s’ added. Examples are photinos, higgsinos, Z-inos, squarks and selectrons. Some supersymmetric particles have the same quantum numbers as each other and therefore can mix together producing particles that are not exact partners of any Standard Model particle. For example, the photino, Higgsino and Z-ino can mix into arbitrary combinations called the neutralinos. Further to the complexity is the fact that there is not one Higgs particle, but five according to supersymmetry.

So far, the super-partner cousins have never been observed in experiments. Thus, it natural to be skeptic about supersymmetry. But the theory passes two important tests: (a) it ties the four fundamental forces together and (b) it stands up at very high energies. Why haven’t we seen any of these SUSY particles yet? The reason could be that they are much heavier than regular particles—the heavier the particles, the shorter are their lifespans. That means they will only be observable for a very short time and our only hope of seeing them is if they appear in powerful particle collisions that the LHC will generate, or in astronomical observations.

Thus, supersymmetry is responsible for doubling the number of particles again. The particle table is quite large (Figure 13-2) even without the antiparticles (not shown here).