The Standard Model has worked beautifully to predict what experiments have shown so far about the basic building blocks of matter, but physicists recognize that it is incomplete. Supersymmetry is an extension of the Standard Model that aims to fill some of the gaps. It predicts a partner particle for each particle in the Standard Model. These new particles would solve a major problem with the Standard Model – fixing the mass of the Higgs boson. If the theory is correct, supersymmetric particles should appear in collisions at the LHC.
At first sight, the Standard Model seems to predict that all particles should be massless, an idea at odds with what we observe around us. Theorists have come up with a mechanism to give particles masses that requires the existence of a new particle, the Higgs boson. However, it is a puzzle why the Higgs boson should be light, as interactions between it and Standard-Model particles would tend to make it very heavy. The extra particles predicted by supersymmetry would cancel out the contributions to the Higgs mass from their Standard-Model partners, making a light Higgs boson possible. The new particles would interact through the same forces as Standard-Model particles, but they would have different masses. If supersymmetric particles were included in the Standard Model, the interactions of its three forces – electromagnetism and the strong and weak nuclear forces – could have the exact same strength at very high energies, as in the early universe. A theory that unites the forces mathematically is called a grand unified theory, a dream of physicists including Einstein.
Supersymmetry would also link the two different classes of particles known as fermions and bosons. Particles like those in the Standard Model are classified as fermions or bosons based on a property known as spin. Fermions all have half of a unit of spin, while the bosons have 0, 1 or 2 units of spin. Supersymmetry predicts that each of the particles in the Standard Model has a partner with a spin that differs by half of a unit. So bosons are accompanied by fermions and vice versa. Linked to their differences in spin are differences in their collective properties. Fermions are very standoffish; every one must be in a different state. On the other hand, bosons are very clannish; they prefer to be in the same state. Fermions and bosons seem as different as could be, yet supersymmetry brings the two types together.
Finally, in many theories scientists predict the lighest supersymmetric particle to be stable and electrically neutral and to interact weakly with the particles of the Standard Model. These are exactly the characteristics required for dark matter, thought to make up most of the matter in the universe and to hold galaxies together. The Standard Model alone does not provide an explanation for dark matter. Supersymmetry is a framework that builds upon the Standard Model’s strong foundation to create a more comprehensive picture of our world. Perhaps the reason we still have some of these questions about the inner workings of the universe is because we have so far only seen half of the picture.