Warped Passages - Lisa Randall [130]
The properties of these particles and their superpartners are rigidly aligned to one another: the bosonic superpartners have the same masses and charges as their fermionic counterparts, and they also have related interactions. For example, if the electron has charge -1, so does the selectron; and if a neutrino interacts via the weak force, so does the sneutrino.
If the universe is supersymmetric, bosons must also have superpartners. The known bosons in the Standard Model are the force carriers: the photon, the charged Ws, the Z, and the gluons, all of which have spin-1. The nomenclature of supersymmetry dictates that the new fermionic superpartners have the same name as the boson with which they are paired with “-ino” tacked on at the end. So the fermionic partners of gauge particles are called gaugino particles, the fermionic partners of gluons are gluinos, and the fermionic partner of the Higgs particle is a Higgsino. As was true for bosonic superpartners, fermionic superpartners have the same charges, the same interactions, and—if supersymmetry is exact—the same mass as the boson with which they are paired (see Figure 64).
You might find it remarkable that physicists take the possibility of supersymmetry as seriously as they do, given that no superpartner has ever been found. I’m sometimes surprised how confident some of my colleagues are about it. But even though supersymmetry has not yet been found in nature, there are several reasons to suspect its presence. Sergio Ferrara, one of the first to work on supersymmetry, expressed the view of many physicists when he told me on our train ride to London that it would be hard to believe that such a surprising and fascinating theoretical construction played no role in the physics of the world.
Figure 64. Particles and their supersymmetric partners.
Other physicists, less taken with the beauty of the symmetry, believe in supersymmetry primarily because of the benefits of the supersymmetric extensions of the Standard Model. Unlike non-supersymmetric theories, they protect the light Higgs particle and the hierarchy of masses.
Supersymmetry and the Hierarchy Problem
The hierarchy problem in the Standard Model was the question of why the Higgs particle is so light. How can there be a light Higgs particle when there are large quantum contributions to its mass from virtual particles? These large contributions tell us that the Standard Model works only if it contains an enormous and unfortunate fudge.
The big advantage of a supersymmetric extension of the Standard Model is that when there are virtual contributions from both particles and superpartners, supersymmetry guarantees the absence of the large quantum contributions to the Higgs particle’s mass that made a light Higgs particle seem so unlikely. Supersymmetric theories can have are correlated. And because of the constraints this imposes, supersymmetric theories don’t have problems with large quantum contributions to particle masses.
In a supersymmetric theory, the virtual Standard Model particles aren’t the only virtual particles that contribute to the Higgs particle’s mass. Virtual superpartners do, too. And because of the remarkable properties of supersymmetry, the two kinds of contribution always add up to zero. The quantum contributions of virtual fermions and bosons to the Higgs particle’s mass are related so precisely that the large contributions made by either bosons or fermions individually are guaranteed to cancel each other out. The value of the fermions’ contribution is negative and exactly cancels the bosons’ contribution.
One such cancellation is illustrated in Figure 65, which shows two diagrams, one with a virtual top quark, and one with a virtual stop squark. Each of the individual diagrams would lead to a large contribution to the Higgs particle’s mass. But because of the special relationships between particles and interactions in supersymmetric theories, the huge quantum contributions to the mass from the top quarks and the stop squarks are obliterated