In addition to the Higgs boson (discussed in the previous entry, below), another type of particle physicists are hoping to find are so-called supersymmetric "superpartners" of the particle of the Standard Model.
To review the Standard Model, we have fermions (matter particles such as quarks, electrons, and neutrinos, which all share the property of half-unit spin) and bosons (force-carrying particles such as photons, for the electromagnetic force, and gluons, for the strong nuclear force, which all have full-unit spins).
As depicted on this page by physics instructor Bram Boroson, supersymmetry involves each standard fermion having a supersymmetric boson partner, and each standard boson having a supersymmetric fermion partner.
For the boson superpartner of a fermion, the letter "s" is placed at the beginning of the fermion's name, thus creating terms such as squark and selectron.
For the fermion superpartner of a boson, the suffix "-ino" is added to the bosonic name, yielding terms such as photino and gluino.
Boroson also raises a very important issue. From the perspective of why physicists might support supersymmetry, he notes that whereas it "doubl[es] the number of kinds of particles. There's got to be some kind of payoff, where this makes the theory simpler in the long run!"
Various introductions to supersymmetry are available. These include online sources such as Boroson's and this web document by Michal Szleper. Dan Hooper's (2006) book Dark Cosmos (which I will review in an upcoming posting) and Gordon Kane's (2000) book Supersymmetry (particularly the section entitled "Some Mysteries Supersymmetry Would Solve", pp. 55-62) also are useful.
Based on these sources, four potential benefits of supersymmetry stand out to me:
1. According to Grand Unification Theories (GUTs), the strong nuclear, weak nuclear, and electromagnetic forces should exhibit the same strength at a given energy level. As shown in Figure 7 of this report by Swagato Banerjee, such convergence does not occur within the Standard Model, but is projected to occur within supersymmetry.
2. Certain types of supersymmetric particles (or even combinations thereof) are possible candidates for something known as dark matter. This topic is discussed extensively in the aforementioned book by Hooper, in the chapter entitled "A Grand Symmetry."
3. According to Hooper, early string theories had a problem with tachyons (something that is "defined as a particle that travels faster than the speed of light," but also can be viewed "in some other frame of reference [as] moving backward in time" (p. 123).
Hooper goes on to note, however, that the first superstring revolution came up with a way to overcome the tachyon problem: "Namely, it was found that if supersymmetry was included in a string theory, the tachyons present in the theory would naturally disappear" (p. 124). The term superstring theory thus derives from this contribution of supersymmetry.
4. Supersymmetry also appears to keep the Higgs boson to a reasonable mass. A May 15, 2007 NY Times article on the Large Hadron Collider states that: “These superpartners cancel out all the quantum effects that make the Higgs mass skyrocket. “Supersymmetry is the only known way to manage this,” Dr. [Joe] Lykken said.”
Book authors Kane and Hooper each discuss detection of supersymmetric superpartners at the particle colliders. Kane writes that:
The superpartners that weigh the least are the ones most likely to be produced first, because it takes less energy to produce them... We don't know for sure which are the lightest ones, but most approaches suggest they might be the photino, Wino, Zino, and higgsinos (p. 157).
Adds Hooper:
Assuming that supersymmetry exists in nature, there should be a slew of new particles with masses between roughly a few tens and a few thousands of giga-electron volts -- perfectly suited for discovery at the LHC (p. 224).
Not all researchers find the idea of supersymmetry convincing. One such skeptic is Lee Smolin, author of the 2006 book The Trouble with Physics. Says Smolin, "My own guess, for what it's worth, is that (at least in the form so far studied) supersymmetry will not explain the observations at the LHC" (p. 78).
Further detail on Smolin's views is available in my review of his book.