Knocking on Heaven's Door - Lisa Randall [140]
A theory with a Higgs boson elegantly avoids high-energy problems. Interactions with the Higgs boson not only change the prediction for high-energy interactions, they exactly cancel the bad high-energy behavior. It’s not a coincidence, of course. It’s precisely what the Higgs mechanism guarantees. We don’t yet know for sure that we have correctly predicted the true implementation of the Higgs mechanism in nature, but physicists are fairly confident that some new particle or particles should appear at the weak scale.
Based on these considerations, we know that whatever saves the theory, be it new particles or interactions, cannot be overly heavy or happen at too high an energy. In the absence of additional particles, flawed predictions would already emerge at energies of about 1 TeV. So not only should the Higgs boson (or something that plays the same role) exist, but it should be light enough for the LHC to find. More precisely, it turns out that unless the Higgs boson is less than about 800 GeV, the Standard Model would make impossible predictions for high-energy interactions.
In reality, we expect the Higgs boson to be a good deal lighter than that. Current theories favor a relatively light Higgs boson—most theoretical clues point to a mass just barely in excess of the current mass bound from the LEP experiments of the 1990s, which is 114 GeV. That was the highest-mass Higgs boson LEP could possibly produce and detect, and many people thought they were on the verge of finding it. Most physicists today expect the Higgs boson mass to be very close to that value, and probably no heavier than about 140 GeV.
The strongest argument for this expectation of a light Higgs boson is based on experimental data—not simply searches for the Higgs boson itself, but measurements of other Standard Model quantities. Standard Model predictions accord with measurements spectacularly well, and even small deviations could affect this agreement. The Higgs boson contributes to Standard Model predictions through quantum effects. If it’s overly heavy, those effects would be too large to get agreement between theoretical predictions and data.
Recall that quantum mechanics tells us that virtual particles contribute to any interaction. They briefly appear and disappear from whatever state you started with and contribute to the net interaction. So even though many Standard Model processes don’t involve the Higgs boson at all, Higgs particle exchange influences all the Standard Model predictions, such as the rate of decay of a Z gauge boson to quarks and leptons and the ratio between the W and Z masses. The size of the Higgs’s virtual effects on these precision electroweak tests depends on its mass. And it turns out the predictions work well only if the Higgs mass is not too big.
The second (and more speculative) reason to favor a light Higgs boson has to do with a theory called supersymmetry that we’ll turn to shortly. Many physicists believe that supersymmetry exists in nature, and according to supersymmetry, the Higgs boson mass should be close to that of the measured Z gauge boson’s and hence relatively light.
So given the expectation that the Higgs boson is not very heavy, you can reasonably ask why we have seen all the Standard Model particles but we have not yet seen the Higgs boson. The answer lies in the Higgs boson’s properties. Even if a particle is light, we won’t see it unless colliders can make it and detect it. The ability to do so depends on its properties. After all, a particle that didn’t interact at all would never be seen, no matter how light it was.
We know