Have we really found the Higgs?
By now you have probably heard the news that CERN has confirmed discovery of “a fundamental scalar” particle, i.e. the Higgs. But as Sean Carroll pointed out on Google+ this morning, nothing in the data suggests it is fundamental. It could be composite. To understand why there could be room for some doubt, consider that the Higgs cannot be directly observed since it has zero spin and zero charge. The only way to “observe” the Higgs is to observe its decay products. The Standard Model (SM) tells us what the Higgs will decay to based on its mass, but it does not predict its actual mass. If the Higgs mass lies somewhere between 115 GeV/c^2 and 180 GeV/c^2 the SM is correct up to the Planck energy. So we look for decay products and depending on which products we see, various conservation laws and the rules of the SM tell us the properties of the particle that produced those products.
There are several assumptions being made here. The first is that the SM is correct, at least to within a certain degree of accuracy. The second then follows from the first: that any particle found within the specified energy range must be the Higgs because that’s what the SM predicts. In other words (and perhaps this is a separate assumption), the SM does not, at present, predict any other particles in this energy range with these decay properties so we assume that it must be the Higgs.
There’s nothing inherently wrong with the above assumptions as long as they are made clear, particularly to a media willing to embellish anything in the name of “selling” news. It should be noted, however, that the SM is far from an unassailable theory. Now before I say more on this, I must emphasize an important point: there is a difference between a theory having problems and a theory having limitations. Newton’s theory has limitations, but for the range of energies for which it is applicable, it is correct. The SM, on the other hand, has some genuine problems. In theory, it should apply to all fundamental particles, but it one of its central tenets is the CPT theorem and recent observations have hinted at CPT violation in neutrinos. This is different from saying we have simply found a limit to the SM. In this way, the SM is more akin to the de Broglie-Bohm theory that serves as a foil to standard quantum theory – it covers almost, but not quite all of the phenomena predicted by standard QM. In the case of SM, it may be that we have simply stumbled on the alternative theory first before we found the actual theory. (Of course, it’s also possible that there simply is no “actual” theory of anything and the best we can do is come up with rough models.)
So, did we find the Higgs? The folks at CERN certainly think so, or at least the media seems to be playing it that way. As a foundations guy, I honestly think it doesn’t matter one way or the other. It’s another particle. It’s existence was predicted by the theory and so we’ve been operating under the assumption that it is real for decades. I suppose if it hadn’t been found, that would have been pretty damning evidence against the SM, but you can’t prove a negative, i.e. there’s no way we could ever say that not finding it was just due to our lack of experimental ability. But finding it still doesn’t solve all the other problems with the SM including the fact that neutrinos and anti-neutrinos might actually have differing masses (some recent data has suggested this is possible). Now, that would be weird and would contradict a central tenet of the SM, the CPT theorem.
One final point I should make, however, is that if the SM does turn out to either need revision or replacement, it doesn’t suddenly invalidate all the amazing discoveries that it has led to over the years. It’s been an immensely useful tool and there’s no denying that it has come close to correctly predicting nearly all the properties of all known fundamental particles. These properties are measurable and thus independent of theory, to some extent (there’s a big caveat there that I will leave for another time). So, we wouldn’t need to suddenly revoke numerous Nobel Prizes if we found a hole in it.