Scientific Method / Science & Exploration

Particle physicists bemoan a perfect theory

LHC hasn't turned up evidence against the Standard Model. What if it never does?

How can anyone hate perfection?

I'm very glad that I'm not a particle physicist. In the excitement of the LHC starting up, breaking, starting up again, performing beautifully, and finding the Higgs Boson, we seem to forget that particle physics is in a really odd situation. In any other field of science, getting experimental results to agree with theory is considered a champagne moment. The most common boast I hear at conferences goes something along the lines of "...a nd the line represents the theory, which is not a fit, it has no free parameters, and you can see that it agrees very well with the experimental data."

Yet in particle physics, smiles turn upside down and presenters shuffle about uncomfortably as they say, "As you can see, the Standard Model accounts for all our data over umpteen gazillion orders of magnitude." That is a magnificent achievement and should be celebrated. Instead, it is being treated like ashes in the mouth. And that was the undercurrent of a session on particle physics that I attended at Physics@FOM.

The session had two experimental talks and two theory talks—luckily, all of them were at a rather high level. Surprisingly, the attitude of the experimentalists and theorists were rather different. On the experimental side, there is a palpable desire to find some data that does not fit the Standard Model. Unfortunately, they haven't done so yet. The Higgs is a very standard Higgs, and at all energies up to and beyond that of the Higgs, particle production has replicated the Standard Model very well. The one hint—and it's only a hint—is that there is some missing energy in some collisions at very high energies. This is most likely a statistical fluctuation, but if not, it could be a signature of dark matter or extra dimensions. However, until the data deviates at a level of five sigma, no one is going to say anything.

The theorists seem to be starting to take the attitude that maybe there simply isn't anything there—or, actually, the two theorists who presented had that opinion (though for different reasons); the majority still thinks the LHC will find something. Their presentations were filled with pre-LHC (and pre-LEP) quotes from people expecting to find great and wonderful things as soon as the power was turned on at the LHC. More seriously, though, the theorists are starting to ponder models that don't have supersymmetry and probably won't produce a whole new zoo of particles.

What does that mean? It means that things like inflation, dark matter, and dark energy, which are currently things not described by the Standard Model, are due to what are described as singlet particles. Singlet particles (like the Higgs) don't fall into some complicated family of particles. The implication is that these particles will be very hard to find for experimentalists.

It's also worth noting that, aside from these problems, there are many other phenomena that have been defined as problems, which may not be as big or fundamental as we think. These include the fine-tuning of fundamental constants, among others.

If you want to stop worrying about fine-tuning, then the easiest way to do that is to propose that our universe is one of many. Each universe has its own set of fundamental constants—and therefore different properties. We exist in this one because it's one that will support the existence of atoms, stars, and galaxies. What I found more intriguing is that a particular version of string theory, called the string landscape, has something to say about this.

The Standard Model uses two things: gauge theories (of which there are an infinite number) and symmetry. The symmetry tells us which gauge theory to use. In the landscape, the idea is that all gauge theories that are physical should be generated by the landscape. Each gauge theory then corresponds to its own universe with its own physics and fundamental constants.

The point is that starting from an arrangement of strings and branes, one can derive not just the Standard Model, but many, many different versions of the Standard Model. This would normally be a bad sign. But if we were looking for an underlying description of a multiverse, this isn't a bad starting place.

The problem, of course, is that the string landscape makes a proposal that is inherently untestable. Not so, according to one of the speakers. And here we come back to the missing energy raised by one of the experimentalists. Missing mass could be due to large(ish) dimensions beyond the four we know and love, which could be taken as evidence for string theory. Or, as many other physicists will tell you, it could be evidence for alternatives. Either way, it will be a difficult job to analyze the data in such a way that the possibilities are narrowed.

It was, despite a lack of sleep and a recent lunch—give me lunch and a warm room and I'm dead to the world in five minutes—a very interesting session. It is, however, also clear that there is a huge amount of experimental work to be done before any progress can be made toward replacing or embedding the Standard Model in some new theoretical framework. The evidence may be lurking in the data awaiting analysis, but more likely, we are going to have to wait for the LHC to turn back on at higher energy and brightness.

Chris Lee attended the Physics@FOM meeting; this is one of a series of reports from that meeting.

Listing image by Flickr user: zigazou76

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