One of the challenges of being a science journalist is conveying not only the content of a new scientific result but also the feel of what it means. The prominent article in the BBC about the new measurement by the LHCb experiment at the Large Hadron Collider [LHC] (reported yesterday at the HCP conference in Kyoto — I briefly described this result yesterday) could have been worse. But it has a couple of real problems characterizing the implications of the new measurement, so I’d like to comment on it.
The measurement is of how often B_s mesons (hadrons containing a bottom quark and a strange anti-quark, or vice versa, along with many quark/anti-quark pairs and gluons) decay to a muon and an anti-muon. This process (which I described last year — only about one in 300,000,000 B_s mesons decays this way) has three nice features:
- it can be well-predicted in the Standard Model (the equations we use to describe the known particles and forces, including the simplest type of Higgs particle)
- it is relatively straightforward to measure, and
- it is very sensitive to effects of as-yet unknown particles and forces.
Yesterday the LHCb experiment reported the evidence for this process, at a rate that is consistent (but see below) with the prediction of the Standard Model.
The worst thing about the BBC article is the headline, “Supersymmetry theory dealt a blow” (though that’s presumably the editor’s fault, as much as or more than the author’s) and the ensuing prose, “The finding deals a significant blow to the theory of physics known as supersymmetry.” What’s wrong with it? It’s certainly true that the measurement means that many variants of supersymmetry (of which there are a vast number) are now inconsistent with what we know about nature. But what does it mean to say a theory has suffered a blow? and why supersymmetry?
First of all, whatever this new measurement means, there’s rather little scientific reason to single out supersymmetry. The rough consistency of the measurement with the prediction of the Standard Model is a “blow” (see below) against a wide variety of speculative ideas that introduce new particles and forces. It would be better simply to say that it is a blow for the Standard Model — the model to beat — and not against any speculative idea in particular. Supersymmetry is by no means the only idea that is now more constrained than before. The only reason to single it out is sociological — there are an especially large number of zealots who love supersymmetry and an equal number of zealots who hate it.
Now about the word “blow”. New measurements usually don’t deal blows to ideas, or to a general theory like supersymmetry. That’s just not what they do. They might deal blows to individual physicists who might have a very particular idea of exactly which variant of the general idea might be present in nature; certain individuals are surely more disappointed than they were before yesterday. But typically, great ideas are relatively flexible. (There are exceptions — the discovery of a Higgs particle was a huge blow to the idea behind “technicolor” — but in my career I’ve seen very few.) It is better to think of each new measurement as part of a process of cornering a great idea, not striking and injuring it — the way a person looking for treasure might gradually rule out possibilities for where it might be located.
Then there’s the LHCb scientist who is quoted as saying that “Supersymmetry may not be dead but these latest results have certainly put it into hospital”; well… Aside from the fact that this isn’t accurate scientifically (as John Ellis points out at the end of the article), it’s just not a meaningful or helpful way to think about what’s going on at the LHC.
Remember what happened with the search for the Higgs particle. Last July, a significant step forward took place; across a large fraction of the mass range for the Standard Model Higgs particle, it was shown that no such particle existed. I remember hearing a bunch of people say that this was evidence against the Standard Model. But it wasn’t: it was evidence against the Standard Model with a Higgs particle whose mass was in a certain range. And indeed, when the rest of the range was explored, a Higgs particle (or something very much like it) turned up. Failure to find one variant of a theory is not evidence against other variants.
If you’re looking for your lost keys, failing to find them in the kitchen, living room and bedroom is not evidence against their being somewhere else in the house.
Similarly, the new result from LHCb is not evidence against supersymmetry. It is evidence against many variants of supersymmetry. We learn from it about what types of supersymmetry cannot be true in nature — we know which rooms don’t have your keys. But this is not evidence against supersymmetry in general — we still don’t know if your keys are elsewhere in the house… and we won’t know until the search is complete. Nature is what it is — your keys are wherever they are — and the fraction of your search that you’ve completed is not logically related to how likely your search is to be successful. It may be related to how optimistic you are, but that’s a statement about human psychology, not about scientific knowledge. The BBC article has confused a blow to the hopes and optimism of supersymmetry zealots with a blow to supersymmetry itself.
It’s also important to understand that despite the fact that some physicists and certainly the science media spend an inordinate amount of time talking about supersymmetry, the particle physics community actually spends a lot of time on other ideas, and also on testing very carefully the hypothesis that the Standard Model is correct. For good reason, a number of the most interesting results presented so far at the HCP conference, not just the one we’ve been talking about, involve precise tests of the Standard Model.
Now, in a bit more detail, here are a few of the scientific issues surrounding the article.
First, it’s important to notice that the measurement quoted yesterday is still very rough. Yes, it agrees with the prediction of the Standard Model, but it is hardly a precise measurement yet: speaking broadly, the fraction of B_s mesons that decay to a muon/anti-muon pair is now known to lie somewhere between 1 in 100,000,000 and 1 in 1,000,000,000. The Standard Model predicts something between 1 in 240,000,000 and 1 in 320,000,000. So the LHCb measurement and the Standard Model prediction are currently consistent, but a more precise measurement in future might change that. Because of this, we should be careful not to draw an overly strong conclusion. Many variants of supersymmetry and of other speculative ideas will cause a deviation from the Standard Model prediction that is too small for this rough measurement to reveal; if that’s what nature is all about, we’ll only become aware of it in a few years time.
One serious scientific problem with the article is that it implies
- that supersymmetry solves the problem of what dark matter is, and
- that if supersymmetry isn’t found, then physicists have no idea what dark matter might be
Both of these are just wrong. Many variants of supersymmetry have at least one proposal as to what dark matter is, but even if supersymmetry is part of nature, none of those proposals may be correct. And even if supersymmetry is not a part of nature, there are plenty of other proposals as to what dark matter might be. So these issues should not be linked together in the way they are in the BBC article; one should not mistake propaganda (sometimes promulgated by supersymmetry zealots) for reality.
Another point worth remembering is that the biggest “blows against” (cornerings of) supersymmetry so far at the LHC don’t come from the LHCb measurement: they come from
- the discovery of a Higgs-like particle whose mass of 125 GeV/c² is largely inconsistent with many, many variants of supersymmetry
- the non-observation so far of any of the superpartner particles at the LHC, effects of which, in many variants of supersymmetry, would have been observed by now
However, though the cornering of supersymmetry is well underway, I still would recommend against thinking about the search for supersymmetry at the LHC as nearly over. The BBC article has as its main title, “Popular physics theory running out of hiding places“. Well, I’m afraid it still has plenty of hiding places. We’re not yet nearing the end; we’re more in the mid-game. [Note added: there were some new results presented today at the HCP conference which push this game a bit further forward; will try to cover this later in the week.]
One more scientific/linguistic problem: left out of this discussion is the very real possibility that supersymmetry might be part of nature but might not be accessible at the LHC. The LHC experiments are not testing supersymmetry in general; they are testing the idea that supersymmetry resolves the scientific puzzle known as the hierarchy problem. The LHC can only hope to rule out this more limited application of supersymmetry. For instance, to rule out the possibility that supersymmetry is important to quantum gravity, the LHC’s protons would need to be millions of billions of times more energetic than they actually are. The same statements apply for other general ideas, such as extra dimensions or quark compositeness or hidden valleys. Is this disappointing? Sure. But that’s reality, folks; we’re only human, and our tools are limited. Our knowledge, even after the LHC, will be limited too, and I expect that our children’s children’s children will still be grappling with some of these questions.
In any case, supersymmetry isn’t in the hospital; many of its variants — more of them than last week — are just plain dead, while others are still very much alive and healthy. The same is true of many other speculative theories. There’s still a long way to go before we’ll really have confidence that the Standard Model correctly predicts all of the phenomena at the LHC.