As reported in BBC News and elsewhere, there have been some results, not yet published, that put new limits on the most promising future theories in particle physics. Some are calling it an arrow to the knee, others saying it has sent the popular theory to the hospital. What’s going on?
First, the theory is question is known as supersymmetry. It is a very popular stepping stone to new physics in the subatomic realm, advanced beyond the Standard Model of particle physics. What supersymmetry (SUSY) does is several-fold. Amongst those, it tries to explain certain things that the Standard Model does not, such as the heirarchy of masses of particles (why do next-generation particle go up in mass in a sort of step-function?), it predicts a whole slew of new particles called supersymmetric partners, and it is the backbone of string theory which promises to unite all the forces of nature. The last bit may be of the greatest public interest, but basically SUSY or something like it is what particle physicists have been looking towards to advance the field.
Like any theory, in its earliest stages it is unclear how it is supposed to work. There were many models of the solar system that were heliocentric before we got to the one we use today that works so well, and there are numerous versions of SUSY. The simplest, called the Minimal Supersymmetric Standard Model (MSSM), is one of the earliest incarnations and basically added the least amount to the equations of the Standard Model (just one term to the Lagrangian hence it is ‘minimal’). It also contained predictions of how MSSM is broken at certain energy levels which will give predictions about how forces and particles will emerge and react. However, MSSM was has problems, and it is not the only version of SUSY out there. There are, for example, Next-to-Minimal Supersymmetric Standard Model (NMSSM), which again as the name implies is just a bit more complicated than MSSM, and there are also other beyond-the-Standard-Model ideas such as Technicolor. There have also been theories that do without the famous Higgs boson, but results from earlier this year suggest that those models will not be necessary.
But what of the recent results? From a conference in Japan, the experiment LHCb, one of the smaller detectors at CERN and the Large Hadron Collider, had shown for the first time the decay of B mesons into muons. B mesons are particles composed of two quarks, one of which is an anti-bottom quark (anti- as in antimatter); there are several versions of the B meson depending on what the other normal-matter quark is, be it up, down, strange, or charm. As for muons, these are basically just like electrons; they have the same charge, but the muon is much heavier than the electron by a factor of about 200.
Now, particles that are heavy and unstable such as B mesons can decay several different ways (called decay channels), and this is the first time scientists have been able to observe B mesons decay into muons. This is important because different theories will predict different decay channels or the probability of a decay channel; to make up numbers, if the Standard Model says that 10% of B mesons decays will be into muons, and if MSSM says it should be 20%, then a good number of observations can distinguish which of these theories is closest to reality. These probabilities tend to be complicated to calculate and depend on energy (some decay channels may be impossible if the energy of the collisions isn’t high enough), so there are a lot of computing that happens, predicting how much decay there should be given various parameters.
All this means is the measurement of the new decay channel means we can but a new set of constraints on any theories that go beyond the Standard Model of particle physics. And this leads to the problems people are seeing. From the results as reported, the decays are inconsistent with many forms of SUSY (though not all), and this will force new constraints on any formation of SUSY that fits the data.
Is this the end to SUSY? Probably not, at least not for a while. For one thing, there are many ways the parameters of SUSY can be adjusted to fit the data, but they have the consequence of making the LHC unable to detect the new particles that SUSY predicts. This can make the theory look more and more ad hoc, having to hide in the places we cannot yet look because we don’t have a powerful enough machine to find them (also look at the concerns highlighted here). But another thing to look at is the confidence level that is being reported. Right now the researchers say they have 3.5-sigma confidence; that basically tells how likely it is that they are getting their results by chance. The higher the sigma confidence level, the more likely the result isn’t a statistical fluctuation. However, in particle physics we tend not to say we have a discovery until reaching 5-sigma, which is about a 1 in a million chance of coincidence. While 3.5-sigma is a good bit of evidence, they are hardly definitive. As one of my professors put it, 90% of 3-sigma results go away with further analysis. And perhaps even more-so with CERN results considering how complex the procedures are. Replications are also desirable where possible. I have neither the knowledge nor the smarts to second-guess the LHCb’s abilities to analyze their data, but they probably also know they aren’t going to change theorists’ minds with a 3.5-sigma result.
How to get to 5-sigma and really make it a problem for SUSY? More time at the collider will supply more data. If the result is real, it will become even more obvious from the detector. However, if there is something else going on, then more collisions can make interesting peaks (or lack of peaks) go away. Only time will tell.
So, there is further excitement to be had in the world of CERN and particle physics. It will probably be years until any strong verdicts can be said about SUSY, positive or negative. But there are other results out or in the pipes that add even more constraints that may bode poorly for SUSY (Peter Woit talks about that some here). No matter what, the theorists will have a lot of work ahead, and perhaps any field theory of particles will be unlike what we have currently conceived. Such is the joy of the sciences that explore the unknown in realms only recently imagined.