The Rise and Fall of Supersymmetry 138
Ethan Siegel at the StartsWithABang blog writes:
"Have you ever wondered why the masses of the fundamental particles have the small values that they do, compared to, say, the Planck scale? Whether the fundamental forces all unify at some high energy? And whether there's a natural, compelling particle candidate for dark matter? Well, in theory supersymmetry (or SUSY, for short) could have solve all three of these problems. In fact, if it solves the first one alone, there will be definitive experimental signatures for it at the Large Hadron Collider. Well, the LHC has completed its first run, and found nothing. What does this mean for theoretical physics, for SUSY in particular, and what are the implications for string theory? A very clear explanation is given here; it might be time to start hammering in those coffin nails."
is there an xkcd comic for this? (Score:3, Interesting)
Is there an xkcd comic that explains this at the level that most of us can understand? Something with an exacerbated physicist trying desperately to explain the experiment with analogies and gestures would be ideal?
SUSY isn't dead yet. (Score:5, Interesting)
An exacerbated physicist (Score:5, Interesting)
The problem SUSY is trying to solve is that nature seems to be performing an amazing balancing act with the Higgs field. Now this is not just some ordinary balancing act that generates a few "oohs" and "aahs" from the audience like Idol Rock [wikipedia.org]. According to the physics we know the chance of the Higgs boson having the mass is does is about one in 10^30. Those are about the same odds as some person winning a national lottery 5 times in a row and getting a lesser prize in the 6th week. By about the third or fourth win the "oohs" and "aahs" are replaced by a call to the serious fraud squad of the local police force with a request to figure out how the person is fixing the results of the lottery because the chance that this person is just "really lucky" are so astronomically small that nobody will believe it is just chance.
This is the situation we are in now with physics and the usual way nature solves balancing problems like this is with a symmetry that requires the balance be perfect. For example it is not just dumb luck that the electrical charge in the universe happens to cancel out so precisely - we were not just "really lucky" with our Big Bang! - there is a symmetry which gives conservation of electric charge which requires that the balance be exact. To solve the problem with the Higgs mass being so tiny the symmetry is called "Supersymmetry" - not because it flies around with a big S on its chest saving us from bad symmetries but because it is an extremely high level symmetry, perhaps even the highest possible in nature. In very simple terms you could describe it as a symmetry between force and matter.
This is also why I would disagree with the article when it says that the LHC must see supersymmtery or else it cannot solve our balance problem. This would be like saying that if you win the lottery twice that's ok but win it a third time and you are automatically guilty of a crime. Winning it 3 times in a row might be very, very unlikely but this is a continuous scale. 10TeV SUSY may be less natural than 1TeV but it is not so incredibly less likely that you know it cannot be right - sometimes 0.1% chances happen e.g. the angular size of the moon being almost exactly the same as the sun on Earth.
Supersymmetry is not a perfect symmetry because otherwise all the super-particles (which have fun names likes squarks, winos and sleptons) would then have the same mass as our Standard Model particles and we would have already seen them. So it has to be broken by some unknown mechanism which gives all the super particles higher masses which is why we have not yet seen them - our colliders do not yet have enough energy.
Another possibility is that the lightest super particle cannot decay. This would give us a high mass, stable particle which is an excellent candidate for dark matter. However this where the article is not correct in saying that the particle should have been seen by direct search experiments because one possibility is that it is a gravitino (a super partner of the graviton). This would mean that it only interacts via gravity and will not be seen in direct search experiments. This would be a real pain for physics because while we would know that we had produced them in the detector (because the other particles we can see will rebound from it) it will be very hard to prove that these were the Dark Matter astronomers see.
Probably out best chance to see supersymmetry, or indeed any new physics, will be the next three year run of the LHC. We will get almost twice the energy and about 5 times the luminosity. Certainly if we do not find supersymmetry or something else then the chances of us every seeing it with the LHC are dramatically lower after this point because increasing numbers of events at the same energy only slowing increase the regions you can search. So fingers crossed!
Re:is there an xkcd comic for this? (Score:5, Interesting)
For one thing, string theory will probably need to be scrapped.
This much has been obvious for quite a while. Too much time has gone by without string theory being able to produce a falsifiable statement and now that finally we have one for SUSY, it failed.
It's been around for about 35 years in its current form and even its best proponent, the distinguished Juan Maldacena, thinks it is still 20 to 30 years before it can be tested experimentally.
p.s. you can count Richard Feynman among the superstring skeptics.
Re:SUSY isn't dead yet. (Score:4, Interesting)
However, the observed Higgs mass of 126GeV is a sweet spot which allows the mass of the lightest SUSY particle to be far greater than the LHC can produce. It'll take a few more colliders before we can dismiss SUSY completely.
The main motivation behind SUSY is that it solves the fine-tuning problem associated with electroweak symmetry breaking. But if SUSY itself is fine-tuned, this solution creates the same problems that it was intended to solve.
BTW: The largest constraint on SUSY partner masses does not come from the $9bn LHC, but from the ACME collaboration's measurement of the electron electric dipole moment, a $6M tabletop atomic physics experiment.
Re:is there an xkcd comic for this? (Score:4, Interesting)
Just because it's ridiculously hard to prove doesn't mean that it's false. For example, most physists believe gravity needs a force carrier which they've called a "graviton", the same way light (electromagnetic radiation) consists of photons. That theory is 80 years old and still totally unproven but as long as nobody has a good competing theory we still kind of assume that's how it works. Not that we're not trying to look for gravitational waves and other clues, but most of it is so far off the scale of what we can experimentally detect that it'll probably still be unproven in a thousand years.
Re:is there an xkcd comic for this? (Score:5, Interesting)
. For one thing, string theory will probably need to be scrapped.
Not because of this. Supersymmetry and string theory address different problems and are more or less independent.
String theory builds on supersymmetry, so evidence of string theory would imply supersymmetry, but evidence of supersymmetry wouldn't imply string theory. Dually, evidence against string theory wouldn't kill supersymmetry, but evidence against supersymmetry would kill string theory.
Until, of course, some string theorist fudges the numbers to make it unfalsifiable again ;)