The Rise and Fall of Supersymmetry

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."

Comments

is there an xkcd comic for this?

[schlachter]

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?

Re:is there an xkcd comic for this?

[flaming error]

I'll try. Supersymmetry predicted the existence of subatomic particles which the LHC would detect. The LHC hasn't detected them.

Re:is there an xkcd comic for this?

[dreamchaser]

I don't think there is one. Simply but probably poorly put, supersymmetry postulates that each particle that we know of and have observed has a heavier 'super-symmetric' partner particle. The significance of this is that if true it explains a whole bunch of how the observed Universe works. If it is not true it's almost as exciting really, at least to me, because it means there are some big missing pieces to our current models and a lot of new and exciting work will need to be done.

The short version of the article is that the LHC should have detected supersymmetric particles by now. There's still a slight chance that the next run will, but the energies that fit the current theories are running out. If they are not detected soon physicists might just have to move on to new theoretical models. For one thing, string theory will probably need to be scrapped.

Re:is there an xkcd comic for this?

[QQBoss]

For one thing, string theory will probably need to be scrapped.

I felt a minor disturbance (you know, like quark-sized) in the force, as if a few hundred physics grad students had their thesis hopes suddenly ended.

That said, I am not a physicist, but I would like to add a hearty 'Huzzah!' to what I am sure is a chorus of many other physicists not present, at least based on my trying to keep up modestly with the goings on.

Re:is there an xkcd comic for this?

[Alomex]

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:is there an xkcd comic for this?

[stevelinton]

. 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.

Re:is there an xkcd comic for this?

[Kjella]

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?

[Warbothong]

. 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 ;)

Re:is there an xkcd comic for this?

[negablade]

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.

Gravity waves have already been proven to exist. The 1993 Nobel Prize in physics was awarded for the study of the Hulse-Taylor binary pulsar that showed indirect confirmation of the existence of gravity waves http://en.wikipedia.org/wiki/H....

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.

Gravity wave detection is expected within the next 20 years from the LIGO programme http://en.wikipedia.org/wiki/G..., http://www.ligo.caltech.edu/ and http://en.wikipedia.org/wiki/L.... It won't require a thousand years, nor is it beyond existing technology. LIGO is already taking measurements in the US, at Hanford and Livingston, and advanced LIGO will increase the sensitivity of the LIGO interferometers by a an order of magnitude, and is expected to increase detection rates from a few per year to 100s per year by increasing the detection volume a thousand fold. If advanced LIGO doesn't detect anything, then it will be time to review the theory.

(I worked for ~6 years at the University of Western Australia in the physics department in collaboration with the Australian LIGO research group)

Re:is there an xkcd comic for this?

[El Puerco Loco]

Maybe they just missed it by skipping over the less glamorous small and medium hadrons and going straight after the large ones.

Re:is there an xkcd comic for this?

[Hognoxious]

If we get a new theory, hopefully it will explain how fucking magnets work.

Re:is there an xkcd comic for this?

[AlterEager]

you can count Richard Feynman among the superstring skeptics.

Appeal to authority.

Re:is there an xkcd comic for this?

[Richard Kirk]

Experiments are real but the results aren't pretty. SUSY is pretty but the results aren't real.

When we look at the tables of known particles, it is tempting to think of the periodic table. We might hope to see patterns in the particles, and then guess at the missing parts of the grid. Unfortunately, we don't have nice families of halogens, alkaline earths, and so on. We started off with electrons, protons, and neutrons which all had sensible masses even if the electron was less than a thousandth of the mass of the others, and photons which had no rest mass at all. Then we have a very irregular family of subatomic particles including things like mesons, and neutrinos, with finite but stupidly tiny mass. They don't seem to form a family at all, but a lot of clever people invented new sub-particles called quarks, and in the end managed to come up with a plausible theory that seemed to fit a lot of these weirder particles into families. But not everything.

The periodic table was backed up by quantum calculations, which showed why the should be two elements per row, then eight, and then all the transition elements. Unfortunately, we don't seem to be able to finish off the table of the subatomic elements in the same way. We can come up with neat SUSY theories that would work if there are lots of symmetric particles that we do not ordinarily see, but might be more common at higher energies, and bend the various graphs into fitting at some point. However, as the guy neatly tabulated, all plausible versions of the SUSY theory seem to come up with things that we ought to be able to see with the LHC and other things, and we don't. So, right now, and after a lot of people had spent most of their lives fooling with this model, it is beginning to look like we may have gone down a dead end.

Re:is there an xkcd comic for this?

[Glock27]

True, but when the "authority" is a Nobel Prize winning physicist specializing in the exact area being discussed it means something. Further, "appeals to authority" invite the reader to investigate the claim, including detailed arguments made by the authority.

So, while not conclusive, an "appeal to authority" in this case is of interest.

Re:is there an xkcd comic for this?

[oh_my_080980980]

Actually those are physicists that have been saying string theory is all hat. Just because you don't like that doesn't make it not true. String theory is dead. Data after data being collected from experiments have been proving String Theories claims as false.

I thought most of the SUSY theories were folded...

[mmell]

into M/String theory, when particles and wavicles fell out of favor in place of open- or closed-loops on the brane?

Never mind - the presence of empirical data which tends to place supersymmetry in doubt is enough to convince me that either we need a better theory, or the existing theory needs a major overhaul.

Re:I thought most of the SUSY theories were folded

[dreamchaser]

That's sort of the point. M-Theory might just be a dead end. If we don't find the supersymmetric partners in the next run of the LHC at the very least string/M theory will need considerable re-vamping if not a total scrapping.

Re:LHC Purpose

[flaming error]

"it not coming through with what was hoped"

I think Science is not about confirming what we want to believe, but more just learning how things work.

You're right that the LHC does it's job; the Higgs Boson is one of the century's biggest discoveries. And, incidentally, that confirmation was exactly what many hoped for. And disproving string theory was also something some hoped for.

So I don't understand where the grumpy comes from. It's been a spectacular success.

Re:LHC Purpose

[joe_frisch]

That's like complaining that the Michelson-Morley failed to measure the presence of the lumineferes ether, something scientists thought was very likely to exist. Science advances when you get a surprising result, not when you see what you expected. If the statistics support this, it is a MUCH more interesting result than finding the Higgs which was pretty much were people expected it.

The best thing science can do...

[Anonymous Coward]

...is disconfirm our beliefs.

SUSY isn't dead yet.

[ITEM-3]

In SUSY, there is no way to predict the masses of supersymmetric particles, but there is a way to predict a range of values that the mass of the lightest SUSY particle must fall within in order for SUSY to be a valid theory. The range is determined by the mass of the Higgs boson. For small Higgs masses (less than ~100GeV, don't quote me on these numbers as it's been a while) and large Higgs masses ( greater than ~140GeV), the range is very small, and our current colliders would have already disproven SUSY. 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.

Re:SUSY isn't dead yet.

[hweimer]

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.

hrm...

[Charliemopps]

I liked this article. The author did a good job of dumbing things down for us mortals. Super symmetry has been dieing since the day the LHC came online. But I have a problem with:

A lot of people have invested their entire careers in SUSY, and if it’s not a part of nature, then a lot of what they’ve invested in is nothing more than a blind alley. For example, if there is no SUSY in nature, at any energy scale (including the Planck Scale, although this will be a challenge to test), then string theory cannot describe our Universe. Plain and simple.

I seriously doubt many of the geniuses that dedicated their entire lives with Super Symmetry would consider it a blind alley. There's been some amazing math, and amazing theoretical work on it. It's a very very good theory. It's rather clear that this point that it's not correct, but whatever the truth really is (something we clearly haven't even imagined yet) will be helped greatly by the work done by those investigating super symmetry. The Wright Bothers didn't just hop in a plane and fly off... There were mountains of work by thousands of failures that they built their success on.

Re:hrm...

[blue trane]

Principles that were unknown to Simon Newcombe and Lord Kelvin, both of whom predicted machines heavier than air would not fly for a very long time before the Wright brothers did it within a few years?

Re:hrm...

[Pino Grigio]

Science progresses one funeral at a time. - Max Planck

Re:I thought most of the SUSY theories were folded

[MildlyTangy]

Insane in the m-brane.

Insane in the Brane...

An exacerbated physicist

[Roger W Moore]

Not that I know of but since I am an exacerbated physicist how about I try to explain our experiment with analogies and you can just imagine appropriate gestures to go with them? First though I should say that while SUSY is in trouble the article paints an overly pessimistic picture and gets a few things wrong.

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. 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:An exacerbated physicist

[PvtVoid]

According to the physics we know the chance of the Higgs boson having the mass is does is about one in 10^30.

Only if you know the measure on the space of parameters for the Higgs. Which you don't.

The question isn't "is there SUSY or not?" That question cannot be answered, because you can always push the SUSY breaking scale up to a little higher than the energy of your collider. SUSY will be a part of quantum gravity for the foreseeable future, since String Theory is not consistent without it as far as anybody can tell.

The real question is: "Does SUSY make useful predictions for detecting physics beyond the Standard Model?" The answer to that seems to be tending very strongly toward "No."

Quality of the actual link

[PC_THE_GREAT]

I don't know about you, but that link goes to an article that has been very cleanly written, no information overload, very well planned. I was surprised that people can write scientific stuffs using such clarity.

Re:LHC Purpose

[Anonymous Coward]

The interesting thing is that Couder's experiments with silicon walkers that replicate on a macroscopic scale the two-slit experiment require an "ether" substrate

Or just a permeating wave function of the object being diffracted... There are plenty of analogs to the wave equations in quantum mechanics in various fluid media, but that doesn't mean quantum mechanics implies the existence of a fluid ether.

String Theory will survive

[anchovy_chekov]

By explaining that those extra supersymmetrical particles are actually packed away in really tiny dimensions that the LHC can't touch. Prove it aint so!

Re:LHC Purpose

[Warbothong]

the Higgs Boson is one of the century's biggest discoveries.

Whilst this discovery is great, I really hope that you're wrong! Roughly 100 years ago Rutherford discovered the nucleus (1911 according to WIkipedia); but in the same century we subsequently discovered the protons and neutrons which make it up, the pions which moderate their interactions, as well as the quarks they're made of and the gluons which moderate their interactions, along with a bunch of other bosons and mesons. We also discovered General Relativity (Special was already known by 1905) and Quantum Mechanics (including the standard model), black holes, neutrinos, W and Z bosons, muon and tau leptons and anti-particles for the above, as well as inferring the Big Bang, dark matter and dark energy and we gave Thermodynamics an information-theoretic renaissance.

That's just in Physics alone!

A century ago there was no heavier-than-air flight, whilst these days we complain that the food on our £30 flight is crappy. Walking on the moon became so routine that the public lost interest and we stopped bothering! A century ago there was no genetics or germ theory. Computers were people that were good at sums. Experimentalists were attempting to transmit sound via radio waves.

I'm very much looking forward to this century's discoveries :)