Tuesday, March 1, 2011

Supersymmetry A Sinking Ship?

I really like supersymmetry, the idea there is some fundamental symmetry between fermions and bosons.  Unfortunately nature doesn't care what I think.  And even more unfortunately, the LHC has been running at very high energies for a while now and nobody is seeing so much as a hint for it.

A few quotes from this Nature article sum up the situation:
The LHC is now rapidly accumulating data at higher energies, ruling out heavier territory for the super particles. This creates a serious problem for SUSY... As the super particles increase in mass, they no longer perfectly cancel out the troubling quantum fluctuations that they were meant to correct. Theorists can still make SUSY work, but only by assuming very specific masses for the super particles — the kind of fine-tuning exercise that the theory was invented to avoid. As the LHC collects more data, SUSY will require increasingly intrusive tweaks to the masses of the particles. 
So far the LHC has doubled the mass limit set by the Tevatron, showing no evidence of squarks at energies up to about 700 gigaelectronvolts. By the end of the year, it will reach 1,000 gigaelectronvolts — potentially ruling out some of the most favoured variations of supersymmetry theory.
So basically, supersymmetry's biggest appeal is that it provides a "natural" solution to many problems in theoretical physics.  (Like why the Higgs mass so small.)  However, enough parameter space is being ruled out by the LHC that is appears that fine-tuning may be required to get SUSY to work correctly.  But the whole point behind SUSY's appeal is that it appeared to be a theory where fine tuning was not needed.

So by saying: "well supersymmetry may still exist if we do a bunch of fine tuning" seems to destroy the whole point for why we thought SUSY was a good idea in the first place.

Privately, a lot of people think that the situation is not good for SUSY... This is a big political issue in our field... For some great physicists, it is the difference between getting a Nobel prize and admitting they spent their lives on the wrong track... [Some have] been working on it for almost 30 years now, and I can imagine that some people might get a little bit nervous.
Now, before I get too hard on the theory, we are only in the first few years of the LHC operating at high energies.  Still, after 30 years you would hope that if SUSY was as "natural" as people have assumed, you would hope by now you would have a hint.  I mean, it's one thing to say we don't have enough to claim discovery but we don't even have enough evidence to suggest a hint!

So, is SUSY a sinking ship?  I think abandoning SUSY is still pre-mature.  However, if the LHC cannot see so much as even a hint in the next few years, I will think things will start looking really bad indeed.



  1. I guess I don't know enough about SUSY to say one way or another, but one thing I am learning in my own field of research is that it is never a good idea to have theories get too far from observations. That is, there are many theorists out there who have spent a long time developing their models and simulations only to have the observations prove them wrong. Some may wish to take that risk, but I prefer to remain at the cutting edge of observations.

  2. Quantumleap42,

    I will second that. Given I like these kids of theories part of me wants to "run wild" and see how far we can push the theory. However, experience may be showing us that, though this sounds fun and intelligent, it may be a grave mistake. I mean, if there is no SUSY there were sure a lot of supposedly smart people who when down a bone-headed path.

  3. I am basically illiterate on the topic at hand, but from what I understand the hint of the higgs boson has been predicted to be at energies exceeding the current limit of the LHC, 14 TeV. This is 14 to 20 times higher than the energies presented in this blog. Also, a particle the mass of the universe squeezed into planck's length, 16.163×10^−36 m, seems unfathomable to reach with any sort of futuristic technology. Currently, it is impossible to observe such tiny dimensions. It would be nice for experimentalists to carry out experiments that could reach high enough energies to possibly find traces of the higgs boson, but funding, like everything else, is entirely political. The Superconducting Supercollider would have been nice to have, and three times bigger than the LHC. Of course the price tag of over twenty billion was too hard to swallow, politics.

  4. Anonymous,

    Thank you for your comment and I agree it would have been nice to have the Superconducting Supercollider.

    I got my numbers from the common numbers thrown around by physicists. For example, if you see the Wikipedia you will find most Higgs models predict the Higgs mass between 115-185 GeV. (And to be honest, many people thought the Higgs mass would be less then 115 GeV until that region was excluded. :)

    So, a machine operating at 14 TeV is just fine, *after* enough data is collected. (Not only do you need high enough energies but you need enough data to see a signal.)

    Anyways, my point is the Higgs should be well within the LHC's grasp unless it is very bizarre. But again, once something has become that bizarre you begin to wonder if it really exists or whether you are on a wild goose chase.

  5. I'm interested in the implications of SUSY's demise for dark matter because it seems like theorists are rapidly running out of particles. Sterile neutrinos have already bitten the dust. If SUSY goes down what is the next leading dark matter candidate? Axions? Kaluza-Klein particles?

  6. Nick,

    Let's just pretend, for sake of argument, that we convince ourselves that supersymmetry is not correct with the LHC. This will be a *HUGE* game changer.

    1. The best candidate for quantum gravity, string theory, gone!

    2. The best dark matter candidates: gone!

    3. The best explanation for the Hierarchy problem: ...etc...

    As for dark matter, yes axions and Kalusa-Klein will be the favored as far as I can tell with axions with the slight edge.

    Although, for me, I won't know what to believe because the same "naturalness" and elegance arguments used to justify SUSY are the main things we use to decide which beyond-the-standard model theories are best. I mean, I think I will immediately start distrusting everything we have no data for as aesthetic arguments will have been what got us into this mess!

    That said, we have lots of data that points to dark matter being a particle so I will still hold to that.

    And again, it is pre-mature to throw out SUSY so just to reiterate the above was just a "for sake or argument" exercise.

  7. "I mean, I think I will immediately start distrusting everything we have no data for as aesthetic arguments will have been what got us into this mess!" -- Excellent! Now you know why I'm an experimentalist! ;)

    But seriously, yes, this would be a major game-changer. However, these types of crises always seem to more than pay for themselves later on. (For example, what appeared to be a minor inconsistency in Maxwell's equations led to the whole theory of Relativity, and finally understanding the "ultraviolet catastrophe" let to quantum mechanics.) All I'm saying is, I think it's healthy to have a few "catastrophes" left in physics from time to time.

    Also, even if SUSY turns out to be wrong (which, fine-tuning involved or not, is still a big if), I wouldn't say all those people dedicating all that time to investigating SUSY and string theory were entirely bone-headed (crackpots, yes, but not bone-headed). Just like Edison "found 2,000 ways not to make a light bulb," sometimes spending some time (or a lot of time) ruling out things that don't work (although less exciting) can be almost as useful as finding things that do work.


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