Is Supersymmetry Ruled Out Yet?

[A Heads Up: I’m giving a public lecture about the LHC on Saturday, April 28th, 1 p.m. New York time/10 a.m. Pacific, through the MICA Popular Talks series, held online at the Large Auditorium on StellaNova, Second Life; should you miss it, both audio and slides will be posted for you to look at later.]

Is supersymmetry, as a symmetry that might explain some of the puzzling aspects of particle physics at the energy scales accessible to the Large Hadron Collider [LHC], ruled out yet? If the only thing you’re interested in is the answer to precisely that question, let me not waste your time: the answer is “not yet”. But a more interesting answer is that many simple variants of supersymmetry are either ruled out or near death.

Still, the problem with supersymmetry — and indeed with any really good idea, such as extra dimensions, or a composite Higgs particle — is that such a basic idea typically can be realized in many different ways. Pizza is a great idea too, but there are a million ways to make one, so you can’t conclude that nobody makes pizza in town just because you can’t smell tomatoes. Similarly, to rule out supersymmetry as an idea, you can’t be satisfied by ruling out the most popular forms of supersymmetry that theorists have invented; you have to rule out all its possible variants. This will take a while, probably a decade.

That said, many of the simplest and popular variants of supersymmetry no longer work very well or at all. This is because of two things: (click here to read the rest of the article.)

17 responses to “Is Supersymmetry Ruled Out Yet?

  1. “Exrtra dimensions – A really good idea” : LOL

  2. Vladimir Kalitvianski

    Supersymmetry is a purely speculative idea without any experimental input, it is born dead. Note, even Electrodynamics is not formulated without contradictions and difficulties.

    • Yes, it is a speculative idea, with a lot of experimental input but with no direct experimental evidence. Have I said otherwise? However, it is not born dead, it is born just the way any good speculative idea is — with the possibility that it might be falsified by data. Nature does not care about either my opinion or yours, which is why we do not need to debate this question; we just need to look at the LHC data.

  3. MATT. : Am i right in understanding that without the 2 extra families of particles and fields which does not contribute to our world , our world would be totally changed.

    • Yes; if you simply remove the muons, strange quarks, charm quarks, taus, top quarks, bottom quarks and two of the neutrinos from our world, the effects of their virtual `particles’ (not really particles at all, but generalized disturbances in the corresponding fields, https://profmattstrassler.com/articles-and-posts/particle-physics-basics/virtual-particles-what-are-they/) would also be removed, with the result that many aspects of the world would change enormously. The proton mass, for instance, would be much larger, by many orders of magnitude. It isn’t clear what would happen to the Higgs field’s average value, but it would change by a great deal.

      Thanks to quantum mechanics and its virtual `particles’, the precise details of our world depend on aspects of each and every particle (and its field) that we know about, and probably on ones we do not yet know about. This is one of great challenges for particle physicists. It also helps to justify studying particles that survive only tiny fractions of a second.

  4. Muhammad Adeel Ajaib

    If no SUSY and extra dimensions, then what in your opinion is the next best way to solve the hierarchy problem?

    • I have not been a big fan of either supersymmetry OR extra dimensions in the past few years, but I have no other good ideas that would replace them. The best other ideas out there involve various forms of compositeness for the Higgs particle. And then there’s the possibility that the hierarchy problem is purely a selection effect. All I can say is that I’m hoping nature will give us a clue soon, because without additional clues I do not know what to think, and have not known what to think for over a decade.

      • Big, big respect from me to professor Matt Strassler here. I wish you a very nice weekend 🙂

  5. Are supersymmetry and extra dimensions here to stay in terms of theoretical physics?
    According to Witten, “Apart from gravity and gauge invariance, the most important general prediction of string theory is supersymmetry, a symmetry between bosons and fermions that string theory requires (at some energy scale). … Whereas in ordinary physics one talks about spacetime and ordinary fields it may contain, in string theory one talks about an auxiliary two-dimensional field theory that encodes the information.”
    http://www.sns.ias.edu/~witten/papers/Reflections.pdf “Reflections On the Fate of Spacetime”, PHYSICS TODAY, 1996
    Witten thinks highly of string theory — he wrote, “The idea of replacing point particles by strings sounds so naïve that it may be hard to believe that it is truly fundamental. But in fact this naïve-sounding step is probably as basic as introducing the complex numbers in mathematics.”
    http://www.sns.ias.edu/~witten/papers/mmm.pdf “Magic, Mystery, and Matrix”, Notices of the AMS, 1998
    Either Witten has overestimated string theory, or supersymmetry is a fundamental part of physics.

    • Supersymmetry and extra dimensions, as tools for doing calculations that are useful in explaining how quantum field theory works, are here to stay, just as quantum field theory, group theory, and string theory itself are here to stay. They are powerful conceptual and calculational tools.

      Supersymmetry and extra dimensions as things that are actually present in nature, are purely speculative at this time. They may or may not show up in data, and they may eventually be abandoned as explanations of aspects of the natural world.

      Some string theorists and gravity theorists would argue that a quantum theory of gravity may require supersymmetry. (It certainly doesn’t require extra dimensions, but without extra dimensions it may still require an infinite number of fields. [I refer to something known as four-dimensional strings, in which you don’t actually have ten dimensions, and the role of the extra dimensions is played instead by a very complicated but perfectly well-defined non-geometric construction; this has been around for decades but for some reason people tend to forget about it. See for example http://arxiv.org/abs/hep-ph/9511456%5D)

  6. Thanks for pointing out Lykken’s paper. Sheldon Glashow says that he sees no way that string theory, extra dimensions, or supersymmetry could be empirically refuted. I think that Prof. Glashow is correct about this. My understanding is that all the string theorists have so far is a “bare bones” perturbative theory of string theory and they have no hypotheses that could sort out what happens when energy-densities approach the Planck scale. Is my understanding wrong?

    • Notice Lykken’s document isn’t really a paper; it is a set of pedagogical lectures that cover an entire subfield of papers.

      Refuting any basic idea is very difficult, because it has so many realizations. I don’t know why Glashow would lump supersymmetry and extra dimensions together with string theory. Glashow co-invented Grand Unification; this hasn’t been refuted either and it isn’t clear that it can be, and moreover it would probably be easier to discover evidence of supersymmetry and extra dimensions than Grand Unification, since the former might show up at the LHC and the latter would not. String theory would be tougher to test than Grand Unification, in its standard form — but it too could in principle show up at the LHC (though this seems increasingly unlikely as the LHC data shows no clear deviations at high energy from the theoretical predictions of the equations governing the known particles and forces.)

      What you say about string theory was only true in the 1980s. String theory is a lot more now than just perturbation theory; an enormous amount is known about some aspects of its non-perturbative properties, thanks to the “3rd string revolution” between 1995 and 2000 or so. In some contexts there are very detailed hypotheses, and even checks of those hypotheses, about what happens at the Planck scale. (I lived and worked through that period; it was quite exciting.) That said, many important non-perturbative questions are not understood, and those include what happens in the early universe if you run the Big Bang backwards in time. Advances have been quite slow since about 2000 or so.

  7. Prof. Strassler, What do you personally think about Burton Richter’s criticism of string theory? In July 2011 I sent Prof. Robert B. Laughlin an email with a number of questions based on some my hypotheses and readings. I wrote, “M-theory predicts Einsteinian gravity, nonabelian gauge symmetry, and
    supersymmetry, but how can M-theory be decisively tested?” He replied:

    ” … M-theory does NOT predict Einsteinian gravity, nonabelian
    gauge symmetry or supersymmetry. It cannot be reliably
    solved at the “low” energy scales at which these things may
    occur. Claims to the contrary are false.” Who agrees with Prof. Laughlin on this? How many professional physicists agree with Stephen Wolfram that his 2002 book “A New Kind of Science” is a book rivaling Darwin’s “Origin of Species” in importance? (To my knowledge Wolfram is the only PhD-level physicist who publicly states this.) I claim that Wolfram is a serious rival to Newton and Einstein if and only if the Rañada-Milgrom effect and the Space Roar Profile Prediction are true, but so far I seem to have been unable to convince Wolfram of this.

    • I don’t know Richter’s precise complaints. I think the problem with string theory is that it is a bit of a political football and that both supporters and detractors ought to say less about it.

      As for Laughlin — he is basically right.

      M-theory is a theory which is conjectured to exist, though the conjecture is extremely plausible — the indirect evidence itself could fill several textbooks. But its full definition is not established. We don’t have a way to write it down as a set of equations. We can only write part of it down. If we assume the theory exists, we can derive a vast array of mathematical consequences that we know are true from other sources, including both standard string theory (perturbative and non-perturbative) and even quantum field theory. This strongly suggests that the assumption that the theory exists is correct. But we don’t have a full definition.

      Without a definition of the theory, and the complete set of equations, and techniques to solve them reliably, you cannot prove that if you solved the equations, you would be guaranteed that Einsteinian gravity and/or nonabelian gauge symmetry and/or supersymmetry would be an automatic characteristic of the universes that you obtain in your solution. Indeed, most universes that you obtained by solving the equations might well *not* have these properties. And strictly speaking it cannot yet be proven that *any* of the solutions have these properties — though there is a lot of evidence that some of them do.

      In fact, even string theory contradicts your statement (though again, it’s not entirely reliable). It would appear that string theory has solutions which do not have supersymmetry and/or nonabelian gauge groups. I have to try to remember if there are any solutions that are known not to have Einsteinian gravity; I suspect so. And since M theory is supposed to contain string theory, that would suggest that your contention is wrong, as stated.

      I think the correct statement is that string theory appears *easily consistent with* (i.e., has some solutions, but not all solutions, with) Einstein’s gravity, non-abelian gauge theory and supersymmetry. But it doesn’t automatically predict any of them — that’s the problem, in fact. And to obtain a universe like ours, with a Big Bang and broken supersymmetry, is not something that we can reliably say we can get from string theory, much less from M theory. It is plausible that you can, but it is quite hard to prove with current understanding of the theory.

  8. David Brown

    “M-theory is a theory which is conjectured to exist …” Yes, M-theory exists mathematically in an incomplete state. Witten, Seiberg, Maldacena, and others assume that some open problems can be solved in a way favorable to M-theory. According to Jaffee and Witten, “Existence theorems that put QFTs on a solid mathematical footing are needed to make the geometrical applications of QFT into a full-fledged part of mathematics.”
    http://www.claymath.org/millennium/Yang-Mills_Theory/yangmills.pdf
    I claim that supersymmetry has already been empirically confirmed. I have 3 hypotheses: the “Milgrom Denial Hypothesis”, the “Seiberg-Witten 100% Successful Hypothesis”, and the “Seiberg-Witten Almost Successful Hypothesis”. By using the “Seiberg-Witten 100% Successful Hypothesis” I think that I might be able to get my “supersymmetry already confirmed” idea published in a refereed journal of physics.