Thursday, February 15, 2018

What does it mean for string theory that the LHC has not seen supersymmetric particles?



The LHC data so far have not revealed any evidence for supersymmetric particles, or any other new particles. For all we know at present, the standard model of particle physics suffices to explain observations.

There is some chance that better statistics which come with more data will reveal some less obvious signal, so the game isn’t yet over. But it’s not looking good for susy and her friends.
Simulated signal of black hole
production and decay at the LHC.
[Credits: CERN/ATLAS]

What are the consequences? The consequences for supersymmetry itself are few. The reason is that supersymmetry by itself is not a very predictive theory.

To begin with, there are various versions of supersymmetry. But more importantly, the theory doesn’t tell us what the masses of the supersymmetric particles are. We know they must be heavier than something we would have observed already, but that’s it. There is nothing in supersymmetric extensions of the standard model which prevents theorists from raising the masses of the supersymmetric partners until they are out of the reach of the LHC.

This is also the reason why the no-show of supersymmetry has no consequences for string theory. String theory requires supersymmetry, but it makes no requirements about the masses of supersymmetric particles either.

Yes, I know the headlines said the LHC would probe string theory, and the LHC would probe supersymmetry. The headlines were wrong. I am sorry they lied to you.

But the LHC, despite not finding supersymmetry or extra dimensions or black holes or unparticles or what have you, has taught us an important lesson. That’s because it is clear now that the Higgs mass is not “natural”, in contrast to all the other particle masses in the standard model. That the mass be natural means, roughly speaking, that getting masses from a calculation should not require the input of finely tuned numbers.

The idea that the Higgs-mass should be natural is why many particle physicists were confident the LHC would see something beyond the Higgs. This didn’t happen, so the present state of affairs forces them to rethink their methods. There are those who cling to naturalness, hoping it might still be correct, just in a more difficult form. Some are willing to throw it out and replace it instead with appealing to random chance in a multiverse. But most just don’t know what to do.

Personally I hope they’ll finally come around and see that they have tried for several decades to solve a problem that doesn’t exist. There is nothing wrong with the mass of the Higgs. What’s wrong with the standard model is the missing connection to gravity and a Landau pole.

Be that as it may, the community of theoretical particle physicists is currently in a phase of rethinking. There are of course those who already argue a next larger collider is needed because supersymmetry is just around the corner. But the main impression that I get when looking at recent publications is a state of confusion.

Fresh ideas are needed. The next years, I am sure, will be interesting.



I explain all about supersymmetry, string theory, the problem with the Higgs-mass, naturalness, the multiverse, and what they have to do with each other in my upcoming book “Lost in Math.”

31 comments:

Bill said...

Thank you for telling us the truth about SUSY, Dr. B. Since 2012 I have wondered how physicists would react to the physics "desert" the LHC is now revealing. Moving the energy goalposts seems to be the only option now for the true believers.

Uncle Al said...

"What does it mean for string theory that the LHC has not seen supersymmetric particles?"

…1) SUSY does not model empirical vacuum.
…2) Third order approximations, new observations within failed paradigms, cherry picking.

"Fresh ideas are needed"

…3) Observe calculated enantiomeric molecule microwave rotational temperature divergence (online pdf, Bee requests no self-URLs). One observation day, perhaps $(USD)100K overall. The worst it can do is succeed.

Elver S.S. said...

I'm very grateful for your efforts to clarify to the layman like me what the state of Physics is today. Thank you; I can't wait for your book to be delivered to my iPad.

For those like me that are not well-versed in the topic, here is a previous post from Dr. B that explains naturalness as it pertains to the Higgs:

http://backreaction.blogspot.com/2016/07/why-lhc-is-such-disappointment-delusion.html

Leibniz said...

Does your book give a simple explanation of what a Landau pole is?

Matthew Rapaport said...

What indeed and yes whatever it is it will be interesting. Thanks for another nice essay Dr. H. Your book due out soon now yes?

Ivan Kirkpatrick said...

Seems like this is the old argument, We need a bigger hammer..... Given the cost of a bigger hammer, I hope they think a lot more on this and come up with some new ideas before we set to building a more powerful collider.

Bruce Rout said...

"String theory requires supersymmetry," How so? Reference please?

Sabine Hossenfelder said...

Liebniz,

No, it does not! I only recalled about this when I was recently reading some papers about asymptotic safety :/ But it's not hard to explain what a Landau pole is, it happens when the strength of an interaction becomes infinitely large. This shouldn't happen. The interaction becomes strong before that happens, so it's hard to say then exactly what goes on, but it's generally recognized that this is an issue which requires a solution. (In contrast to the value of the Higgs-mass which is well explained by it just having some value.) Best,

B

Sabine Hossenfelder said...

Bruce,

You need supersymmetry to keep the vacuum stable. For a longer explanation, I recommend Conlon's book (review here).

Reimond said...


Sabine´s “You need supersymmetry to keep the vacuum stable” reminds me of just another reasoning that uses supersymmetry.
I vaguely remember that since the vacuum energy for fermions is negative and for bosons positive the hope was that for every fermion there is a bosonic partner with about the same mass to cancel these contributions. Thus, supersymmetry could be used for a theory with gravitons, which would couple to these loops to get out of the cosmological constant problem.
Well, this reasoning assumes that the cosmological constant has anything to do with vacuum energy and is not just a free parameter of general relativity. The later I prefer.
Thus, no supersymmetry could imply: no gravitons and no quantization of spacetime. Which is also reasonable, since gravity is not renormalizable anyway.

John Fredsted said...

I was under the impression that the no-findings of the LHC is problematic for the notion of supersymmetry, at least for supersymmetric extentions of the standard model, in the sense that the masses of the yet hypothetical supersymmetric particles would need to be so large, via some process of spontaneous symmetry breaking, that it clashes with the eletroweak scale itself. In other words: the 'natural' models of supersymmetry do increasingly seem to be ruled out empirically. I know, you are not a big fan about 'naturalness', so perhaps your assertion should be seen in that context.

Sabine Hossenfelder said...

John,

Yes, as I wrote, the results are in conflict with the idea of naturalness. Exactly what about my explanation is it that you don't like?

John Fredsted said...

Sabine,

I apologize, I think I was too fast in posting, having just read your blog entry again. Your discussion of naturalness in connection with the Higgs mass is, of course, more or less tantamount to a discussion of naturalness in connection with the weak scale itself.

Larry said...

Has anyone proffered the idea that String Theory is bunch of nonsense and is based upon "mathics" and not observation. Considering there are thousands of string theories out there and more arriving everyday, that maybe the methods and scientist is flawed and "new particles" are "discovered" because of errors in the theory and math causing the physicist to think they have discovered a new particle, when in fact they have discovered the fact string theory is wrong. There is plenty of reason for physicists to keep string theory alive--MONEY, grants, reputation. My bullshit detector is on overdrive when I read some of the cockamamie crap the comes out of 20th century physics. It might be time to abandon most of 20th century physics, ban young physicists form using computers and force them to make observations again rather than playing with math.

C. M. said...


@Larry

"abandoning most of the 20th century physics, ban young physicists form using computers"

This would be equivalent to prohibiting the successors of Democritus, Leucippus or Epicurus from working on the atomic hypothesis. The reality of atoms was only demonstrated 2400 years later by Jean Perrin.
2400 years and we are complaining about 30, 40 years !!!???
People must be able to work on string theory, LQG, non-commutative geometry etc. with or without computers.

Regards

Eusa said...

Is it only by random that the energy of Higgs boson is (20^2)/3 x the energy of free proton? And that the vacuum expectation value is round two times Higgs boson energy?

Unknown said...

An inability to distinguish between string theory and the bulk of 20th century physics is a gross lack of discernment.

Jonathan Starr said...

Sabine,
I understand your objection to theories that are really just an excuse to ask for a bigger collider (read more money)instead of admitting that strings are great for guitars but less then great for producing testable predictions (yes I'm in the anti sting theory camp my self). but isn't that exactly what we have on the books as the path forward? See "International Linear Collider" aka make the Japanese pay for it.
And I heard somewhere that a circular collider is for statistics on gobs and gobs of particles at once but can't probe individual reactions where linear can for some reason. Is this more-a-less correct?
What I really want to know is if playing with math isn't the answer (I agree it's not/come at me string bro's) and "higher energy/bigger collider/just throw money at it" isn't the answer then aren't you just proposing we wait for the next genius to bail us out by theorizing a new avenue? Then again maybe I agree and don't want to admit we are in a no win scenario to my self... sigh

Not trying to be critical just want a working TOE, like all the other crazy ppls
Are there other avenues that I don't understand? where should we send the cash?

A silly engineer

Louis Tagliaferro said...

Sabine said, “Fresh ideas are needed.”

Max Plank – “As an older friend, I must advise against it, In the first place, you won’t succeed, and even if you do, no one will believe you.” Advice he gave to Einstein after Einstein wrote him of his intention to work on the problem of gravity.

Getting past established thinking can be as difficult as the problems that need to be solved, in my opinion. It must start with looking at one's own established thinking first.

Nick Maiorino said...

Hi Sabine,

Shantanu posted in a comment, over at Peter Woit's blog, a link to an article in "The Economist" titled, "Fundemental physics is frustrating physists". Although Shantanu seemed to be having a problem reading the entire article through whatever device he was using, I had no problem reading it on my mobile. You are quoted quite extensively at several points. Perhaps others, on your blog, will enjoy the read as well.

-Nick

Sabine Hossenfelder said...

Jonathan,

I find it rather peculiar how often people assign opinions to me that I never put forward. You for example seem to assume that I am opposed to the idea of building a larger collider. That isn't so. I am opposed to lying to the public about what such a collider could achieve.

Particle colliders (lepton colliders in particular) are a particularly obvious way to probe physics that has never been tested before, and to do so in a clean and controlled environment (as opposed to cosmic rays whose reconstruction is difficult). While I think naturalness is silly, it is correct of course that the Higgs is different from all the other particles we know so far, so poking it and try to understand it better seems a good idea to me. Best,

B.

MichaelG said...

Dr. H, aren't neutrino oscillations another problem for the standard model?

Sabine Hossenfelder said...

MichaelG,

I wrote about that here. The LHC has little to say about neutrino oscillations.

Jarek Stolarz said...

Low Energy SUSY is going bye,bye.. but still we need SUSY for finite and consistent QG theory. Let's make Planck scale a SUSY breaking scale. Long life for SUSY. One problem solved.
Next pappers will predict many unvisible sterile scalars and common requirement: We need a bigger machine. Much bigger.
Any other problems to be solved?
Why so serious?
Good luminosity of LHC. May is close. Hope never dies.

Reimond said...

Neutrino oscillation can also be explained without masses via Lorentz violation (standard model extension). And the good news is that KATRIN will have results in about 2 years.

Bruce Rout said...

Thank you.

Bruce Rout said...

Apparently your review of tgis biok is a little conservative. Is this the only reference to such a claim: that string theory must have SUSY to survive? That is a pretty bold statement. I found this reference contradicting you, for what it's worth. https://arxiv.org/abs/1012.5091

BTW: your article is very good.

MarkusM said...

If you think about it, if supersymmetry is true, its breaking is fine-tuned such that all the non-supersymmetric particles have masses just right such that we can observe any one of them, whereas the supersymmetric partners have masses just right such that we cannot see a single one of them. Looks like a big cosmic conspiracy, doesn't it? What makes this situation even worse is that the non-supersymmetric particle masses have a broad scatter (e.g. neutrinos, top quark), so one would expect a broad scatter of masses in case of the supersymmetric particles as well.

Jarek Stolarz said...

Can the SM survive all the way to a Planck scale? Probably Yes. See LHC results.
What we have is a kind of desert - no particles, but maybe something else is missing too.
Maybe is a big desert from EW to PL - no Landau poles, no intermediate scales, etc...
Desert is a place for many interesting ideas.
Few of them can be tested in LHC. I guess.






Lawrence Crowell said...

Supersymmetry is really a framework. It tells us how a transformation between fermions and bosons gives spacetime boosts. Hence the statistics of quantum mechanics are associated with spacetime symmetries. There is nothing in this that tells us anything about the masses of superpartners. In fact the pure theory requires the particles and their SUSY masses be equal. To make sense of this with particle physics we then hang all sorts of phenomenology on supersymmetry, and in particular this has been the standard model.

The idea has been to look for light supersymmetric partners. The reason really has to do with the fact our particle physics can only probe for low energy, low compared to Planck/string scale physics, and so we are the person looking for their lost keys under the street lamp because at least there is light there. The Higgs mass with the quartic potential is not stable, and SUSY requires additional Higgs particles. If these are not found then the quartic interaction means there is RG flow up to GUT or Planck scale for these Higgses. It would seem to me this implies supersymmetry is an extreme high energy physics and not observable at the low energy scales we can muster.

This association between particle statistics and spacetime symmetries SUSY offers is a neat trick. I also suspect it is a facet of the current concepts of spacetime as emergent from entanglements. I think there is some general framework of which SUSY is just one part of.

Nichol Brummer said...

I don't quite agree about the 'fresh ideas are needed'. I think it is more important to get fresh empirical evidence of a problem. Or new thinking about the existing problems. The greatest recent progress has come from neutrino mixing and cosmology finding that the universe expands, so we have both a 'dark matter' and a 'dark energy' problem to describe. The other 'problem' in particle physics is what to do with the CKM matrix, and the neutrino mass matrix. On that, more data should also help. What is the origin of CP violation?

Don't forget that the LHC was proposed in ~1990 by Rubbia, who 'predicted' that it could be built by ~1998. He was wrong by about a decade. But the original radical idea that a collider with 10-20 interactions per bunch crossing at incredibly high frequencies was a challenge that people could deal with, once Moore's law would bring us the computing power .. that has worked out.

The challenge always is to design the right experiments for the right time. And we have more than one frontier to explore.