Wednesday, November 16, 2016

A new theory SMASHes problems

Most of my school nightmares are history exams. But I also have physics nightmares, mostly about not being able to recall Newton’s laws. Really, I didn’t like physics in school. The way we were taught the subject, it was mostly dead people’s ideas. On the rare occasion our teacher spoke about contemporary research, I took a mental note every time I heard “nobody knows.” Unsolved problems were what fascinated me, not laws I knew had long been replaced by better ones.

Today, mental noting is no longer necessary – Wikipedia helpfully lists the unsolved problems in physics. And indeed, in my field pretty much every paper starts with a motivation that names at least one of these problems, preferably several.

A recent paper which excels on this count is that of Guillermo Ballesteros and collaborators, who propose a new phenomenological model named SM*A*S*H.
    Unifying inflation with the axion, dark matter, baryogenesis and the seesaw mechanism
    Guillermo Ballesteros, Javier Redondo, Andreas Ringwald, Carlos Tamarit
    arXiv:1608.05414 [hep-ph]

A phenomenological model in high energy particle physics is an extension of the Standard Model by additional particles (or fields, respectively) for which observable, and potentially testable, consequences can be derived. There are infinitely many such models, so to grab the reader’s attention, you need a good motivation why your model in particular is worth the attention. Ballesteros et al do this by tackling not one but five different problems! The name SM*A*S*H stands for Standard Model*Axion*Seesaw*Higgs portal inflation.

First, there are the neutrino oscillations. Neutrinos can oscillate into each other if at least two of them have small but nonzero masses. But neutrinos are fermions and fermions usually acquire masses by a coupling between left-handed and right-handed versions of the particle. Trouble is, nobody has ever seen a right-handed neutrino. We have measured only left-handed neutrinos (or right-handed anti-neutrinos).

So to explain neutrino oscillations, there either must be right-handed neutrinos so heavy we haven’t yet seen them. Or the neutrinos differ from the other fermions – they could be so-called Majorana neutrinos, which can couple to themselves and that way create masses. Nobody knows which is the right explanation.

Ballesteros et al in their paper assume heavy right-handed neutrinos. These create small masses for the left-handed neutrinos by a process called see-saw. This is an old idea, but the authors then try to use these heavy neutrinos also for other purposes.

The second problem they take on is the baryon asymmetry, or the question why matter was left over from the Big Bang but no anti-matter. If matter and anti-matter had existed in equal amounts – as the symmetry between them would suggest – then they would have annihilated to radiation. Or, if some of the stuff failed to annihilate, the leftovers should be equal amounts of both matter and anti-matter. We have not, however, seen any large amounts of anti-matter in the universe. These would be surrounded by tell-tale signs of matter-antimatter annihilation, and none have been observed. So, presently, nobody knows what tilted the balance in the early universe.

In the SM*A*S*H model, the right-handed neutrinos give rise to the baryon asymmetry by a process called thermal leptogenesis. This works basically because the most general way to add right-handed neutrinos to the standard model already offers an option to violate this symmetry. One just has to get the parameters right. That too isn’t a new idea. What’s interesting is that Ballesteros et al point out it’s possible to choose the parameters so that the neutrinos also solve a third problem.

The third problem is dark matter. The universe seems to contain more matter than we can see at any wavelength we have looked at. The known particles of the standard model do not fit the data – they either interact too strongly or don’t form structures efficiently enough. Nobody knows what dark matter is made of. (If it is made of something. Alternatively, it could be a modification of gravity. Regardless of what xkcd says.)

In the model proposed by Ballesteros, the right-handed neutrinos could make up the dark matter. That too is an old idea and it’s working very well: The more massive of the right-handed neutrinos can decay into lighter ones by emitting a photon and this hasn’t been seen. The problem here is getting the mass range of the neutrinos to both work for dark matter and the baryon asymmetry. Ballesteros et al solve this problem by making up dark matter mostly from something else, a particle called the axion. This particle has the benefit of also being good to solve a fourth problem.

Fourth, the strong CP problem. The standard model is lacking a possible interaction term which would cause the strong nuclear force to violate CP symmetry. We know this term is either absent or very tiny because otherwise the neutron would have an electric dipole moment, which hasn’t been observed.

This problem can be fixed by promoting the constant in front of this term (the theta parameter) to a field. The field then will move towards the minimum of the potential, explaining the smallness of the parameter. The field however is accompanied by a particle (dubbed the “axion” by Frank Wilczek) which hasn’t been observed. Nobody knows whether the axion exists.

In the SMASH model, the axion gives rise to dark matter by leaving behind a condensate and particles that are created in the early universe from the decay of topological defects (strings and domain walls). The axion gets its mass from an additional quark-like field (denoted with Q in the paper), and also solves the strong CP problem.

Fifth, inflation, the phase of rapid expansion in the early universe. Inflation was invented to explain several observational puzzles, notably why the temperature of the cosmic microwave background seems to be almost the same in every direction we look (up to small fluctuations). That’s surprising because in a universe without inflation the different parts of the hot plasma in the early universe which created this radiation had never been in contact before. They thus had no chance to exchange energy and come to a common temperature. Inflation solves this problem by blowing up an initially small patch to gigantic size. Nobody knows, however, what causes inflation. It’s normally assumed to be some scalar field. But where that field came from or what happened to it is unclear.

Ballesteros and his collaborators assume that the scalar field which gives rise to inflation is the Higgs – the only fundamental scalar which we have so far observed. This too is an old idea, and one that works badly. To make Higgs inflation works, one needs to introduce an unconventional coupling of the Higgs field to gravity, and this leads to a breakdown of the theory (loss of unitarity) in ranges where one needs it to work (ie the breakdown can’t be blamed on quantum gravity).

The SM*A*S*H model contains an additional scalar field which gives rise to a more complicated coupling and the authors claim that in this case the breakdown doesn’t happen until at the Planck scale (where it can be blamed on quantum gravity).

So, in summary, we have three right-handed neutrinos with their masses and mixing matrix, a new quark-like field and its mass, the axion field, a scalar field, the coupling between the scalar and the Higgs, the self-coupling of the scalar, the coupling of the quark to the scalar, the axion decay constant, the coupling of the Higgs to gravity, and the coupling of the new scalar to gravity. Though I might have missed something.

In case you just scrolled down to see if I think this model might be correct. The answer is almost certainly no. It’s a great model according to the current quality standard in the field. But when you combine several speculative ideas without observational evidence, you don’t get a model that is less speculative and has more evidence speaking for it.

29 comments:

David Schroeder said...

From the description of the theory that you have so excellently outlined, it sounds more like a mishmash of ideas glued together, rather than something that is derived from a simpler conceptual foundation from which solutions of the major problems of contemporary physics/astrophysics naturally fall out.

akidbelle said...

Hi Sabine,

thanks, I laughed a lot. Once more it looks to me like the only recipe at hand is to piling-up fields and particles. Maybe nobody knows the answers, but many know how to word it, including syntax and grammar! Is that all there is in theoretical physics?

J.

Rob van Son (Not a physicist, just an amateur) said...

This SM*A*S*H. model has a strong "wheels inside wheels" feel.

Uncle Al said...

Chirp a gravity wave (ocean not vacuum), get a tsunami[1]. Clap hands before a New World stepped pyramid, Quetzalcoatl chirps back (sonic diffraction grating)[2]. Spring winds' chirping sounds are birds.

The neutrino see-saw mechanism is Wesley Crusher brain lint. Axions are easily detected but unseen. Double beta-decay experiments are sterile. Ice Cube says "three generations of neutrinos." The "Higgs" is insufficiently massive to stabilize the universe.

SM*A*S*H sums the worst of physical theory curve fittings. The universe is what it appears to be. A geometric Eötvös experiment opposing enantiomeric alpha-quartz looks. Look.

[1] http://onlinelibrary.wiley.com/doi/10.1029/2005GL023783/full
[2] http://news.nationalgeographic.com/news/2002/12/1206_021206_TVMayanTemple.html

Xerxes said...

I'm confused. All the ideas "proposed" in this paper are just Standard Model (plus axions). Right-handed fermions? SM. Inflaton is an existing field? SM. Strong-CP problem solved by axions? Not the SM but certainly a standard solution.

Is the objection that the adjustable parameters won't actually work out to solve the problems it purports to solve?

N said...

How about proton decay?

Sabine Hossenfelder said...

N,

Don't know what you mean. For all I can tell there are none of the troublesome operators in this model. Note that it's not a grand unification.

naivetheorist said...

sabine:

"The name SM*A*S*H stands for Standard Model*Axion*Seesaw*Higgs portal inflation.". well, the model meets the first requirement for any model in theoretical physics - a clever or cute acronym.

richard

Sesh Nadathur said...

Alternatively, it could be a modification of gravity. Regardless of what xkcd says.

Theories of modified gravity without any non-baryonic dark matter component fail to fit the CMB power spectrum. In fact they are guaranteed to fail to fit the CMB, unless they are also non-local - in that the gravitational force somehow points in a different direction to the location of baryonic mass. This is a well-known fact, acknowledged even by prominent MOND proponents like McGaugh.

Every attempt to get modified gravity theories to fit the CMB had either failed, or has introduced another form of dark matter anyway (and generally still failed).

So in fact XKCD was literally correct: the idea that there is no dark matter and it is all a modification of gravity simply does not fit the data on the largest scales.

Sesh Nadathur said...

(Note that I am not claiming the CMB proves dark matter must be CDM, or that it must be WIMPS, or anything other than that it must be dark - not coupled to photons - and it must exist, even in theories of modified GR.)

Maro said...

How many (new) parameters does this model have?

Sabine Hossenfelder said...

Maro,

I don't know, it doesn't say so explicitly in the paper. Please see the second to last paragraph for my counting of what the model adds. It brings me to at least 12 new parameters, but I'm not sure that's a complete count. The reason is that the paper doesn't explain much about the Higgs portal thing and it's not a topic I know very much about. Best,

B.

Maro said...

Sabine, do you know if it's possible to make new predictions / experiments that can be used to verify/disprove this model? I'm asking because you write "... if I think this model might be correct. The answer is almost certainly no.", which implies to me you think this is the sort of model that _does_ make testable predictions, so it's (fortunately) possible to talk about correct / not correct.

Another angle is, I presume it's possible to pick those >10 params in a way to fit existing observations?

Sabine Hossenfelder said...

Maro,

It's a phenomenological model. The whole point of making phenomenological models is that you can test them. So, yes, you can test that, in principle. Look for the axion, look for the signatures of Higgs inflation, and so on. Look at the paper??

Even if you'd find anything though, I suspect it would be very hard to find out it's this specific model. You'd need a tremendous amount of data for all kinds of things. Best,

B.

Uncle Al said...

http://www.mpbio.com/images/product-images/molecular-structure/05211962.png
http://www.essentialchemicalindustry.org/images/stories/610_Polymethyl/PolyM2MP_01.JPG

Phthalo Blue hugely self-associates like a deck of cards. Even a dimer changes the optical absorption spectrum. It dissolves in 338 °C boiling concentrated sulfuric acid. Dissolve it in Plexiglas.

One free sample later, the VP R&D held 500 grams of 30 wt-% Phthalo Blue single molecules in Plexiglas, a masterbatch dye. Pour components into a desk top thingie, out comes stuff. (It usually makes jet turbine high temperature nickel superalloys.)

Physical theory has an empirically incomplete postulate. When derivation fails, look elsewhere.

Bill said...

Thanks for the post and comments, Sabine.

It seems that particle physicists today are relying more and more on theories with clever pop culture titles (MADMAX, ORPHEUS, [Hulk] SMASH) based on hypothetical unseen or undetectable particles and fields. If a theory can never be tested (especially one based on Planck-level energies or ad hoc piled-on parameters), then it's no different from a religious belief. As you are wont to say, "it's an old idea," but it seems to be very much in vogue nowadays. Perhaps the idea is that if you can get enough people to believe in it (string theory, inflation), then it becomes true. (And I say that as an disillusioned American watching his country adopt lies as the truth).

piein skee said...

No, because he's transformed x stand-alone ideas, each requiring effectivrly arbitrary parameter setting decisions. into a partially self-referential equation. As such the individual speculations, which would have to be summed, are exchanged for one level of speculation applicable to the system as a wh9ole. That hugely enhances the potential of the solution, compared with the sum of those same solutions run in isolation. Lots of mutually independent ways to demonstrate the enhancement e.g. Occam.

piein skee said...

hi I'm just reposting the above comment because the excerpt from your post dropped out somehow. It's not really readable without the excerpt because the context for certain words is taken from there (e.g. 'speculation')

Doc Hossenfelder's concluding prognosis was "But when you combine several speculative ideas without observational evidence, you don’t get a model that is less speculative and has more evidence speaking for it. "

No, because he's transformed x stand-alone ideas, each requiring effectivrly arbitrary parameter setting decisions. into a partially self-referential equation. As such the individual speculations, which would have to be summed, are exchanged for one level of speculation applicable to the system as a wh9ole. That hugely enhances the potential of the solution, compared with the sum of those same solutions run in isolation. Lots of mutually independent ways to demonstrate the enhancement e.g. Occam.

Sabine Hossenfelder said...

piein,

I think you misunderstand my point. I think you are saying that by throwing n ideas together you don't necessarily get Sum_n parameters. Fine. Though in this case for all I can tell you still indeed have all the parameters, you just remove some problems that previously were entirely fatal. What I said however was merely that the probability that a conjunction of two untested hypothesis is correct is always smaller than each of them separately, even if less than Sum_n. It goes under the name conjunction fallacy. Best,

B.

piein skee said...

Sure that's right for the situation he strings together several independent ideas into a sequence where the subsequent depends the preceding being correct, because in that scenario the probabilities multiply. And that is what he's doing on one side hence it seems a very plausible first-approximation

But what he's actually doing is marrying from two different conceptually opposite directions, one being from problems, the other being from solutions. And that changes the equation in this particular sense relating to robustness

Sabine Hossenfelder said...

piein,

And what makes you think that your argument "changes the equation"? Exactly what about it supposedly changes it? We have here a conjunction of ideas that supposedly explain a conjunction of problems (several of which didn't need solving in the first place). How do you think this makes this multi-model any more likely to be correct than right-handed neutrinos to be correct, just to pick a random example?

Best,

B.

piein skee said...

xerxes says: I'm confused. All the ideas "proposed" in this paper are just Standard Model (plus axions). Right-handed fermions? SM. Inflaton is an existing field? SM. Strong-CP problem solved by axions? Not the SM but certainly a standard solutio

That's right, though I think you're reacting to the negative signalling from commenters more than doc hossenfelder's piece. The comments are considerably are absolutely in the negative whereas her piece was actually positive.

But you are right, what is not to like about solving a bunch of problems in terms of each-other? That is not curve-fitting uncle-al! This is like simultaneous equations. It's not curve fitting.

Doesn't mean it's right. I don't actually think it's right. But the methodology is definitely right.

piein skee said...

hi doc hossenfelder - I'm not making those mistakes but I'm not being very clear either. What I'll do is assemble my best eloquence (which isn't great) and try again later. The reality is that you already will know the distinction - I'd take poison on that. I've seen you say it. It's just a case of knowledge that is general in of itself, does not automatically rise to the surface in a given context. Brains just don't provide that service...if they did the world wouldn't be so messed up. But they don't. I'll be back.

Uncle Al said...

@piein skee "This is like simultaneous equations. It's not curve fitting. "

A swing pendulum measures weight. It fails in vacuum free fall. Its equation omits the bob. A torsion pendulum measures mass. It works in vacuum free fall. Its equation has the bob. Given equal periods, equate the equations. Gravitation/length is then inversely proportional to inertial moment. Run the units. Really?

piein skee said...

Hi Uncle-Al - you might be right, but just to say that being 'like' something leaves a large amount to the context in play. Clearly, there's no way to write equations and benefit from mathematical certainty, at that sort of stage. But it is 'like' simultaneous equations in that you are trying to solve a set of problems in terms of each other. I'm going to try to capture the benefit in a short comment but I obviously accept the main reason I'm doing that is so that people are able to explain what's wrong with it. But to that end, will you mention why you see curve-fitting?

Ali Lezeik said...

I don't know why people publish papers based on speculations, this way you'll get a new speculation which is based on a speculation, and others will use this new paper to publish a newer one, so it'll be speculation based on speculation based on speculation !! Speculationception !! I mean it's interesting and all, but it makes no sense to do all that work when there is no solid infrastructure to the whole theory. And I believe it's sad to see all these minds "wasting" their time on that rather than really trying to go from A to B and come up with a bright clear conclusion. Well it might not be a complete waste of time since you could find "Something" while doing the research, but I guess my point is clear enough.

Uncle Al said...

@ piein skee Science is rigor that survives empirical falsification (Galileo, Popper). The penultimate sentence of my prior post is crap. Don’t believe proclaimed authority. Look.

If the equations’ periods are equal, then time = time. LOOK at the two equations’ dimensions, run the math (torque constant). Bee knows physics (uncomfortably) has never observed whether opposite shoes diverge at the required sensitivity level, whether reality is fundamentally not mirror symmetric as postulated. Gravitation theory would violently swerve, or not. Look.

Mr Roboto said...

I like the general approach of the model: bottom-up rather than top-down. We've had forty years of supersymmetric, top-down models, each usually adding dozens of new fields and hundreds of new parameters, all in the name of more symmetry. But now that it's starting to look like supersymmetry isn't going to pan out--at least not in the energy ranges accessible to the LHC--I think it's time that more effort be put into these sort of modest, conservative extensions of the Standard Model. Such a minimal extension of the SM coupled to asymptotically safe gravity may in fact be the final theory, regardless of how satisfying physicists may find it.

David Schroeder said...

This paper underscores the fact that we seem to be confronting a new 'aether' conundrum. The world awaits another Einstein, whether individual woman or man, or research team, who will illuminate the darkness, and perhaps usher in a major paradigm shift in our thinking.