## The First Particle Physics Evidence of Physics Beyond the Standard Model? 97

StartsWithABang writes

*It's the holy grail of modern particle physics: discovering the first smoking-gun, direct evidence for physics beyond the Standard Model. Sure, there are unanswered questions and unsolved puzzles, ranging from dark matter to the hierarchy problem to the strong-CP problem, but there's no experimental result clubbing us over the head that can't be explained with standard particle physics. That is, the physics of the Standard Model in the framework of quantum field theory. Or is there? Take a look at the evidence from the muon's magnetic moment, and see what might be the future of physics.*
## Desensationalised (Score:5, Interesting)

I admit not having read the clickbait (this is /. after all), but I presume that the real story behind it is that an experiment to measure the muon magnetic moment has recently moved [sciencemag.org] from Brookhaven to Fermilab to get access to more energetic muons. They're hoping to start measuring data in 2.5 years.

## Goddammit! (Score:2, Interesting)

## No, there is no evidence of BSM yet (Score:5, Interesting)

I am a particle physicist, and I have worked directly on this problem. The uncertainty in the hadronic contributions to the vacuum polarization and light-by-light scattering are large enough that the supposed BSM signal is not significant.

That is, you can do nice high-order paper-and-pencil calculations of Feynman diagrams when the particles involved are electrons and muons, but there are important cases where the particles contributing to this effect are composite: hadrons (which are made of quarks). Since you cannot do calculations on hadrons without considering how the hadron is composed of quarks, you can't avoid getting into strongly coupled quantum chromodynamics (QCD). See here for further discussion: Hadronic Light-by-Light [washington.edu].

That means you can't do your calculation on paper, you have to use a supercomputer and something called lattice QCD. Unfortunately, it's easier to crank out a thousand crappy model calculations of BSM that is supposedly showing up than to properly fund studies of the theory uncertainties. As a result, the precision of the theory values are not good enough to establish whether the muon magnetic moment is consistent with the Standard Model or not.

That said, it's still an interesting place to look, and somebody will work out all the uncertainties eventually. In a few years, there might be something to talk about seriously.

## Mixed feelings (Score:4, Interesting)

Honestly, I really love the Starts With a Bang blog, and have been reading it for years.

But I do have to wonder why every single post is announced on /. these days.

We already have RSS, we subscribe, we know about it. By the time /. has caught up I've already read it, usually 24 hours ago.

Is /. getting a paid for these posts? Inquiring minds want to know.

## Re:Why gravity is treated as a force? (Score:2, Interesting)

Because it is a force? A force is anything which transfers momentum and energy around. Which gravity does.

Moreover, what seems so obvious to you that gravity is the curvature of space time? What does that

mean? Because it is in no way obvious. For example, if gravity is spacetime curvature, then it doesn't really pull on things in 4D spacetime since we've already defined it away. So why do things appear to move down gravitational wells? Are they elastically colliding with a sheet of space time? Why aren't they normally deflected by it?Finally, it doesn't matter what new theory shows something is or isn't. It has to verify old theory. And old theory says that gravity looks and acts, in the human range of experience, like a conventional force identical to any other. So whatever it is, it has to be simply back down to confirming our everyday experience.

## Re:Goddammit! (Score:5, Interesting)

so without reading the article, the answer is going to be no, there's no evidenceNothing to see here (if you have no soul whatsoever), move along (and let the real scientists do their thing so you can have your hoverboards and replicators 50 years from now).

I do have a question for the

seriousparticipants in this discussion, however... Since the Muon counts as 40,000 (200^2) times more sensitive to unexpected effects than the electron, why not work with the Tau instead, which should have a whopping 1.2e7 times more sensitivity?## Re:Why gravity is treated as a force? (Score:3, Interesting)

It is confusing isn't it? Again and again someone demonstrates gravity with a sheet and a ball, and again and again there is someone looking for or talking about the 'graviton'.

Another one: The presenter starts off with an illustration of space and time being -the same thing: "space-time". But then goes on to explain space only, or time only, or both but each in their own silo.

My approach to understanding this has been to watch every documentary I can, distill the common, repeated 'truths' and extrapolate a mental image from that. I think space and time are -literally- the same thing (the perpetual expansion of space -is- the passage of time), gravity is -not- a force, there was no big bang, nor was there inflation.

Ok, the last bit is somewhat radical so I'll explain a bit. The universe (space) is always expanding, slower when there is nearby matter with mass, faster when there is not. When galaxies are so far away from each other space expands so rapidly there is a "breach", covering a huge astronomical area, in which energy/matter rushes in uniformly, slowing down the expansion.

I have no math skills and might be completely wrong, but it feels right and I'll probably hold on to this mental model until/unless there is some clear irrefutable proof otherwise. Really, I don't see that happening because (lol) no one is every going to investigate the scenario I just described.

## Whoa there: many corrections! (Score:5, Interesting)

Since Einstein, we know gravity is the curvature of space-time. It may be represented as a force in calculations but in reality there is no force.

How about I turn that around and say that Einstein showed gravity can be modelled by the curvature of space-time but in reality it is a force? The fact of the matter is that, at a fundamental level, we have no clue what gravity is. However you can represent it very well by a spin-2, mass-less particle which couples to a particle's 4-momentum (the caveat being that you cannot make this theory work without an energy cut-off at some scale for which there is no justification). Until we solve quantum gravity we simply do not know what gravity really is but, if I were to bet, I suspect the latter is closer to the truth but needs some correction for the quantum structure of space-time which is something we have no clue about.

If gravity is not a force then do we really have a hierarchy problem?

Yes, and if anything it would be worse. The current problem comes about because we cannot scale the Higgs corrections up to the Planck-scale where we know there is new physics. If we remove that scale then we have a theory which has no upper scale limit and so should generate infinitely large corrections to the Higgs mass i.e. we go from an incredibly unlikely 1 in ~10^34 chance of the corrections giving such a light Higgs to a zero percent chance of the theory giving a light Higgs, or any Higgs with a non-infinite mass.

Obviously, if this is the case, G has nothing to do with Fermi's constant and we should not compare the two.

You are getting your 'g's and 'G's confused. In the muon g-2 experiment the 'g' is the muon's anomalous magnetic dipole moment. This is a precision test of Quantum Electrodynamics. The high order corrections to this will involve Fermi's constant (G_F) due to W and Z loops but these contributions will be incredibly small and were this any other experiment I would have said negligible but perhaps not in this case given the incredibly high precision involved. Neither of these constants have anything to do with the gravitational constant (G) nor the local acceleration due to gravity (g). So we are not comparing the two.

## Re:Goddammit! (Score:4, Interesting)

so without reading the article, the answer is going to be no, there's no evidenceWell, yeah okay there's that. But as others have said, that's not new information. I should have said there's no

newevidence.The point of my moan is that this is yet another puff piece from StartsWithABang that frames itself as "Holy shit! Earth shattering breakthrough occurs!" when it actually hasn't. I find that annoying and something that seems to be happening more and more lately.

## Re:Neutrino Mass (Score:5, Interesting)

Lattice gauge theorist here -- we're working on that.

I agree with your interpretation -- this very well may just mean that the QCD part is hard and the theorists didn't correctly estimate systematic errors from it. However, there's quite a push in the lattice community to actually calculate the messy nonperturbative parts, so there's hope that this will be sorted out from the theory side alongside the new Fermilab experiment.