[gus_massa]

> Have you had a good look?

It's difficult to explain, but they[1] tried very hard.

For example the electron has an electric charge but it's also like a small magnet. In an ideal elementary particle, the value of the magnet is 2 * something. In a real elementary particle the value is almost-2 * something, so they are measuring the almost-2, and it's call g [2].

For an electron, the measured value of g is

  2.00231930436256(35)

, there are is an uncertainty of 0.000000000002%. The problem is that it agree with the current theoretical prediction.

The muon is very similar to an electron, but the experimental g is [3]

  2.0023318416(13)

 and the current theoretical prediction is 

  2.00233183620(86)
  

It's a difference of 0.000000001%. Most people will be happy with that disagreement and forget about it. But They are happy because there is a disagreement and perhaps they can use that to discover a new particle. It still may be a long lived statistical fluke, but it already survived many years. Other team claimed that there is a small error in one of the experimental numbers used in the theoretical calculation, but I'm not sure if they are genius or crackpots or something in between.

And there are many other experiments. I like for example the IceCube [4] that is just a giant chunk of ice in the Antarctica. They try to detect neutrinos from stars. It has many experiments, but in particular some experiments are useful to measure the difference of mass of the neutrinos that is a not very clear part of the Standard Model.

[1] Not my area of research. They live in the next corridor.

[2] https://en.wikipedia.org/wiki/G-factor_(physics)

[3] https://en.wikipedia.org/wiki/Muon_g-2

[4] https://en.wikipedia.org/wiki/IceCube_Neutrino_Observatory

[pfdietz]

There have been various amazing "tabletop" experiments looking for effects that would imply new particles/forces. Electric dipole moment of the electron, electric dipole moment of the neutron, "fifth force" (deviations from 1/r^2 gravity at very short ranges), neutron oscillation. The cleverness of some of these experiments is astounding.

[abhv]

gus_massa: Since you are likely an expert, could you recommend a resource that explains how you use the Lagrangian equation for the standard model [1] to actually compute a predicted value for the electron's g ?

An elementary resource that goes through basic steps for a computer scientist (non expert in QFT) would be a great. A simpler particle than electron is also ok, but I'd love to understand how you mess with that equation.

[1] http://nuclear.ucdavis.edu/~tgutierr/files/stmL1.html

[gus_massa]

Sadly not an expert in that area. I only took a course of Nuclear Physics for a Major in Physics [1]. So I can read and understand that stuff, but the fine details pass over my head.

Looking at a recent page of that course, the recomended books are

* F. Halzen, A. Martin, “Quarks and Leptons: An introductory course in modern particle physics” (Wiley 1984)

* D. Griffiths, “Introduction to elementary particles” (Wiley 1987)

(and a few more)

The calculation for g=2 is quite easy (for an advanced Physics student). I remember the general idea, but not the details. I think I can reconstruct the details if necessary. It may be explainable in a blog post skipping some details.

The first correction g=2+1/137.036 is also humanly compresible, and can also be explained with some graphics. It would be very hard for me, but if I have a week to seach and rehearsal it is possible.

As the sibling comment says, the following corrections g=2+1/137.036+g=2+?/137.036^2 get harder and harder. And there are too many technical details and problems. I can only see the graphics and get a shallow understanding, but how they are transformed to integral and how to calculate all of them efficiently is too much for my knowledge.

[1] I never finished my Major in Physics, but I finished the one in Math.

[lr1970]

> Looking at a recent page of that course, the recomended books are * F. Halzen, A. Martin, “Quarks and Leptons: An introductory course in modern particle physics” (Wiley 1984) * D. Griffiths, “Introduction to elementary particles” (Wiley 1987)

It is telling that for a recent course the recommended books are over 35 years old. Consistent with the OP proposition.

[wrycoder]

Rather like Jackson is still a standard electrodynamics text after 60 years. Classical EM is finished. But, quantum field theory is not.

[gus_massa]

The complete list is

* F. Halzen, A. Martin, “Quarks and Leptons: An introductory course in modern particle physics” (Wiley 1984)

* D. Griffiths, “Introduction to elementary particles” (Wiley 1987)

* J.J. Sakurai, “Advanced quantum mechanics” (Addison Wesley 1967).

* P.E. Hodgson, et al., “Introductory nuclear physics” (Oxford 1997).

* H. Frauenfelder, E.M. Henley, “Sub-atomic Physics” (Prentice Hall 1992)

IIRC the Sakurai book is more about generic quantum mechanics, but he has two books, I'm not sure if this has more about particle physics. The other two are more modern, but I don't remember them. I also tried to keep the list short, because usually the main book of the course cover most of the topics.

Anyway, it's a mandatory undergraduate course for everyone that want to be a Physics. If you want to learn cutting edge particle physics, you should take one or two optative course about the topic, then make a one year undergraduate thesis, then take a 5 years PhD, and then perhaps 2 years of a postdocs. So the cutting edge is like 8 years away.

[l33tman]

The paper describing the theoretical steps necessary to compute g for the muon is hundreds of pages of condensed math, theorems and approximations etc.

The SM Lagrangian is not computable, so a big part of theoretical physics is about finding tricks to actually compute it.

Incidentally this is why there is disagreement on the muon g-2 discrepancy, at least two theory groups have calculated different values using different approximations.

[sigmoid10]

It should be noted that the anomalous electron g-2 is computable analytically (at least to very good approximation) which makes the theoretical value much less controversial. The anomalous muon g-2 however depends more heavily on interactions of quantum chromodynamics, which can only be computed using numerical lattice QCD simulations. This is notoriously hard and has only become practical in recent years, hence why theorists don't yet fully agree on the value.

Also, computing even just one part of this value is basically on the level of a theoretical particle physics dissertation. Don't expect to be able to do this without several years of research experience in this specific field.

[kkylin]

It may be worth first understanding why g=2 (if you haven't before). This can be done on the basis of special relativity + quantum mechanics, i.e., the Dirac equation:

https://en.wikipedia.org/wiki/Dirac_equation

The "g" is the Lande g factor:

https://en.wikipedia.org/wiki/Land%C3%A9_g-factor

(As I recall nonrelativistic QM gives g=1.)

PS Not a physicist, but learned some of this at some point. Only ever learned about electrons, though; don't know how any of this translates to other particles.

[gaze]

You mess with it by doing diagrammatic perturbation theory, that is, calculating Feynman diagrams. Zee or Weinberg could be good references. There’s also lattice QFT but you generally want to learn the perturbative methods first

[jwuphysics]

I have two recommendations you might find useful. The first is QED, a series of lectures by Richard Feynman. This text covers the qualitative nature of the perturbation theory used for quantum electrodynamics. The second is Quantum Field Theory for the Gifted Amateur by Lancaster and Blundell. It's nicely written and accessible at the advanced undergrad level, building up QFT from the basics.

Caveat-- I work in astronomy but have a PhD in physics and have taken graduate QFT.

[orbifold]

You can look at “QFT in a nutshell” by Zee, a highly recommended and pretty accessible book (to the degree a book on QFT can be accessible), for the computation of g for the electron to one loop order. That calculation can also be found in “Quantum field theory and the Standard Model” by Schwartz in Chapter 17 (p. 321). I’m not aware of a textbook exposition of the calculations relevant for the muon g.