Muon g-2 and the Standard Model: Updates and Questions

[mathman]

A few years ago BNL did some experiments measuring muon g-2. At the time there was a significant enough difference from the calculations based on the Standard Model to raise questions about SM. Since then I've seen papers indicating that SM was probably OK, with the discrepancy being due to the difficulty in calculation, particularly the hadronic contribution. Where do things stand today? Has it been resolved to the community's satisfaction or is the question still open?

 

[arivero]

A few years ago BNL did some experiments measuring muon g-2. At the time there was a significant enough difference from the calculations based on the Standard Model to raise questions about SM. Since then I've seen papers indicating that SM was probably OK, with the discrepancy being due to the difficulty in calculation, particularly the hadronic contribution. Where do things stand today? Has it been resolved to the community's satisfaction or is the question still open?

The theoretical community rebuilt the calculations, finding some signs here and there and adding some new higher order terms until they entered into agreement within the experimental error gaussian. Then, the experimentalist did a new round of experiments reducing the experimental error, and the question was left open.

 

[mathman]

Are there any recent papers (e.g. in ArXiv) which discuss these issues?

 

[arivero]

Actually all the stuff is in the ArXiV, but I can not tell which the good ones are. Use SPIRES instead of the arxiv to navegate across the citation tree.

In our paper hep-ph/0503104 we quote hep-ph/0406325 and hep-ph/9810512 as sources for the theoretical calculation.

The most recent experimental measurement is still, I believe, http://arxiv.org/abs/hep-ex/0401008

The theoretical calculation keeps giving papers; last review from Kino****a at http://arxiv.org/abs/hep-ph/0512330

A recent "independent" review of the state of the question could be hep-ph/0509372 by M. Passera, last updated 10/Oct/2005

 

[arivero]

It is funny to look table 1 of http://arxiv.org/abs/hep-ph/0509372 from our (Hans and me) point of view. We consider instead of $$a_\mu$$ the difference $$a_\mu-a_e$$, to which we add the surviving therms of a_e in the substraction, namely $$a^{vp}_e$$, the so called vacuum polarisation terms, so that the whole expresion is really $$a_{\mu}^{vp}$$ Then we put table 1 in terms of mass units by mapping

$$a_\mu \to {m_e \over (a_\mu-a_e+a^{vp}) }$$

And we get the table

80.418 GeV
80.417 GeV
80.420 GeV
80.417 GeV
80.396 GeV
80.407 GeV

Now, the most current evaluation from LEP EW group for W mass is 80.392 GeV, and so the result closest to M_W in our remapped table happens to be also the result closest to $$a^{exp}_\mu$$ in Passera's table.