### BSM

This morning brought about another suggestive (if I may be so bold so say that) experimental hint of new physics in the leptonic sector in the form of a paper from the MiniBooNE collaboration: “Unexplained Excess of Electron-Like Events From a 1-GeV Neutrino Beam (arXiv:0812.2243).”

Recall that the past two months have also brought us a speculative “multi-muon anomaly” at CDF (arXiv:0810.5357, see also Tommaso’s summary), the publication of the PAMELA cosmic-ray positron excess (arXiv:0810.4995, ), and related publications by ATIC4 (Nature) and HESS (arXiv:0811.3894) on the electron/positron spectrum. Apparently the leptonic sector has decided to be kind (if coy) to model-builders in light of LHC delays. Now, MiniBooNE joins in on the fun.

MiniBooNE neutrino low-energy excess. Image from arXiv:0812.2243.

For an excellent summary of the MiniBooNE experiment, see Heather Ray’s post on Cosmic Variance. (Unfortunately their TeX didn’t transfer over well since they moved to Discover… hopefully someone over there will fix up all the LaTeX tags that are now garbled?)

As I’m writing this Symmetry Breaking has published a post on the result that summarizes the recent news. Here’s my own quick-and-dirty summary as I understand it:

In April 2007, MiniBooNE published results that showed no signs of the LSND anomaly (hep-ex/0104049), leading many model-builders to immediately jump off the neutrino band-wagon (see Jester’s theory report). They noted, however, a curious excess in their data at lower energies, in an energy region that was not (at least on face value) related to the unrequited LSND hint for new physics. This was left for further investigation and data analysis.

Now after more than a year of said investigation and analysis, the excess is still there. (See image above.) What’s even more interesting, is that the bump does not appear as pronounced in the antineutrino sector, according to a recent report (see image below). LSND and the fresh-on-the-arXiv MiniBooNE paper were analyses based on neutrinos. It’s a bit surprising that the MiniBooNE antineutrino analysis doesn’t have a similar feature

MiniBooNE antineutrino data showing a much weaker signal at low energies compared to the neutrino data. Image from Fermilab.

I hope to spend some time reading up on this over the holidays, I should then be able to give a more coherent summary.

The Standard Model is a chiral theory; left- and right-handed fermions (i.e. -/+ eigenstates of the chirality operator $\gamma_5$) live in different representations of the SM gauge group. This poses a rather rigid constraint on what kind of model becomes effective at the TeV scale.
Chirality prevents the use of low-scale models with multiple supersymmetries ($\mathcal N>1$), since this means one would be able to take a spin +1/2 fermion $\psi$ and expect to find a spin -1/2 fermion $Q_1Q_2\psi$ in the same supermultiplet (i.e. with the same gauge quantum numbers).
In extra dimensional models, the lack of a chiral operator in 5 dimensions (and more generally for most higher dimensions) stunted the development of KK models until the 80s. In a nutshell, there exists no chirality operator in five dimensions ($\gamma_5$ is just an ‘ordinary’ gamma matrix) and hence all fermions are Dirac rather than Weyl. This has led to lots of work with orbifolds and boundary conditions. [It might be neat to think about how such boundary conditions for different backgrounds could come from string theory.]