Particle physics minimalism for the 2010s

Physics today has a standard idea of what lies beyond the standard model - supersymmetry, grand unification, string theory - and such models tend to be very complicated. But occasionally someone constructs what I call a neo-minimalist theory of everything - adding the bare minimum to the standard model that is required to incorporate new data from cosmology, astrophysics, and neutrino physics.

To my mind, the two leading neo-minimal models are the "new minimal standard model" (NMSM) and the "neutrino minimal standard model" (nuMSM). To the standard model, the NMSM adds two right-handed neutrinos and two scalars (an inflaton and a dark matter particle). The nuMSM just adds three right-handed neutrinos, and proposes to use the Higgs as the inflaton.

There is essentially just one paper about the NMSM, but it describes how the NMSM addresses all the data. The nuMSM is elaborated across many papers.

It's probably time that new minimal models were made, in the light of new data. The observed Higgs is just below the range of allowed masses in the NMSM (when considered as a theory valid all the way to the Planck scale), while the nuMSM managed to predict the Higgs mass, on the hypothesis of special boundary conditions at that scale. We now have a variety of possible dark-matter signals, though none of them are confirmed. (I find it intriguing that CDMS-II may be seeing something of mass O(10 GeV), like one of the unstable heavy neutrinos in the nuMSM.) We have new cosmological constraints on inflation and on the composition of the universe.

Both NMSM and nuMSM explain dark energy via a cosmological constant. I think minimalism should also consider quintessence - dark energy from a new scalar field.

Galactic problems

I've started to list the possible signals of dark matter physics, and have even put forward a schematic model. What about these galactic issues? Wikipedia lists two basic problems for cold dark matter, the cuspy halo problem and the dwarf galaxy problem.

Pavel Kroupa goes into some detail about the problems for cold dark matter at scales of 10 megaparsecs and less - a scale which he characterizes as being a success for MOND (modified Newtonian gravity) and a failure for CDM. Nonetheless, I expect that for now, I will instead be interested in whether "partly interacting dark matter" can deal with the problems. This is the idea that, while most of the dark matter is homogeneous, a sector of it - perhaps as big as the baryonic sector - also forms structures: "dark atoms", or even larger objects.

Ingredients II

To kick things off, here are some arbitrary speculations about how to match all this data.

For the positron excess, I shall assume a supersymmetrized version of "A Theory of Dark Matter". i.e. MSSM + a new dark sector.

For the 130 GeV gamma-ray line, we could try a higgsino LSP with a mass mysteriously close to that of the Higgs.

And for CDMS's 8 GeV particle... how about a right-handed neutrino? There are right-handed neutrinos with O(10 GeV) mass in the "neutrino minimal standard model".

Having started with that combination of ideas, a serious model-builder would then sit down privately and see if that combination is even possible. But for now I'll just put it out there, and add comments as they occur to me.

Ingredients for a model

If I was a dark-sector model builder, right now I would be trying to make a model which accounts for CDMS's 8 GeV events, Weniger's 130 GeV gamma-ray line, and PAMELA/AMS's positron excess - along with the complexities of galactic dark matter, which I do not have a handle on, but which I keep hearing are problematic for simple models of the dark matter halo. I suppose the model-building starting point would be a combination of "A Theory of Dark Matter" and "Double-Disk Dark Matter".

And models, too

I will also want to make a list of relevant theoretical options and considerations. For example, "A Theory of Dark Matter" has been a popular paper, describing a nontrivial, nontraditional dark sector. It might be an interesting exercise to second-guess Nima Arkani-Hamed, by combining "simply unnatural supersymmetry" with "dark matter with Sommerfeld enhancement" (two ideas to which he contributed), and then trying to fit all the data.

Dark data and neutrino data

Some day I will start summarizing all the data and maybe-data that we have about dark matter and about neutrinos.

There's Weniger's 130 GeV line (and maybe a 111 GeV line too); the positron flux detected by PAMELA and confirmed by AMS-02; the constraints from LHC and from various direct-detection experiments; the astrophysical data on rotation curves of different galaxies, on the history of structure formation, even CMB should have something to say about DM... and then there's neutrino-world, including the apparently contradictory ground-based experiments looking for a fourth neutrino species.

But today is not the day when I will start collating all those considerations. That's a task for the future.