Scalars, neutrinos, and their roles

Last week's idea was to get dark matter and dark energy from the excitations and the VEV of the scalar field in a scalar-tensor theory of gravity. However, we could return to the WDM idea of dark matter as keV-mass neutrinos, and still have a use for this scalar field.

In fact, let me list some of the uses of scalars. They can provide mass to fermions (Higgs mechanism). They can provide dark matter (scalar particles). They can provide dark energy (VEV). They can drive inflation (inflaton field).

An ultimate minimalism might be a scalar-tensor gravity in which the scalar field is simultaneously the Higgs field, the inflaton field, and the source of dark matter and dark energy. At the other extreme we could have a separate scalar for each of the following: scalar component of gravity, providing mass to SM fermions, providing mass to sterile neutrinos, inflation, dark matter, dark energy. And in between are multi-scalar theories in which some or all of the scalars perform multiple functions.

So what I'm thinking is that there could be a whole spectrum of conservative extensions of (standard model + gravity), defined by number of sterile neutrinos, number of scalars, and assignment of roles to these new fields.

Dark minimalism

I was talking with a friend about how the Bullet Cluster is supposed to favor dark matter theories over modified gravity theories like MOND, and opined that the distinction isn't absolute. I started with the example of supergravity: the gravitino is part of the gravitational superfield, yet it is also a candidate for DM particle. Then it occurred to me that perhaps the scalar in a scalar-tensor theory of gravity could also be the dark matter. And just now, I've thought: what if dark energy is the VEV of some field, and dark matter is the quanta of the field?

So this unified minimal model of the dark sector is, a quantized scalar-tensor theory of gravity, in which the scalar component has a VEV (the dark energy) and its quanta are the dark matter. It wouldn't surprise me if this exact model already exists in the literature. The question is whether there are some obvious or non-obvious reasons why it's wrong...

3+3

Having pondered LUX's null result for a while, I am inclined to think that keV-scale warm dark matter is the simplest model of dark matter now, and that the simplest model of that, would be a keV-mass right-handed neutrino.

I see that back in June I talked about a "3+4" neutrino paradigm for dark matter. If the CDMS signal was just background, I guess the paradigm should shrink to 3+3.

I was never too committed to this approach in the first place. It was just a starting point, with many details lacking. For example, I had no model for how the O(100) and O(300) GeV signals might be produced by right-handed neutrinos. I still don't. But until I have a better idea of comparable concreteness, it stands as the official theory of this blog...

Naturalino dark matter from top partners

Naturalino is my name for dark matter which also makes the extended standard model "natural" - a play on the common dark matter candidate, "neutralino". And a top partner is a new particle that is somehow associated with the top quark - for example, the stop squarks of the MSSM.

A few days ago I began to contemplate the possibility of naturalinos as top partners, ever since I realized that the top or its partners are the common element to otherwise dissimilar schemes for explaining the mass of the Higgs.

Today on the arxiv we have a paper on dark matter models containing "a singlet dark matter particle with cubic renormalizable couplings between standard model particles and 'partner' particles with the same gauge quantum numbers as the standard model particle.. We focus on the case of dark matter interactions with colored particles."

And today we also have a second paper aiming to obtain naturalness from a scale-invariant extension of the standard model where the Higgs is coupled to a dark matter multiplet.

Neither is quite what I had in mind, but if you jam them together and rearrange the parts...

I will also report a recent paper adding new neutrinos to Lambda CDM, producing the "nu-LCDM".

Conformally coupled Higgs inflation

This is another possibility which should interest minimalists: In a theory that contains gravity and the Higgs, a "non-minimal coupling" between the two fields, usually designated by the coefficient "xi", is possible. By default, xi is normally set equal to zero. In theories of Higgs inflation, xi is normally set to a large value of several thousand. But for a special, small but nonzero, value of xi, the Higgs-curvature coupling becomes conformal. Is it possible, perhaps with the right UV physics, to get successful Higgs inflation from conformal coupling to gravity? The main interest of this question lies in the possibility that the underlying fundamental theory has a conformal symmetry broken by quantum corrections, perhaps as contemplated by Meissner and Nicolai, and Bars, Steinhardt, and Turok. In this discussion thread, I have collected some technical resources on Higgs inflation which may help to clarify the issue.

Towards a minimal natural standard model

Apart from neutrino masses and dark matter, the standard model matches most of the particle data. But it has a "naturalness" problem (and a hierarchy problem and a finetuning problem; I need to understand the nuances of their relations better).

When discussing minimalism, I mentioned the "new minimal standard model". The authors say explicitly that they will not care whether their parameters are finetuned, they will only care about matching the data.

In this era of absent supersymmetry, revisionist perspectives on naturalness and the hierarchy problem are gaining ground. But it's also true that finetuning can be eliminated without introducing the whole supersymmetric spectrum. All you need are "top partners" and maybe higgsinos.

In the discussion of Weniger's 130 GeV gamma ray line, the higgsino is one of the least popular candidates; nonetheless, there are a handful of papers trying to get dark matter, and the line, from higgsinos. So today's question is this: Can we construct a natural extension of a minimal beyond-standard-model theory, in which a higgsino-like particle both supplies the dark matter, and helps to stabilize the mass of the Higgs?

Partly interacting warm dark matter

Perhaps this is a better way to approach it: the dark matter is mostly "warm dark matter" with a keV-scale mass, but there is an interacting dark-matter subsector about as big as the baryonic sector, which is the source of all the GeV-scale signals.

Towards a 3+4 minimal model

I have a framework now: standard model, plus four right-handed neutrinos, plus a new scalar for inflation.

The four new neutrinos are there to provide dark matter on four scales: keV for the "warm dark matter" that may be the bulk of it, O(10) GeV for the CDMS signal (Dan Hooper also argues that there are galactic-center emissions consistent with a 10 GeV particle), O(100) GeV for the Fermi line, O(300) GeV for the PAMELA/AMS positron excess.

At this point I have little idea how well this will fare. I am especially unsure of whether GeV-mass sterile neutrinos are suitable for producing any of the DM signals; but there's lots more to worry about too. The point is just that it's a place to start, a framework to test. It can be replaced with something more complex if it just can't work.

Expect progress in developing this framework to be slow-to-nonexistent.

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.

Postmortem on CDF's Wjj anomaly

CDF's Wjj anomaly was the occasion for the creation of this blog. But its failure to show up at D0 quickly led people to assume it was a mistake. Now via Jester we learn that the postmortem is almost done - papers giving the official analysis are on their way. For now, see this talk - discussion starts around slide 49, a summing up appears on slide 75.

The new austerity

First the LHC tells us that the Higgs has no properties inconsistent with the standard model, and now Planck tells us that it looks like the universe began with the simplest sort of inflation. Is it apt that this conceptual austerity is coming from Europe, as it also struggles with the new economic austerity?

A new hope for orthodoxy?

The plan was, build the LHC, observe beyond-standard-model particles, and then happily figure out the new physics. It could still happen... but for now, the LHC is just saying "Higgs".

Andrew Oh-Willeke has a post summarizing different responses to this situation, especially as it pertains to supersymmetry. For the hardbitten supersymmetry skeptic, it's confirmation of what was suspected all along. For traditional supersymmetry diehards, it's just incremental progress, excluding some more parameter space. And if you're Nima Arkani-Hamed, it means it's time to think about explaining the hierarchy problem anthropically, while retaining supersymmetry for other reasons.

Peter Woit speculates that, if no new particles show up, the future of physics could be a new dogma combining split supersymmetry with anthropic finetuning, according to which the superpartners like gluinos are out there, but at undetectably high energies. As for myself, the Shaposhnikov-Wetterich prediction of the Higgs mass impresses me - perhaps that is the future, and the anthropic Higgs will just be a fad.

Meanwhile, however, it seems that astronomy is yielding so much data, that the phenomenologists will be able to justify their work without too much of a paradigm shift. Early-universe cosmology already provides a formidable constraint on particle models. We have dark matter to account for. The "130 GeV gamma ray line" might be a direct sign of dark matter annihilation.

Now two new papers suggest to me that there is just so much data coming from astrophysics and cosmology, that it can begin to play the role that the LHC was supposed to play - as the main source of new physics data, that constrains the baroque constructions of the phenomenologists. (The LHC also constrains their work, but so long as it shows nothing but standard model, it plays only a negative role.)

First, Archidiacono et al have produced new fits of both "3+1" and "3+2" extensions of the SM's neutrino sector, to a combination of cosmological and more traditional observations. I don't know if it's the first time, but my previous impression had been that, on the one hand, you could constrain neutrino physics using cosmological data, and on the other hand, the "short base-line" observations were yielding a vexed, contradictory set of results. Apparently it's possible to aggregate everything after all.

Second, Hooper and Slatyer claim that the "low-galactic-latitude" emissions from the "Fermi Bubbles" that lie above and below the plane of the Milky Way, require some new physical mechanism, possibly dark matter annihilation. But it's coming at an order of magnitude lower than the 130 GeV line, so it would have to be some other aspect of dark-sector physics that's on display here.

Astrophysical observation has been a factor in particle physics theory for a long time now. But what I'm claiming here, is that there is now so much to explain, that the future dystopia of untested anthropic dogma as the new physics consensus may be averted, because even the builders of anthropic, split-supersymmetric models will want to account for this increasingly complex-looking dark sector.

Perhaps we can say that the two great empirical sources of fundamental new information, bearing on particle physics, are now neutrino physics and dark matter observation - with CMB observation a close third. CMB observation is entirely cosmological; dark matter is originally and mostly a matter of astronomical observation, though we are trying to directly detect it on Earth as well; and in neutrino physics, as we have seen, cosmology and traditional methods are playing an equal role.

There may be other surprises in store. I have a somewhat frivolous blog which toys with particle "numerology" of a sort which "alternative physicists" love, but which the mainstream rejects as coincidence, for its own reasons. Conceivably, patterns in the existing data, such as the particle masses, will yield a new stage of progress in physics, all by themselves. And of course, new particles may yet turn up at the LHC, in its next period of activity, starting two years from now. But my chief claim here is that, even if neither of those things happen, the wealth of data from the universe will be enough to keep theoretical physics an empirical discipline - even its anthropic and supersymmetric "sector".

Note: In a discussion of Hooper and Slatyer's paper, Lubos Motl dubs the two regions of the Fermi Bubbles "polar" and "tropical". "Polar" corresponds to high galactic latitudes, "tropical" to low galactic latitudes. The possible dark matter signal is "tropical" in origin... which fits the original theme of this blog.