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Tag Archives: sterile neutrinos

Axions, Inflation and Baryogenesis: It’s a SMASH (pi)

Searches for direct detection of dark matter have focused primarily on WIMPs (weakly interacting massive particles) and more precisely on LSPs (the lightest supersymmetric particle). These are hypothetical particles such as neutralinos that are least massive members of the hypothesized family of supersymmetric partner particles.

But supersymmetry may be dead. There have been no supersymmetric particles detected at the Large Hadron Collider at CERN as of yet, leading many to say that this is a crisis in physics.

At the same time as CERN has not been finding evidence for supersymmetry, WIMP dark matter searches have been coming up empty as well. These searches keep increasing in sensitivity with larger and better detectors and the parameter space for supersymmetric WIMPs is becoming increasingly constrained. Enthusiasm unabated, the WIMP dark matter searchers continue to refine their experiments.

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LUX dark matter detector in a mine in Lead, South Dakota is not yet detecting WIMPs. Credit: Matt Kapust/ Sanford Underground Research Facility

What if there is no supersymmetry? Supersymmetry adds a huge number of particles to the particle zoo. Is there a simpler explanation for dark matter?

Alternative candidates under consideration for dark matter, including sterile neutrinos, axions, and primordial black holes, and are now getting more attention.

From a prior blog I wrote about axions as dark matter candidates:

Axions do not require the existence of supersymmetry. They have a strong theoretical basis in the Standard Model as an outgrowth of the necessity to have charge conjugation plus parity conserved in the strong nuclear force (quantum chromodynamics of quarks, gluons). This conservation property is known as CP-invariance. (While CP-invariance holds for the strong force, the weak force is CP violating).

In addition to the dark matter problem, there are two more outstanding problems at the intersection of cosmology and particle physics. These are baryogenesis, the mechanism by which matter won out over antimatter (as a result of CP violation of Charge and Parity), and inflation. A period of inflation very early on in the universe’s history is necessary to explain the high degree of homogeneity (uniformity) we see on large scales and the near flatness of the universe’s topology. The cosmic microwave background is at a uniform temperature of 2.73 Kelvins to better than one part in a hundred thousand across the sky, and yet, without inflation, those different regions could never have been in causal contact.

A team of European physicists have proposed a model SMASH that does not require supersymmetry and instead adds a few particles to the Standard Model zoo, one of which is the axion and is already highly motivated from observed CP violation. SMASH (Standard Model Axion Seesaw Higgs portal inflation) also adds three right-handed heavy neutrinos (the three known light neutrinos are all left-handed). And it adds a complex singlet scalar field which is the primary driver of inflation although the Higgs field can play a role as well.

The SMASH model is of interest for new physics at around 10^11 GeV or 100 billion times the rest mass of the proton. For comparison, the Planck scale is near 10^19 GeV and the LHC is exploring up to around 10^4 GeV (the proton rest mass is just under 1 GeV and in this context GeV is short hand for GeV divided by the speed of light squared).

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Figure 1 from Ballesteros G. et al. 2016. The colored contours represent observational limits from the Planck satellite and other sources regarding the tensor-to-scalar power ratio of primordial density fluctuations (r, y-axis) and the spectral index of these fluctuations (ns, x-axis). These constraints on primordial density fluctuations in turn constrain the inflation models. The dashed lines ξ = 1, .1, .01, .001 represent a key parameter in the assumed slow-roll inflation potential function. The near vertical lines labelled 50, 60, 70, 80 indicate the number N of e-folds to the end of inflation, i.e. the universe inflates by a factor of e^N in each of 3 spatial dimensions during the inflation phase.

So with a single model, with a few extensions to the Standard Model, including heavy right-handed (sterile) neutrinos, an inflation field, and an axion, the dark matter, baryogenesis and inflation issues are all addressed. There is no need for supersymmetry in the SMASH model and the axion and heavy neutrinos are already well motivated from particle physics considerations and should be detectable at low energies. Baryogenesis in the SMASH model is a result of decay of the massive right-handed neutrinos.

Now the mass of the axion is extremely low, of order 50 to 200 μeV (millionths of an eV) in their model (by comparison, neutrino mass limits are of order 1 eV), and detection is a difficult undertaking.

There is currently only one active terrestrial axion experiment for direct detection, ADMX. It has its primary detection region at lower masses than the SMASH model is suggesting, and has placed interesting limits in the 1 to 10 μeV range. It is expected to push its range up to around 30 μeV in a couple of years. But other experiments such as MADMAX and ORPHEUS are coming on line in the next few years that will explore the region around 100 μeV, which is more interesting for the SMASH model.

Not sure why the researchers didn’t call this the SMASHpie model (Standard Model Axion Seesaw Higgs portal inflation), because it’s a pie in the face to Supersymmetry!

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It would be wonderfully economical to explain baryogenesis, inflation, and dark matter with a handful of new particles, and to finally detect dark matter particles directly.

Reference

“Unifying inflation with the axion, dark matter, baryogenesis and the seesaw mechanism” Ballesteros G., Redondo J., Ringwald A., and Tamarit C. 2016  https://arxiv.org/abs/1608.05414

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Dark Matter: Made of Sterile Neutrinos?

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Composite image of the Bullet Group showing galaxies, hot gas (shown in pink) and dark matter (indicated in blue). Credit: ESA / XMM-Newton / F. Gastaldello (INAF/IASF, Milano, Italy) / CFHTLS 

What’s more elusive than a neutrino? Why a sterile neutrino, of course. In the Standard Model of particle physics there are 3 types of “regular” neutrinos. The ghost-like neutrinos are electrically neutral particles with 1/2 integer spins and very small masses. Neutrinos are produced in weak interactions, for example when a neutron decays to a proton and an electron. The 3 types are paired with the electron and its heavier cousins, and are known as electron neutrinos, muon neutrinos, and tau neutrinos (νe, νμ, ντ).

A postulated extension to the Standard Model would allow a new type of neutrino, known as a sterile neutrino. “Sterile” refers to the fact that this hypothetical particle would not feel the standard weak interaction, but would couple to regular neutrino oscillations (neutrinos oscillate among the 3 types, and until this was realized there was consternation around the low number of solar neutrinos detected). Sterile neutrinos are more ghostly than regular neutrinos! The sterile neutrino would be a neutral particle, like other neutrino types, and would be a fermion, with spin 1/2. The number of types, and the respective masses, of sterile neutrinos (assuming they exist) is unknown. Since they are electrically neutral and do not feel the standard weak interaction they are very difficult to detect. But the fact that they are very hard to detect is just what makes them candidates for dark matter, since they still interact gravitationally due to their mass.

What about regular neutrinos as the source of dark matter? The problem is that their masses are too low, less than 1/3 of an eV (electron-Volt) total for the three types. They are thus “too hot” (speeds and velocity dispersions too high, being relativistic) to explain the observed properties of galaxy formation and clumping into groups and clusters. The dark matter should be “cold” or non-relativistic, or at least no more than “warm”, to correctly reproduce the pattern of galaxy groups, filaments, and clusters we observe in our Universe.

Constraints can be placed on the minimum mass for a sterile neutrino to be a good dark matter candidate. Observations of the cosmic microwave background and of hydrogen Lyman-alpha emission in quasar spectra have been used to set a lower bound of 2 keV for the sterile neutrino’s mass, if it is the predominant component of dark matter. A sterile neutrino with this mass or larger is expected to have a decay channel into a photon with half of the rest-mass energy and a regular (active) neutrino with half the energy.

A recent suggestion is that an X-ray emission feature seen at 3.56 keV (kilo-electron Volts) from galaxy clusters is a result of the decay of sterile neutrinos into photons with that energy plus active (regular) neutrinos with similar energy. This X-ray emission line has been seen in a data set from the XMM-Newton satellite that stacks results from 73 clusters of galaxies together. The line was detected in 2 different instruments with around 4 or 5 standard deviations significance, so the existence of the line itself is on a rather strong footing. However, it is necessary to prove that the line is not from an atomic transition from argon or some other element. The researchers argue that an argon line should be much, much weaker than the feature that is detected.

In addition, a second team of researchers, also using XMM-Newton data have claimed detection of lines at the same 3.56 keV energy in the Perseus cluster of galaxies as well as our neighbor, the Andromeda galaxy.

There are no expected atomic transition lines at this energy, so the dark matter decay possibility has been suggested by both teams. An argon line around 3.62 KeV is a possible influence on the signal, but is expected to be very much weaker. Confirmation of these XMM-Newton results are required from other experiments in order to gain more confidence in the reality of the 3.56 keV feature, regardless of its cause, and to eliminate with certainty the possibility of an atomic transition origin. Analysis of stacked galaxy cluster data is currently underway for two other X-ray satellite missions, Chandra and Suzaku. In addition, the astrophysics community eagerly awaits the upcoming Astro-H mission, a Japanese X-ray astronomy satellite planned for launch in 2015. It should be able to not only confirm the 3.56 keV X-ray line (if indeed real), but also detect it within our own Milky Way galaxy.

Thus the hypothesis is for dark matter composed primarily of sterile neutrinos of a little over 7.1 keV in mass (in E = mc^2 terms), and that the sterile neutrino has a decay channel to an X-ray photon and regular neutrino. Each decay product would have an energy of about 3.56 keV. Such a 7 keV sterile neutrino is plausible with respect to the known density of dark matter and various cosmological and particle physics constraints. If the dark matter is primarily due to this sterile neutrino, then it falls into the “warm” dark matter domain, intermediate between “cold” dark matter due to very heavy particles, or “hot” dark matter due to very light particles.

The abundance of dwarf satellite galaxies found in the Milky Way’s neighborhood is lower than predicted from cold dark matter models. Warm dark matter could solve this problem. As Dr. Abazajian puts in in his recent paper “Resonantly Produced 7 keV Sterile Neutrino Dark Matter Models and the Properties of Milky Way Satellites”

the parameters necessary in these models to produce the full dark matter density fulfill previously determined requirements to successfully match the Milky Way galaxy’s total satellite abundance, the satellites’ radial distribution, and their mass density profile..