Tag Archives: neutrinos

Dark Matter: Made of Sterile Neutrinos?


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..



Antimatter is not Dark Matter: Antimatter secrets uncovered with nuclear reactors

Dark matter is much more abundant than ordinary matter. On the other hand, antimatter is much rarer in the universe than ordinary matter. Antimatter is not dark matter. In general dark matter refers to something other than antimatter, although it would be possible to have dark matter that is anti-dark matter. See this prior post https://darkmatterdarkenergy.com/2011/06/04/dark-matter-powered-stars/

Antimatter refers to matter that is similar to ordinary matter but has the opposite electrical charge from what is seen in regular matter. Electrons have a negative charge of -1, positrons, which are anti-electrons, have a positive charge of +1. Similarly, protons posses a charge of +1, and antiprotons have a charge of -1. Inside protons and neutrons there are quarks. There can also be antiquarks and so on.

When a particle and its associated anti-particle get too close to one another they mutually annihilate and all of their rest mass energy is converted to radiation or other particles, in accordance with E = mc2. For example, the electron has a rest mass of 511 keV (1 keV is one thousand electron-Volts, where the energy of 1 eV is that of moving an electron through a potential of one Volt.) When an electron and positron (anti-electron) annihilate, two gamma rays are produced each with energy around 511 keV. See the figure below, which is the Feynman diagram for the interaction. In the case of electron-positron annihilation, this is the only outcome possible due to the low energy of the two annihilating particles.


Mutual annhilation of an electron and positron yielding two gamma rays at 511 keV each

The big mystery is why there is matter in the universe at all! Why did not the Big Bang produce equal amounts of matter and anti-matter? In such a case the matter and anti-matter mutual annihilation process could have left little or no matter behind, and stars, galaxies, planets and people could not have formed. Cosmologists and particle physicists believe there was some small excess of matter over anti-matter, such that our present amount of matter remained after all the annihilation processes were finished.

This excess of matter over anti-matter is thought to be due to some asymmetry in the laws of physics. In general the laws are highly symmetric. Particle physicists look to understand the degree and nature of any putative asymmetries. One way to do this is by studying neutrinos, very low mass electrically neutral particles which are signatures of the weak nuclear force and products of radioactive decay. The neutrino mass is less than 2 eV, much, much less than the already small electron mass. There are believed to be 3 types of neutrinos – electron neutrinos, muon neutrinos and tau neutrinos – which are in turn associated with the electron, muon and tau particles; the muon and tau are ‘heavy’ members of the electron family.

If the neutrino has non-zero mass, then through a quantum effect known as “neutrino oscillation”, the different types of neutrinos mix together. This is due to the wave nature of all particles in quantum mechanics. Neutrinos have been detected from the Sun for many years, but at a much lower rate than initially expected, which was an outstanding puzzle. The “neutrino oscillation” mechanism resolves the discrepancy. Also, differences in neutrino and antineutrino interactions, which are due to neutrino oscillation, are thought by many particle physics to be related to the excess of matter over antimatter in the universe.

There are 3 parameters of the “neutrino oscillation” theory, which are known as ‘mixing angles’, and two of these, θ12 and θ23, have been reasonably well measured. The third mixing angle, known as θ13, is has not been well measurable until very recently.

Particle physicists working as part of the US-Chinese collaboration at the Daya Bay experiment have announced in March 2012 a positive result for the third mixing angle. It is based on measurements made near two nuclear reactors in China, one at Daya Bay and one at Ling Ao. Nuclear reactors are strong sources of antineutrinos. Another similar experiment, known as RENO, is based at a six-reactor nuclear power site in Korea. As of April 2012 the RENO physicists are also claiming a positive measurement of the θ13 mixing angle parameter, with a similar level of statistical confidence in excluding the zero value hypothesis.

Both experiments are indicating a value of around 0.10 for the mixing angle parameter, satisfying the expression sin2 (2θ13 ) = 0.1.

Other experiments include T2K in Japan, MINOS in the US and the Double Chooz international collaboration based in France. All three are seeing hints of a positive value of θ13 as well, but none have reached the statistical confidence level of the Daya Bay and RENO experiments.

The value being measured is surprisingly large, and thus very supportive of the neutrino oscillation theory for the matter vs. anti-matter discrepancy. These are exciting times for oscillating neutrinos and these experiments are moving us to closer to solving the antimatter quandry!



http://www.nu.to.infn.it/exp/all/reno/ – RENO neutrino experiment, Korea

http://theory.fnal.gov/jetp/talks/RENO-results-seminar-new.pdf – Presentation on RENO results

Wikipedia articles on antimatter, annhilation and the neutrion oscillation: