I Are neutrinos definitely ruled out as Dark Matter?

[snorkack]

What is the net lepton number of the universe?

 

[Orodruin]

I think it's of order a percent, so we expect n_f = 3.03 or so for 3 families of neutrinos. The neutrino density is ~100/flavor/cm^3, and taking 0.2 eV for the sum of the neutrino masses (a guess on the high side), that means its 20 eV/cm^3. The dark matter density is around 600 MeV/cm^3, so to make neutrinos work, you need to increase their density by 30 million. Not only does this impose, as you say, a serious production problem, even second order effects get large.

As I said, the bound on the number of neutrino species from BBN comes from the effect on extra relativistic degrees of freedom affecting the expansion rate during BBN. If there were a non-relativistic component of neutrinos during BBN, they would not contribute to N_eff. Now there are several problems with having such a component, not only would you have to produce it - it would somehow have to avoid coming into thermal equilibrium (neutrinos do not freeze out until around 2 MeV).

 

[stoomart]

The neutrino density is ~100/flavor/cm^3, and taking 0.2 eV for the sum of the neutrino masses (a guess on the high side), that means its 20 eV/cm^3. The dark matter density is around 600 MeV/cm^3, so to make neutrinos work, you need to increase their density by 30 million. Not only does this impose, as you say, a serious production problem, even second order effects get large.

So if I understand this right, SM neutrinos are ruled out as a significant contributor for the observed effects of dark matter, leaving the hypothetical right-handed (sterile) neutrino, which is thought to have an extreme mass range:

The Phenomenology of Right Handed Neutrinos
Neutrino dark matter candidate in fourth generation scenarios

 

[Orodruin]

So if I understand this right, SM neutrinos are ruled out as a significant contributor for the observed effects of dark matter, leaving the hypothetical right-handed (sterile) neutrino, which is thought to have an extreme mass range:

The Phenomenology of Right Handed Neutrinos
Neutrino dark matter candidate in fourth generation scenarios

Additional fermion singlets can essentially have masses anywhere within an extremely large mass range - from tiny masses that effectively make neutrinos pseudo-Dirac particles to the typical high mass implementations of the seesaw mechanism. The phenomenology is quite rich in many parts of the mass range. With respect to dark matter, it is possible that a sterile neutrino around the keV range could be a significant dark matter contributor. It depends a lot on the sterile-active mixing and the right-handed masses. Other mass ranges gives different phenomenology.

 

[nikkkom]

So if I understand this right, SM neutrinos are ruled out as a significant contributor for the observed effects of dark matter

Strictly speaking, SM neutrinos are ruled out in general. Meaning, we know that real neutrinos have mass, and thus they are not SM neutrinos. SM needs to be extended to match neutrino mass observations. This almost always adds new neutrino-like particles.

If you meant "left-handed neutrinos are ruled out as DM", then yes, they are ruled out by cosmological data.

 

[Orodruin]

This almost always adds new neutrino-like particles.

I think "almost always" is too strong to use here. There is no actual need to introduce new standard model singlet fermions. You can just as well add an SU(2) triplet scalar that effectively takes a small vev. This is the type-II seesaw mechanism. Essentially, what new states you need to add would typically depend on how you open the Weinberg operator at higher energies.

 

[nikkkom]

I think "almost always" is too strong to use here. There is no actual need to introduce new standard model singlet fermions.

Yes. That's why I used "almost always" phrase, not "always".