Sterile neutrino in Planck 2013 analysis

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Ana Vasile
Posts: 25
Joined: March 26 2006
Affiliation: Institute for Space Sciences
Contact:

Sterile neutrino in Planck 2013 analysis

Post by Ana Vasile » June 25 2013

Dear all,

I am having trouble understanding the sterile neutrino analysis performed in CosmoMC March 2013 (Planck 2013 results).

More specifically:
In CMB_Cls_simple.f90 what is the physical significance of the comparison: If \Omega_{\nu}h^2 > \Omega_{\nu}h^2_{sterile} (or else ...)?

CMB_Cls_simple.f90

Code: Select all

if (CMB%omnuh2>CMB%omnuh2_sterile) then
            neff_massive_standard = num_massive_neutrinos*default_nnu/3
            P%Num_Nu_Massive = num_massive_neutrinos
            P%Nu_mass_eigenstates=P%Nu_mass_eigenstates+1
            if (CMB%nnu > neff_massive_standard) then
                P%Num_Nu_Massless = CMB%nnu - neff_massive_standard
            else
                P%Num_Nu_Massless = 0
                neff_massive_standard=CMB%nnu
            end if
            P%Nu_mass_degeneracies(P%Nu_mass_eigenstates) = neff_massive_standard
            P%Nu_mass_fractions(P%Nu_mass_eigenstates) = (CMB%omnuh2-CMB%omnuh2_sterile)/CMB%omnuh2
else
            neff_massive_standard=0
end if
Moreover, the case when \Omega_{\nu}h^2 > \Omega_{\nu}h^2_{sterile} seems to be valid all the time when a sterile neutrino is included, since in Params_CMB.f90, \Omega_{\nu}h^2_{sterile} has been added to \Omega_{\nu}h^2


Params_CMB.f90:

Code: Select all

CMB%omnuh2 = CMB%omnuh2 + CMB%omnuh2_sterile


I would really appreciate some help to better understand the physics behind this.

Thank you,

Ana

Antony Lewis
Posts: 1379
Joined: September 23 2004
Affiliation: University of Sussex
Contact:

Re: Sterile neutrino in Planck 2013 analysis

Post by Antony Lewis » June 25 2013

Given the values of the total neutrino density, sterile density and total N_eff it is trying to assign values to the standard neutrinos. By default the standard netrinos are assumed to have standard thermal history (i.e. heated slightly at electron-positron annihilation, so N_eff = 3.046/3 per species). And the analysis assumes that the standard neutrinos have the minimal hierarchy, so there's one eigenstate with 0.06eV mass (so \Omega_nu h^2 is greater than just the sterile contribution). The steriles are assumed not to be heated at electron-positron annihilation.

The non-integer N_eff (slight heating) of massive neutrinos is handled by assignining it to the eigenstate degeneracy parameter: one eigenstate with non-integer degeneracy factor is equivalent to changing N_eff because everything is a function of m/T.

Ana Vasile
Posts: 25
Joined: March 26 2006
Affiliation: Institute for Space Sciences
Contact:

Sterile neutrino in Planck 2013 analysis

Post by Ana Vasile » June 25 2013

Thanks Antony!

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