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Sterile neutrino in Planck 2013 analysis
 
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Ana Vasile



Joined: 26 Mar 2006
Posts: 25
Affiliation: Institute for Space Sciences

PostPosted: June 25 2013  Reply with quote

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 Ωνh2 > Ωνh2sterile (or else ...)?

CMB_Cls_simple.f90

Code:
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 Ωνh2 > Ωνh2sterile seems to be valid all the time when a sterile neutrino is included, since in Params_CMB.f90, Ωνh2sterile has been added to Ωνh2


Params_CMB.f90:

Code:
CMB%omnuh2 = CMB%omnuh2 + CMB%omnuh2_sterile




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

Thank you,

Ana
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Antony Lewis



Joined: 23 Sep 2004
Posts: 1278
Affiliation: University of Sussex

PostPosted: June 25 2013  Reply with quote

Given the values of the total neutrino density, sterile density and total Neff 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 Neff = 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 Ωnu h2 is greater than just the sterile contribution). The steriles are assumed not to be heated at electron-positron annihilation.

The non-integer Neff (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 Neff because everything is a function of m/T.
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Ana Vasile



Joined: 26 Mar 2006
Posts: 25
Affiliation: Institute for Space Sciences

PostPosted: June 25 2013  Reply with quote

Thanks Antony!
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