CAMB and CLASS disagree on lensing-polarization cross spectrum

[hersle]

Hi!

I am unable to make CAMB and CLASS to agree on $C_l$ for lensing-polarization with default-ish precision parameters. Here is a script to reproduce my comparison (run in an empty directory, and modify class and camb to executable paths if not in $PATH):

echo '
output_root = camb
scalar_covariance_output_file = cl.dat
feedback_level = 1
get_scalar_cls = T
get_transfer = F
do_lensing = T
l_max_scalar = 2500
CMB_outputscale = 7.42835025e12

# recommended in inifiles/params.ini
k_eta_max_scalar = 10000
accuracy_boost = 2
l_accuracy_boost = 2

# same minimal model parameters as CLASS
hubble = 70.0
temp_cmb = 2.7
ombh2 = 0.02
omch2 = 0.12
omk = 0
massless_neutrinos = 0.0
omnuh2 = 0.0
massive_neutrinos = 0
scalar_amp = 2e-9
scalar_spectral_index = 1.0
helium_fraction = 0.25
reionization = F
recombination_model = Recfast
' > camb.ini

echo '
root = class
overwrite_root = yes
background_verbose = 2
output = tCl,pCl,lCl
format = camb

# same minimal model parameters as CAMB
H0 = 70.0
T_cmb = 2.7
omega_b = 0.02
omega_cdm = 0.12
omega_k = 0.0
N_ur = 0
N_ncdm = 0
A_s = 2e-9
n_s = 1.0
YHe = 0.25
reio_parametrization = reio_none
recombination = recfast
' > class.ini

echo "Running CLASS:"
class class.ini

echo -e "\nRunning CAMB:"
camb camb.ini

echo -e "\nPlotting CLASS vs CAMB to out.png:"
gnuplot -e "
	set terminal png size 1600, 800;
	set output 'out.png';
	set xlabel 'l';
	set ylabel 'Dₗ (lensing × polarization)';
	plot 'camb_cl.dat' using 1:9 with lines, 'class_cl.dat' using 1:8 with lines;
"
xdg-open out.png

Output:

Running CLASS:
Running CLASS version v3.2.3
Computing background
Chose ndf15 as generic_evolver
 -> age = 13.648033 Gyr
 -> conformal age = 14230.579653 Mpc
 -> N_eff = 0 (summed over all species that are non-relativistic at early times) 
 -> radiation/matter equality at z = 5877.103771
    corresponding to conformal time = 86.574974 Mpc
 ---------------------------- Budget equation ----------------------- 
 ---> Nonrelativistic Species 
-> Bayrons                        Omega = 0.0408163       , omega = 0.02           
-> Cold Dark Matter               Omega = 0.244898        , omega = 0.12           
 ---> Relativistic Species 
-> Photons                        Omega = 4.86065e-05     , omega = 2.38172e-05    
 ---> Other Content 
-> Cosmological Constant          Omega = 0.714237        , omega = 0.349976       
 ---> Total budgets 
 Radiation                        Omega = 4.86065e-05     , omega = 2.38172e-05     
 Non-relativistic                 Omega = 0.285714        , omega = 0.14            
 Other Content                    Omega = 0.714237        , omega = 0.349976        
 TOTAL                            Omega = 1               , omega = 0.49            
 -------------------------------------------------------------------- 

Running CAMB:
Om_b h^2             =  0.020000
Om_c h^2             =  0.120000
Om_nu h^2            =  0.000000
Om_darkenergy        =  0.714237
Om_K                 =  0.000000
Om_m (inc Om_u)      =  0.285714
100 theta (CosmoMC)  =  1.195719
Age of universe/GYr  =  13.648
zstar                =  1101.75
r_s(zstar)/Mpc       =  165.07
100*theta            =  1.186388
DA(zstar)/Gpc        =  13.91371
zdrag                =  1064.92
r_s(zdrag)/Mpc       =  168.56
k_D(zstar) Mpc       =  0.1307
100*theta_D          =  0.172743
z_EQ (if v_nu=1)     =  5877.11
k_EQ Mpc (if v_nu=1) =  0.013532
100*theta_EQ         =  0.622227
100*theta_rs_EQ      =  0.350612
tau_recomb/Mpc       =  317.21  tau_now/Mpc =  14230.6

Plotting CLASS vs CAMB to out.png:

Do you know the reason for this disagreement and/or how it can be resolved? Thank you!

[hersle]

Also cross-posted in cmbant/CAMB#186.