Photometric neutrino-response regression#

Summary for collaborators#

The large new photometric neutrino discrepancies are explained, for the tested case, by a change in the CLASS nonlinear cold-dark-matter-plus-baryon (cb) prescription. CLASS was switched to its native nonlinear cb spectrum, while CAMB continued to use a reconstruction from total nonlinear matter power and linear neutrino terms. This was a change of model convention, not just a solver accuracy setting.

Small differences in fiducial power do not guarantee small differences in its neutrino-mass derivative. In our local control, fiducial spectra differ by only about 0.02–0.08% in median, but native CLASS cb has substantially different mass responses on nonlinear scales. Those responses enter the Fisher matrix and change parameter degeneracies.

Restoring the matched reconstruction in CLASS, together with its previous mass-to-density convention, reduces the maximum marginalized-error discrepancy in case 03.2.0 from 21.38% to 5.5266%, reproducing the historical passing forecast. We retained the new supported-grid checks throughout this test.

Interpretation: the comparison must use a declared cb prescription on both sides. The experiment does not establish that native CLASS cb is physically incorrect or that the reconstruction is more accurate. It establishes why the new forecast no longer reproduced the earlier matched-backend validation. The remaining affected Fisher cases still need confirmation on Frey.

What are the three spectra being compared?#

Let \(f_{cb}=\Omega_{cb}/\Omega_m\) and \(f_\nu=\Omega_\nu/\Omega_m\). The historical reconstruction is

\[ P_{cb}^{\mathrm{rec}}(k,z) = \frac{P_{mm}^{\mathrm{NL}}(k,z) - 2 f_{cb} f_\nu P_{cb,\nu}^{\mathrm{L}}(k,z) - f_\nu^2 P_{\nu,\nu}^{\mathrm{L}}(k,z)}{f_{cb}^2}. \]

The superscripts L and NL denote linear and nonlinear quantities. The linear cross and neutrino-auto terms are obtained from each solver’s own outputs. This reconstruction approximates those neutrino terms as linear; it is not the same prescription as directly requesting native nonlinear cb.

Curve	Total matter / neutrino inputs	cb prescription
CAMB reconstruction (reference)	CAMB outputs	Equation above
CLASS reconstruction	CLASS outputs	Same equation
CLASS native cb	CLASS nonlinear cb API	`get_pk_cb_array(..., nonlinear=True)`

The main three-way comparison uses density factor 94.07 for both solvers to isolate the prescription change. A separate control repeats CLASS with 93.14. All comparisons hold the input total-matter sigma8 fixed at 0.81: they are not derivatives at fixed primordial amplitude As.

The interactive comparison notebook plots fiducial spectra, percent residuals, \(\partial P_{cb}/\partial M_\nu\), \(\partial\ln P_{cb}/\partial M_\nu\), and derivative differences at four redshifts. It uses the saved nine-run spectrum control by default and offers explicit solver regeneration from the committed case configuration.

Local diagnosis (12 October 2026)#

The October CAMB/CLASS campaign changed passing photometric neutrino cases into large discrepancies, while CAMB errors remained consistent with the historical dashboard. Two CLASS changes in commit 399ae0a were isolated:

native nonlinear cb spectra replaced the matched linear-neutrino subtraction approximation still used by CAMB;
the default mass-to-density factor changed from 94.07 to 93.14.

These are distinct numerical/physical conventions, not interchangeable ways of computing the same derivative. The diagnosis does not establish that native CLASS cb is physically wrong; it establishes that it is not the matched recipe used by the passing comparison.

Spectrum control#

Using saved 03.2.0 settings, fixed sigma8 and masses 0.054, 0.060, 0.066 eV, the local experiment sampled four redshifts (0.5, 1, 1.5, 2) and 60 physical wavenumbers from 0.001 to 10 inverse Mpc. Both CLASS prescriptions were evaluated from each solver run. The reference is CAMB’s subtraction prescription. The central response is the finite difference of log power with respect to mass. Here \(M_\nu\) denotes the summed neutrino mass, represented by the mnu parameter:

\[ \left.\frac{\partial\ln P}{\partial M_\nu}\right|_{\sigma_8} \simeq \frac{\ln P(0.066)-\ln P(0.054)}{0.012\ \mathrm{eV}}. \]

The notebook also computes the direct finite difference of P, without taking its logarithm. This is one finite-difference step, not a step-convergence study. Both k and power use physical units: inverse Mpc and cubic Mpc, respectively.

CLASS prescription	Density factor	Median fiducial difference	Median response difference	Maximum response difference
Native cb	93.14	0.07482%	3.31424%	100.69628%
Subtraction	93.14	0.02405%	2.78695%	5.52321%
Native cb	94.07	0.03983%	3.10270%	100.35201%
Subtraction	94.07	0.01940%	1.82190%	4.82212%

Relative derivative differences can amplify near a small reference response. However, the discrepancy is not just one such point: with factor 94.07, the native prescription has median response differences of 18.83% over 0.1–1 inverse Mpc and 55.23% over 1–10 inverse Mpc. The corresponding subtraction differences are 3.02% and 3.51%. The restored result reproduces the earlier spectrum-response diagnostic.

One-Fisher confirmation#

Only CLASS case 03.2.0 was rerun, preserving the saved fiducial parameters, free parameters, solver profile and survey settings. Machine-specific paths were relocated. Its CAMB reference is the existing October run, not a new CAMB calculation. The corrected CLASS run took approximately 738 seconds with four OpenMP threads.

Maximum marginalized error deviation: 21.38% before → 5.5266% after.
Sigma8 error deviation after correction: 2.2720%.
The largest remaining deviation is mnu (5.5266%).
Corrected CLASS errors reproduce the historical passing dashboard to displayed precision; this case is below its 10% acceptance threshold.

The supported nonlinear-grid hardening was retained in this confirmation. The remaining affected cases have not yet been rerun with the correction.

Backend convention#

For CLASS-native nonlinear models, the default NONLINEAR.cb_prescription is neutrino_subtraction: total nonlinear matter power minus linear neutrino cross and auto terms, divided by the squared cb fraction. This matches CAMB’s existing approximation. To deliberately request native CLASS nonlinear cb instead:

NONLINEAR:
  model: class_native
  cb_prescription: class_native

The default CLASS PARAMETER_TRANSLATION.neutrino_mass_fac is again 94.07, matching CAMB. Explicit profile overrides (including 93.14 in matched SYREN/EE2 profiles) are preserved. The EE2 cb boost approximation is unchanged.

Reproduction and remaining campaign#

The isolated diagnostic is scripts/validation/diagnose_neutrino_response.py:

OMP_NUM_THREADS=4 OPENBLAS_NUM_THREADS=1 MKL_NUM_THREADS=1 \
uv run python scripts/validation/diagnose_neutrino_response.py \
  --specs /path/to/saved_CLASS_FM_specs.json \
  --outdir scripts/benchmark_results/neutrino_isolation

It saves spectra, translated CLASS parameters, the input hash and numerical summary. Local evidence is under scripts/benchmark_results/ in neutrino_isolation_20261012b/ and neutrino_fixed_03.2.0b/; these generated files are ignored by Git and have not been uploaded to the DVC archive. The archived reference is compare_photo_camb_vs_class_cfg_13fddbddf6_20261007_175905 in cjfp-data (see Validation data and examples (cjfp-data) for access).

After transferring the corrected package code to Frey, rerun the affected cases and the fixed-neutrino/Pmm controls before claiming campaign-wide resolution:

bash scripts/validation/run_parallel_validations.sh \
  --cases 03.2.0,03.2.1,04.2.0,04.2.1,05.2.0,06.2.0,07.2.0,07.2.0.1,08.2.0 \
  --jobs 2 --omp-threads 4 --force

Case 07.2.0.1 is the deliberate Pmm teaching control; it is not expected to pass merely because the matched cb prescription is restored. Review reports before updating the DVC manifest and uploading results.