Too small chi2 in CMS_TTBAR_8TEV_2L_DIF_MTTBAR-YT-NORM

[jacoterh]

The new commentata implementation of dataset CMS_TTBAR_8TEV_2L_DIF_MTTBAR-YT-NORM seems to overestimate experimental uncertainties in the new commondata implementation resulting in a super low chi2=0.0274 per datapoint (ndat = 15). The old commondata implementation did a much better job, giving chi2=1.28 per datapoint. Both chi2 are computed with the fully NNLO TTBAR FK tables from MATRIX.

• Old commondata data-theory comparison
• New commondata data-theory comparison

In these comparisons, current fit refers to the NNLO TTBAR FK tables, while the reference fit refers to theory 708, i.e. NLO + K-factors.

[scarlehoff]

@kamillaurent please have a look at the implementation of the dataset (check that all correlations are ok / bin widths taken into account, etc)