WG2 fid/diff/STXS meeting: ggH stage 1.2 uncertainties

Edward Scott (Imperial College (GB)) , Frank Tackmann, Hongtao Yang (Lawrence Berkeley National Lab. (US))

Please join using zoom link: https://lbnl.zoom.us/j/3455792796

    • 14:00 14:20
      CMS status 20m
      Speaker: Jonathon Mark Langford (Imperial College (GB))
      • Haider asks if CMS correlates the type A and type B uncertainties. Ans: no (at the moment, can be changed). 

      • Kerstin asks a technical question about whether we re-derive the type A and type B uncertainties for each differences signal parameterisation. Ans: no, the only thing that changes between parameterisations is which nuisances are included in the fit. 

      • Some general discussion about the dashed boundaries, their purpose, and the differences of using them wrt the current CMS approach. It is commented that the ATLAS and CMS approaches are for the most part very similar.

    • 14:20 14:40
      Update from ATLAS I 20m
      Speaker: Hui-Chi Lin (National Tsing Hua University (TW))

      Hui-Chi’s talk:

      • Jonathon comments that this confirms that the way CMS and ATLAS derive their uncertainties is very similar

      • Frank makes the point that having some defined source of an uncertainty, such as the migration around a certain boundary, is much more well-defined and therefore more likely to be able to attract e.g. improved calculations from theorists in future. 

      • Frank also asks a technical question about considering >=1 jet bins for the pTH uncertainties on S11. Ans: this has been tried and is being considered, possibility as (or similar to) method 2 as described on the slide.

      Subsequent discussion on the synchronisation:

      • ATLAS proposes a single nuisance for migrations across the pT(Hjj) boundary, whilst CMS currently has one type A nuisance in the mjj < 350 region and two type B nuisances in the mjj > 350 region, one for each of 350-700 and >700. It is pointed out by Haider that these latter two already have very similar values, and Frank says clearly the underlying source is the same for both and also the type A nuisance. Jonathon & Ed agree, and it would be simple for CMS to modify this to correlate the existing three sources. It seems to be agreed that given we expect the main source of type A uncertainty in these bins to arise from the pT(Hjj) distribution, this CMS type A nuisance can be treated as an effective pT(Hjj) boundary migration nuisance and as such the correlation both within CMS and also across experiments makes sense. It should be noted that the CMS and ATLAS methods and magnitudes of the uncertainties here seem to be compatible. 

      • For the mjj boundary nuisances, ATLAS proposes one for each boundary so four in total (two solid, two dashed). CMS currently has type B nuisances corresponding to the two solid boundaries, but then just one type A uncertainty for the > 700 region. There was no clear consensus on what should be done here. The existing CMS nuisance could simply be correlated with whichever of the ATLAS nuisances is seen to have the greater impact. Alternatively, CMS could split its existing nuisance into two (with different nuisances affecting different analysis categories), and thereby produce two nuisances that could be appropriately correlated with ATLAS’ sources. This would probably need some follow-up discussion on the details.

    • 14:40 15:00
      Update from ATLAS II 20m
      Speaker: Robin Hayes (University of British Columbia (CA))

      Robin’s talk:

      • Frank explains that the theorists have this preference for a linear addition for various reasons but since the true sources are different (and e.g. the m_top dependence is very variable with pTH) he thinks they should be uncorrelated parameters in the final prescription. 


      Subsequent discussion on the synchronisation: 

      • There are some slight differences in the detail of how CMS and ATLAS derive their high-pTH migration nuisances. The ATLAS numbers seem to be slightly larger - CMS is happy to change to these numbers, although it could be useful to discuss in slightly more detail. 

      • The overall magnitudes of the uncertainties applied to each bin seem consistent between ATLAS and CMS, although the exact nuisance parameterisation scheme differs. CMS has kept e.g. the yield and res nuisances, and has a migration nuisance at 200 which is slightly different from ATLAS’ in that it (very slightly) affects the bins below 200 also. CMS has a generic boosted nuisance motivated by the old m_top scheme - this can easily be replaced with the updated calculations for the m_top nuisance as presented by ATLAS. Finally, ATLAS proposes a single nuisance to account for the pT(Hj)/pTH boundary, whilst CMS treats this as a single type A nuisance. It’s not discussed in detail but it seems here a similar thing can be done as for pT(Hjj), where we assume the main source of the CMS type A uncertainty is this boundary and thus it can be correlated with the ATLAS nuisance. Again this could be properly discussed in a follow-up meeting.