Top Mass Systematic Uncertainties

Towards common Tevatron top mass systematic uncertainties

The top group at CDF has a new and improved prescription to evaluate systematic uncertainties in top mass measurements. We would like to have a common understanding with D0 of the systematic uncertainties as well as a common way to evaluate them. We have documentation about what CDF is currently doing at:
Prescriptions for Summer 08
Work to do for final prescription Winter 09
Following the current CDF prescription and the available public notes from D0 top mass measurements we want to resume our past discussions with D0 by asking what is not clear to us.

Prescriptions for evaluating systematic uncertanties in top mass analyses for 2008 Winter conferences are described below. A list of tasks to be finalized before the next publication round is available here. Again, volunteer to help!

Jet Energy Scale
Residual Jet Energy Scale (for in-situ JES measurements)
PDF (updated on April 30, 2008)
ISR/FSR
MC Generator (Use updated prescription)
Background fraction and shape
b-tagging
b-jet energy scale (Use updated prescription)
Lepton p_T
Multiple Interactions (Use updated prescription)

Other
Interpretation of shifts UPDATED

Note that the files named here are gen6.

(1/2) Jet Energy Scale / Residual JES

Is D0 considering the systematic uncertainties due to energy left out side the cone ?
Since you correct to parton level, we would like to know if there is a systematic uncertainty added for mismodeling between parton and particle level ?

(3) PDF

It seems that D0 similarly to CDF adds in quadrature the difference between CTEQ6M 20 pairs of eigenvectors
This part is not clear if D0 does it:
- add in quadrature the difference between MRST with two different values of Λ_QCD (e.g. MRST72 Λ_QCD=228MeV vs. MRST75 Λ_QCD=300MeV)
- Check CTEQ5L and MRST central values are consistent
- Add in quadrature the effect of reweighting gg contribution from 5% (LO) to 20% (=15+5) (NLO)

(4) ISR/FSR

Was D0 succesful in running the Pythia samples with our parameters ?
D0 and CDF are clearly evalluating different uncertainties.

(5) MC Generator

Is D0 planning to add this uncertainty? (Herwig-Pythia)
We use Alpgen as a cross check but learned that Alpgen has zero width for the top quark and W boson. We're not sure what the implications of this are. What did D0 find?
We want to add an NLO generator to this uncertainty soon. Could be MC@NLO or POWHEG.

(6) Background fraction and shape

This uncertainty is evaluated differently but since it depends on the type of analysis and channel we are not sure we can have an universal prescription for it. That being said, we could agree in the different bg shapes would be (we use different settings of Q2).
Note that what D0 calls W+jets HF syst. uncer. is included in CDF's background fraction uncertianty.
The multijet background uncertainty from D0 is (should) part of the bg shape uncertainty.
Is trigger efficiency included as part of the CDF bg uncertainty ?

(7) b-tagging efficiency

CDF only uses this one in the Lxy.
D0 seems to be adding it to all the analyses

(8) b-jet energy scale

We think we evaluate this uncertainty in the same way as D0. We do have soem extra components.
It would be good to further understand why D0 sees a large effect related to the difference between Pythia default B fragmentation and the LEP-tuned parameters (while CDF doesn't).

(9) Lepton P_T

D0 doesn't include this uncertainty. How well is the lepton pT known ?

(10) Multiple Interactions or Pile-up

This uncertainty takes into accouont the fact that our MC doesn't have a good modelling of the high luminosity runs.
We can't find anything like this in the D0 documentation.

(11) Other

Some specific analysis uncertainties :

MC statistics
Method calibration

Possible sources of additional systematics (see to do list for more details):

JES resolution: D0 includes it, CDF has a prescription to include it in the future too.
NLO systematics
Color Reconnection
- What is the plan?