Mass systematics from generator

I met with generator experts at CERN last week

HERWIG : Webber, Seymour(MC@NLO), Richardson

PYTHIA : Peter Skands

Others : Alexander Flossodorf + one more I did not catch his name

I will report on what transpired from the meetings

systematics from hadronization

systematics from perturbative effects

systematics from color reconnection

Top Mass meeting, April 23-2008, Lina Galtieri

Systematics from Hadronization

Top Mass meeting, April 23-2008, Lina Galtieri

Seymour says that comparison of PYTHIA and HERWIG

only probes the hadronization part of the process as the ISR and FSR are the same. Both are NLO

I will use results from MTM3 to illustrate the point

Bottom line about PYTHIA and HERWIG difference:

given the same tune : ΔM = -0.14 +- 0.37 GeV

old tune is acceptable as well : ΔM = -0.01 +- 0.37 GeV

With no Jimmy we get : ΔM = -0.85 +- 0.37 GeV

Systematics from extra radiation

Top Mass meeting, April 23-2008, Lina Galtieri

Extra loops, ISR, FSR, extra jets uncertainties can be obtained in many ways. Seymour suggests that we take the largest of the difference for

the following cases:

Systematics from perturbative effects: ΔM = 0.50 GeV

Comments:

should increase the statistics

MC@NLO was run with the old tune (PT >2.50)

HERWIG was not much affected by use of new tune (PT>3.25)

(it is likely that MC@NLO will not be affected much)

PT(ttbar)

The PT(ttbar) system for PYTHIA and HERWIG are very different, as observed long ago by many people (see below).

In spite of PT(ttbar) difference, the top mass between PYTHIA and HERWIG is ΔM = -0.14 +- 0.37 GeV

Top Mass meeting, April 23-2008, Lina Galtieri

PT(top) distributions are the same

MW and Mtop

~ 1 GeV different

JES =1.0036 Pythia

JES = 0.9925 Herwig

What changes the PT(ttbar)?

Mousumi 's talk: ISR is one of the effects

Top Mass meeting, April 23-2008, Lina Galtieri

Color Reconnection

Peter Skands and Daniel Wicke have studied this problem .

P. Skands and D. Wicke hep-ph/0703081v1 (March 2007)

Their new tune includes variations on UE, ISR, FSR, CR and beam renmants.

They fit the Nch and AVG( PT) -vs- Nch distributions of min bias events at the Tevatron

They seem to prefer Rick's DW tune (includes UE data and Drell- Yan data) rather then Tune A.

With this new tune they reproduce the HERWIG PT(ttbar) distribution.

They get uncertainties on the top mass from generators to be:

ΔM = +- 0.5 GeV from non-perturbative effects (we have +-0.37)

ΔM = +- 1.0 GeV from perturbative effects (we have +-0.50)

Top Mass meeting, April 23-2008, Lina Galtieri

Color Reconnection (cont.)

How to proceed:

HERWIG : run it with CLRECO=.TRUE., RECO=.9 (default)

(it has been there since LEP's W mass measurement)

PYTHIA : use V6.408 or V6.413 (we use V6.216, 2003)

call PYTUNE, set MSTP(5) = 300

this is the S0 solution shown in their paper

Sasha Golassonov apparantly is working on this

Comments: HERWIG is easy, we can try to get it done by regular MC

production

PYTHIA: we need a more modern version (Sasha?)

Top Mass meeting, April 23-2008, Lina Galtieri

Summary and Conclusions

So far, systematics from generator are:

ΔM = 0.37 GeV from hadronization

ΔM = 0.50 GeV from perturbative effects

To do:

add more statistics to the MC samples

run HERWIG to evaluate Color Reconnection effects

run PYTHIA for CR effects (should this be left for the summer?)

Top Mass meeting, April 23-2008, Lina Galtieri

PT(ttbar)

Top Mass meeting, April 23-2008, Lina Galtieri

Courtesy of Mousumi Datta

2002-04-21T14:25:46