# Presentation: 009433

## Top MCsfrom Tevatron to LHC

- Un-ki Yang
- University of Chicago

- TEV4LHC workshop, Fermilab,  Sep 16-18, 2004

## Many exciting programs using top

- Top physics programs at the Tevatron is now in the phase of precision measurements.
- Time to explore top quark physics potential from many different angles
- At the LHC, even more precision measurements, however, major backgrounds in Higgs and new physics (ttH/h, VBF etc)

- MC Modeling!!!

## MC modeling for top pair production

- Hard scattering: qq/gg->tt
- pQCD calculation with spin corr.
- Top decay: t->bW(->l,qq)

- Well predicted

- Start to wonder!

- pp->tt+X -> l+2 bjets + 2 W jets + extra jets    (multi-jets and multi-scale process)
- pp->tt : (Q2 scale/PDFs)
- extra gluon: NLO hard emission, ISR, FSR
- Beam-beam remnants, multiple parton interactions
- Fragmentations (color singlet Wjet, non-singlet bjet, ISR/FSR jets)

## Various MCs for top production 

- ME: ALPGEN/MadGraph/ComHep/TopRex etc
-             (t=0)

|  | Pythia | Herwig | ME | MC@NLO |
| --- | --- | --- | --- | --- |
| Hard scattering | LO tt | LO tt | LO tt+n  | NLO tt
(hard gluon) |
| PS shower (ISR/FSR) | coherent branching
(LO DGLAP) | coherent branching
(LO DGLAP) | Pythia 
or Herwig interface
(double counting problem, can be fixed 
by the CKKW) | Herwig
 |
| Hadronization
 | LUND string | cluster model |  |  |
| beam-beam remants, MPI | all | No MPI
(Yes, v605) |  |  |
| Spin corr | NO | Yes | Yes | No |
| Comments
 | Good for inclusive tt 
but poor in tt+njets |  | Good for multi-jets, but still LO | Good for tt multi jets |

## Kinematics in tt events

- Rec: Pt(lep)

- Gen: Pt(b parton)

- Gen: Pt(lep)

-  W,b,lepton from top decay are reasonably well described by various MCs.
- But detailed modeling of multi-objects structure are now required for next phase of analysis

## NLO effect on the leading jet Et

-  NLO effect reduce the leading Jet Et by 4% at the Tevatron

## Modeling extra jets in tt 

-  Extra jets from ISR/FSR/hard gluon emissions:
-     tt + Njets, Pt (tt), and (tt)

- Tevatron
-    Top mass
-     - ISR: wrong comb.=> Mtop=1.3 GeV RunI CDF (0.4 in RunII)
-     - Pt(tt) is used to select a correct combination.
- LHC
-    tt+1/2j major bkgs to ttH/h and VBF
-    -  top Yukawa coupling, Higgs and new physics searches

- Hbb
-       WW

## Angular correlations between top and anti-top

- Tevatron

- LHC

- LHC plot from ATL-COM-PHYS-03-043

- More back-to-back events in Pythia!!!

- tune-A

- default

## N jets and eta for extra jets

- Gen: extra n jets (Et>12 GeV, |eta|<3)

- Gen: eta for extra jets

- MC@NLO has more extra jets than Pythia and Herwig

- Herwig and MC@NLO: more extra jets in forward region than Pythia: same feature
- in -jet between Herwig and Pythia

- 0.8
- 0.9
- 1.0

## Pt(tt) 

- Even at Tevatron: Pythia is very different from Herwig

- MC@NLO effect is shown up at high-tail

- Tevatron

- Gen: Pt(tt)

- LHC

-  Pt(tt) from intrinsic Kt and ISR (extra jets)

## How to tune ISR and it’s uncertainty?

- +

- -

- qq -> tt vs 

- ISR effects are governed  by DGALP eq.
-    ( Q2,QCD, splitting functions, PDFs )
- Average Pt of the DY  [ Q2 ~M(DY)2 ]
-    - measure the slope :allows us to estimate
-      the size of ISR at top production region.

- Mt2 +Pt2

- The prediction at Q2=Mt2+Pt2 is
- slightly higher than Pythia

## ISR uncertainty 

- +

- -

- Q2max: K PARP(67):

- Qmin :  PARP(62)

- Kt2 = PARP(64)(1-z)Q2
-  QCD = PARP (61)

| Pythia | ISR more  | ISR less |
| --- | --- | --- |
| PARP(61) (D=0.192 GeV) | 0.384 | 0.100 |
| PARP(64)(D=1.0) | 0.25 | 2.0 |

- ISR uncertainty is only due to uncertainty in shower processing,
- PDF, factorization scale uncertainties are  not treated as a part of the ISR uncertainty

- No effect
- on s-ch resonance

- Conservative

## ISR  effect on qq vs gg channels

- Pythia: extra n jets (Et>12 GeV, |eta|<3)

- Pythia Pt(tt): qq vs gg

- Herwig Pt(tt): qq vs gg

- gg channel has more extra jets than qq channel higher Pt(tt), different peaks in Herwig
- Extrapolation from Tevatron(qq) to LHC(gg): can be risky if you use wrong tuning parameter
- Problem in LO Pythia/Herwig tt only 5% gg (compared to the 15% NLO), bias in kinematic analysis & top mass (DLM: Mt(qq-gg)=4 GeV)

## Few thoughts on tuning  - Underlying events, ISR, FSR, Q2 scale, PDFs - 

- All correlated, so almost impossible to have an universal tune for underlying events, ISR etc. But perhaps we can select system such that other factors have very small correlations.
- PDFs:
- DIS e/-N, W/Z, jet data from pp ( Wu-ki Tung)
- Beam-beam remnants, multiple interactions
- back-to-back dijet events (less ISR): (Rick Field),
- but even back-to-back DY events (less ISR, no Q2 scale dep)
- ISR: now with tuned PDFs, underlying events
- DY events as a function of M(ll)2, Pt(ll), Njet, (ll): but diff. ISR for qq vs gg channel

## Tuning ISR using  [ dijet, DY() ]

- Pythia with higher ISR using PARP(67)=4 describes the data like NLO, HERWIG.
- Will be very interesting to look at same quantity (using the DY data!!!, PARP(67) no longer plays here.

## Wish lists

- Need to resolve the difference in extra jets between Pythia and Herwig (any improvement in newer version?)
- Develop a coherent scheme to tune underlying/ISR/FSR/Q2 scale for both Tevatron and LHC
- Understanding of fragmentations (especially, color non-singlet b-jet) coherent work with LEP too.
- MC@NLO and ME+PS CKKW/MLM give us a great opportunity for top physics, are these good enough?
- more MC@NLO processes (W/Z+jets, single top etc)
- NLO DGLAP shower evolution?
- ME+PS multi-jets with one K factor is not good enough, more NLO/NNLO calculations.

- Coherent studies of  the Tevatron, HEP, LEP, and LHC
- theorists/experimentalists
-  are really crucial in order to explore new area of physics

## Problem in LO single top t-channel

- “second-b” ( b not from top decay) is not properly described in LO.
- Solution:
- Low-PT from LO sample
- High-PT from NLO sample
- Low-High threshold 18 GeV/c.

## ISR variations due to QCD, K factor

- ISR variations due to QCD, K factor

- ISR uncertainty samples
- (conservative)

- More ISR  : QCD = 384, K = 0.5
- Less ISR   : QCD = 100,  K = 2.0
-     Run I: no ISR: K = infinite