# Presentation: 778590

## tt~ Production at ATLAS and tt~ Monte Carlo Generators

- Borut Paul Kersevan
- Jozef Stefan Institute
- and Faculty of Mathematics and Physics, Univ. of Ljubljana
- On behalf of the ATLAS collaboration

## NLO Cross-section for tt~ production at LHC is (tt)~830 ± 100 pb-1(LO about 1⁄2 of this at the same scale)
About 90% contribution from gg colissions 
the remaining 10% from qq~ collisions  
Main sources of uncertainties are the scale choice and the PDF uncertainties
Two sequential resonant (Breit-Wigner type) decays of tops and W-s
Angular (spin) correlations between the decay products.

- About 90% contribution from gg colissions
- the remaining 10% from qq~ collisions
- Main sources of uncertainties are the scale choice and the PDF uncertainties
- Two sequential resonant (Breit-Wigner type) decays of tops and W-s
- Angular (spin) correlations between the decay products.

- What is tt~ production at LHC?

## Simplifications

- Monte Carlo generation of the full 2  6 process (gg/qq~  bb~f1f2f3f4) turns out to be difficult.
- To simplify the generation procedure approximations are often used:

## Progress in tt~ Monte Carlos

- To schematically illustrate the development of tt~ Monte Carlo generators  (ATLAS uses all of them):

- Pythia

- MC@NLO

- TopReX

- AcerMC

- Alpgen

- NLO calculation

- Additional final state jets

- Polarised top decays

- Full 2  6 process

## Comparison between tt~ generators 

- Pythia has: 22 LO ME, BW top smearing, unpolarised top decays
- TopRex has: 22 LO ME, BW top smearing, polarised top decays
- Alpgen has: 22 LO ME, no BW top and W smearing, polarised top decays, up to four additional jets (quarks and gluons)
- MC@NLO has: 22 NLO ME, BW top smearing, unpolarised top decays
- AcerMC has: full 26 LO ME and phase space (polarisations implicit).

- What we want at ATLAS is clearly all of it!
- :-)

## Full 2 6 process: Why is it so difficult?

- One thing is the matrix element itself; today automatic tools like Madgraph/HELAS exist that do it for you.
- There are however still some issues!
- First and foremost however, the efficient Phase Space sampling is difficult to achieve!
- Experimentalists want unweighted events to pass through the complex detector simulation/digitization/reconstruction!
- The complexity of the problem increases with the number of Feynman diagrams for a certain process.
- Difficulty level also increases with the number of particles in the final state.
- Also hard to efficiently describe are the invariant mass. distributions at the threshold of heavy particle production.

## AcerMC 2.x Monte Carlo Generator

- The Monte Carlo generator for a select list of processes at ATLAS/LHC.
- Current version is AcerMC 2.4
- Code and documentation available from the web:http://cern.ch/borut

- Currently implemented processes

**Top quark production**

## AcerMC 2.x Monte Carlo Generator

- LO Matrix elements obtained from modified MADGRAPH/HELAS3 code:T. Stelzer and W. F. Long, Comput. Phys. Commun. **81 **(1994) 357.
- Parton density functions from LHAPDF or PDFLIB804.
- Phase space sampling done by native AcerMC routines based on:
- Adaptive multi-channel approach,R. Kleiss, and R. Pittau, Comput. Phys. Commun.** ****83** (1994) 141.
- Revised Kajantie-Byckling phase space factorisation,E. Byckling and K. Kajantie, Nucl. Phys **B9** (1969) 568.B. Kersevan and E. Richter-Was, Eur. Phys. J. **C39** (2005) 439.
- AcerMC native ‘massive’ importance sampling functions of particle four-momenta.B. Kersevan and E. Richter-Was, hep-ph/0405247.
- Additional Ac-VEGAS adaptive grids/algorithms:G. P. Lepage, J. Comput. Phys. **27** (1978) 192.

**The final unweigh****t****ing efficiency is between ****10-40%, depending on the process!**

## AcerMC tt~ 2  6 process

- Generation (unweighing) efficiency ~ 15% achieved.
- The inclusive cross-section matches other LO predictions.
- The spin correlations are certainly there as shown by the helicity angle distribution:

- <CosΘ+ · CosΘ->

- <CosΘ+ · CosΘ->

- <CosΘ+ · CosΘ->

- No helicity correlation

- Helicity correlation

- AcerMC Helicity correlation

## AcerMC tt~ 2 6 process

- AcerMC tt~ 2 6 process

- The question is of course, whether there are any observable differences?
- Results from F. Hubaut et al doing W polarisation studies in tt~ events

- ATLAS scientific note SN-ATLAS-2005-052

- Toprex is taken as input,
- agrees reasonably well with Alpgenwhich uses similar strategy

- AcerMC shows a small disagreement(hard process scales not matched)

## We can go one step further: Adding non-resonant contributions

- AcerMC handles well 6 particle final states:
- We expand the tt~ matrix element to include all WWbb intermediate states: 45 Feynman diagrams
- The contributions to the total cross-section quite small, however turn out to be vital to the Higgs searches at LHC: Studies show an up to a factor 2 increase in backgrounds!
- N. Kauer Phys.Rev.D67:054013,2003
- Further studies needed!

- A representative ‘non-resonant’ diagram

## Interface to showering/fragmentation Generators

- ATLAS uses Herwig 6.507.2 with ‘native’ (ATLAS) fixes - a must for MC@NLO; in addition Jimmy 4.2 is used for UE simulation.
- ATLAS also uses Pythia 6.324 with the new showering and underlying event modeling: some changes ahead!

- Studies done by the ATLAS NIKHEF group

- ‘Old’ Pythia ISR

- ‘New’ Pythia ISR

- PT(bISR) [GeV]

- ISR

## ATLAS UE/MB tunings 

- Using new energy dependent tunings of Jimmy 4.1
-  (A. Moraes *et. al.*)

**PTJIM=4.9 **
**          ****= 2.8 ****x (14 / 1.8)****0.27**

**E****nergy dependent PTJIM generates UE predictions similar to the ones generated by PYTHIA6.2 – ATLAS. **

## ATLAS UE/MB tuning:tt~ events

- ‘Data Challenge 3` tuning:** **tuning based on comparisons to UE data leads to a natural agreement to PYTHIA’s prediction for particle density in tt~ events at LHC.

## Just a note on backgrounds:Commissioning studies

- An interesting ATLAS commissioning study (assuming no b-tag, no jet calibration but good lepton ID) done by the ATLAS NIKHEF group uses the ALPGEN W+jets - where the strength of ALPGEN becomes obvious (exact ME for W+4 jets, MLM matching of W+n jets).

- Δσstat(t-tbar) ~ 2% (1 week) !!!

**Semi-leptonic event selection****:**
**Isolated lepton with P****T****>20 GeV**
**Exactly 4 jets with P****T****>40 GeV**** and ****η****<2.5**
**Missing E****T****>20 GeV**
**Reconstruct top ****from ****3 jets with**** maximal resulting P****T**** **
**Fit the distribution with basic shapes**
**Estimated ****ε**** ~ 1%**

## Just a note on backgrounds:Commissioning studies

- An interesting ATLAS commissioning study (assuming no b-tag, no jet calibration but good lepton ID) done by the ATLAS NIKHEF group uses the ALPGEN W+jets - where the strength of ALPGEN becomes obvious (exact ME for W+4 jets, MLM matching of W+n jets).

- Δσstat(t-tbar) ~ 2% (1 week) !!!

**Semi-leptonic event selection****:**
**Isolated lepton with P****T****>20 GeV**
**Exactly 4 jets with P****T****>40 GeV**** and ****η****<2.5**
**Missing E****T****>20 GeV**
**Reconstruct top ****from ****3 jets with**** maximal resulting P****T**** **
**Fit the distribution with basic shapes**
**Estimated ****ε**** ~ 1%**

- The next step is then of course to tune andtest our tt~ Monte-Carlo on these distributions (as well as to include b-tag, jet calibration etc..)

## Conclusions

- At ATLAS we use a wide variety of Monte Carlo generators for tt~ production.
- Each generator has its strong points.
- For systematics studies and comparisons it is advantageous to use all of them.
- We believe to have a certain edge with the AcerMC Monte Carlo generator of full 2  6 tt~ production processes with resonant and non-resonant contributions.
- We also use different showering and fragmentation as well as UE/MB models with promising tuning procedures being implemented.
- Advanced studies of different Monte Carlo tools for generation of background processes are also of the essence.
- A lot of work still to be done but we strongly believe we will be well prepared for the first top events next year!