Study of tt production at NLO

Stan BentvelsenEdwin Bos

MC@NLO generator Since few months MC@NLO available in

Atlas repository: Using 6.5.0 release of Generators Using tag 00-00-00 of package McAtNLO_i

What is MC@NLO? What consequences for

top-production and top-reconstruction?

Developed by S Frixione and B Webber,www.hep.phy.cam.ac.uk/theory/webber/MCatNLO/

The MC@NLO generator ‘New generation’ event generator

Matching NLO calculations of QCD process with parton shower MC simulation Fully exclusive events generated

Hard emissions treated as in NLO Soft emissions handled by MC shower (Herwig) No ‘double counting’ between these two

Running in ATLAS: Create event file using ‘runNLO’ program

Contains kinematic of hard NLO process

Interface to Herwig via McAtNLO_i

Matching NLO with parton shower

Concequence: events weights can become negative

Weights and double counting Born term (LO) Virtual NLO

corrections Real NLO

corrections

Virtual and real NLO corrections both infinite → their sum is finite.

Real gluon emission in NLOReal gluon emission in parton shower

Problem of ‘double counting’!

Clever subtraction terms, slightly different for real and virtual NLO correctionscompensate for additional parton shower gluon emission: avoid double counting

MC@NLO

Weights Weights: ±w

‘unweighted’ events, up to a sign!(practically weight ±1)

Efficient event generation possible NLO distributions (without

MC showering) are non-physical

86.5%13.5%

tt production cross sectionMCatNLO: 842 pbHERWIG: 458 pbPYTHIA: 490 pb

(nb: no consistent pdf’s!)

‘standard’ tt productionprocess -1706

Matched with MC: distribution is positive

for all values of Pt

Pt(tt-system) non-zero due to real gluon emission(both via NLO and PS)

NLO and MC matching

NLO results by itself give ‘nonsense’ results Pt(tt-system) from NLO

small Pt: negativelarge Pt: positive

Comparison to LO generators

All distributions normalised to 1

Pt(tt system) Herwig & MCatNLO agree at

low Pt, At large Pt MCatNLO ‘harder’ PYTHIA completely off

Same distribution on linear scale

Comparison to LO generators

Azimuthal opening angle top-topb

Rapidity of tops(2 entries/event)

Pt – distribution individual top

Pt distributions topidentical for all MC

Before ISR/FSR

After ISR/FSR

Top reconstruction Use ATLFast package (00-01-24)

StandardNtupleMaker algorithm Modified slightly to

Get access to event weights Get access to partons after ISR in Herwig Get partons from W-decays

Use calibrated ‘atlfastB jets’ Mimic Top reconstruction Mimic Top reconstruction w/o b-tagging

Reconstructed jets

Pt-distribtion of jet with highest Pt

Very hard gluon jets in MC@NLO

Second ‘hardest’ Pt jet:effect of hard emission gone

Kinematic reconstruction t-mass

Reconstruct W by considering all jet-jet permutations, take combination with minimum mass distance to 80.3 GeV

Reconstruct top for 1 b-jet:if R(b-jet,W) < R(b-jet,electron)

No spectacular differences!

Reconstructed t-mass

Some of the entries are still negative

In case of 2 b-tagged jets:Take b-jet which lead to highest Pt of reconstructed top

Top-mass for top with Pt>250:

Top mass All generators give

rather equivalent results No Pt differences in

reconstructed top No big mass

differences Quantitative

differences have not been determined yet

Pt distribution of reconstructed top(for 2 b-jet)

‘Commissioning’ top

Extra ‘hard’ gluon can effect reconstructionof top-mass when no b-tag is assumed(Pessimistic commissioning scenario)

Top reconstructed by selecting 3 jets which give the highest resulting Pt

Also for resulting top-Pt > 250 GeV

Again no significant differences between Herwig and MCatNLO

‘Commissioning’ top

Reconstructed W-mass

Top-pt:

Top-pt

Top polarization

In di-lepton channel Angle between 2 leptons ‘true’ information Is this the observable for

polarization? (I am not expert!)

MCatNLO does not have polarization included Can be implemented

according to authors

Conclusions MCatNLO beautiful ‘next

generation’ generator Practicalities:

Negative event weights: store in full simulation?

Read ‘pre’-generated’ NLO event file I do not know how to

‘skip’ events in this file Generated many NLO

event files, each 50k events.

No polarization included yet.

Spectrum for pt of lepton has problems:

Maybe problem with Ntuples, investigating…

In previous plots spikes are removed.

AlpGen generator tt production – with extra n-jets (‘light jets’)

Exact (LO) matrix element Matching n-jets for various n left to user (?) Only started very recently…

Alpgen generates file with hard scattering To be fed into Herwig/Pythia shower MC’s Generation of (unweighted) events time consuming

Problems: (Herwig_i-00-01-18) Top decays done in Alpgen (incl polarization) –

Event record not recognized in Herwig interface Top decays as well done in Herwig

For now force alpgen to leave top stable

AlpGen tt+1-jet production

tt-system alpgen affected by extra gluon

Extra jet: Pt-min = 10 GeV |η| < 2.5 R>0.4

Need a bit more study….

Outlook Proceed with backgrounds to top

MC@NLO for background studies?? Use AlpGen for background, W+ n-jet Estimation of QCD background

NIKHEF has broader interest in top: Single top (theory and measurements) Top cross section at ‘commissioning stage’ Top and extra dimensions Top analyses at D0