Download - Study of tt production at NLO
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