Документ взят из кэша поисковой машины. Адрес оригинального документа : http://qfthep.sinp.msu.ru/talks/Bunichev_singletop_tWb_LHC.pdf Дата изменения: Sat Sep 18 19:59:36 2010 Дата индексирования: Mon Oct 1 19:54:49 2012 Кодировка:

Modeling of single top tW+b signal at the LHC
Slava Bunichev Moscow State University In collaboration with E. Boos and L. Dudko

http://comphep.sinp.msu.ru


Single top quark production at the LHC:

leading order diagrams for single top production

t-channel

s-channel

associated tW production


Single Top tWb processes at the LHC:

tWb channel gives a significant contribution to the Single Top signal at the LHC SingleTop tWb is one of the main backgrounds for ttbar

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Single top quark tWb production at the LHC:
Diagrams for leading order 2 2 tW production

O(1/log(mt /mb) ) 2 3 processes


Problems of tWb modeling:

1. Discriminate single top tWb and tt-bar events 2. Matching 2->2 +ISR and 2->3 events (Treatment of the double counting: combining of the Wt+ISR and complete tWb processes)

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Methods of discriminating single top tWb and tt-bar
Naive Removing tt-bar resonant diagrams in the tWb amplitude

This method IS NOT SUITABLE for real modelling !
1 2 3 4 . . . . No interference between SingleTop and tt-bar Wrong rate kinematic distributions are wrong Wrong spin correlations

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Methods of discriminating single top tWb and tt-bar
tW is distinguished by cuts:

|M(Wb)-Mt| 7


Methods of discriminating single top tWb and tt-bar
Local cancellation of resonant tt-bar contribution
gg t W b
singletop

total- k 2 gg tW = gg tW b b
narrow

Narrow Width Approach

k = 0.01

= 0.0001

t

In the region where invariant mass of Wb system is close to top quark mass the behaviour of partonic cross section may be expressed as (Tait, hep-ph/9909352):

k gg tW b

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Narrow Width Approach

= gg t Br t W t b

Pro: gauge invariant, correct total rate, correct spin correlation, complete set of interference terms, Contra: slow speed of calculations there are some events with negative weights,


Kinematic distributions after applying local subtraction procedure

PT of top-quark

Pseudo-rapidity of top-quark

Wb-invariant mass = 3.1 pb

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PT of b-quark

pseudo-rapidity of b-quark


Method of "Local cancellation of resonant tt-bar contribution" is good for accurate calculations of cross section,
but it leads to events with negative weights, to additional singularities and increases the computation time.

We need a more efficient method for generating MC events

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Removing tt-bar resonant squared diagrams in the matrix element

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Kinematic distributions after removing tt-bar resonant squared diagrams in the matrix element

PT of top-quark

Pseudo-rapidity of top-quark Wb-invariant mass

= 3.1 pb

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PT of b-quark

pseudo-rapidity of b-quark


Method of "Removing tt-bar resonant squared diagrams" is rather good,
It keeps interference, spin correlations and correct rate but it leads to additional singularities.

Method needs some corrections ->

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Applying wide width approach procedure to the interference terms after removing tt-bar resonant squared diagrams
gg t W S b = k 2
Wide Width Approach Interference SingleTop t t

ingleTop

SingleTop Resonant

k = 10



wide

=10000 GeV

Pro: correct total rate, correct spin correlation, correct kinematic distributions complete set of interference terms, there are no events with negative weights, high speed of calculations, straightforward way to produce events beyond SM (with anomalous couplings etc...).


Kinematic distributions after removing tt-bar resonant squared diagrams and applying wide width approach

PT of top-quark

Pseudo-rapidity of top-quark

Wb-invariant mass

= 3.2 pb

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PT of b-quark

pseudo-rapidity of b-quark


Treatment of the double counting: combining of the Wt+ISR and complete tWb processes (Matching 22 +ISR and 23 events ).
Monte-Carlo generator SingleTop (NLO approach). (the same method as for t-channel NLO simulations)
(Boos, Bunichev, Dudko, Savrin, Sherstnev, Phys.Atom.Nucl.69 (2006) 1317)

MC Events with PT(b)> PT0 (hard region) modelled in the CompHEP. MC Events with PT(b)< PT0 (soft region) modelled with ISR simulation in the Pythia.

Combining events from CompHEP at the Pythia level:
The relative contributions of the processes of the Pythia and CompHEP determined from the normalization conditions to the total NLO cross section



NLO

= K

0 2 2 ISR P T b PT





2 3 P T b P0 T



.

PT0 -some value of the transverse momentum of additional b-quark -factor is chosen from the condition of smoothness of the distribution PT(b)

Generator SingleTop correctly simulates the NLO correction. No events with negative weights and there is no double counting of events


Concluding Remarks
Developed a new algorithm for accurate modelling the single top tWb signal using CompHEP. First results of calculations is obtained. Remains to be done Producing tWb events with anomalous couplings (in progress). Compare distributions from CompHEP-based generator SingleTop with other generators.

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