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Programs

The known event generators for single top quark production include: ONETOP, PYTHIA, COMPHEP, MADGRAPH.

ONETOP

Author(s): C.P.Yuan, D. Carlson, S. Mrenna, Barringer, B. Pineiro, R. Brock

This is a stand-alone event generator which calculates weighted and unweighted events for the 2->2; 2->3; s-channel signal processes. It also calculates LO W+bbar background and ttbar background events. The unique feature about ONETOP is due to the matrix element calculation within the helicity basis. The complete density matrix is preserved in the signal and ttbar background events.

As used in ATLAS: the initial and final state partons are passed to PYTHIA for partlcle generation, including ISR and FSR. Standard PYTHIA and/or HEP COMMON records can be written. PYTHIA common is passed to ATLFAST for detector simulation.

Beatirz Pineiro has prepared documentation on the use of the ntuples produced with ONETOP.

PYTHIA

Author: Torbjörn Sjöstrand

Pythia has its own version of single top quark production.

Implementation in ATLAS:

Dubna:

 

Protvino

 

COMPHEP

Author(s): P.A.Baikov et al., Physical Results by means of CompHEP,in Proc.of X Workshop on High Energy Physics and Quantum Field Theory (QFTHEP-95), ed.by B.Levtchenko, V.Savrin, Moscow, 1996, p.101 , hep-ph/9701412

 

MADGRAPH

Authors: T. Stelzer and W.F. Long, Comp. Phys. Comm. 81, 357 (1994).

This is not an event generator, but rather a program for calculating the matrix elements of user-defined processes.

 

Calculations

11/17 We have been attempting to compare cross section calculations, This is a compilation to date:

---

footnotes.

^a 2->2 process passed to pythia; pdf-CTEQ2L

^e bbbar content not yet filtered. Totals are:

q qbar -> W (e nu) g (q q) 20,000 pb


q glue -> W (e nu) q 37,400 pb

^f specifically the process g g & q qbar -> t(->b W(->e nubar)) tbar(->b W(->q qbar))

¹ private calculation pdf-CTEQ2? ; QCD scales in pdf, m_t or M_W ; m_b = 4.5 GeV/c² ;

² specifically the process

³ specifically the process g b -> W(->e nu) t(->b W(->e nu))

⁴ specifically the process

back to table

⁵ specifically the process g g & q qbar -> t(->b W(->e nubar)) tbar(->b W(->q qbar))

¹⁰ ONETOP pdf-CTEQ2L ;

¹¹ specifically the process u b -> d t(->b W(->e nu))

¹² specifically the process u dbar -> b t(->b W(->e nu))

back to table

¹³ specifically the process g b -> W(->e nu) t(->b W(->e nu))

¹⁴ specifically the process u dbar -> W(->e nu) g(->b bbar) (2 graphs)

¹⁵ specifically the process g g & q qbar -> t(->b W(->e nubar)) tbar(->b W(->ebar nu))

²³ From Dugan's mail message of 11/17: generated Wplus jets without ISR or FSR in pythia and filtered results for bbbar final states got

1.85e4pb * (35 events with bbbar/300000 total events) = 2.16pb where W->e,nu

²⁴ with radiation for both graphs:

3.26e4pb * (319 events with bbbar/300000 total events) = 34.7pb where W->e,nu

³⁰ analytic calculations ; pdf CTEQ4L ; scale in g-f -SQRT(pT^2 + mb^2) - NLO calculations - k-factors at work!

³⁶ NLO - so should not agree with LO calculations

⁴⁰ comphep calculation...almost imposible to unambiguously figure out what they are doing.

⁴¹ cross section taken from Willenbrock

 

back to table

Proposal for calculational parameters:

We agreed that I would make a proposal for everyone for common parameters to be used in the MC calculations. I'm late, but here they are:

• General plan of 'attack'

• We should only use the 2->2 process in all calculations, relying on Pythia to take care of the other b
• Use CTEQ2L as the default pdf
• Use appropriate "k factors" to get the cross sections to the following values:
  • g-f: 245 pb (t + tb)
  • s channel 10 pb (t + tb)
  • Wt 60 pb (t + tb)
  • ttbar 830 pb
  • Wbbbar Gosh, I don't know. How about 300 pb?

• Scales

  I propose that the scale for pdf's be the same as that for alph_s. After discussing this admitedly flexible subject with Yuan, I suggest:

• W-gluon fusion (like deep inelastic scattering)

The 2-2 Process - use M_W

• s-channel

 

Use s-hat

 

• Wt

 

Use M_W

 

• ttbar

 

Use m _t

 

• Wbb-bar

 

Use M_W

 

• constants
  • quark masses

    mb = 4.5 GeV

    mt = 175 GeV

   

• alpha_s
• Here is the code for alpha_s from ONETOP.f

       AL4 = QCDL                          (lambda)
       AL5=AL4*(AL4/AXMBOT)**(2./23.)

       AL6=AL5*(AL5/XMTOP)**(2./21.)

       XRATMIN=5.

       B0=12.D0*3.14159/(33.-8.)

       B1=6.D0*(153.-19.*4.)/(33.-8.)**2

       ISTART=.FALSE.

      END IF

      XRAT=QQ**2

      IF(XRAT.LT.XRATMIN)XRAT=XRATMIN

      IF(SQRT(XRAT).GT.XMTOP)THEN

       RAT=LOG(XRAT/AL6**2)

       B0=12.D0*3.14159/21.

       B1=6.D0*(153.-19.*6.)/21.**2

      ELSEIF(SQRT(XRAT).GT.AXMBOT)THEN

       RAT=LOG(XRAT/AL5**2)

       B0=12.D0*3.14159/23.

       B1=6.D0*(153.-19.*5.)/23.**2

      ELSE

       RAT=LOG(XRAT/AL4**2)

       B0=12.D0*3.14159/25.

       B1=6.D0*(153.-19.*4.)/25.**2

      END IF

      ALPHAS=B0/RAT

last modified: Thursday, December 10, 1998

Maintained by Chip Brock
Raymond Brock, Professor of Physics and Chairperson, Department of Physics and Astronomy, Michigan State University, East Lansing, MI 48824,