cross section error 4%~5% compared with pythia

Hi Karo,

Sorry for the late answer.

With the information that you provide to me, I don't see any reason to be worry about, such variation can be linked to the way to determine the scale for alpha_s. Did you run in fix scale for both?

Also did you have remove (increase to large value) the BW_cut? Or did you include the same kind of cut in Pythia8? (I know this is possible). This can reduces the differences.

Note that the correct way to compute such cross-section is anyway via merging/matching.

Cheers,

Olivier

Hi Olivier,

    I think maybe you are right.

   But I use different MG5 version with the same parameter, the result is different. Eg,
   p p > t j, t > l+ vl b (three generations)

   The result of MG5_V1_4_8_4 is 9.9 pb. But the result of MG5_V1_5_11 is 11.05 pb. The error about 10% is normal ?

Cheers,
Karo

Hi Karo,

I think I know the reason for that (the treatment of the mass of the b in the initial state) but let me check that carefully.

Cheers,

Olivier

Hi Karo,

I confirm that this is indeed related to the mass of the b quark.
If those one are set as zero (as you should have it since you assume one of the b coming from the beam)
and set maxjetflavor on 5 (to have proper cut in the final state) Then you have perfect agreement:
 22.67 +- 0.06211 pb
against
 22.71 +- 0.0735 pb

Additionally, the reason between the value is 1.4.x and 1.5.x is due to a change introduce in 1.5.5:
1.5.5 (18/11/12) JA: Ensure that t-channel single top gives non-zero cross section
                      even if maxjetflavor=4 (note that if run with matching,
                      maxjetflavor=5 is necessary for correct PDF reweighting).

Cheers,

Olivier

Hi Olivier,

     I set the mass of b quark on zero and maxjetflavor on 5, then the results of two MG5 version is equal. The result is like that,

     maxjetflavor m_{b} MG5.1.4.X MG5.1.5.X
           4 0 9.969 11.08
           5 0 10.55 10.55

    <<1.5.5 (18/11/12) JA: Ensure that t-channel single top gives non-zero cross section
                      even if maxjetflavor=4 (note that if run with matching,
                      maxjetflavor=5 is necessary for correct PDF reweighting).

     That means the result with maxjetflavor on 4 should equal the result with maxjetflavor on 5 both in MG5.1.5.X ? But the above results show the result of maxjetflavor on 4 is different with maxjetflavor on 5 in MG5.1.5.X. Which result and which maxjetflavor is right in MG5.1.5.X ? 11.08 or 10.55 ? I don't know.

All the bests,
Karo

Hi Olivier,

     I set the mass of b quark on zero and maxjetflavor on 5, then the results of two MG5 version is equal. The result is like that,

     maxjetflavor~~m_{b}~~ MG5.1.4.X~~MG5.1.5.X
           4~~~~~ 0 ~~~~9.969 ~~~~11.08
           5~~~~~ 0 ~~~~10.55 ~~~~10.55

    <<1.5.5 (18/11/12) JA: Ensure that t-channel single top gives non-zero cross section
                      even if maxjetflavor=4 (note that if run with matching,
                      maxjetflavor=5 is necessary for correct PDF reweighting).

     That means the result with maxjetflavor on 4 should equal the result with maxjetflavor on 5 both in MG5.1.5.X ? But the above results show the result of maxjetflavor on 4 is different with maxjetflavor on 5 in MG5.1.5.X. Which result and which maxjetflavor is right in MG5.1.5.X ? 11.08 or 10.55 ? I don't know.

All the bests,
Karo

Hi Karo,

The difference is linked to the cut on the b, which are different when you set maxjetflavor on a different value
when you set maxjetflavor on 5, the cut use for the b jet are ptj and not ptb anymore.
Therefore it is expected to have two different value since the phase-space cut are not the same.

Cheers,

Olivier

On Aug 22, 2013, at 3:41 AM, karo <email address hidden> wrote:

> Question #233788 on MadGraph5 changed:
> https://answers.launchpad.net/madgraph5/+question/233788
>
> Status: Answered => Open
>
> karo is still having a problem:
> Hi Olivier,
>
> I set the mass of b quark on zero and maxjetflavor on 5, then the
> results of two MG5 version is equal. The result is like that,
>
> maxjetflavor m_{b} MG5.1.4.X MG5.1.5.X
> 4 0 9.969 11.08
> 5 0 10.55 10.55
>
>
> <<1.5.5 (18/11/12) JA: Ensure that t-channel single top gives non-zero cross section
> even if maxjetflavor=4 (note that if run with matching,
> maxjetflavor=5 is necessary for correct PDF reweighting).
>
> That means the result with maxjetflavor on 4 should equal the
> result with maxjetflavor on 5 both in MG5.1.5.X ? But the above
> results show the result of maxjetflavor on 4 is different with
> maxjetflavor on 5 in MG5.1.5.X. Which result and which maxjetflavor
> is right in MG5.1.5.X ? 11.08 or 10.55 ? I don't know.
>
>
> All the bests,
> Karo
>
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