Fix problems with MC with user-defined minimum partonic energy

[cschwan]

When running processes which with a custom and large tau min (the partonic centre-of-mass energy), for instance NNPDF_TTB_14TEV_40_PHENO, the generation is stuck at step 0 of the MC. Running one of the stuck processes manually, the standard output is:

tau_min   1     2 : 0.00000E+00      --       0.34000E+03
tau_min   2     2 : 0.34000E+03  0.34000E+03  0.34000E+03
tau_min   3     2 : 0.34000E+03  0.34000E+03  0.34000E+03
tau_min   4     2 : 0.00000E+00      --       0.34000E+03
tau_min   5     2 : 0.00000E+00      --       0.34000E+03
tau_min   6     2 : 0.00000E+00      --       0.34000E+03
tau_min   7     2 : 0.00000E+00      --       0.34000E+03
 REAL 4: keeping split order            1
 REAL 2: keeping split order            1
 REAL 3: keeping split order            1
 REAL 5: keeping split order            1
tau_min   1     3 : 0.00000E+00      --       0.34000E+03
tau_min   2     3 : 0.34000E+03  0.34000E+03  0.34000E+03
tau_min   3     3 : 0.34000E+03  0.34000E+03  0.34000E+03
tau_min   4     3 : 0.00000E+00      --       0.34000E+03
tau_min   5     3 : 0.00000E+00      --       0.34000E+03
tau_min   6     3 : 0.00000E+00      --       0.34000E+03
tau_min   7     3 : 0.00000E+00      --       0.34000E+03
tau_min   1     4 : 0.00000E+00      --       0.34000E+03
tau_min   2     4 : 0.34000E+03  0.34000E+03  0.34000E+03
tau_min   3     4 : 0.34000E+03  0.34000E+03  0.34000E+03
tau_min   4     4 : 0.00000E+00      --       0.34000E+03
tau_min   5     4 : 0.00000E+00      --       0.34000E+03
tau_min   6     4 : 0.00000E+00      --       0.34000E+03
tau_min   7     4 : 0.00000E+00      --       0.34000E+03
 Error #2 in function Lambda:   241.93210601806643        29756.250000000000        29756.250000000000      -0.42601990979335702

Apparently there's a numerical problem when calculating the lambda function in genps_fks.f, which could cause this problem.

[marcozaro]

Why is tau=340? this is even lower than 2*mt ...

Marco Zaro

On Tue, Dec 28, 2021 at 9:31 PM Christopher Schwan @.***> wrote:

Assigned #120 https://github.com/NNPDF/runcards/issues/120 to @marcozaro https://github.com/marcozaro.

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[cschwan]

@marcozaro because I set it manually to that value. Is that bad? Is it the minimum centre-of-mass energy automatically set to the correct value in the case of top-pair production?

[marcozaro]

Hi, in the case of massive particles, the three tau_min's are always set to the sum of masses. The fact that it is hardcoded below may introduce bugs or unwanted behaviours

cheers,

Marco

On 10 Jan 2022, at 13:32, Christopher Schwan @.***> wrote:

@marcozaro https://github.com/marcozaro because I set it manually to that value. Is that bad? Is it the minimum centre-of-mass energy automatically set to the correct value in the case of top-pair production?

— Reply to this email directly, view it on GitHub https://github.com/NNPDF/runcards/issues/120#issuecomment-1008831142, or unsubscribe https://github.com/notifications/unsubscribe-auth/AHECN5D66GZKDDSPWA5LFRLUVLGUDANCNFSM5K4XQWUA. Triage notifications on the go with GitHub Mobile for iOS https://apps.apple.com/app/apple-store/id1477376905?ct=notification-email&mt=8&pt=524675 or Android https://play.google.com/store/apps/details?id=com.github.android&referrer=utm_campaign%3Dnotification-email%26utm_medium%3Demail%26utm_source%3Dgithub. You are receiving this because you were mentioned.

[cschwan]

OK in that case removing it should fix the problem.