# Polarized cross-section issue when including QED

Dear,

we are computing ttbar production cross-section for many polarized cases: ++, +-, -+ and --. We are following the guideline in https:/

When using only QCD,

pp > t t~ QED=0

we find that the polarized cross-sections correctly sum the total cross-section within the uncertainty.

However, when simulating using

pp > t t~ QED=2

we find that there is mismatch between computing polarized cross-sections separately and adding them, that computing altogether. This disagreement is far beyond the uncertainty.

We find, for instance, that (using QED=2 for LHAPDF 262000 and a dynamical renormalization scale of transverse mass / 4)

sigma(++) = 229.5 pb

sigma(- -) = 230.1 pb

sigma (++ plus - - ) = 430.6 pb

sigma (+ - ) = 115.9 pb

sigma (- + ) = 116.6 pb

sigma ( + - plus - + ) = 205.1 pb

sigma (pp > tt~ total) = 605.5 pb

All uncertainties below 1pb.

Any ideas of why this could be? Do you see a workaround ?

Thank you so much! Ezequiel.

## Question information

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- Last query:
- 2019-08-26

- Last reply:
- 2019-09-11

This is a scale issue pass infix scale computation and the problem will disappear (or another dynamic scale than the default

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From: <email address hidden> <email address hidden> on behalf of Ezequiel Alvarez <email address hidden>

Sent: Saturday, August 24, 2019 1:08:33 AM

To: Olivier Mattelaer <email address hidden>

Subject: [Question #683242]: Polarized cross-section issue when including QED

New question #683242 on MadGraph5_aMC@NLO:

https:/

Dear,

we are computing ttbar production cross-section for many polarized cases: ++, +-, -+ and --. We are following the guideline in https:/

When using only QCD,

pp > t t~ QED=0

we find that the polarized cross-sections correctly sum the total cross-section within the uncertainty.

However, when simulating using

pp > t t~ QED=2

we find that there is mismatch between computing polarized cross-sections separately and adding them, that computing altogether. This disagreement is far beyond the uncertainty.

We find, for instance, that (using QED=2 for LHAPDF 262000 and a dynamical renormalization scale of transverse mass / 4)

sigma(++) = 229.5 pb

sigma(- -) = 230.1 pb

sigma (++ plus - - ) = 430.6 pb

sigma (+ - ) = 115.9 pb

sigma (- + ) = 116.6 pb

sigma ( + - plus - + ) = 205.1 pb

sigma (pp > tt~ total) = 605.5 pb

All uncertainties below 1pb.

Any ideas of why this could be? Do you see a workaround ?

Thank you so much! Ezequiel.

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Ezequiel Alvarez (sequi76) said : | #2 |

Dear Olivier,

thank you very much for your prompt answer!

We have modified run_card.dat relevant lines to what we copy below this message. We still have the same problem. For instance, now we have (always ttbar at QED=2 and the other setting remained unchanged except for the ren and fact scales)

sigma(++) = 242.4 pb

sigma(- -) = 241.3 pb

sigma (++ plus - - ) = 450.4 pb

sigma (+ - ) = 128.6 pb

sigma (- + ) = 128.2 pb

sigma ( + - plus - + ) = 227.1 pb

sigma (pp > tt~ total) = 648 pb

Therefore, following what we understood from your message, i.e. setting fixed ren. and fact. scales, we are still having the same problem.

Thank you very much, Ezequiel.

# Renormalization and factorization scales *

#******

True = fixed_ren_scale ! if .true. use fixed ren scale

True = fixed_fac_scale ! if .true. use fixed fac scale

91.188 = scale ! fixed ren scale

91.188 = dsqrt_q2fact1 ! fixed fact scale for pdf1

91.188 = dsqrt_q2fact2 ! fixed fact scale for pdf2

3 = dynamical_

1.0 = scalefact ! scale factor for event-by-event scales

Launchpad Janitor (janitor) said : | #3 |

This question was expired because it remained in the 'Open' state without activity for the last 15 days.

Sorry for the delay on this.

I have run this just now and I do not reproduce your issue. (I used 100GeV as fixed scale)

RL: 118.6 ± 0.21

LR:118.8 ± 0.2

RR: 227.7 ± 0.32

LL: 227.7 ± 0.32

SUM: 692.5999999999999

ALL: 692.7 ± 1.3

Therefore you should have made a mistake in the hacking of the files.

Cheers,

Olivier