identifying the source for a change in the cross section - S3 leptoquark and SM

Asked by Ophir Turetz on 2019-06-18

Dear MadGraph experts,
I'm trying to understand the source for a change in a cross-section I'm calculating - whether the change comes from an interference? or something else.
I consider the process 'p p > mu+ mu-' with up to 2 jets in the 5 flavor scheme (5fs). One time when only SM is involved, and a second time when there is a leptoquark S3 particle in addition to the SM.
I'm using a S3 leptoquark model, 'LO_LQ_S3', that can be downloaded from 'https://lqnlo.hepforge.org/downloads/?f=LQ_NLO-0.0.1.zip'.
I've created two restriction files: 'restrict_no_b_mass.dat' and 'restrict_no_b_mass_LQ_YUK_OFF.dat', which content is listed below. In the 'restrict_no_b_mass.dat' file I put the b mass and the b yukawa term with the higgs to be zero (5fs), and set the yukawa terms of the leptoquark with the b and \mu and with the s and \mu to be 0.01. In the 'restrict_no_b_mass_LQ_YUK_OFF.dat' I put all of the above parameters to be zero - essentialy I set the 5fs and decouple the LQ from the SM.
I then use the scripts which are listed below to calculate the cross-section in each case. The results I get are:
1. no_b_mass_LQ_YUK_OFF
   -----------------------------------------
 Cross-section (pb) Events Data
 ============== ===== ====
  1128 +/- 1.2 50000 parton
 1043 +/- 1.8 46237 pythia8

2. no_b_mass
   ------------------
 Cross-section (pb) Events Data
 ============== ===== ====
 1107 +/- 1.3 50000 parton
 1019 +/- 1.8 46010 pythia8

So, obviously there is a change in the cross-section.
Next, I wanted to understand the source for this difference. Hence, I entered the results.html page and went to the table of the leading process 'P0_qq_ll'. This is what I see:
1. no_b_mass_LQ_YUK_OFF
   -----------------------------------------
Graph Cross-Section Error Events (K) Unwgt Luminosity
G2 726.3 0.798 551.474 44246.0 60.9
G1 249.1 0.686 178.74 21987.0 88.3

2. no_b_mass
   ------------------
Graph Cross-Section Error Events (K) Unwgt Luminosity
G2 725.4 0.787 558.72 44925.0 62
G1 220.5 0.802 116.76 14618.0 66.3
G4 9.754e-08 4.53e-09 7.01 29.0 2.97e+08
G3 7.56e-09 2.96e-10 7.038 118.0 1.56e+10

Then, I followed 'https://answers.launchpad.net/mg5amcnlo/+faq/2944' to identify the diagrams that contribute to each integration channel.
Suprisingly I've found out that in both cases the G2 integration channel gets contributions from the Z propogator *only*, and the G1 from the photon propogator *only*.
Thus - why do we see a change in the G1 integration channel cross-section? does it come from an interference with the LQ diagrams? and if so, why are these diagrams not included in the G1 integration channel as well (the diagrams that include a LQ appear only in the G4 and G3 integration channels)? if it's not interference then what is the cause for this change?

Thank you,
Ophir

======================================================================================
restrict_no_b_mass.dat content:
======================================================================================

###################################
## INFORMATION FOR SMINPUTS
###################################
Block SMINPUTS
    1 1.279000e+02 # aEWM1
    2 1.166370e-05 # Gf
    3 1.184000e-01 # aS

###################################
## INFORMATION FOR MASS
###################################
Block MASS
    5 0.000000e+00 # MB
    6 1.720000e+02 # MT
   15 1.777000e+00 # MTA
   23 9.118760e+01 # MZ
   25 1.250000e+02 # MH

###################################
## INFORMATION FOR DECAY
###################################
DECAY 6 1.50833649E+00
DECAY 23 2.4952e+00
DECAY 24 2.085e+00
DECAY 25 4.07e-03

###################################
## INFORMATION FOR YUKAWA
###################################
Block YUKAWA
    5 0.000000e+00 # ymb
    6 1.720000e+02 # ymt
   15 1.777000e+00 # ymtau

###################################
## INFORMATION FOR LQPARAM
###################################
Block lqparam
    1 3.000000e+02 # MS3

###################################
## INFORMATION FOR YUKS3LL
###################################
Block yuks3ll
    1 1 0.000000e+00 # yLL1x1
    1 2 0.000000e+00 # yLL1x2
    1 3 0.000000e+00 # yLL1x3
    2 1 0.000000e+00 # yLL2x1
    2 2 1.000000e-02 # yLL2x2
    2 3 0.000000e+00 # yLL2x3
    3 1 0.000000e+00 # yLL3x1
    3 2 2.000000e-02 # yLL3x2
    3 3 0.000000e+00 # yLL3x3

======================================================================================
restrict_no_b_mass_LQ_YUK_OFF.dat content:
======================================================================================

###################################
## INFORMATION FOR SMINPUTS
###################################
Block SMINPUTS
    1 1.279000e+02 # aEWM1
    2 1.166370e-05 # Gf
    3 1.184000e-01 # aS

###################################
## INFORMATION FOR MASS
###################################
Block MASS
    5 0.000000e+00 # MB
    6 1.720000e+02 # MT
   15 1.777000e+00 # MTA
   23 9.118760e+01 # MZ
   25 1.250000e+02 # MH

###################################
## INFORMATION FOR DECAY
###################################
DECAY 6 1.50833649E+00
DECAY 23 2.4952e+00
DECAY 24 2.085e+00
DECAY 25 4.07e-03

###################################
## INFORMATION FOR YUKAWA
###################################
Block YUKAWA
    5 0.000000e+00 # ymb
    6 1.720000e+02 # ymt
   15 1.777000e+00 # ymtau

###################################
## INFORMATION FOR LQPARAM
###################################
Block lqparam
    1 3.000000e+02 # MS3

###################################
## INFORMATION FOR YUKS3LL
###################################
Block yuks3ll
    1 1 0.000000e+00 # yLL1x1
    1 2 0.000000e+00 # yLL1x2
    1 3 0.000000e+00 # yLL1x3
    2 1 0.000000e+00 # yLL2x1
    2 2 0.000000e+00 # yLL2x2
    2 3 0.000000e+00 # yLL2x3
    3 1 0.000000e+00 # yLL3x1
    3 2 0.000000e+00 # yLL3x2
    3 3 0.000000e+00 # yLL3x3

======================================================================================
The script which is using 'no_b_mass' content:
======================================================================================

import model LO_LQ_S3-no_b_mass
define p = g u c d s u~ c~ d~ s~ b b~
define j = g u c d s u~ c~ d~ s~ b b~
generate p p > mu+ mu- QED<=99 @0
add process p p > mu+ mu- j QED<=99 @1
add process p p > mu+ mu- j j QED<=99 @2
output LQ_YUK_bmu_smu_pp2mumu

launch

shower=Pythia8
analysis=off

set pdlabel lhapdf
set lhaid 263000
set maxjetflavor 5
set asrwgtflavor 5
set nevents=50000
set ebeam1 6500
set ebeam2 6500
set use_syst False
set lqparam ms3 300
set yuks3ll yll2x2 0.01
set yuks3ll yll3x2 0.01
set ickkw 1
set xqcut 50
set ptj 0
set ptb 0
set drjj 0
set drbj 0
set drbb 0

done

======================================================================================
The script which is using 'no_b_mass_LQ_YUK_OFF' content:
======================================================================================

import model LO_LQ_S3-no_b_mass_LQ_YUK_OFF
define p = g u c d s u~ c~ d~ s~ b b~
define j = g u c d s u~ c~ d~ s~ b b~
generate p p > mu+ mu- @0
add process p p > mu+ mu- j @1
add process p p > mu+ mu- j j @2
output LQ_NO_YUK_pp2mumu

launch

shower=Pythia8
analysis=off

set pdlabel lhapdf
set lhaid 263000
set maxjetflavor 5
set asrwgtflavor 5
set nevents=50000
set ebeam1 6500
set ebeam2 6500
set use_syst False
set ickkw 1
set xqcut 50
set ptj 0
set ptb 0
set drjj 0
set drbj 0
set drbb 0

done

Question information

Language:
English Edit question
Status:
Answered
For:
MadGraph5_aMC@NLO Edit question
Assignee:
No assignee Edit question
Last query:
2019-06-18
Last reply:
2019-06-18

>Suprisingly I've found out that in both cases the G2 integration channel gets contributions from the Z propogator *only*, and the G1 from the photon propogator *only*.

This statement is not actually not True. It would be True ONLY if they are no interference at all between any pair of diagram and if the number of channel of integration equals the number of Feynman Diagram.

The diagram enhacement method (hep-ph/0208156) still integrates the full matrix-element but with a pre-factor that favour a given Feynman diagram. So I'm not surprised that G1 and G2 are changed, I would say that this could happen even if the interference term is zero with the LQ diagram. (As long as one interference exists between the four diagram --which should always be the case--)

Cheers,

Olivier

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