Change helicity option in MG5aMC re-weighting

Asked by Alexandros Marantis on 2021-02-14

Dear experts,
I'm writing you regarding the MadGraph re-weighting tool and more specifically, the use of the "change helicity" flag in the re-weighting procedure.
I am using an EFT dim-8 QGC model (http://feynrules.irmp.ucl.ac.be/wiki/AnomalousGaugeCoupling#no1) to generate samples with one operator non-zero each time. Then, I re-weight the sample to another value of the same operator or to another operator, with two different ways:
1. with the MG re-weighting tool,
2. by-hand using the ratio of the Matrix Elements of each event following the formula in Olivier's paper (https://arxiv.org/pdf/1607.00763.pdf)
W_new = |M_new|^2 / |M_orig|^2 * W_orig [eq.1]
The Matrix Element is calculated for each event by using the MG tool (check_sa.f) with a) the original value (ME_orig) and b) the value in which we want to re-weight (ME_new).

When I re-weight between values of the same operator, these two methods of re-weighting give identical results. For example https://docs.google.com/presentation/d/1qTqSTlthH6zOcKoiS66TglF4xOM-8m7lVSq7GOYLbko/edit?usp=sharing (slide 1), I re-weight a sample generated with Ft0 = 1 TeV^-4 to Ft0 = 0.3 TeV^-4. The cross sections are close to the cross section of the original sample generated to Ft0 = 0.3 TeV^-4.

Nevertheless, when I re-weight to another operator, the results of the two methods differ. In slides 2-3 are two examples (Ft0->Ft1, Ft9->Ft8).
In my understanding, the second method, which I'm performing by hand calculating the Matrix Element for each event , is the first method but with "change helicity False". The option "change helicity True" is the default for the re-weighting, but in the cases where we re-weight between different operators, the mode "change helicity False" gives us cross sections closer to the cross section of an "original" sample (generated).
In the paper mentioned above, we read about the helicity: "The helicity state of the external states of a parton-level event is optional in the LHEF convention, yet some programs use this information to decay the heavy state with an approximated spin-correlation matrix. In this case it is easy to modify Eq.1 to correctly take into account the helicity information".
I don't intend to use programs like BRIDGE which is mentioned in the paper. My purpose for the re-weighted samples is to make distributions of observables like the invariant mass of the system, the p_T of leading lepton or boson etc.
So, my question is, if I don't want to use the helicity of the events is it safe to re-weight with the "change helicity False" flag when I'm reweighting between different operators?

Thank you in advance,
Alexandros

Question information

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MadGraph5_aMC@NLO Edit question
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Solved by:
Alexandros Marantis
Solved:
2021-02-14
Last query:
2021-02-14
Last reply:
2021-02-14

As long as you are sure that you do not use such information then they are no problem.

Cheers,

Olivier

> On 14 Feb 2021, at 23:41, Alexandros Marantis <email address hidden> wrote:
>
> New question #695538 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/695538
>
> Dear experts,
> I'm writing you regarding the MadGraph re-weighting tool and more specifically, the use of the "change helicity" flag in the re-weighting procedure.
> I am using an EFT dim-8 QGC model (http://feynrules.irmp.ucl.ac.be/wiki/AnomalousGaugeCoupling#no1) to generate samples with one operator non-zero each time. Then, I re-weight the sample to another value of the same operator or to another operator, with two different ways:
> 1. with the MG re-weighting tool,
> 2. by-hand using the ratio of the Matrix Elements of each event following the formula in Olivier's paper (https://arxiv.org/pdf/1607.00763.pdf)
> W_new = |M_new|^2 / |M_orig|^2 * W_orig [eq.1]
> The Matrix Element is calculated for each event by using the MG tool (check_sa.f) with a) the original value (ME_orig) and b) the value in which we want to re-weight (ME_new).
>
> When I re-weight between values of the same operator, these two methods of re-weighting give identical results. For example https://docs.google.com/presentation/d/1qTqSTlthH6zOcKoiS66TglF4xOM-8m7lVSq7GOYLbko/edit?usp=sharing (slide 1), I re-weight a sample generated with Ft0 = 1 TeV^-4 to Ft0 = 0.3 TeV^-4. The cross sections are close to the cross section of the original sample generated to Ft0 = 0.3 TeV^-4.
>
> Nevertheless, when I re-weight to another operator, the results of the two methods differ. In slides 2-3 are two examples (Ft0->Ft1, Ft9->Ft8).
> In my understanding, the second method, which I'm performing by hand calculating the Matrix Element for each event , is the first method but with "change helicity False". The option "change helicity True" is the default for the re-weighting, but in the cases where we re-weight between different operators, the mode "change helicity False" gives us cross sections closer to the cross section of an "original" sample (generated).
> In the paper mentioned above, we read about the helicity: "The helicity state of the external states of a parton-level event is optional in the LHEF convention, yet some programs use this information to decay the heavy state with an approximated spin-correlation matrix. In this case it is easy to modify Eq.1 to correctly take into account the helicity information".
> I don't intend to use programs like BRIDGE which is mentioned in the paper. My purpose for the re-weighted samples is to make distributions of observables like the invariant mass of the system, the p_T of leading lepton or boson etc.
> So, my question is, if I don't want to use the helicity of the events is it safe to re-weight with the "change helicity False" flag when I'm reweighting between different operators?
>
> Thank you in advance,
> Alexandros
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Alexandros Marantis (amaranti) said : #2

Hello Olivier,
thank you for the quick response.