Change helicity option in MG5aMC re-weighting

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
>
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Hello Olivier,
thank you for the quick response.