First event weight not nominal
Hi MG5_aMC team,
We have observed in the ATLAS software in v2.6.1 and have reproduced with a standalone setup using v2.6.6 that the first weight in the LHE file is not the nominal weight. E.g. with:
[--mur=0.5,1,2', '--muf=0.5,1,2', '--pdf=errorset'] = systematics_
We see something like the following in the LHEF:
<initrwgt>
<weightgroup name="Central scale variation" combine="envelope">
<weight id="1" MUR="0.5" MUF="0.5" PDF="260000" > MUR=0.5 MUF=0.5 </weight>
<weight id="2" MUR="0.5" MUF="0.5" DYN_SCALE="1" PDF="260000" > MUR=0.5 MUF=0.5 dyn_scale_
<weight id="3" MUR="0.5" MUF="0.5" DYN_SCALE="2" PDF="260000" > MUR=0.5 MUF=0.5 dyn_scale_choice=HT </weight>
...
<event>
4 1 +4.5600700e-02 1.74785200e+02 7.54677100e-03 1.07475900e-01
21 -1 0 0 501 502 +0.0000000000e+00 +0.0000000000e+00 +9.0884757352e+01 9.0884757352e+01 0.0000000000e+00 0.0000e+00 1.0000e+00
21 -1 0 0 502 503 -0.0000000000e+00 -0.0000000000e+00 -3.7554094781e+02 3.7554094781e+02 0.0000000000e+00 0.0000e+00 1.0000e+00
6 1 1 2 501 0 +1.5198609831e+01 -1.9745396748e+01 -6.6786755993e+01 1.8711052672e+02 1.7300000000e+02 0.0000e+00 1.0000e+00
-6 1 1 2 0 503 -1.5198609831e+01 +1.9745396748e+01 -2.1786943447e+02 2.7931517844e+02 1.7300000000e+02 0.0000e+00 1.0000e+00
<mgrwt>
<rscale> 2 0.17478524E+
<asrwt>0</asrwt>
<pdfrwt beam="1"> 1 21 0.13982270E-01 0.17478524E+
<pdfrwt beam="2"> 1 21 0.57775531E-01 0.17478524E+
<totfact> 0.14259007E+
</mgrwt>
<rwgt>
<wgt id='1'> +5.7582231e-02 </wgt>
<wgt id='2'> +7.0903054e-02 </wgt>
<wgt id='3'> +4.5600700e-02 </wgt>
...
This is pretty dangerous since it puts one of the scale variations as the first weight which is the default weight for the entire analysis chain... Also, I believe it breaks the LHEFv3 convention: https:/
Could you check this and in future ensure the nominal weight is at "id=1"?
Best,
Josh
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