Negative weights in LO MLM matched MC sample

Asked by Sung Hak Lim on 2017-06-19

Dear MG5 team,

I'm just playing with LO MLM matched MC samples and I noticed that there are some events with negative weight.

Process:
   generate p p > t t~
   add process p p > t t~ j
   add process p p > t t~ j j

Additiona command:
   set run_card pdlabel lhapdf
   set run_card lhaid 261000
   set run_card xqcut 80.0
   set run_card sys_pdf 261000

I found two events having negative weights among 10000 events. Here is one of lhe output:

<event>
 6 3 -6.4592046e+02 3.01567200e+02 7.54677100e-03 9.99998300e-02
       21 -1 0 0 502 503 +0.0000000000e+00 +0.0000000000e+00 +7.5275943115e+02 7.5275943115e+02 0.0000000000e+00 0.0000e+00 1.0000e+00
       -1 -1 0 0 0 501 -0.0000000000e+00 -0.0000000000e+00 -3.2013906455e+03 3.2013906455e+03 0.0000000000e+00 0.0000e+00 -1.0000e+00
        6 1 1 2 502 0 +1.1403921787e+02 -1.3429936222e+02 +2.5588744069e+02 3.5559618138e+02 1.7300000000e+02 0.0000e+00 1.0000e+00
       -6 1 1 2 0 501 -3.1363459834e+02 +2.9217926105e+02 +2.9444846113e+02 5.4805499553e+02 1.7300000000e+02 0.0000e+00 1.0000e+00
       21 1 1 2 504 503 +8.3675008823e+01 -1.2766593668e+02 -2.1348620803e+02 2.6244325007e+02 0.0000000000e+00 0.0000e+00 -1.0000e+00
       -1 1 1 2 0 504 +1.1592037164e+02 -3.0213962140e+01 -2.7854809082e+03 2.7880556497e+03 0.0000000000e+00 0.0000e+00 -1.0000e+00
<scales pt_clust_3="13000.00000" pt_clust_4="13000.00000" pt_clust_5="208.77878" pt_clust_6="119.79322"></scales>
<mgrwt>
<rscale> 2 0.30156721E+03</rscale>
<asrwt> 2 0.11979322E+03 0.20877878E+03</asrwt>
<pdfrwt beam="1"> 1 21 0.11580914E+00 0.30156721E+03</pdfrwt>
<pdfrwt beam="2"> 1 -1 0.49252165E+00 0.11979322E+03</pdfrwt>
<totfact>-0.40072237E-05</totfact>
</mgrwt>
<clustering>
<clus scale=" 119.793"> 2 6 1 -1</clus>
<clus scale=" 208.779"> 2 5 1 -1</clus>
<clus scale=" 301.567"> 2 4 1 -1</clus>
<clus scale=" 301.567"> 1 3 2 -1</clus>
</clustering>
<rwgt>
<wgt id='1'> -1.1139163e+03 </wgt>
<wgt id='2'> -5.8886622e+02 </wgt>
<wgt id='3'> -5.7381283e+02 </wgt>
<wgt id='4'> -7.8855949e+02 </wgt>
<wgt id='5'> -3.5521572e+02 </wgt>
<wgt id='6'> -9.0882155e+02 </wgt>
<wgt id='7'> -4.8044391e+02 </wgt>
<wgt id='8'> -4.6816215e+02 </wgt>
<wgt id='9'> -6.4336956e+02 </wgt>
<wgt id='10'> -2.8981324e+02 </wgt>
<wgt id='11'> -7.5616239e+02 </wgt>
<wgt id='12'> -3.9974142e+02 </wgt>
<wgt id='13'> -3.8952269e+02 </wgt>
<wgt id='14'> -5.3529966e+02 </wgt>
<wgt id='15'> -2.4113190e+02 </wgt>
<wgt id='16'> -9.5241617e+02 </wgt>
<wgt id='17'> -5.1341283e+02 </wgt>
<wgt id='18'> -5.0062564e+02 </wgt>
<wgt id='19'> -6.8225836e+02 </wgt>
<wgt id='20'> -3.1366583e+02 </wgt>
<wgt id='21'> -7.7705688e+02 </wgt>
<wgt id='22'> -4.1888303e+02 </wgt>
<wgt id='23'> -4.0845022e+02 </wgt>
<wgt id='24'> -5.5664064e+02 </wgt>
<wgt id='25'> -2.5591354e+02 </wgt>
<wgt id='26'> -6.4653087e+02 </wgt>
<wgt id='27'> -3.4852122e+02 </wgt>
<wgt id='28'> -3.3984086e+02 </wgt>
<wgt id='29'> -4.6313901e+02 </wgt>
<wgt id='30'> -2.1292650e+02 </wgt>
<wgt id='31'> -8.2203756e+02 </wgt>
<wgt id='32'> -4.5103622e+02 </wgt>
<wgt id='33'> -4.4006512e+02 </wgt>
<wgt id='34'> -5.9523944e+02 </wgt>
<wgt id='35'> -2.7873550e+02 </wgt>
<wgt id='36'> -6.7068364e+02 </wgt>
<wgt id='37'> -3.6799123e+02 </wgt>
<wgt id='38'> -3.5904013e+02 </wgt>
<wgt id='39'> -4.8564369e+02 </wgt>
<wgt id='40'> -2.2741459e+02 </wgt>
<wgt id='41'> -5.5802565e+02 </wgt>
<wgt id='42'> -3.0617796e+02 </wgt>
<wgt id='43'> -2.9873042e+02 </wgt>
<wgt id='44'> -4.0406776e+02 </wgt>
<wgt id='45'> -1.8921466e+02 </wgt>
<wgt id='46'> -9.2593138e+02 </wgt>
<wgt id='47'> -5.0183402e+02 </wgt>
<wgt id='48'> -4.8954850e+02 </wgt>
<wgt id='49'> -6.6321731e+02 </wgt>
<wgt id='50'> -3.0889167e+02 </wgt>
<wgt id='51'> -7.5544848e+02 </wgt>
<wgt id='52'> -4.0943612e+02 </wgt>
<wgt id='53'> -3.9941261e+02 </wgt>
<wgt id='54'> -5.4110544e+02 </wgt>
<wgt id='55'> -2.5201840e+02 </wgt>
<wgt id='56'> -6.2855213e+02 </wgt>
<wgt id='57'> -3.4066115e+02 </wgt>
<wgt id='58'> -3.3232134e+02 </wgt>
<wgt id='59'> -4.5021334e+02 </wgt>
<wgt id='60'> -2.0968564e+02 </wgt>
<wgt id='61'> -7.9168605e+02 </wgt>
<wgt id='62'> -4.3753236e+02 </wgt>
<wgt id='63'> -4.2710884e+02 </wgt>
<wgt id='64'> -5.7381283e+02 </wgt>
<wgt id='65'> -2.7276035e+02 </wgt>
<wgt id='66'> -3.5697370e+02 </wgt>
<wgt id='67'> -3.4846937e+02 </wgt>
<wgt id='68'> -4.6816215e+02 </wgt>
<wgt id='69'> -2.2253959e+02 </wgt>
<wgt id='70'> -5.3742207e+02 </wgt>
<wgt id='71'> -2.9701110e+02 </wgt>
<wgt id='72'> -2.8993528e+02 </wgt>
<wgt id='73'> -3.8952269e+02 </wgt>
<wgt id='74'> -1.8515854e+02 </wgt>
<wgt id='75'> -6.8331019e+02 </wgt>
<wgt id='76'> -3.8437477e+02 </wgt>
<wgt id='77'> -3.7544162e+02 </wgt>
<wgt id='78'> -5.0062564e+02 </wgt>
<wgt id='79'> -2.4238532e+02 </wgt>
<wgt id='80'> -5.5749881e+02 </wgt>
<wgt id='81'> -3.1360351e+02 </wgt>
<wgt id='82'> -3.0631514e+02 </wgt>
<wgt id='83'> -4.0845022e+02 </wgt>
<wgt id='84'> -1.9775722e+02 </wgt>
<wgt id='85'> -4.6385303e+02 </wgt>
<wgt id='86'> -2.6092601e+02 </wgt>
<wgt id='87'> -2.5486190e+02 </wgt>
<wgt id='88'> -3.3984086e+02 </wgt>
<wgt id='89'> -1.6453898e+02 </wgt>
<wgt id='90'> -7.8227600e+02 </wgt>
<wgt id='91'> -4.3293762e+02 </wgt>
<wgt id='92'> -4.2273455e+02 </wgt>
<wgt id='93'> -5.6582394e+02 </wgt>
<wgt id='94'> -2.7115133e+02 </wgt>
<wgt id='95'> -6.3824299e+02 </wgt>
<wgt id='96'> -3.5322495e+02 </wgt>
<wgt id='97'> -3.4490047e+02 </wgt>
<wgt id='98'> -4.6164418e+02 </wgt>
<wgt id='99'> -2.2122682e+02 </wgt>
<wgt id='100'> -5.3103422e+02 </wgt>
<wgt id='101'> -2.9389204e+02 </wgt>
<wgt id='102'> -2.8696587e+02 </wgt>
<wgt id='103'> -3.8409957e+02 </wgt>
<wgt id='104'> -1.8406628e+02 </wgt>
<wgt id='105'> -6.6885842e+02 </wgt>
<wgt id='106'> -3.7746388e+02 </wgt>
<wgt id='107'> -3.6881670e+02 </wgt>
<wgt id='108'> -4.8954850e+02 </wgt>
<wgt id='109'> -2.3943453e+02 </wgt>
<wgt id='110'> -5.4570790e+02 </wgt>
<wgt id='111'> -3.0796506e+02 </wgt>
<wgt id='112'> -3.0091000e+02 </wgt>
<wgt id='113'> -3.9941261e+02 </wgt>
<wgt id='114'> -1.9534973e+02 </wgt>
<wgt id='115'> -4.5404270e+02 </wgt>
<wgt id='116'> -2.5623468e+02 </wgt>
<wgt id='117'> -2.5036469e+02 </wgt>
<wgt id='118'> -3.3232134e+02 </wgt>
<wgt id='119'> -1.6253589e+02 </wgt>
<wgt id='120'> -5.7729673e+02 </wgt>
<wgt id='121'> -3.3160426e+02 </wgt>
<wgt id='122'> -3.2420106e+02 </wgt>
<wgt id='123'> -4.2710884e+02 </wgt>
<wgt id='124'> -2.1277071e+02 </wgt>
<wgt id='125'> -4.7100460e+02 </wgt>
<wgt id='126'> -2.7054913e+02 </wgt>
<wgt id='127'> -2.6450902e+02 </wgt>
<wgt id='128'> -3.4846937e+02 </wgt>
<wgt id='129'> -1.7359527e+02 </wgt>
<wgt id='130'> -3.9188767e+02 </wgt>
<wgt id='131'> -2.2510368e+02 </wgt>
<wgt id='132'> -2.2007816e+02 </wgt>
<wgt id='133'> -2.8993528e+02 </wgt>
<wgt id='134'> -1.4443563e+02 </wgt>
<wgt id='135'> -6.4592046e+02 </wgt>
<wgt id='136'> +4.8737257e+02 </wgt>
<wgt id='137'> -5.0323189e+02 </wgt>
<wgt id='138'> -1.6386290e+03 </wgt>
<wgt id='139'> -1.5989304e+03 </wgt>
<wgt id='140'> -9.2676826e+02 </wgt>
<wgt id='141'> -2.8853283e+02 </wgt>
<wgt id='142'> -1.0062295e+03 </wgt>
<wgt id='143'> -1.6506252e+03 </wgt>
<wgt id='144'> +1.7255055e+02 </wgt>
<wgt id='145'> -9.0984388e+01 </wgt>
<wgt id='146'> +4.0993576e+02 </wgt>
<wgt id='147'> -6.0562481e+02 </wgt>
<wgt id='148'> -8.4948641e+02 </wgt>
<wgt id='149'> +3.1883291e+02 </wgt>
<wgt id='150'> -1.1049902e+03 </wgt>
<wgt id='151'> -1.1959257e+03 </wgt>
<wgt id='152'> -4.3249898e+01 </wgt>
<wgt id='153'> -3.7486750e+02 </wgt>
<wgt id='154'> -2.9306541e+03 </wgt>
<wgt id='155'> -1.2718238e+03 </wgt>
<wgt id='156'> -2.2608081e+02 </wgt>
<wgt id='157'> +5.0891409e+01 </wgt>
<wgt id='158'> -5.1265770e+02 </wgt>
<wgt id='159'> +2.8941238e+02 </wgt>
<wgt id='160'> -6.0211538e+02 </wgt>
<wgt id='161'> +3.1028151e+02 </wgt>
<wgt id='162'> -6.8215556e+02 </wgt>
<wgt id='163'> +1.4104334e+02 </wgt>
<wgt id='164'> -4.7916982e+02 </wgt>
<wgt id='165'> -1.0559768e+03 </wgt>
<wgt id='166'> -1.7099236e+03 </wgt>
<wgt id='167'> -1.0442194e+03 </wgt>
<wgt id='168'> -1.9237848e+03 </wgt>
<wgt id='169'> -4.4079193e+02 </wgt>
<wgt id='170'> -7.6346762e+02 </wgt>
<wgt id='171'> -1.4291308e+03 </wgt>
<wgt id='172'> -6.8483737e+02 </wgt>
<wgt id='173'> -4.1995218e+02 </wgt>
<wgt id='174'> -2.2661533e+02 </wgt>
<wgt id='175'> +2.8852919e+02 </wgt>
<wgt id='176'> -2.6579120e+02 </wgt>
<wgt id='177'> -6.3252097e+02 </wgt>
<wgt id='178'> -1.0238816e+03 </wgt>
<wgt id='179'> -1.5416082e+03 </wgt>
<wgt id='180'> -4.2291052e+02 </wgt>
<wgt id='181'> -2.6101943e+02 </wgt>
<wgt id='182'> -5.5902035e+01 </wgt>
<wgt id='183'> -3.6772813e+02 </wgt>
<wgt id='184'> +4.6481857e+02 </wgt>
<wgt id='185'> -6.6669643e+02 </wgt>
<wgt id='186'> -8.0543815e+02 </wgt>
<wgt id='187'> -4.9155148e+02 </wgt>
<wgt id='188'> +8.9879205e+02 </wgt>
<wgt id='189'> +3.6226317e+02 </wgt>
<wgt id='190'> -1.2256650e+03 </wgt>
<wgt id='191'> -2.3195069e+03 </wgt>
<wgt id='192'> +2.3768053e+02 </wgt>
<wgt id='193'> +3.9529091e+02 </wgt>
<wgt id='194'> -1.8996454e+03 </wgt>
<wgt id='195'> -1.8186350e+03 </wgt>
<wgt id='196'> -3.8857898e+02 </wgt>
<wgt id='197'> -2.1777700e+03 </wgt>
<wgt id='198'> -4.5997124e+02 </wgt>
<wgt id='199'> -1.3749764e+03 </wgt>
<wgt id='200'> -5.2982787e+02 </wgt>
<wgt id='201'> -3.3017525e+02 </wgt>
<wgt id='202'> -4.8711169e+02 </wgt>
<wgt id='203'> -5.0578035e+02 </wgt>
<wgt id='204'> -7.2565567e+02 </wgt>
<wgt id='205'> +9.2447450e+01 </wgt>
<wgt id='206'> -5.5588809e+02 </wgt>
<wgt id='207'> -3.7479926e+01 </wgt>
<wgt id='208'> -2.0569868e+03 </wgt>
<wgt id='209'> -1.1435902e+03 </wgt>
<wgt id='210'> -1.8443050e+03 </wgt>
<wgt id='211'> -6.6486324e+02 </wgt>
<wgt id='212'> -2.1077821e+03 </wgt>
<wgt id='213'> -1.3276187e+03 </wgt>
<wgt id='214'> -6.7330735e+02 </wgt>
<wgt id='215'> -3.2154356e+02 </wgt>
<wgt id='216'> +2.3119580e+01 </wgt>
<wgt id='217'> -3.0857749e+02 </wgt>
<wgt id='218'> -1.3801350e+02 </wgt>
<wgt id='219'> +6.6368143e+02 </wgt>
<wgt id='220'> -8.9433767e+01 </wgt>
<wgt id='221'> -1.4253144e+02 </wgt>
<wgt id='222'> -7.3266384e+02 </wgt>
<wgt id='223'> -1.2742273e+03 </wgt>
<wgt id='224'> -1.1878992e+03 </wgt>
<wgt id='225'> -8.7450714e+02 </wgt>
<wgt id='226'> -7.5166062e+02 </wgt>
<wgt id='227'> +1.5398289e+01 </wgt>
<wgt id='228'> -3.4708738e+02 </wgt>
<wgt id='229'> -9.0113730e+02 </wgt>
<wgt id='230'> -6.3348298e+02 </wgt>
<wgt id='231'> -8.0245297e+02 </wgt>
<wgt id='232'> -6.6128191e+02 </wgt>
<wgt id='233'> -9.4782979e+02 </wgt>
<wgt id='234'> -1.8999447e+02 </wgt>
<wgt id='235'> -4.6293795e+02 </wgt>
</rwgt>
</event>

I roughly have idea that unweighted LO sample have positive definite weights. The only possiblity that I can imagine is that such events are located at matching boundary and it can be vetoed or not vetoed depending on PDF replica and scale choice.

  === Results Summary for run: run_01 tag: tag_1 ===

     Cross-section : 645.4 +- 1.327 pb
     Nb of events : 10000
     Pythia8 merged cross-sections are:
      > Merging scale = 105 : 445.17 +/- 0.67 [pb]
      > Merging scale = 157.5 : 454.28 +/- 0.68 [pb]
      > Merging scale = 210 : 449.82 +/- 0.67 [pb]
     Notice that because Systematics computation is turned on, the merging did not veto events but modified their weights instead.
     The resulting hepmc/stdhep file should therefore be use with those weights.

However, if some of events having weight zero then it looks okay but still sign inversion somewhat hard to understand at a glance. May I know what's happening behind scene?

Have a nice day!

Best wishes,
Sung Hak Lim

Question information

Language:
English Edit question
Status:
Solved
For:
MadGraph5_aMC@NLO Edit question
Assignee:
No assignee Edit question
Solved by:
Olivier Mattelaer
Solved:
2017-06-19
Last query:
2017-06-19
Last reply:
2017-06-19

Hi,

Which PDF set are you using?
If your PDF set is not definite positive (like a lot of NLO ones) then indeed you can have negative contribution.

Cheers,

Olivier

> On 19 Jun 2017, at 08:59, Sung Hak Lim <email address hidden> wrote:
>
> New question #644186 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/644186
>
> Dear MG5 team,
>
> I'm just playing with LO MLM matched MC samples and I noticed that there are some events with negative weight.
>
> Process:
> generate p p > t t~
> add process p p > t t~ j
> add process p p > t t~ j j
>
> Additiona command:
> set run_card pdlabel lhapdf
> set run_card lhaid 261000
> set run_card xqcut 80.0
> set run_card sys_pdf 261000
>
> I found two events having negative weights among 10000 events. Here is one of lhe output:
>
> <event>
> 6 3 -6.4592046e+02 3.01567200e+02 7.54677100e-03 9.99998300e-02
> 21 -1 0 0 502 503 +0.0000000000e+00 +0.0000000000e+00 +7.5275943115e+02 7.5275943115e+02 0.0000000000e+00 0.0000e+00 1.0000e+00
> -1 -1 0 0 0 501 -0.0000000000e+00 -0.0000000000e+00 -3.2013906455e+03 3.2013906455e+03 0.0000000000e+00 0.0000e+00 -1.0000e+00
> 6 1 1 2 502 0 +1.1403921787e+02 -1.3429936222e+02 +2.5588744069e+02 3.5559618138e+02 1.7300000000e+02 0.0000e+00 1.0000e+00
> -6 1 1 2 0 501 -3.1363459834e+02 +2.9217926105e+02 +2.9444846113e+02 5.4805499553e+02 1.7300000000e+02 0.0000e+00 1.0000e+00
> 21 1 1 2 504 503 +8.3675008823e+01 -1.2766593668e+02 -2.1348620803e+02 2.6244325007e+02 0.0000000000e+00 0.0000e+00 -1.0000e+00
> -1 1 1 2 0 504 +1.1592037164e+02 -3.0213962140e+01 -2.7854809082e+03 2.7880556497e+03 0.0000000000e+00 0.0000e+00 -1.0000e+00
> <scales pt_clust_3="13000.00000" pt_clust_4="13000.00000" pt_clust_5="208.77878" pt_clust_6="119.79322"></scales>
> <mgrwt>
> <rscale> 2 0.30156721E+03</rscale>
> <asrwt> 2 0.11979322E+03 0.20877878E+03</asrwt>
> <pdfrwt beam="1"> 1 21 0.11580914E+00 0.30156721E+03</pdfrwt>
> <pdfrwt beam="2"> 1 -1 0.49252165E+00 0.11979322E+03</pdfrwt>
> <totfact>-0.40072237E-05</totfact>
> </mgrwt>
> <clustering>
> <clus scale=" 119.793"> 2 6 1 -1</clus>
> <clus scale=" 208.779"> 2 5 1 -1</clus>
> <clus scale=" 301.567"> 2 4 1 -1</clus>
> <clus scale=" 301.567"> 1 3 2 -1</clus>
> </clustering>
> <rwgt>
> <wgt id='1'> -1.1139163e+03 </wgt>
> <wgt id='2'> -5.8886622e+02 </wgt>
> <wgt id='3'> -5.7381283e+02 </wgt>
> <wgt id='4'> -7.8855949e+02 </wgt>
> <wgt id='5'> -3.5521572e+02 </wgt>
> <wgt id='6'> -9.0882155e+02 </wgt>
> <wgt id='7'> -4.8044391e+02 </wgt>
> <wgt id='8'> -4.6816215e+02 </wgt>
> <wgt id='9'> -6.4336956e+02 </wgt>
> <wgt id='10'> -2.8981324e+02 </wgt>
> <wgt id='11'> -7.5616239e+02 </wgt>
> <wgt id='12'> -3.9974142e+02 </wgt>
> <wgt id='13'> -3.8952269e+02 </wgt>
> <wgt id='14'> -5.3529966e+02 </wgt>
> <wgt id='15'> -2.4113190e+02 </wgt>
> <wgt id='16'> -9.5241617e+02 </wgt>
> <wgt id='17'> -5.1341283e+02 </wgt>
> <wgt id='18'> -5.0062564e+02 </wgt>
> <wgt id='19'> -6.8225836e+02 </wgt>
> <wgt id='20'> -3.1366583e+02 </wgt>
> <wgt id='21'> -7.7705688e+02 </wgt>
> <wgt id='22'> -4.1888303e+02 </wgt>
> <wgt id='23'> -4.0845022e+02 </wgt>
> <wgt id='24'> -5.5664064e+02 </wgt>
> <wgt id='25'> -2.5591354e+02 </wgt>
> <wgt id='26'> -6.4653087e+02 </wgt>
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> <wgt id='182'> -5.5902035e+01 </wgt>
> <wgt id='183'> -3.6772813e+02 </wgt>
> <wgt id='184'> +4.6481857e+02 </wgt>
> <wgt id='185'> -6.6669643e+02 </wgt>
> <wgt id='186'> -8.0543815e+02 </wgt>
> <wgt id='187'> -4.9155148e+02 </wgt>
> <wgt id='188'> +8.9879205e+02 </wgt>
> <wgt id='189'> +3.6226317e+02 </wgt>
> <wgt id='190'> -1.2256650e+03 </wgt>
> <wgt id='191'> -2.3195069e+03 </wgt>
> <wgt id='192'> +2.3768053e+02 </wgt>
> <wgt id='193'> +3.9529091e+02 </wgt>
> <wgt id='194'> -1.8996454e+03 </wgt>
> <wgt id='195'> -1.8186350e+03 </wgt>
> <wgt id='196'> -3.8857898e+02 </wgt>
> <wgt id='197'> -2.1777700e+03 </wgt>
> <wgt id='198'> -4.5997124e+02 </wgt>
> <wgt id='199'> -1.3749764e+03 </wgt>
> <wgt id='200'> -5.2982787e+02 </wgt>
> <wgt id='201'> -3.3017525e+02 </wgt>
> <wgt id='202'> -4.8711169e+02 </wgt>
> <wgt id='203'> -5.0578035e+02 </wgt>
> <wgt id='204'> -7.2565567e+02 </wgt>
> <wgt id='205'> +9.2447450e+01 </wgt>
> <wgt id='206'> -5.5588809e+02 </wgt>
> <wgt id='207'> -3.7479926e+01 </wgt>
> <wgt id='208'> -2.0569868e+03 </wgt>
> <wgt id='209'> -1.1435902e+03 </wgt>
> <wgt id='210'> -1.8443050e+03 </wgt>
> <wgt id='211'> -6.6486324e+02 </wgt>
> <wgt id='212'> -2.1077821e+03 </wgt>
> <wgt id='213'> -1.3276187e+03 </wgt>
> <wgt id='214'> -6.7330735e+02 </wgt>
> <wgt id='215'> -3.2154356e+02 </wgt>
> <wgt id='216'> +2.3119580e+01 </wgt>
> <wgt id='217'> -3.0857749e+02 </wgt>
> <wgt id='218'> -1.3801350e+02 </wgt>
> <wgt id='219'> +6.6368143e+02 </wgt>
> <wgt id='220'> -8.9433767e+01 </wgt>
> <wgt id='221'> -1.4253144e+02 </wgt>
> <wgt id='222'> -7.3266384e+02 </wgt>
> <wgt id='223'> -1.2742273e+03 </wgt>
> <wgt id='224'> -1.1878992e+03 </wgt>
> <wgt id='225'> -8.7450714e+02 </wgt>
> <wgt id='226'> -7.5166062e+02 </wgt>
> <wgt id='227'> +1.5398289e+01 </wgt>
> <wgt id='228'> -3.4708738e+02 </wgt>
> <wgt id='229'> -9.0113730e+02 </wgt>
> <wgt id='230'> -6.3348298e+02 </wgt>
> <wgt id='231'> -8.0245297e+02 </wgt>
> <wgt id='232'> -6.6128191e+02 </wgt>
> <wgt id='233'> -9.4782979e+02 </wgt>
> <wgt id='234'> -1.8999447e+02 </wgt>
> <wgt id='235'> -4.6293795e+02 </wgt>
> </rwgt>
> </event>
>
> I roughly have idea that unweighted LO sample have positive definite weights. The only possiblity that I can imagine is that such events are located at matching boundary and it can be vetoed or not vetoed depending on PDF replica and scale choice.
>
> === Results Summary for run: run_01 tag: tag_1 ===
>
> Cross-section : 645.4 +- 1.327 pb
> Nb of events : 10000
> Pythia8 merged cross-sections are:
>> Merging scale = 105 : 445.17 +/- 0.67 [pb]
>> Merging scale = 157.5 : 454.28 +/- 0.68 [pb]
>> Merging scale = 210 : 449.82 +/- 0.67 [pb]
> Notice that because Systematics computation is turned on, the merging did not veto events but modified their weights instead.
> The resulting hepmc/stdhep file should therefore be use with those weights.
>
> However, if some of events having weight zero then it looks okay but still sign inversion somewhat hard to understand at a glance. May I know what's happening behind scene?
>
> Have a nice day!
>
> Best wishes,
> Sung Hak Lim
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Sung Hak Lim (sunghak-lim) said : #2

Hi,

I'm currently using NNPDF30_nnlo_as_0118, ( lhaid: 261000).

For each replica, I agree this could happen. But does mean value of replicas have negative values also? I got negative weights for default weight slot in lhe also.

It sounds like I need to be aware of negative weights in LO case also..

Best wishes,
Sung Hak

Hi,

> But does mean value of
> replicas have negative values also?

Yes it can.

Cheers,

Olivier

> On 19 Jun 2017, at 09:47, Sung Hak Lim <email address hidden> wrote:
>
> Question #644186 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/644186
>
> Sung Hak Lim posted a new comment:
> Hi,
>
> I'm currently using NNPDF30_nnlo_as_0118, ( lhaid: 261000).
>
> For each replica, I agree this could happen. But does mean value of
> replicas have negative values also? I got negative weights for default
> weight slot in lhe also.
>
> It sounds like I need to be aware of negative weights in LO case also..
>
> Best wishes,
> Sung Hak
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Sung Hak Lim (sunghak-lim) said : #4

Hi, I just calculated pdf values for the event with negative weight

-----------------------------------------------------------------------

import lhapdf

p1 = lhapdf.mkPDF(261000)

Q = 2.37834600e+02
x1=2989./6500.
x2=50.68/6500.

print p1.xfxQ(-1,x1,Q)
print p1.xfxQ(21,x1,Q)

------------------------------------------------------------------

output:
-0.00272492225813
0.0131673473687

The PDF value was negative as you said.

Thanks for help!

Best wishes,
Sung Hak

Sung Hak Lim (sunghak-lim) said : #5

Thanks Olivier Mattelaer, that solved my question.