dijet mjj distribution @ 8TeV LHC

Hi guys,

I am trying to compute the mjj spectrum of the dijet production @ 8TeV LHC using MG5 (version 1.5.9) and I seem to be running into some sort of issue that I dont really understand.

I am looking only at partonic level analysis for now and only Standard Model. All I am trying to do is to get x-sections sigma_i for p p > j j process where "i" denotes the i-th m_jj bin. To get this I have simply implemented the m_jj lower and higher cut in the run_card.dat.

My issue is the following. I choose the m_jj bin to be for instance [2100 GeV, 2200 GeV] and set the cuts in the run_card.dat accordingly. I then compute the sigma_i x-section. The precision I use is set to 10.000 events. As MG5 computes the sigma_i it gives me the result and the estimated error which is well below 1%. But then I repeat this procedure 100 times - computing the same thing every time and I get results which seem to have significant fluctuations.

In particular based on these 100 computations of sigma_i, i get avg(sigma_i) = 1905 pb, with standard deviation of this sample std(sigma_i) = 276 pb which is of the order 15%! Further, the spread {sigma_i^min, sigma_i^max} = {836.8, 2560} which seems really high.

I am trying to understand why I am seeing such substantial fluctuations which are clearly not compatible with the MG estimated precision which well below 1% level.

My definition of jets includes the b quark. The light quarks are massless and I am using the following cuts in the run_card.dat
30 = ptj ! minimum pt for the jets
2.5 = ej ! minimum E for the jets
5 = etaj ! max rap for the jets
0.0 = drjj ! min distance between jets
2100 = mmjj ! min invariant mass of a jet pair
2200 = mmjjmax ! max invariant mass of a jet pair

I am using adaptive ren/fact scale with scalefact set to 0.707 and cteq6_m pdf.

Please let me know if you have any idea of what I might be causing these fluctuations?
I will be happy to supply any additional information/cards.

Best Regards
Jure Drobnak