Generated events depend on pT cut

Dear MadGraph experts,

I'm looking into the process e+ e- > l+ l- j j / h , as a background to single-Higgs production via ZBF. At the cuts.f level, I require the product of the leptons' rapidities to be negative in order to ensure that they are in opposite regions of the detector. To make sure that as much PS is covered as possible, at the run_card level I set the minimum pT of the final-state objects (leptons and jets) to be 0.5 GeV, and their \eta < 5.0. A further cut on the invariant mass of the dilepton system is required for several centre-of-mass energies, namely \sqrt{s} = {1, 3} TeV => mmll = {300, 1000} GeV respectively. The hard_survey flag is set to 1.

With this minimal set of cuts, I find that MadGraph either gets stuck refining events, or simply fails to reach target by a huge margin. After playing a while, I noticed that hardening the pT cut of the jets solves the issue (say, for 1 TeV I had to set ptj = 20 GeV, and to 60 GeV for 3 TeV), but on doing so I'm skewing the samples and hence introducing unwanted bias to the analysis. Please find below the banner of the latter case. NOTE: This is not the case for other backgrounds, such as lla, where the looser cuts (pta = 0.5 GeV) give no trouble.

Any ideas of why such a loose cut might be troublesome for MadGraph, and any tips on how to overcome this situation without skewing the samples?

Best wishes,

Oscar.

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