ttZ matrix element error

[Generic User (don't modify these fields)]

Dear Sherpa experts,

I've come across an error message when creating ttZ -> lnu qq bb tau tau events. The job options (attached) is similar to the LHC_TTH example job options.

On running I receive messages like:

METS_Scale_Setter::CalculateScale(): Momentum not conserved. \sum p = (0.03101,-0.000305173,7.85008e-05,-0.0309608) in (0x9720ed0)[0]: 2 -> 2 {

\mu_r = 0, \mu_f = 0 x_1 = 3.89108e-09, x_2 = 0.00314646 k_{T,QCD} = 1.71626, (R/B)_{max} = 1 oew = 0, oqcd = 0 swap = 0, new = (<no entry>) (0,4,5,6,7,8,9) ub (-0.0628883,5.58284e-13,-1.74331e-12,-0.0628883) (0,3) [0|0,0]

(1) u (-84.6131,-1.42053e-10,4.42148e-10,84.6131) (3,0) [0|0,0] (2) tau- (25.3457,-0.662691,-2.0042,-25.2575) (0,0) [0|0,0] (3) tau+ (59.3613,0.662386,2.00428,-59.3237) (0,0) [0|0,0]

}

but it later completes the calculation. Does this sound correct?

Cheers, Chris Boddy

[Stefan Hoeche]

Dear Chris,

the error you encounter is not really a serious one. What happens is that the inernal scale-setting algorithm for the inclusive Matrix-Element Parton-Shower merging produces some momentum violation in projecting a multi-particle final state onto a 2->2 process. This is most likely due to numerical instabilities. It affects the proper choice of the renormalisation scale at this particular phase-space point, however the kinematics for evaluating the hard matrix element are still well-defined. If this error happens too often (say more than 10 times during one iteration step), please let us know.

Thank you for pointing out this problem

Stefan

[Stefan Hoeche]

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