error specifying jet multiplicity in W/Z+jet production

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

Dear Sherpa experts, We wish to generate W/Z+ 2 or 3 jets. Modifying the example LHC_Zjets Run.dat to be

(run){

EVENTS = 10000 HEPMC2_GENEVENT_OUTPUT=sherpa.Zqqq SPIN_CORRELATIONS = 1

}(run)

(beam){

BEAM_1 = 2212; BEAM_ENERGY_1 = 7000; BEAM_2 = 2212; BEAM_ENERGY_2 = 7000;

}(beam)

(processes){

Process 93 93 -> 11 -11 93 93 93{1} Order_EW 2; CKKW sqr(30/E_CMS) Integration_Error 0.02 {5}; End process;

}(processes)

(selector){

Mass 11 -11 66 116

}(selector)

(me){

ME_SIGNAL_GENERATOR = Internal Comix

}(me)

(mi){

MI_HANDLER = None # None or Amisic

}(mi)

I receive this error:

Process_Group::CalculateTotalXSec(): Calculate xs for '2_4jje-e+jj' (Comix) Starting the calculation. Lean back and enjoy ... . Spinor<Scalar>::Construct(): \sqrt{p^+p-} = 7.04584e-07 vs. |p_\perp| = 8.50061e-07, rel. diff. 0.206472. Sherpa: Spinor<Scalar>::Construct throws fatal error:

Cannot construct massive two-component spinor.

Exception_Handler::GenerateStackTrace(..): Generating stack trace {

0x285450 in 'ATOOLS::Exception_Handler::GenerateStackTrace(std::ostream&, bool, std::string const&)'

from '/data/atlas/atlasdata3/boddy/SHERPA-MC-1.2.0/lib/SHERPA-MC/libToolsOrg.so.0'

0x286d14 in 'ATOOLS::Exception_Handler::Terminate()'

from '/data/atlas/atlasdata3/boddy/SHERPA-MC-1.2.0/lib/SHERPA-MC/libToolsOrg.so.0'

0x28886c in 'ATOOLS::Terminate()'

from '/data/atlas/atlasdata3/boddy/SHERPA-MC-1.2.0/lib/SHERPA-MC/libToolsOrg.so.0'

0x0 in '<unknown>'

from '/usr/lib/libstdc++.so.6'

0x0 in '<unknown>'

from '/usr/lib/libstdc++.so.6'

0x80494f4 in 'main'

from './Sherpa'

0x9ebd20 in 'libc_start_main'

from '/lib/tls/libc.so.6'

0x80493ec in 'gxx_personality_v0'

from './Sherpa'

}

The example produced events without incident with the original LHC+Zjets example Run.dat, producing Z + up to 4 jets. Is ther another way to speecify how many jets will be in the final state?

Cheers, Chris Boddy

[Frank Siegert]

Dear Chris,

You have to decide, whether you want to generate a CKKW merged sample with up to N jets, or whether you want an exclusive sample of "N or M" jets (which is then a pure matrix-element level calculation, and not hooked up to the shower).

So for the proper CKKW sample, you can't specify 93 93 93{1}, because it would lead to an inconsistent sample to leave out the 0- and 1-jet ME (mainly because our definition of a jet is different from the one you are using in your analysis). Of course I understand where you are coming from, not wanting to generate 0 and 1 jet events, most of which you throw away in your analysis. But since we can't rule out that some of them will end up in *your* 2 or 3 jet region, this is highly dangerous. A better solution also used by other experiments would be the usage of enhancement factors to basically (but properly!) eliminate production of 0- and 1-jet events. Of course, this introduces a weight for each event, which is constant though for each type of ME (0jet, 1jet, 2jet, 3jet, ...). If you are interested in how to do this, please let me know and I can elaborate.

In case you want a pure matrix-element level 2+3 jet sample, you will have to deal with the regularisation of the cross section yourself, i.e. you have to define the jets you want for example by using a kT jet finder as a matrix element selector. The different matrix element selectors are documented in our manual in ​http://projects.hepforge.org/sherpa/doc/SHERPA-MC-1.2.0.html#SEC78

In any case, please let us know whether one of the proposed solutions works for you, or if not, which other questions you run into, because this is a very tricky issue.

Frank

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

Dear Frank,

Thanks for your helpful reply. If you could elaborate on the use of enhancement factors to eliminate 0 and 1 jet events I would be most grateful. I had tried to do this already with setting the enhancement factors for 2 and 3 jet events to be very high (by adding Enhance_Factor 100000 {2}; Enhance_Factor 100000 {3}; to the process section) but the hepmc output mainly consisted of 0 or 1 jet events regardless of this. How would one use the enhancement factors to properly remove 0 and 1 jet events from these processes?

Cheers, Chris

[Frank Siegert]

Dear Chris,

Great, so you have already discovered the Enhance_Factors. What you did sounds correct to me, so I'm surprised that it didn't work. Could you please attach the full run card that you used for that? Then I'll check what is going wrong.

Cheers, Frank

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

Hi Frank, Sure, I've attached the Run.dat and in the tarball are the first 5 events of the hepmc output.

Cheers, Chris

[Frank Siegert]

Hi Chris,

Now I realise what's wrong: The {N} behind the Enhance_Factor referes to the full final state multiplicity! I.e. you have been enhancing the 2->2 and 2->3 processes. So no surprise, your first events contained 0- and 1-jet events ;)

Please try {4} and {5} and see whether that helps.

Frank

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

Hi Frank,

Ah, that was it. It is behaving as expected now.

Thanks! Chris

[Stefan Hoeche]

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