[EvtGen] Continuum Background Model

[John Back]

Hello,

The JSCONT model has been discontinued since it uses JetSet/Pythia6, and 
EvtGen only supports Pythia8; we need to update the documentation to 
mention this.

However, I think you will be able to generate the events you want by 
using a Pythia 8 decay mode which generates onium to 3 gluons, or 2 
gluons and a photon, and hadronises them:

Decay vpho
1.0 g g g PYTHIA 92;
Enddecay

or

Decay vpho
1.0 gamma g g PYTHIA  92;
Enddecay

where vpho is the (parent) virtual photon. Since this has zero mass, you 
need to define its energy to the e+e- beam energy (per event). For 
example, in your C++ program where you call EvtGen you need to do 
something like:

// Define virtual photon parent particle 4-momentum (GeV)
double E0 = 10.8;
EvtVector4R pInit(E0, 0.0, 0.0, 0.0);

EvtParticle* theParent = EvtParticleFactory::particleFactory(theId, pInit);
// Vector particle spin
if (theParent->getSpinStates() == 3){theParent->setVectorSpinDensity();}

// Generate the event
evtGen.generateDecay(theParent);

See validation/genRootDecayChain.cc and 
validation/UpsilonFiles/UpsilonTo3g.dec as examples.

 From the old JSCONT model documentation, the angular distribution of 
the jet was 1 + cos^2Theta. I don't think this will be done for the new 
way shown above.

Another limitation is that you can't specify the quark content of the 
virtual photon decay; all quarks are allowed, and the integer parameter 
used for the JSCONT model to define the quark flavour can't be used anymore.

If you find that the generated events contain problems, or the wrong 
physics content, then we can try to see if we can convert and upgrade 
the JSCONT model to work in Pythia 8, although this will not be an easy 
nor quick process.

Regards,
	John.

On 14/01/18 20:46, Yongcheng Wu wrote:
> Dear EvtGen Authors:
>
>     I'm trying to use EvtGen to generate continuum background events at
> Upsilon4S. According to your documentation (maybe it is not up to date
> ), JSCONT model should be used. However, in the newest version of
> EvtGen, it seems this model has been removed. Can you please tell me
> that which model should I use for a continuum background events
> generation? Thanks so much!
>
> Best!
>