## Vertex Information

While the setting of secondary production vertices of unstable hadrons and leptons is a standard part of the particle decay rotines, no corresponding standardized handling is in place for the evolution in the partonic or hadronization phases of the event generation. The intention is to provide such methods in due course.

There are some cases where such information is needed already now, specifically for the Rope Hadronization framework. Therefore the beginning of a framework is available, that can be used to set vertices for partonic production by MPI, FSR and ISR. This is done in the `PartonVertex` class. This is a base class, with a default implementation, but the user can replace it with a derived class that does a more sophisticated handling.

Note that currently the parton-level vertices are expressed in fm, unlike the normal mm scale. This will be fixed as the methods evolve. Also other improvements and extensions are likely to come. So, while people are welcome to write their own derived classes, it is likely that these may need to be modified in later PYTHIA versions.

### Rope Hadronization Parameters

Currently the base class implements two alternative approaches to picking a partonic vertex, for use inside the rope hadronization framework. There are also some free parameters in the models.

`flag` PartonVertex:setVertex (`default = off`)
Master switch to allow the setting of partonic vertices.

`mode` PartonVertex:modeVertex (`default = 2`; `minimum = 1`; `maximum = 2`)
Choice between alternative procedures to select vertex.
`option` 1 : Proton profile is a uniform black disc.
`option` 2 : Proton profile is a two-dimensional Gaussian.

`parm` PartonVertex:ProtonRadius (`default = 0.7`; `minimum = 0.`; `maximum = 10.`)
The proton radius and shape depends on collision energy. At LHC collision energies, say 14 TeV, the profile corresponds roughly to a Gaussian with a with of around 0.7 fm, according to the DIPSY model [Fle11].

`parm` PartonVertex:EmissionWidth (`default = 0.1`; `minimum = 0.`; `maximum = 10.`)
Vertices of ISR+FSR partons are smeared relative to their mother by a Gaussian distribution with a width of `EmissionWidth`/pT, where pT is the transverse momentum of the emission (in GeV). This parameter thus determined the overall strength of the transverse space smearing.

`parm` PartonVertex:pTmin (`default = 0.2`; `minimum = 0.05`; `maximum = 1.`)
The parton transverse smearing is assumed proportional to 1 / pT, but pT is limited to be above this parameter so as to avoid unreasonable values.

### External models

A derived class for setting parton vertex information can be provided to PYTHIA with the
`bool Pythia::setPartonVertexPtr( PartonVertex* partonVertexPtrIn)`
method. The methods in the derived `PartonVertex` class can then be used to add vertex information to produced particles, at creation time, in MPI, FSR and ISR. The assigned vertex information will afterwards be accessible as properties of the individual particles. Particles produced in other types of processes than the ones mentioned above will not have vertex information assigned (e.g. hard process, beam remnants etc.), neither will particles produced in the weak shower.

virtual void init()
can be used to initialize various parameters of the model or precalculate common numbers. Note that a separate non-virtual method will already have provided pointers to the `Info`, `Settings` and `Rndm` classes, so that these are available in all derived classes.

virtual void vertexMPI( int iBeg, int nAdd, double bNow, Event& event)
Method to assign a production vertex to a particle produced in the MPI framework. Should set the vertices `vProd` of the particles concerned.
`argument` iBeg : is the index of the first parton of a MPI.
`argument` nAdd : is the number of partons involved in the MPI, currently always four: two in and two out.
`argument` bNow : is the impact parameter of the event. It is not expressed in physical units (like fm), but rescaled such that the average is unity for MPI events. See the section on Multiparton Interactions for a description of choices for the b dependence.
`argument` event : reference to the whole event, to read information from and set the relevant `vProd` values into.

virtual Vec4 vertexFSR( int iNow, Event& event)
Method to assign production vertex to a particle produced in the FSR (`TimeShower`). Should set the vertex `vProd` of the particle concerned.
`argument` iNow : is the index of the parton concerned. In a branching the daughters automatically inherit the vertex of the mother, if it has one, and similarly for the recoiler. This method is called specifically for what is considered the emitted parton of the process, i.e. the gluon in a q → q g branching, and allows the vertex of this parton to be modified.
`argument` event : reference to the whole event, to read information from and set the relevant `vProd` values into.

virtual Vec4 vertexISR( int iNow, Event& event)
Method to assign production vertex to a particle produced in the ISR (`SpaceShower`). Should set the vertices `vProd` of the particle concerned.
`argument` iNow : is the index of the parton concerned. This method is called three times for each ISR branching, for the daughter, the new recoiler and the sister.
`argument` event : reference to the whole event, to read information from and set the relevant `vProd` values into.