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.