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Herwig::UA5Handler Class Reference
Inheritance diagram for Herwig::UA5Handler:
This is the class definition for the UA5Handler.
This class is designed to generate an underlying event based on the UA5 model. This is intended as a basic underlying event model which will be superceded by a new model in Herwig.
The Hadronization is responsible for the formation of the beam clusters. In this step the colour connection between the spectators and the initial-state parton showers is cut by the forced emission of a soft quark-antiquark pair. The underlying soft event in a hard hadron-hadron collision is then assumed to be a soft collision between these two beam clusters.
The model used for the underlying event is based on the minimum-bias event generator of the UA5 Collaboration, UA5 Collaboration, G.J. Alner et al., Nucl. Phys. B291 (1987) 445, modified to make use of our cluster fragmentation algorithm.
The parameter ProbSoft enables one to produce an underlying event in only a fraction ProbSoft of events (default=1.0).
The UA5 model starts from a parametrization of the inelastic charged multiplicity distribution as a negative binomial distribution,
The parameter is given by
and is given by
As an option, for underlying events the value of used to choose the multiplicity may be increased by a factor EnhanceCM to allow for an enhanced underlying activity in hard events.
Once a charged multiplicity has been selected from the above distribution, `softclusters' are generated with flavours by drawing randomly from the vacuum. Soft cluster masses are assigned as
where are random numbers, which gives a (shifted) exponential distribution of . The parameters and control the distribution and are the masses of the quarks in the cluster.
As each soft cluster is generated, it is decayed to stable hadrons using the cluster hadronization model (without cluster fission) and the accumulated charged multiplicity is computed. Once the preselected charged multiplicity is exactly reached, cluster generation is stopped. If it is exceeded, all clusters are rejected and new ones are generated until the exact value is reached. In this way the multiplicity distribution of stable charged hadrons is generated exactly as prescribed.
At this stage (to save time) the kinematic distribution of the soft clusters has not yet been generated. The decay products of each cluster are stored in its rest frame. Now the transverse momenta of the clusters are generated with the distribution
where the slope parameter depends as indicated on the flavour of the quark or diquark pair created in the primary cluster decay, for light quarks, for the strange and charm quarks and for diquarks. Next the clusters are given a flat rapidity distribution with Gaussian shoulders. The `reduced rapidities' are generated first by drawing from a distribution
where is the normalization. Next a scaling factor is computed such that the scaled cluster rapidities , their masses and transverse momenta satisfy momentum conservation when compared to the total energy of the underlying event. Thus the soft cluster rapidity distribution retains its overall shape but becomes higher and wider as the energy of the underlying event increases.
Finally the decay products of each cluster are boosted from its rest frame into the lab frame and added to the event record.
Member Function Documentation
Make a simple clone of this object.
Decay a cluster to two hadrons is sufficiently massive and to one if not.
Make a clone of this object, possibly modifying the cloned object to make it sane.
Reimplemented from ThePEG::InterfacedBase.
template<typename T >
Various methods to generate random distributions.
The implementation of the cylindrical phase space.
Members to generate the momenta of the clusters.
This generates the momentum of the produced particles according to the cylindrical phase space algorithm given in Computer Physics Communications 9 (1975) 297-304 by S. Jadach.
This is the routine that starts the algorithm.
The standard Init function used to initialize the interfaces.
Called exactly once for each class by the class description system before the main function starts or when this class is dynamically loaded.
Recursively add particle and decay products to the step.
The value of the mean multiplicity for a given energy E.
This is wher , and are parameters.
Generates a multiplicity for the energy E according to the negative binomial distribution.
Members to generate the multiplicity according to a negative binomial distribution.
Calculate the negative binomial probability given the mean , the multiplicity and .
This is never defined and since it can never be called it isn't needed.
The prototype is defined so the compiler doesn't use the default = operator.
Members to decay the clusters and hadrons produced in their decay, and insert the output in the event record.
Perform the decay of an unstable hadron.
Function used to read in object persistently.
Function used to write out object persistently.
template<typename T >
Generates a random azimuthal angle and puts x onto px and py TODO: Should move this to Utilities.
This returns random number from distribution, where .
It uses Newton's method to solve
This returns a random number with a flat distribution [-A,A] plus gaussian tail with stdev B TODO: Should move this to Utilities.
Member Data Documentation
The documentation for this class was generated from the following file:
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