Particle Decays

The ParticleDecays class performs the sequential decays of all unstable hadrons produced in the string fragmentation stage, i.e. up to and including b hadrons and their decay products, such as the tau lepton. It is not to be used for the decay of more massive resonances, such as top, Z^0 or SUSY, where decays must be performed already at the ProcessLevel of the event generation. The decay description essentially copies the one present in PYTHIA since many years, currently with some simplifications for some charm and bottom channels, and updates will need to be introduced eventually.

Variables determining whether a particle decays

Before a particle is actually decayed, a number of checks are made.

(i) Decay modes must have been defined for the particle kind; tested by the canDecay() method of Event (and ParticleData).

(ii) The main switch for allowing this particle kind to decay must be on; tested by the mayDecay() method of Event (and ParticleData).

(iii) Particles may be requested to have a nominal proper lifetime tau0 below a threshold.

flag name="ParticleDecays:limitTau0" default="off"
When on, only particles with tau0 < tau0Max are decayed.

parameter name="ParticleDecays:tau0Max" default="1." min="0."
The above tau0Max, expressed in mm/c.

(iv) Particles may be requested to have an actual proper lifetime tau below a threshold.

flag name="ParticleDecays:limitTau" default="off"
When on, only particles with tau < tauMax are decayed.

parameter name="ParticleDecays:tauMax" default="1." min="0."
The above tauMax, expressed in mm/c.
In order for this and the subsequent tests to work, a tau is selected and stored for each particle, whether in the end it decays or not. (If each test would use a different temporary tau it would lead to inconsistencies.)

(v) Particles may be requested to decay within a given distance of the origin.

flag name="ParticleDecays:limitRadius" default="off"
When on, only particles with a decay within a radius r < rMax are decayed. There is assumed to be no magnetic field or other detector effects.

parameter name="ParticleDecays:rMax" default="1." min="0."
The above rMax, expressed in mm/c.

(vi) Particles may be requested to decay within a given cylidrical volume around the origin.

flag name="ParticleDecays:limitCylinder" default="off"
When on, only particles with a decay within a volume limited by rho = sqrt(x^2 + y^2) < xyMax and |z| < zMax are decayed. There is assumed to be no magnetic field or other detector effects.

parameter name="ParticleDecays:xyMax" default="1." min="0."
The above xyMax, expressed in mm/c.

parameter name="ParticleDecays:zMax" default="1." min="0."
The above zMax, expressed in mm/c.

Mixing

flag name="ParticleDecays:mixB" default="on"
Allow or not B^0 - B^0bar and B_s^0 - B_s^0bar mixing.

parameter name="ParticleDecays:xBdMix" default="0.771" min="0.70" max="0.85"
The mixing parameter x_d = Delta(m_B^0)/Gamma_B^0 in the B^0 - B^0bar system.

parameter name="ParticleDecays:xBsMix" default="25.0" min="15.0"
The mixing parameter x_s = Delta(m_B_s^0)/Gamma_B_s^0 in the B_s^0 - B_s^0bar system.

Other variables

parameter name="ParticleDecays:mSafety" default="0.001" min="0." max="0.01"
Minimum mass difference required between the decaying mother mass and the sum of the daughter masses, kept as a safety margin to avoid numerical problems in the decay generation.

When a decay mode is defined in terms of a partonic content, a random multiplicity (and a random flavour set) of hadrons is to be picked, especially for some charm and bottom decays. This is done according to a Gaussian distribution, for n_q quarks centered at c + n_q/4 and with a width sqrt(c), with the final number rounded off to the nearest integer. Here
c = multIncrease * ln ( mDiff / multRefMass)
with mDiff the difference between the decaying particle mass and the sum of the constituent quark masses. For gluonic systems multGoffset offers and optional additonal term to the multiplicity. There is always some lower limit for the allowed multiplicity; if a number smaller than this is picked the choice is repeated.

parameter name="ParticleDecays:multIncrease" default="4.5" min="3." max="6."
The above multIncrease parameter.

parameter name="ParticleDecays:multRefMass" default="0.7" min="0.2" max="2.0"
The above multRefMass parameter.

parameter name="ParticleDecays:multGoffset" default="0.0" min="0.0" max="2.0"
The above multGoffset parameter.

parameter name="ParticleDecays:colRearrange" default="0.5" min="0." max="1.0"
When a decay is given as a list of four partons to be turned into hadrons (i.e. for mode 11 - 30), it is assumed that they are listed in pairs, as a first and a second colour singlet, which could give rise to separate sets of hadrons. Here colRearrange is the probability that this original assignment is not respected, and default corresponds to no memory of this original colour topology.

flag name="ParticleDecays:FSRinDecays" default="true"
When a particle decays to q qbar, g g, g g g or gamma g g, allow or not a shower to develop from it, before the partonic system is hadronized. (The typical example is Upsilon decay.) In addition, some variables defined for string fragmentation and for flavour production are used also here.