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.