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.
parm 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.
parm 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.
parm 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.
parm name="ParticleDecays:xyMax" default="1." min="0."
The above xyMax, expressed in mm/c.
parm 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.
parm 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.
parm 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
parm 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.
parm name="ParticleDecays:multIncrease" default="4.5"
min="3." max="6."
The above multIncrease parameter.
parm name="ParticleDecays:multRefMass" default="0.7"
min="0.2" max="2.0"
The above multRefMass parameter.
parm name="ParticleDecays:multGoffset" default="0.0"
min="0.0" max="2.0"
The above multGoffset parameter.
parm 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.
Modes for Matrix Element Processing
Some decays can be treated better than what pure phase space allows,
by reweighting with appropriate matrix elements. The presence of such
corrections is signalled by a nonvanishing meMode()
value
for a decay mode in the
particle
data table. The list of allowed values almost agrees with the
PYTHIA 6 ones, but several obsolete choices have been removed, and
a few moved for better consistency. Here is the list of currently
allowed meMode()
codes:
- 0 : pure phace space of produced particles ("default")
- 1 : omega and phi -> pi+ pi- pi0
- 2 : Dalitz decay of pi0 and eta
- 3 : polarization in V -> PS + PS (V = vector,
PS = pseudoscalar), when V is produced by
PS -> PS + V or PS -> gamma + V
- 11 : turn partons into hadrons and distribute in pure phase
space
- 12 : ditto, but with higher average multiplicity
(imagined q -> g)
- 13 - 15 : as 11, but require at least
code
- 10 number
of particles in the decay channel
- 22 - 30 : as 11, but fixed multiplicity
code
- 20
- 32 : decay to q qbar or g g, which should shower
and hadronize
- 33 (was 4) : decay onium to g g g or g g gamma
(with matrix element), which should shower and hadronize
- 41 : decay to phase space, but weight up neutrino_tau spectrum
in tau decay
- 42 : weak decay; if there is a quark spectator system it collapses to
one hadron; for leptonic/semileptonic decays the V-A matrix element
is used, for hadronic decays simple phase space (gives somewhat softer
B -> D spectrum than PYTHIA 6; to be improved!??)
- 43 (was 48) : as 42, but require at least three particles
in decay