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, but with some improvements, e.g. in the decay
tables and the number of decay models available. Recently a more
sophisticated handling of tau decays has also been introduced.
Some issues may need further polishing.
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
). By default this is defined as true for
all particles with tau0 below 1000 mm, and false for ones above,
see the Particle Data Scheme.
This means that mu^+-, pi^+-, K^+-,
K^0_L and n/nbar always remain stable unless decays
are explicity switched on, e.g. 211:mayDecay = true
.
(iii) Particles may be requested to have a nominal proper lifetime
tau0 below a threshold.
flag
ParticleDecays:limitTau0
(default = off
)
When on, only particles with tau0 < tau0Max are decayed.
parm
ParticleDecays:tau0Max
(default = 10.
; minimum = 0.
)
The above tau0Max, expressed in mm/c.
(iv) Particles may be requested to have an actual proper lifetime
tau below a threshold.
flag
ParticleDecays:limitTau
(default = off
)
When on, only particles with tau < tauMax are decayed.
parm
ParticleDecays:tauMax
(default = 10.
; minimum = 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
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
ParticleDecays:rMax
(default = 10.
; minimum = 0.
)
The above rMax, expressed in mm.
(vi) Particles may be requested to decay within a given cylindrical
volume around the origin.
flag
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
ParticleDecays:xyMax
(default = 10.
; minimum = 0.
)
The above xyMax, expressed in mm.
parm
ParticleDecays:zMax
(default = 10.
; minimum = 0.
)
The above zMax, expressed in mm.
Mixing
flag
ParticleDecays:mixB
(default = on
)
Allow or not B^0 - B^0bar and B_s^0 - B_s^0bar mixing.
parm
ParticleDecays:xBdMix
(default = 0.776
; minimum = 0.74
; maximum = 0.81
)
The mixing parameter x_d = Delta(m_B^0)/Gamma_B^0 in the
B^0 - B^0bar system. (Default from RPP2006.)
parm
ParticleDecays:xBsMix
(default = 26.05
; minimum = 22.0
; maximum = 30.0
)
The mixing parameter x_s = Delta(m_B_s^0)/Gamma_B_s^0 in the
B_s^0 - B_s^0bar system. (Delta-m from CDF hep-ex-0609040,
Gamma from RPP2006.)
Tau decays
Decays of tau leptons can be performed using helicity
information from the tau production process and with the
hadronic current of the tau decay modelled using form factors
fit to data. The tau decay framework is largely based on the
corresponding Herwig++ implementation [Gre07], with some
input from Tauola [Jad90]. A short summary can be found in
[Ilt12], while the complete writeup is in [Ilt14].
The decays of tau leptons are categorized
as correlated
, where a tau pair is produced from
a single process, or uncorrelated
, where only
one tau is produced. Currently internally
supported tau production mechanisms include correlated decays
from gamma, Z^0, Z'^0, gamma^*/Z^0/Z'^0,
and Higgs bosons (CP-even, odd, or mixed) and uncorrelated decays
from W^+-, W'^+-, B/D hadrons, and charged
Higgs bosons. For all mechanisms except B/D hadrons, both the
full process, e.g. q qbar → Z^0 → tau^+ tau^-, as
well as just the decay of the boson with a given initial polarization,
e.g.
Z^0 → tau^+ tau^-, can be handled. The axial and vector
couplings of the Z'^0 and W'^0 are set from the
relevant parameters in New Gauge
Boson Processes. Note that the CP of the various Higgs bosons
can be set with the
options HiggsX:parity
, HiggsX:etaParity
,
and HiggsX:phiParity
as described
in Higgs Processes
where X
is either H1
, H2
,
or A3
.
The tau polarization and tau decay correlation
mechanism can be determined either using internal
matrix
elements or external
SPINUP information provided in the
event, e.g. via Les Houches Event Files (LHEF). For internal
determination any tau pair or single tau from the
processes of the previous list can be handeled. For external
determination of a single uncorrelated tau, its polarization
is set to its SPINUP information. When the SPINUP for the tau
is not valid, e.g. when FSR is applied, the SPINUP for the first copy
of that tau is used instead unless also invalid. For the
external determination of a correlated tau pair the following
options are available.
mode
TauDecays:externalMode
(default = 2
; minimum = 0
; maximum = 2
)
Choice of the external polarization and correlation mechanism for
correlated tau pairs.
option
0 : all correlated pairs are treated as single
uncorrelated tau leptons. Their polarization is still set via
SPINUP.
option
1 : the mother of the tau pair is found. If the
mother is from the list of available internal correlated processes, a
correlated decay is performed. If the SPINUP for the mother is valid,
this is used to set the mother polarization, otherwise the mother is
assumed to be unpolarized.
option
2 : nothing is done.
Note: option 1
has limited functionality as
SPINUP is intended primarily for particles with 2 spin states. For
massive vector bosons SPINUP is interpreted here as the transverse
polarization and so the diagonal for the vector boson helicity density
matrix is set as [(1 - SPINUP)/3, 1/3, (1 + SPINUP)/3].
A default behaviour is defined when the polarization and decay
mechanism cannot be determined using either the internal or external
methods. If the tau is known to be produced from
a W^+-, gamma, or Z^0, the tau
or tau pair is assumed to be produced from an unpolarized
boson of this type. If the mediator is unknown but there is a
correlated tau pair, the pair is assumed to be produced from
an unpolarized photon and a warning is issued. Finally, if
the tau is uncorrelated, an unpolarized and uncorrelated
decay is performed and a warning is issued.
Both the internal and external determination have advantages and
disadvantages. For example, if an LHEF Z^0 → tau^+ tau^-
event is passed with SPINUP provided for both taus but
without SPINUP for the Z^0 then
with TauDecays:externalTau
set to 0
the
decays of the taus will be uncorrelated. Using 1
instead will result in correlations, assuming an
unpolarized Z^0. If using internal determination, then the
correlation and polarization will be fully calculated using the
correct production mechanism for the Z^0. Consequently, a
variety of options on how to determine polarization and correlation
are available, with a sensible default in place which should catch most
everything.
mode
TauDecays:mode
(default = 1
; minimum = 0
; maximum = 5
)
Choice of tau decay model.
option
0 : old decay model, with isotropic decays.
option
1 : sophisticated decays where external and then
internal determination is applied.
option
2 : sophisticated decays as above, but now taus
with a mother TauDecays:tauMother
are forced into an
uncorrelated decay with a polarization set
by TauDecays:tauPolarization
.
option
3 : sophisticated decays where all taus,
regardless of mother, are forced into an uncorrelated decay with a
polarization set
by TauDecays:tauPolarization
.
option
4 : sophisticated decays where only internal
determination is applied.
option
5 : sophisticated decays where only external (SPINUP)
determination is applied.
Warning 1: options 2
and 3
,
to force a specific tau polarization, only affect the decay
of the tau. The angular distribution of the tau itself,
given by its production, is not modified by these options. If you want, e.g.,
a righthanded W, or a SUSY decay chain, the kinematics should
be handled by the corresponding cross section class(es), supplemented by
the resonance decay one(s). The options here could then still be used
to ensure the correct polarization at the tau decay stage.
Warning 2: for options 1
through 5
, if the polarization and correlation mechanism
for the tau cannot be determined (internally or externally)
then the default behaviour described above is applied.
parm
TauDecays:tauPolarization
(default = 0
; minimum = -1.
; maximum = 1.
)
Polarization of the tau when mode 2
or 3
of
TauDecays:mode
is selected.
mode
TauDecays:tauMother
(default = 0
; minimum = 0
)
Mother of the tau for forced polarization when mode 2
of
TauDecays:mode
is selected. You should give the
positive identity code; to the extent an antiparticle exists it will
automatically obtain the inverse polarization.
QED radiation
So far PYTHIA does not have any generic machinery for handling QED radiation
in normal particle decays. In order to include this, a program like Photos
[Bar94, Dav10] could be used as an afterburner. In a few cases,
however, the existing shower machinery can be used also here: for
two-body decays to a lepton pair (l^+ l^- or l^+- nu_l).
Such decays are mediated by gamma^*/Z^0/W^+- exchange, for
which PYTHIA does have an existing machinery that can be applied,
including first-order matrix-element corrections for the first (hardest)
photon emission.
flag
ParticleDecays:allowPhotonRadiation
(default = off
)
Allow or not photon radiations in decays to a lepton pair, see above.
Note: The current default is to have radiation switched off,
in order to avoid double-counting of emissions if you link to an external
QED-radiation program, as is the norm in many collaborations.
Other variables
parm
ParticleDecays:mSafety
(default = 0.0005
; minimum = 0.
; maximum = 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.
parm
ParticleDecays:sigmaSoft
(default = 0.5
; minimum = 0.2
; maximum = 2.
)
In semileptonic decays to more than one hadron, such as
B → nu l D pi, decay products after the first three are
dampened in momentum by an explicit weight factor
exp(-p^2/sigmaSoft^2), where p is the
three-momentum in the rest frame of the decaying particle.
This takes into account that such further particles come from the
fragmentation of the spectator parton and thus should be soft.
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 Poissonian distribution, for n_p normal particles and
n_q quarks the average value is chosen as
n_p/ 2 + n_q/4 + multIncrease * ln ( mDiff / multRefMass)
with mDiff the difference between the decaying particle mass
and the sum of the normal-particle masses and the constituent quark masses.
For gluon systems multGoffset offers and optional additional
term to the multiplicity. The lowest possible multiplicity is
n_p + n_q/2 (but at least 2) and the highest possible 10.
If the picked hadrons have a summed mass above that of the mother a
new try is made, including a new multiplicity. These constraints
imply that the actual average multiplicity does not quite agree with
the formula above.
parm
ParticleDecays:multIncrease
(default = 4.
; minimum = 2.
; maximum = 6.
)
The above multIncrease parameter, except for
meMode = 23
.
parm
ParticleDecays:multIncreaseWeak
(default = 2.5
; minimum = 1.
; maximum = 4.
)
The above multIncrease parameter, specifically for
meMode = 23
. Here the weak decay implies that only the
virtual W mass should contribute to the production of new particles,
rather than the full meson mass.
parm
ParticleDecays:multRefMass
(default = 0.7
; minimum = 0.2
; maximum = 2.0
)
The above multRefMass parameter.
parm
ParticleDecays:multGoffset
(default = 0.5
; minimum = 0.0
; maximum = 2.0
)
The above multGoffset parameter.
parm
ParticleDecays:colRearrange
(default = 0.5
; minimum = 0.
; maximum = 1.0
)
When a decay is given as a list of four partons to be turned into
hadrons (primarily for modes 41 - 80) 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
ParticleDecays:FSRinDecays
(default = on
)
When a particle decays to q qbar, g g, g g g
or gamma g g, with meMode > 90
, 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. In others a partonic
content has to be converted to a set of hadrons. The presence of such
corrections is signaled by a nonvanishing meMode()
value
for a decay mode in the particle
data table. The list of allowed possibilities almost agrees with the
PYTHIA 6 ones, but several obsolete choices have been removed,
a few new introduced, and most have been moved for better consistency.
Here is the list of currently allowed meMode()
codes:
- 0 : pure phase space of produced particles ("default");
input of partons is allowed and then the partonic content is
converted into the minimal number of hadrons (i.e. one per
parton pair, but at least two particles in total)
- 1 : omega and phi → pi+ pi- pi0
- 2 : polarization in V → PS + PS (V = vector,
PS = pseudoscalar), when V is produced by
PS → PS + V or PS → gamma + V
- 11 : Dalitz decay into one particle, in addition to the
lepton pair (also allowed to specify a quark-antiquark pair that
should collapse to a single hadron)
- 12 : Dalitz decay into two or more particles in addition
to the lepton pair
- 13 : double Dalitz decay into two lepton pairs
- 21 : decay to phase space, but weight up neutrino_tau spectrum
in tau decay
- 22 : 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
- 23 : as 22, but require at least three particles in decay
- 31 : decays of type B → gamma X, very primitive simulation where
X is given in terms of its flavour content, the X multiplicity is picked
according to a geometrical distribution with average number 2, and
the photon energy spectrum is weighted up relative to pure phase space
- 42 - 50 : turn partons into a random number of hadrons, picked according
to a Poissonian with average value as described above, but at least
code
- 40 and at most 10, and then distribute then in pure
phase space; make a new try with another multiplicity if the sum of daughter
masses exceed the mother one
- 52 - 60 : as 42 - 50, with multiplicity between
code
- 50
and 10, but avoid already explicitly listed non-partonic channels
- 62 - 70 : as 42 - 50, but fixed multiplicity
code
- 60
- 72 - 80 : as 42 - 50, but fixed multiplicity
code
- 70,
and avoid already explicitly listed non-partonic channels
- 91 : decay to q qbar or g g, which should shower
and hadronize
- 92 : decay onium to g g g or g g gamma
(with matrix element), which should shower and hadronize
- 100 - : reserved for the description of partial widths of
resonances
Three special decay product identity codes are defined.
- 81: remnant flavour. Used for weak decays of c and b hadrons, where the
c or b quark decays and the other quarks are considered as a spectator
remnant in this decay. In practice only used for baryons with multiple
c and b quarks, which presumably would never be used, but have simple
(copied) just-in-case decay tables. Assumed to be last decay product.
- 82: random flavour, picked by the standard fragmentation flavour
machinery, used to start a sequence of hadrons, for matrix element
codes in 41 - 80. Assumed to be first decay product, with -82 as second
and last. Where multiplicity is free to be picked it is selected as for
normal quarkonic systems. Currently unused.
- 83: as for 82, with matched pair 83, -83 of decay products. The
difference is that here the pair is supposed to come from a closed gluon
loop (e.g. eta_c → g g) and so have a somewhat higher average
multiplicity than the simple string assumed for 82, see the
ParticleDecays:multGoffset
parameter above.