- Resonance Decays and Cross Sections
- Special properties and methods for resonances
- Modes for Matrix Element Processing
- Filter for Resonance Decays
ResonanceDecays class performs the sequential decays of
all resonances formed in the hard process. Note the important distinction
between "resonances" and other "particles" made in PYTHIA.
There is one ambiguous case in this classification, namely the photon.
The gamma^*/Z^0 combination contains a low-mass peak when
produced in a hard process. On the other hand, photons can participate
in shower evolution, and therefore a photon originally assumed
massless can be assigned an arbitrarily high mass when it is allowed
to branch into a fermion pair. In some cases this could lead to
double-counting, e.g. between processes such as
f fbar → (gamma^*/Z^0) (gamma^*/Z^0),
f fbar → (gamma^*/Z^0) gamma and
f fbar → gamma gamma. Here it make sense to limit the
lower mass allowed for the gamma^*/Z^0 combination,
The list of resonances contains gamma^*/Z^0, W^+-, top,
the Higgs, and essentially all new particles of Beyond-the-Standard-Model
physics: further Higgs bosons, sfermions, gauginos, techniparticles, and
so on. The partial widths to different decay channels are perturbatively
calculable, given the parameters of the respective model, and branching
ratios may be allowed to vary across a (reasonably broad) resonance peak.
Usually resonances are short-lived, and therefore it makes sense to consider
their decays immediately after the primary hard process has been set up.
Furthermore, in several cases the decay angular distributions are encoded
as part of the specific process, e.g. the W decays differently in
f fbar → W^+-, f fbar → W^+ W^- and
h^0 → W^+ W^- . All of these particles are (in PYTHIA) only
produced as part of the hard process itself, i.e. they are not produced
in showers or hadronization processes. Therefore the restriction to
specific decay channels can be consistently taken into account as a
corresponding reduction in the cross section of a process. Finally, note
that all of these resonances have an on-shell mass above 20 GeV, with the
exception of some hypothetical weakly interacting and stable particles
such as the gravitino.
The other particles include normal hadrons and the Standard-Model leptons,
including the tau^+-. These can be produced in the normal
hadronization and decay description, which involve unknown nonperturbative
parameters and multistep chains that cannot be predicted beforehand:
a hard process like g g → g g can develop a shower with a
g → b bbar branching, where the b hadronizes to a
B^0bar that oscillates to a B^0 that decays to a
tau^+. Therefore any change of branching ratios - most of which
are determined from data rather than from first principles anyway -
will not be taken into account in the cross section of a process.
Exceptions exist, but most particles in this class are made to decay
isotropically. Finally, note that all of these particles have a mass
below 20 GeV.
23:mMin, to be the same as the upper limit allowed
for an off-shell photon in the shower evolution, in
TimeShower:mMaxGamma. By default this matching is done
at 10 GeV.
In spite of the above-mentioned differences, the resonances and the
other particles are all stored in one common
particle data table, so as to offer a
uniform interface to setting and
getting properties such as name, mass, charge and decay modes,
also for the particle properties
in the event record. Some methods are specific to resonances, however,
in particular for the calculation of partial widths and thereby of
branching ratio. For resonances these can be calculated dynamically,
set up at initialization for the nominal mass and then updated to the
current mass when these are picked according to a Breit-Wigner resonance
Resonance Decays and Cross Sections
As already hinted above, you have the possibility to set the allowed
decay channels of resonances, see
Particle Data Scheme description.
For instance, if you study the process q qbar → H^0 Z^0
you could specify that the Z^0 should decay only to
lepton pairs, the H^0 only to W^+ W^-, the
W^+ only to a muon and a neutrino, while the W^-
can decay to anything. Unfortunately there are limits to the
flexibility: you cannot set a resonance to have different properties
in different places of a process, e.g. if instead
H^0 → Z^0 Z^0 in the above process then the three
Z^0's would all obey the same rules.
The restrictions on the allowed final states of a process is directly
reflected in the cross section of it. That is, if some final states
are excluded then the cross section is reduced accordingly. Such
restrictions are built up recursively in cases of sequential decay
chains. The restrictions are also reflected in the compositions of
those events that actually do get to be generated. For instance,
the relative rates of H^0 → W^+ W^- and
H^0 → Z^0 Z^0 are shifted when the allowed sets of
W^+- and Z^0 decay channels are changed.
We remind that only those particles that Pythia treat as resonances
enjoy this property, and only those that are considered as part of the
hard process and its associated resonance decays.
There is one key restriction on resonances:
default = 1.97e-19;
minimum = 1e-30)
Minimal allowed width of a resonance, in GeV. If the width falls below
this number the resonance is considered stable and will not be allowed
to decay. This is mainly intended as a technical parameter, to avoid
disasters in cases where no open decay channels exists at all. It could
be used for real-life decisions as well, however. The default value
corresponds to an average lifetime c * tau of 1 km, meaning
such a particle would only rarely decay inside a typical detector.
Special caution would be needed if coloured resonance particles were
made stable, since the program would not necessarily know how to
hadronize them, and therefore fail at that stage (see further the
In spite of this technical parameter choice, it is possible to set
a lifetime for a resonance, and thereby to obtain displaced vertices.
If a resonance is allowed to decay it will do so, irrespective of
the location of the decay vertex. This is unlike
normal particle decays,
where it is possible to define some region around the primary
vertex within which all decays should happen, with particles
leaving that region considered stable. The logic is that resonances
as a rule are too short-lived for secondary vertices,
so if you pick a scenario with a long-lived but unstable resonance
it is because you want to study secondary vertices.
How to interface those decays to a detector simulation program then
is another story, to be solved separately. Do note that a special
treatment is needed for coloured long-lived resonances, that form
R-hadrons, and where charge and flavour
may change between the production and decay vertices.
Special properties and methods for resonances
ParticleData::isResonance(id) allows you to
query whether a given particle species is considered a resonance or not.
You can also change the default value of this flag in the normal way,
pythia.readString("id:isResonance = true").
Resonances come in two kinds.
Sometimes experimentalists want to modify the physical width of a
resonance, to understand how sensitive analyses are to this width,
if at all. For the second, simpler kind of resonances, the
The standard built-in ones include Z^0, W^+-,
t, h^0, and many more. These have explicit decay-widths
formulae encoded, in classes derived from the
base class. The formulae are used, e.g., to calculate all the partial
widths as a function of the resonance masses you choose, and at
initialization the existing total width values are overwritten.
This is especially convenient for hypothetical states, like a Z',
where the mass is not known and therefore routinely changed. Often the
partial widths are associated with parameters that can be changed by the
user, e.g. for MSSM Higgs states.
If a resonance does not have a class of its own, with hardcoded equations
for all relevant partial widths, then a simpler object will be created
at initialization. This object will take the total width and branching
ratios as is (with the optional variations explained in the next section).
When you set a particle to be a resonance, and do not provide any
class to go with its width calculations, this is where it will end up.
id:mWidth can be changed right away, but for the first
kind any change will be overwritten at initialization. To circumvent
this problem, the
id:doForceWidth flag can be changed from
on. Then the width stored
id:mWidth is strictly used to describe the Breit-Wigner
of the resonance. Partial widths are still recalculated to set the
mass-dependent branching ratios, but then uniformly rescaled to the
requested total width. The width can also run across the lineshape,
so that it deviates from the nominal one in the wings of the
For processes that contain interference terms between resonances,
notably gamma^*/Z^0 or gamma^*/Z^0/Z'^0, it is not
obvious how these contributions should be modified consistently.
Therefore it is necessary to set
WeakZ0:gmZmode = 2
Zprime:gmZmode = 3, to have a pure Z^0 or
Z'^0, respectively, for width forcing to be allowed in
A warning is that the different processes have cross sections
that rescale in different ways when the resonance width is varied.
This depends on them not having been implemented in a guaranteed
uniform way. To illustrate the point, consider the case of an
s-channel resonance, where the cross section dependence
on the width can be written as
sigmaHat(sHat) = constant * Gamma_in * Gamma_out
/ ((sHat - m^2)^2 + m^2 * Gamma^2)
doForceWidth = on option ensures that the
Gamma in the denominator is rescaled by some factor
k relative to the natural width, but does not guarantee
that Gamma_in and Gamma_out are rescaled as well.
If all three are rescaled by the same factor k, as they
should, then the integrated cross section also scales like k,
assuming that the peak is reasonably narrow, so that the variation
of PDF's across the Breit-Wigner can be neglected. This is the case
for some processes. But in others either or both of the production
and decay vertices can have been hardcoded, based on the coupling
structure, and thus not scale with k. If only
Gamma_out scales with k, say, the cross section
remains (approximately) constant, and if neither scales the cross
section will even go like 1/k. Such obvious normalization
imperfections have to be corrected by hand.
Mainly for internal usage, the
some special methods that are only meaningful for resonances:
These methods actually provide an interface to the classes derived from
resInit(...) to initialize a resonance, possibly
including a recalculation of the nominal width to match the nominal
resWidth(...) to calculate the partial and total widths
at the currently selected mass;
resWidthOpen(...) to calculate the partial and total
widths of those channels left open by user switches, at the currently
resWidthStore(...) to calculate the partial and total
widths of those channels left open by user switches, at the currently
selected mass, and store those as input for a subsequent selection of
resOpenFrac(...) to return the fraction of the total
width that is open by the decay channel selection made by users (based on
the choice of
for the various decay channels, recursively calculated for sequential
resWidthRescaleFactor(...) returns the factor by which
the internally calculated PYTHIA width has to be rescaled to give the
resWidthChan(...) to return the width for one particular
channel (currently only used for Higgs decays, to obtain instate coupling
from outstate width).
ResonanceWidths base class, to describe various
Modes for Matrix Element Processing
meMode() value for a decay mode is used to specify
nonisotropic decays or the conversion of
a parton list into a set of hadrons in some channels of normal
particles. For resonances it can also take a third function, namely
to describe how the branching ratios and widths of a resonance should
be rescaled as a function of the current mass of the decaying resonance.
The rules are especially useful when new channels are added to an
existing particle, or a completely new resonance added.
- 0 : channels for which hardcoded partial-width expressions are
expected to exist in the derived class of the respective resonance.
Should no such code exist then the partial width defaults to zero.
- 1 - 99 : same as 0, but normally not used for resonances.
- 100 : calculate the partial width of the channel from its stored
branching ratio times the stored total width. This value remains unchanged
when the resonance fluctuates in mass. Specifically there are no
threshold corrections. That is, if the resonance fluctuates down in
mass, to below the nominal threshold, it is assumed that one of the
daughters could also fluctuate down to keep the channel open. (If not,
there may be problems later on.)
- 101 : calculate the partial width of the channel from its stored
branching ratio times the stored total width. Multiply by a step threshold,
i.e. the channel is switched off when the sum of the daughter on-shell
masses is above the current mother mass.
- 102 : calculate the partial width of the channel from its stored
branching ratio times the stored total width. Multiply by a smooth
beta = sqrt( (1 - m_1^2/m_2 - m_2^2/m^2)^2 - 4 m_1^2 m_2^2/m^4)
for two-body decays and sqrt(1 - Sum_i m_i / m) for multibody
ones. The former correctly encodes the size of the phase space but
misses out on any nontrivial matrix-element behaviour, while the latter
obviously is a very crude simplification of the correct phase-space
expression. Specifically, it is thereby assumed that the stored branching
ratio and total width did not take into account such a factor.
- 103 : use the same kind of behaviour and threshold factor as for
102 above, but assume that such a threshold factor has been used when
the default branching ratio and total width were calculated, so that one
should additionally divide by the on-shell threshold factor. Specifically,
this will give back the stored branching ratios for on-shell mass,
unlike the 102 option. To avoid division by zero, or in general
unreasonably big rescaling factors, a lower limit
minThreshold (see below) on the value of the on-shell
threshold factor is imposed. (In cases where a big rescaling is
intentional, code 102 would be more appropriate.)
default = 0.1;
minimum = 0.01)
Used uniquely for
meMode = 103 to set the minimal value
assumed for the threshold factor,
sqrt( (1 - m_1^2/m_2 - m_2^2/m^2)^2 - 4 m_1^2 m_2^2/m^4)
for two-body decays and sqrt(1 - Sum_i m_i / m) for multibody
ones. Thus the inverse of this number sets an upper limit for how
much the partial width of a channel can increase from the on-shell
value to the value for asymptotically large resonance masses. Is mainly
intended as a safety measure, to avoid unintentionally large rescalings.
All of these
meMode's may coexist for the same resonance.
This would be the case e.g. if you want to add a few new channels to an
already existing resonance, where the old partial widths come hardcoded
while the new ones are read in from an external file. The typical example
would be an MSSM Higgs sector, where partial widths to SM particles are
meMode = 0, while decay rates to sparticles
are read in from some external calculation and maybe would be best
approximated by using
meMode = 103. Indeed the default
particle table in PYTHIA uses 103 for all channels that are expected
to be provided by external input.
Some further clarification may be useful. At initialization the existing
total width and on-shell branching ratios will be updated. For channels
meMode < 100 the originally stored branching ratios
are irrelevant, since the existing code will anyway be used to calculate
the partial widths from scratch. For channels with
meMode = 100
or bigger, instead the stored branching ratio is used together with the
originally stored total width to define the correct on-shell partial width.
The sum of partial widths then gives the new total width, and from there
new branching ratios are defined.
In these operations the original sum of branching ratios need not be
normalized to unity. For instance, you may at input have a stored total
width of 1 GeV and a sum of branching ratios of 2. After initialization
the width will then have been changed to 2 GeV and the sum of branching
ratios rescaled to unity. This might happen e.g. if you add a few channels
to an existing resonance, without changing the branching ratios of the
existing channels or the total width of the resonance.
In order to simulate the Breit-Wigner shape correctly, it is important
that all channels that contribute to the total width are included in the
above operations. This must be kept separate from the issue of which
channels you want to have switched on for a particular study, to be
In the event-generation process, when an off-shell resonance mass has been
selected, the width and branching ratios are re-evaluated for this new mass.
At this stage also the effects of restrictions on allowed decay modes are
taken into account, as set by the
onMode switch for each
separate decay channel. Thus a channel may be on or off, with different
choices of open channels between the particle and its antiparticle.
In addition, even when a channel is on, the decay may be into another
resonance with its selection of allowed channels. It is these kinds of
restrictions that lead to the Gamma_out possibly being
smaller than Gamma_tot. As a reminder, the Breit-Wigner for
decays behaves like Gamma_out / ((s - m^2)^2 + s * Gamma_tot^2),
where the width in the numerator is only to those channels being studied,
but the one in the denominator to all channels of the particle. These
ever-changing numbers are not directly visible to the user, but are only
stored in a work area.
Filter for Resonance Decays
include/Pythia8Plugins provides a convenient
way to select a specific final state from resonance decays. For
example, this could be applied to provide events from top pair
production that include leptons + jets. The example
main103 demonstrates how to use this user hook. Further
documentation on the available settings can by found in the
ResonanceDecayFilterHooks.h header file.