Particle corresponds to one entry/slot in the
event record. Its properties therefore is a mix of ones belonging
to a particle-as-such, like its identity code or four-momentum,
and ones related to the event-as-a-whole, like which mother it has.
What is stored for each particle is
From these, a number of further quantities may be derived.
- the identity code,
- the status code,
- two mother indices,
- two daughter indices,
- a colour and an anticolour index,
- the four-momentum and mass,
- the scale at which the particle was produced (optional),
- the polarization/spin/helicity of the particle (optional),
- the production vertex and proper lifetime (optional),
- a pointer to the particle kind in the particle data table, and
- a pointer to the event the particle belongs to.
Basic output methods
The following member functions can be used to extract the most important
the identity of a particle, according to the PDG particle codes
status code. The status code includes information on how a particle was
produced, i.e. where in the program execution it was inserted into the
event record, and why. It also tells whether the particle is still present
or not. It does not tell how a particle disappeared, whether by a decay,
a shower branching, a hadronization process, or whatever, but this is
implicit in the status code of its daughter(s). The basic scheme is:
In detail, the list of used or foreseen status codes is:
- status = +- (10 * i + j)
- + : still remaining particles
- - : decayed/branched/fragmented/... and not remaining
- i = 1 - 9 : stage of event generation inside PYTHIA
- i = 10 -19 : reserved for future expansion
- i >= 20 : free for add-on programs
- j = 1 - 9 : further specification
- 11 - 19 : beam particles
- 11 : the event as a whole
- 12 : incoming beam
- 13 : incoming beam-inside-beam (e.g. gamma
- 14 : outgoing elastically scattered
- 15 : outgoing diffractively scattered
- 21 - 29 : particles of the hardest subprocess
- 21 : incoming
- 22 : intermediate (intended to have preserved mass)
- 23 : outgoing
- 24 : outgoing, nonperturbatively kicked out in diffraction
- 31 - 39 : particles of subsequent subprocesses
- 31 : incoming
- 32 : intermediate (intended to have preserved mass)
- 33 : outgoing
- 34 : incoming that has already scattered
- 41 - 49 : particles produced by initial-state-showers
- 41 : incoming on spacelike main branch
- 42 : incoming copy of recoiler
- 43 : outgoing produced by a branching
- 44 : outgoing shifted by a branching
- 45 : incoming rescattered parton, with changed kinematics
owing to ISR in the mother system (cf. status 34)
- 46 : incoming copy of recoiler when this is a rescattered
parton (cf. status 42)
- 47 : a W or Z gauge boson produced in the
- 51 - 59 : particles produced by final-state-showers
- 51 : outgoing produced by parton branching
- 52 : outgoing copy of recoiler, with changed momentum
- 53 : copy of recoiler when this is incoming parton,
with changed momentum
- 54 : copy of a recoiler, when in the initial state of a
different system from the radiator
- 55 : copy of a recoiler, when in the final state of a
different system from the radiator
- 56 : a W or Z gauge boson produced in a
shower branching (special case of 51)
- 61 - 69 : particles produced by beam-remnant treatment
- 61 : incoming subprocess particle with primordial kT
- 62 : outgoing subprocess particle with primordial kT
- 63 : outgoing beam remnant
- 71 - 79 : partons in preparation of hadronization process
- 71 : copied partons to collect into contiguous colour singlet
- 72 : copied recoiling singlet when ministring collapses to
one hadron and momentum has to be reshuffled
- 73 : combination of very nearby partons into one
- 74 : combination of two junction quarks (+ nearby gluons)
to a diquark
- 75 : gluons split to decouple a junction-antijunction pair
- 76 : partons with momentum shuffled to decouple a
- 77 : temporary opposing parton when fragmenting first two
strings in to junction (should disappear again)
- 78 : temporary combined diquark end when fragmenting last
string in to junction (should disappear again)
- 81 - 89 : primary hadrons produced by hadronization process
- 81 : from ministring into one hadron
- 82 : from ministring into two hadrons
- 83, 84 : from normal string (the difference between the two
is technical, whether fragmented off from the top of the
string system or from the bottom, useful for debug only)
- 85, 86 : primary produced hadrons in junction fragmentation of
the first two string legs in to the junction,
in order of treatment
- 91 - 99 : particles produced in decay process, or by Bose-Einstein
- 91 : normal decay products
- 92 : decay products after oscillation B0 ↔ B0bar or
B_s0 ↔ B_s0bar
- 93, 94 : decay handled by external program, normally
or with oscillation
- 99 : particles with momenta shifted by Bose-Einstein effects
(not a proper decay, but bookkept as an 1 → 1 such,
happening after decays of short-lived resonances but before
decays of longer-lived particles)
- 101 - 109 : particles in the handling of R-hadron production and
decay, i.e. long-lived (or stable) particles containing a very heavy
- 101 : when a string system contains two such long-lived particles,
the system is split up by the production of a new q-qbar
pair (bookkept as decay chains that seemingly need not conserve
flavour etc., but do when considered together)
- 102 : partons rearranged from the long-lived particle end to prepare
for fragmentation from this end
- 103 : intermediate "half-R-hadron" formed when a colour octet particle
(like the gluino) has been fragmented on one side, but not yet on
- 104 : an R-hadron
- 105 : partons or particles formed together with the R-hadron during
the fragmentation treatment
- 106 : subdivision of an R-hadron into its flavour content, with
momentum split accordingly, in preparation of the decay of
the heavy new particle, if it is unstable
- 107 : two temporary leftover gluons joined into one in the formation
of a gluino-gluon R-hadron.
- 111 - 199 : reserved for future expansion
- 201 - : free to be used by anybody
Note: a clarification on the role of the "hardest" vs. the
"subsequent" subprocesses, the 20'ies and 30'ies status code series,
respectively. Most events contain exactly one "hardest"
2 → n interaction, and then an arbitrary number of
"subsequent" softer 2 → 2 ones generated by the
MPI framework. In the
SoftQCD:nonDiffractive event class
also the "hardest" is generated by the MPI machinery, and can be
arbitrarily soft, but still with 20'ies codes. Diffractive systems
span a broad mass range, where the higher masses admit a perturbative
description with "hard" and "subsequent" subprocesses, like for
nondiffractive events. A double diffractive event can contain up to two
such "hardest" interactions, one per diffractive system. A nonperturbative
diffractive system does not contain any 2 → n subprocesses,
but there is a kicked-out quark or gluon with status 24, combined with a
beam remnant of one or two partons with status 63, that together define
the mass and longitudinal axis of the diffractive system, for use in the
subsequent hadronization. An event may also contain two 20'ies
perturbative subcollisions if you use the
Second Hard Process
the indices in the event record where the first and last mothers are
stored, if any. There are six allowed combinations of
mother1 = mother2 = 0: for lines 0 - 2, where line 0
represents the event as a whole, and 1 and 2 the two incoming
mother1 = mother2 > 0: the particle is a "carbon copy"
of its mother, but with changed momentum as a "recoil" effect,
e.g. in a shower;
mother1 > 0, mother2 = 0: the "normal" mother case, where
it is meaningful to speak of one single mother to several products,
in a shower or decay;
mother1 < mother2, both > 0, for
abs(status) = 81 - 86: primary hadrons produced from the
fragmentation of a string spanning the range from
mother2, so that all partons in this range should be
considered mothers; and analogously for
abs(status) = 101 - 106, the formation of R-hadrons;
mother1 < mother2, both > 0, except case 4: particles
with two truly different mothers, in particular the particles emerging
from a hard 2 → n interaction.
mother2 < mother1, both > 0: particles with two truly
different mothers, notably for the special case that two nearby partons are
joined together into a status 73 or 74 new parton, in the
g + q → q case the q is made first mother
to simplify flavour tracing.
Note 1: in backwards evolution of initial-state showers,
the mother may well appear below the daughter in the event record.
Note 2: the
motherList() method below returns
a vector of all the mothers, providing a uniform representation for all
the indices in the event record where the first and last daughters
are stored, if any. There are five allowed combinations of
daughter1 = daughter2 = 0: there are no daughters
daughter1 = daughter2 > 0: the particle has a
"carbon copy" as its sole daughter, but with changed momentum
as a "recoil" effect, e.g. in a shower;
daughter1 > 0, daughter2 = 0: each of the incoming beams
has only (at most) one daughter, namely the initiator parton of the
hardest interaction; further, in a 2 → 1 hard interaction,
like q qbar → Z^0, or in a clustering of two nearby partons,
the initial partons only have this one daughter;
daughter1 < daughter2, both > 0: the particle has
a range of decay products from
daughter2 < daughter1,
both > 0: the particle has two separately stored decay products (e.g.
in backwards evolution of initial-state showers).
Note 1: in backwards evolution of initial-state showers, the
daughters may well appear below the mother in the event record.
Note 2: the mother-daughter relation normally is reciprocal,
but not always. An example is hadron beams (indices 1 and 2), where each
beam remnant and the initiator of each multiparton interaction has the
respective beam as mother, but the beam itself only has the initiator
of the hardest interaction as daughter.
Note 3: the
daughterList() method below
returns a vector of all the daughters, providing a uniform representation
for all five cases. With this method, also all the daughters of the beams
are caught, with the initiators of the basic process given first, while
the rest are in no guaranteed order (since they are found by a scanning
of the event record for particles with the beam as mother, with no further
the colour and anticolour tags, Les Houches Accord [Boo01]
style (starting from tag 101 by default, see below).
Note: in the preliminary implementation of colour sextets
(exotic BSM particles) that exists since PYTHIA 8.150, a negative
anticolour tag is interpreted as an additional positive colour tag,
and vice versa.
the particle four-momentum components.
the particle four-momentum vector, with components as above.
the particle mass, stored with a minus sign (times the absolute value)
for spacelike virtual particles.
the scale at which a parton was produced, which can be used to restrict
its radiation to lower scales in subsequent steps of the shower evolution.
Note that scale is linear in momenta, not quadratic (i.e. Q,
the polarization/spin/helicity of a particle. Currently Pythia does not
use this variable for any internal operations, so its meaning is not
uniquely defined. The LHA standard sets
SPINUP to be the
cosine of the angle between the spin vector and the 3-momentum of the
decaying particle in the lab frame, i.e. restricted to be between +1
and -1. A more convenient choice could be the same quantity in the rest
frame of the particle production, either the hard subprocess or the
mother particle of a decay. Unknown or unpolarized particles should
be assigned the value 9.
the production vertex coordinates, in mm or mm/c.
The production vertex four-vector. Note that the components of a
Vec4 are named
px(), py(), pz() and e()
which of course then should be reinterpreted as above.
the proper lifetime, in mm/c. It is assigned for all hadrons with
positive nominal tau, tau_0 > 0, because it can be used
by PYTHIA to decide whether a particle should or should not be allowed
to decay, e.g. based on the decay vertex distance to the primary interaction
The same method names as above are also overloaded in versions that
set values. These have an input argument of the same type as the
respective output above, and are of type
There are also a few alternative methods for input:
sets the status sign positive or negative, without changing the absolute
void Particle::statusCode(int code)
changes the absolute value but retains the original sign.
void Particle::mothers(int mother1, int mother2)
sets both mothers in one go.
void Particle::daughters(int daughter1, int daughter2)
sets both daughters in one go.
void Particle::cols(int col, int acol)
sets both colour and anticolour in one go.
void Particle::p(double px, double py, double pz, double e)
sets the four-momentum components in one go.
void Particle::vProd(double xProd, double yProd, double zProd, double tProd)
sets the production vertex components in one go.
Further output methods
In addition, a number of derived quantities can easily be obtained,
but cannot be set, such as:
the absolute value of the particle identity code.
the absolute value of the status code.
true for a remaining particle, i.e. one with positive status code,
else false. Thus, after an event has been fully generated, it
separates the final-state particles from intermediate-stage ones.
(If used earlier in the generation process, a particle then
considered final may well decay later.)
true for particles with a status code -34, -45, -46 or -54, else false.
This singles out partons that have been created in a previous
scattering but here are bookkept as belonging to the incoming state
of another scattering.
production vertex has been set; if false then production at the origin
squared mass, which can be negative for spacelike partons.
(squared) mass calculated from the four-momentum; should agree
m(), m2() up to roundoff. Negative for spacelike
energy calculated from the mass and three-momentum; should agree
e() up to roundoff. For spacelike partons a
positive-energy solution is picked. This need not be the correct
one, so it is recommended not to use the method in such cases.
(squared) transverse momentum.
(squared) transverse mass. If m_T^2 is negative, which can happen
for a spacelike parton, then
-sqrt(-m_T^2), by analogy with the negative sign used to store
(squared) three-momentum size.
(squared) transverse energy,
eT = e * sin(theta) = e * pT / pAbs.
polar and azimuthal angle.
angle in the (p_x, p_z) plane, between -pi and
+pi, with 0 along the +z axis
E +- p_z.
rapidity and pseudorapidity.
the decay vertex coordinates, in mm or mm/c. This decay vertex is
calculated from the production vertex, the proper lifetime and the
four-momentum assuming no magnetic field or other detector interference.
It can be used to decide whether a decay should be performed or not,
and thus is defined also for particles which PYTHIA did not let decay.
Not part of the
Particle class proper, but obviously tightly
linked, are the two methods
double m(const Particle& pp1, const Particle& pp2)
double m2(const Particle& pp1, const Particle& pp2)
the (squared) invariant mass of two particles.
Properties of the particle species
Each Particle contains a pointer to the respective
ParticleDataEntry object in the
particle data tables.
This gives access to properties of the particle species as such. It is
there mainly for convenience, and should be thrown if an event is
written to disk, to avoid any problems of object persistency. Should
an event later be read back in, the pointer will be recreated from the
id code if the normal input methods are used. (Use the
if your persistency scheme bypasses the normal methods.) This pointer is
used by the following member functions:
the name of the particle.
as above, but for negative-status particles the name is given in
brackets to emphasize that they are intermediaries.
2 *spin + 1 when defined, else 0.
charge, and three times it to make an integer.
charge different from or equal to 0.
0 for colour singlets, 1 for triplets,
-1 for antitriplets and 2 for octets. (A preliminary implementation of
colour sextets also exists, using 3 for sextets and -3 for antisextets.)
the nominal mass of the particle, according to the data tables.
the width of the particle, and the minimum and maximum allowed mass value
for particles with a width, according to the data tables.
the mass of the particle, picked according to a Breit-Wigner
distribution for particles with width. It is different each time called,
and is therefore only used once per particle to set its mass
will give the constituent masses for quarks and diquarks,
else the same masses as with
the nominal lifetime tau_0 > 0, in mm/c, of the particle species.
It is used to assign the actual lifetime tau.
flag whether particle has been declared unstable or not, offering
the main user switch to select which particle species to decay.
flag whether decay modes have been declared for a particle,
so that it could be decayed, should that be requested.
particles that are decayed by an external program.
particles where the decay is to be treated as part of the hard process,
typically with nominal mass above 20 GeV (W^+-, Z^0, t, ...).
particles with strong or electric charge, or composed of ones having it,
which thereby should be considered visible in a normal detector.
true for a lepton or an antilepton (including neutrinos).
true for a quark or an antiquark.
true for a gluon.
true for a diquark or an antidiquark.
true for a gluon, a quark or antiquark up to the b (but excluding top),
and a diquark or antidiquark consisting of quarks up to the b.
true for a hadron (made up out of normal quarks and gluons,
i.e. not for R-hadrons and other exotic states).
a reference to the ParticleDataEntry.
Methods that may access the event the particle belongs to
A particle can be created on its own. When inserted into an event record,
it obtains a pointer to that event-as-a-whole. It is then possible to
use methods that do not make sense for a particle in isolation. These
methods are listed below. Whenever the pointer to the event is not
defined, these will return an appropriate "null" value, this being -1
for an integer, false for a bool, and empty for a vector, unless otherwise
the index of the particle itself in the event record.
returns the status code according to the HepMC conventions agreed in
February 2009. This convention does not preserve the full information
provided by the internal PYTHIA status code, as obtained by
Particle::status(), but comes reasonably close.
The allowed output values are:
- 0 : an empty entry, with no meaningful information and therefore
to be skipped unconditionally;
- 1 : a final-state particle, i.e. a particle that is not decayed
further by the generator (may also include unstable particles that
are to be decayed later, as part of the detector simulation);
- 2 : a decayed Standard Model hadron or tau or mu lepton, excepting
virtual intermediate states thereof (i.e. the particle must undergo
a normal decay, not e.g. a shower branching);
- 3 : a documentation entry (not used in PYTHIA);
- 4 : an incoming beam particle;
- 11 - 200 : an intermediate (decayed/branched/...) particle that does
not fulfill the criteria of status code 2, with a generator-dependent
classification of its nature; in PYTHIA the absolute value of the normal
status code is used.
Note: for a particle without a properly set pointer to its
event, codes 1 and 4 can still be inferred from its status code, while
everythg else is assigned code 0.
are used to trace carbon copies of the particle up to its top mother
or down to its bottom daughter. If there are no such carbon copies,
the index of the particle itself will be returned. A carbon copy is
when the "same" particle appears several times in the event record, but
with changed momentum owing to recoil effects.
also trace top mother and bottom daughter, but do not require carbon
copies, only that one can find an unbroken chain, of mothers or daughters,
with the same flavour
id code. When it encounters ambiguities,
say a g → g g branching or a u u → u u hard
scattering, it will stop the tracing and return the current position.
It can be confused by nontrivial flavour changes, e.g. a hard process
u d → d u by W^+- exchange will give the wrong
answer. These methods therefore are of limited use for common particles,
in particular for the gluon, but should work well for "rare" particles.
vector<int> Particle::sisterList(bool traceTopBot = false)
returns a vector of all the sister indices of the particle, i.e. all the
daughters of the first mother, except the particle itself. If the argument
traceTopBot = true the particle is first traced up with
iTopCopy() before its mother is found, and then all the
particles in the
daughterList() of this mother are traced down
iBotCopy(), omitting the original particle itself.
The method is not meaningful for the 0 entry, with status code -11, and
there returns an empty list.
bool Particle::isAncestor(int iAncestor)
traces the particle upwards through mother, grandmother, and so on, until
either iAncestor is found or the top of the record is reached.
Normally one unique mother is required, as is the case e.g. in decay chains
or in parton showers, so that e.g. the tracing through a hard scattering
would not work. For hadronization, first-rank hadrons are identified with
the respective string endpoint quark, which may be useful e.g. for
b physics, while higher-rank hadrons give
Currently also ministrings that collapsed to one single hadron and
junction topologies give
removes the decay chain of the particle and thus restores
it to its undecayed state. It is only intended for "normal" particle
decay chains, and will return false in other cases, notably if
the particle is coloured. The procedure would not work if non-local
momentum shifts have been performed, such as with a Bose-Einstein
shift procedure (or for a dipole shower recoiler). As the decay products
are erased from the event record, mother and daughter indices are
updated to retain a correct history for the remaining particles.
Methods that perform operations
There are some further methods, some of them inherited from
Vec4, to modify the properties of a particle.
They are of little interest to the normal user.
void Particle::rescale3(double fac)
multiply the three-momentum components by
void Particle::rescale4(double fac)
multiply the four-momentum components by
void Particle::rescale5(double fac)
multiply the four-momentum components and the mass by
void Particle::rot(double theta, double phi)
rotate three-momentum and production vertex by these polar and azimuthal
void Particle::bst(double betaX, double betaY, double betaZ)
boost four-momentum and production vertex by this three-vector.
void Particle::bst(double betaX, double betaY, double betaZ, double gamma)
as above, but also input the gamma value, to reduce roundoff errors.
void Particle::bst(const Vec4& pBst)
boost four-momentum and production vertex by
beta = (px/e, py/e, pz/e).
void Particle::bst(const Vec4& pBst, double mBst)
as above, but also use gamma> = e/m to reduce roundoff errors.
void Particle::bstback(const Vec4& pBst)
void Particle::bstback(const Vec4& pBst, double mBst)
as above, but with sign of boost flipped.
void Particle::rotbst(const RotBstMatrix& M)
combined rotation and boost of the four-momentum and production vertex.
void Particle::offsetHistory( int minMother, int addMother, int minDaughter, int addDaughter))
add a positive offset to the mother and daughter indices, i.e.
mother1 is above
addMother is added to it, same with
daughter1 is above
addDaughter is added to it, same with
void Particle::offsetCol( int addCol)
add a positive offset to colour indices, i.e. if
addCol is added to it, same with
Constructors and operators
Normally a user would not need to create new particles. However, if
necessary, the following constructors and methods may be of interest.
constructs an empty particle, i.e. where all properties have been set 0
Particle::Particle(int id, int status = 0, int mother1 = 0, int mother2 = 0, int daughter1 = 0, int daughter2 = 0, int col = 0, int acol = 0, double px = 0., double py = 0., double pz = 0., double e = 0., double m = 0., double scale = 0., double pol = 9.)
constructs a particle with the input properties provided, and non-provided
ones set 0 (9 for
Particle::Particle(int id, int status, int mother1, int mother2, int daughter1, int daughter2, int col, int acol, Vec4 p, double m = 0., double scale = 0., double pol = 9.)
constructs a particle with the input properties provided, and non-provided
ones set 0 (9 for
Particle::Particle(const Particle& pt)
constructs an particle that is a copy of the input one.
Particle& Particle::operator=(const Particle& pt)
copies the input particle.
void Particle::setEvtPtr(Event* evtPtr)
sets the pointer to the
Event object the particle
belongs to. This method is automatically called when a particle
is appended to an event record. Also calls
void Particle::setPDEPtr(ParticleDataEntry* pdePtr = 0)
sets the pointer to the
ParticleDataEntry object of the
particle, based on its current
id code. If the particle
belongs to an event there is no need to provide the input argument.
As explained above, a valid
is needed for the methods that provide information generic to the