Particle Properties

A 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

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 production vertex and proper lifetime (optional),
a pointer to the particle kind in the particle data table, and
a pointer to the whole particle data table.

From these, a number of further quantities may be derived.

Basic output methods

The following member functions can be used to extract the most important information:

int Particle::id()
the identity of a particle, according to the PDG particle codes [Yao06].

int Particle::status()
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:

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

In detail, the list of used or foreseen status codes is:

11 - 19 : beam particles

11 : the event as a whole
12 : incoming beam
13 : incoming beam-inside-beam (e.g. gamma inside e)
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

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)

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

61 - 69 : particles produced by beam-remnant treatment

61 : incoming subprocess particle with primordial kT included
62 : outgoing subprocess particle with primordial kT included
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 junction-antijunction pair
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 frogmentation 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 effects

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 - 199 : reserved for future expansion
201 - : free to be used by anybody

int Particle::mother1()
int Particle::mother2()
the indices in the event record where the first and last mothers are stored, if any. There are five allowed combinations of mother1 and mother2:

mother1 = mother2 = 0: for lines 0 - 2, where line 0 represents the event as a whole, and 1 and 2 the two incoming beam particles;
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 mother1 to mother2, so that all partons in this range should be considered mothers;
mother1 < mother2, both > 0, except case 4: particles with two truly different mothers, in particular the particles emerging from a hard 2 -> n interaction.

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(i) method of the Event class returns a vector of all the mothers, providing a uniform representation for all five cases.

int Particle::daughter1()
int Particle::daughter2()
the indices in the event record where the first and last daughters are stored, if any. There are five allowed combinations of daughter1 and daughter2:

daughter1 = daughter2 = 0: there are no daughters (so far);
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 daughter1 to daughter2;
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 multiple 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(i) method of the Event class 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 information).

int Particle::col()
int Particle::acol()
the colour and anticolour tags, Les Houches Accord [Boo01] style (starting from tag 101 by default, see below).

double Particle::px()
double Particle::py()
double Particle::pz()
double Particle::e()
the particle four-momentum components.

Vec4 Particle::p()
the particle four-momentum vector, with components as above.

double Particle::m()
the particle mass, stored with a minus sign (times the absolute value) for spacelike virtual particles.

double Particle::scale()
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, not Q^2).

double Particle::xProd()
double Particle::yProd()
double Particle::zProd()
double Particle::tProd()
the production vertex coordinates, in mm or mm/c.

Vec4 Particle::vProd()
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.

double Particle::tau()
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 vertex.

Input methods

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 void.

There are also a few alternative methods for input:

void Particle::statusPos()
void Particle::statusNeg()
sets the status sign positive or negative, without changing the absolute value.

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:

int Particle::idAbs()
the absolute value of the particle identity code.

int Particle::statusAbs()
the absolute value of the status code.

bool Particle::isFinal()
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.)

bool Particle::isRescatteredIncoming()
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.

bool Particle::hasVertex()
production vertex has been set; if false then production at the origin is assumed.

double Particle::m2()
squared mass, which can be negative for spacelike partons.

double Particle::mCalc()
double Particle::m2Calc()
(squared) mass calculated from the four-momentum; should agree with m(), m2() up to roundoff. Negative for spacelike virtualities.

double Particle::eCalc()
energy calculated from the mass and three-momentum; should agree with 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.

double Particle::pT()
double Particle::pT2()
(squared) transverse momentum.

double Particle::mT()
double Particle::mT2()
(squared) transverse mass. If m_T^2 is negative, which can happen for a spacelike parton, then mT() returns -sqrt(-m_T^2), by analogy with the negative sign used to store spacelike masses.

double Particle::pAbs()
double Particle::pAbs2()
(squared) three-momentum size.

double Particle::eT()
double Particle::eT2()
(squared) transverse energy, eT = e * sin(theta) = e * pT / pAbs.

double Particle::theta()
double Particle::phi()
polar and azimuthal angle.

double Particle::thetaXZ()
angle in the (p_x, p_z) plane, between -pi and +pi, with 0 along the +z axis

double Particle::pPos()
double Particle::pNeg()
E +- p_z.

double Particle::y()
double Particle::eta()
rapidity and pseudorapidity.

double Particle::xDec()
double Particle::yDec()
double Particle::zDec()
double Particle::tDec()
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.

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 Event::restorePtrs() method if your persistency scheme bypasses the normal methods.) This pointer is used by the following member functions:

string Particle::name()
the name of the particle.

string Particle::nameWithStatus()
as above, but for negative-status particles the name is given in brackets to emphasize that they are intermediaries.

int Particle::spinType()
2 *spin + 1 when defined, else 0.

double Particle::charge()
int Particle::chargeType()
charge, and three times it to make an integer.

bool Particle::isCharged()
bool Particle::isNeutral()
charge different from or equal to 0.

int Particle::colType()
0 for colour singlets, 1 for triplets, -1 for antitriplets and 2 for octets.

double Particle::m0()
the nominal mass of the particle, according to the data tables.

double Particle::mWidth()
double Particle::mMin()
double Particle::mMax()
the width of the particle, and the minimum and maximum allowed mass value for particles with a width, according to the data tables.

double Particle::mass()
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 m().

double Particle::constituentMass()
will give the constituent masses for quarks and diquarks, else the same masses as with m0().

double Particle::tau0()
the nominal lifetime tau_0 > 0, in mm/c, of the particle species. It is used to assign the actual lifetime tau.

bool Particle::mayDecay()
flag whether particle has been declared unstable or not, offering the main user switch to select which particle species to decay.

bool Particle::canDecay()
flag whether decay modes have been declared for a particle, so that it could be decayed, should that be requested.

bool Particle::doExternalDecay()
particles that are decayed by an external program.

bool Particle::isResonance()
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, ...).

bool Particle::isVisible()
particles with strong or electric charge, or composed of ones having it, which thereby should be considered visible in a normal detector.

bool Particle::isLepton()
true for a lepton or an antilepton (including neutrinos).

bool Particle::isQuark()
true for a quark or an antiquark.

bool Particle::isGluon()
true for a gluon.

bool Particle::isHadron()
true for a hadron (made up out of normal quarks and gluons, i.e. not for R-hadrons and other exotic states).

ParticleDataEntry& particleDataEntry()
a reference to the ParticleDataEntry.

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.

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 fac.

void Particle::rescale4(double fac)
multiply the four-momentum components by fac.

void Particle::rescale5(double fac)
multiply the four-momentum components and the mass by fac.

void Particle::rot(double theta, double phi)
rotate three-momentum and production vertex by these polar and azimuthal angles.

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. if mother1 is above minMother then addMother is added to it, same with mother2, if daughter1 is above minDaughter then addDaughter is added to it, same with daughter2.

void Particle::offsetCol( int addCol)
add a positive offset to colour indices, i.e. if col is positive then addCol is added to it, same with acol.

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.

Particle::Particle()
constructs an empty particle, i.e. where all properties have been set 0 or equivalent.

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.)
constructs a particle with the input properties provided, and non-provided ones set 0.

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.)
constructs a particle with the input properties provided, and non-provided ones set 0.

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::setPDTPtr()
sets the pointer to the ParticleData objects, i.e. to the full particle data table. Also calls setPDEPtr below.

void Particle::setPDEPtr()
sets the pointer to the ParticleDataEntry object of the particle, based on its current id code.

Final notes

The Event class also contains a few methods defined for individual particles, but these may require some search in the event record and therefore cannot be defined as Particle methods.

Currently there is no information on polarization states.