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

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

Basic methods

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

method name="id()"
the identity of a particle, according to the PDG particle codes [Eid04].

method name="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:

In detail, the list of foreseen status codes is:

method name="mother1(), 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:

  1. 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;
  2. 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;
  3. 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;
  4. 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;
  5. 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.

method name="daughter1(), 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:

  1. daughter1 = daughter2 = 0: there are no daughters (so far);
  2. 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;
  3. 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;
  4. daughter1 < daughter2, both > 0: the particle has a range of decay products from daughter1 to daughter2;
  5. 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).

method name="col(), acol()"
the colour and anticolour tags, Les Houches Accord [Boo01] style (starting from tag 101 by default, see below).

method name="px(), py(), pz(0), e()"
the particle four-momentum components, alternatively extracted as a Vec4 p().

method name="m()"
the particle mass.

method name="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).

method name="xProd(), yProd(), zProd(), tProd()"
the production vertex coordinates, in mm or mm/c, alternatively extracted as a Vec4 vProd(). The initial process is assumed to occur at the origin.
Note:the Vec4 has components px(), py(), pz() and e(), which of course should be reinterpreted as above.

method name="tau()"
the proper lifetime, in mm/c; is assigned for all hadrons with positive nominal tau, tau_0 > 0, even if not decayed by PYTHIA (because of one veto or another).

The same method names are overloaded to take an argument, in which case the corresponding property is set accordingly.

Further methods

There are a few alternative methods for input:

method name="statusPos(), statusNeg()"
sets the status sign positive or negative, without changing the absolute value.

method name="statusCode(code)"
changes the absolute value but retains the original sign.

method name="mothers(m1, m2)"
sets both mothers in one go.

method name="daughters(d1, d2)"
sets both daughters in one go.

method name="cols(c, ac)"
sets both colour and anticolour in one go.

method name="p( px, py, pz, e)"
sets the four-momentum in one go; alternative input as a Vec4 object.

method name="vProd( xProd, yProd, zProd, tProd)"
sets the production vertex in one go; alternative input as a Vec4 object.

In addition, a number of derived quantities can easily be obtained (but cannot be set), such as:

method name="statusAbs"
the absolute value of the status code.

method name="remains()"
true for a remaining particle, i.e. one with positive status code, else false.

method name="isQ(), isNotQ()"
true for a quark or an antiquark, or the reverse.

method name="isG(), isNotG()"
true for a gluon, or the reverse.

method name="isQorG()"
true for a quark, an antiquark or a gluon.

method name="isQQ()"
true for a diquark or antidiquark.

method name="isQorQQ()"
true for a (anti)quark or (anti)diquark.

method name="isL()"
true for a lepton (charged or neutrino).

method name="hasCol()"
true when either of the colour or anticolour indices are nonvanishing.

method name="m2()"
squared mass.

method name="mCalc(), m2Calc()"
(squared) mass calculated from the four-momentum; should agree with m(), m2() up to roundoff.

method name="eCalc()"
energy calculated from the mass and three-momentum; should agree with e() up to roundoff.

method name="pT(), pT2()"
(squared) transverse momentum.

method name="mT(), mT2()"
(squared) transverse mass.

method name="pAbs(), pAbs2()"
(squared) three-momentum size.

method name="theta(), phi()"
polar and azimuthal angle.

method name="thetaXZ()"
angle in the (p_x, p_z) plane, between -pi and +pi, with 0 along the +z axis

method name="pPlus(), pMinus()"
E +- p_z.

method name="y(), eta()"
rapidity and pseudorapidity.

method name="xDec(), yDec(), zDec(), tDec()"
the decay vertex coordinates, in mm or mm/c, alternatively extracted as a Vec4 vDec(); 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. This pointer is used by the following member functions:

method name="name()"
the name of the particle, as a string.

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

method name="m0()"
the nominal mass of the particle, according to the data tables.

method name="mass()"
the mass of the particle, picked according to a Breit-Wigner distribution for particles with width, and thus different each time called.

method name="constituentMass()"
will give the constituent masses for quarks and diquarks, else the same masses as normal.

method name="colType()"
0 for colour singlets, 1 for triplets, -1 for antitriplets and 2 for octets.

method name="charge(), icharge()"
charge, and three times it to make an integer.

method name="isCharged(), isNeutral()"
charge different from or equal to 0.

method name="isVisible(), isInvisible()"
particles with/without strong or electromagnetic interactions, which thereby are or are not visible in a normal detector.

method name="spinType()"
2 *spin + 1; still incomplete implementation.

method name="canDecay()"
flag whether decay modes have been declared for a particle, so that it could be decayed, should that be requested.

method name="mayDecay()"
flag whether particle has been declared unstable or not, offering the main user switch to select which particle species to decay.

method name="particleData()"
a reference to the ParticleDataEntry.

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

There are some further methods, inherited from Vec4, to rotate and boost the four-momentum, and a << method to list a single particle.

Not part of the event class proper, but obviously tightly linked, are the metods m(Particle, Particle) and m2(Particle, Particle) to calculate the (squared) invariant mass of two particles.

Currently there is no information on polarization states.