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

method  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 used or foreseen status codes is:

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

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

method  m()  
the particle mass.

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

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

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

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

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

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

method  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  idAbs()  
the absolute value of the particle identity code.

method  statusAbs()  
the absolute value of the status code.

method  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.)

method  m2()  
squared mass.

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

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

method  pT(), pT2()  
(squared) transverse momentum.

method  mT(), mT2()  
(squared) transverse mass.

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

method  theta(), phi()  
polar and azimuthal angle.

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

method  pPlus(), pMinus()  
E +- p_z.

method  y(), eta()  
rapidity and pseudorapidity.

method  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 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:

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

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

method  spinType()  
2 *spin + 1 when defined, else 0.

method  charge(), chargeType()  
charge, and three times it to make an integer.

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

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

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

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

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

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

method  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, ...).

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

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

method  doExternalDecay()  
particles that are decayed by an external program.

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

method  doForceWidth()  
resonances that have code to recalculate the width in mWidth from the nominal mass value m0, but where nevertheless the stored mWidth value is used.

method  isLepton()  
true for a lepton or an antilepton (including neutrinos).

method  isQuark()  
true for a quark or an antiquark.

method  isGluon()  
true for a gluon.

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

method  particleData()  
a reference to the ParticleDataEntry.

There are some further methods, inherited from Vec4, to rotate and boost the four-momentum.

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.

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 a Particle method.

Currently there is no information on polarization states.