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 output methods

The following member functions can be used to extract the most important information: the identity of a particle, according to the PDG particle codes Yao06. 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: 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; and analogously for abs(status) = 101 - 106, the formation of R-hadrons;
  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. 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 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(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). 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, not Q^2). 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 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: sets the status sign positive or negative, without changing the absolute value. changes the absolute value but retains the original sign. sets both mothers in one go. sets both daughters in one go. sets both colour and anticolour in one go. sets the four-momentum components in one go. 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 is assumed. squared mass, which can be negative for spacelike partons. (squared) mass calculated from the four-momentum; should agree with m(), m2() up to roundoff. Negative for spacelike virtualities. 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. (squared) transverse momentum. (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. (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.

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

Not part of the Particle class proper, but obviously tightly linked, are the two methods 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. multiply the three-momentum components by fac. multiply the four-momentum components by fac. multiply the four-momentum components and the mass by fac. rotate three-momentum and production vertex by these polar and azimuthal angles. boost four-momentum and production vertex by this three-vector. as above, but also input the gamma value, to reduce roundoff errors. boost four-momentum and production vertex by beta = (px/e, py/e, pz/e). as above, but also use gamma> = e/m to reduce roundoff errors. as above, but with sign of boost flipped. combined rotation and boost of the four-momentum and production vertex. 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. 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. constructs an empty particle, i.e. where all properties have been set 0 or equivalent. constructs a particle with the input properties provided, and non-provided ones set 0 (9 for pol). constructs a particle with the input properties provided, and non-provided ones set 0 (9 for pol). constructs an particle that is a copy of the input one. copies the input particle. sets the pointer to the ParticleData objects, i.e. to the full particle data table. Also calls setPDEPtr below. 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.