The structure and operation of the particle data table is described
here. That page
also describes how default data properties can be changed. The
current page provides the actual default values.
Main settings
Apart from the data itself, the particle data table only contains
a few aspects that are available to change:
Selection of particle masses when the mass(id) is called
to provide a new mass:
Note: this mode only applies to normal hadronic
resonances like the rho. The more massive states of the
isResonance() type, like Z^0 or top, are
considered separately.
The modifications in options 2 and 4 above enhance the large-mass tail
of the Breit-Wigners (the mass spectrum develops a dm/m divergence).
However, we expect form factors to dampen this tail at masses some distance
above the nominal one, so cut off the rise by requiring the actual
Breit-Wigner weight not to be more than a factor maxEnhanceBW
above the one obtained with options 1 or 3, respectively. This also
opens up for a simpler technical handling of mass selection in options
2 and 4, by using standard hit-and-miss Monte Carlo.
Since running masses are only calculated for the six quark flavours,
e.g. to obtain couplings to the Higgs boson(s), there is not an entry
in the normal tables for each particles, but only the six MSbar mass
values below, used as starting point for the running. In addition you
can pick an alpha_s(M_Z), which is converted into a first-order
five-flavour Lambda that is used to determine the rate of the running.
(Without any match to four flavours below m_b; if desired, this
can be fixed by slightly shifted default mass values, since the routines
never should be called below the m_b scale anyway.)
the d quark MSbar mass at 2 GeV scale.
the u quark MSbar mass at 2 GeV scale.
the s quark MSbar mass at 2 GeV scale.
the c quark MSbar mass at the mass scale itself.
the b quark MSbar mass at the mass scale itself.
the t quark MSbar mass at the mass scale itself.
the alpha_s(M_Z) value used to define tha rate at which MSbar
masses run.
Comments on the data
The starting point for the current data is the 2006 Review of Particle
Physics Yao06. All known particle masses, widths and lifetimes
have been set accordingly, while not-yet-measured particles are kept at
their values from PYTHIA 6. Decay channels and their branching
ratios remain a major worry: many particles do not have one single solidly
measured branching ratio, and many further do not have known branching
ratios that add up to (the neighbourhood of) unity.
Uncertainties are especially big for the scalar, pseudovector and tensor
L = 1 multiplets available in PYTHIA. We note that
some distributions become better described when these multiplets are
included in the generation, while others become worse. It is tempting to
associate this lackluster performance with the primitive knowledge.
Not even the multiplets themselves are particularly well known.
It used to be that the a_0(980) and f_0(980)
were considered to be members of the scalar multiplet. Nowadays they are
commonly assumed to be either four-quark states or of some other exotic
character. This means that the PYTHIA 8 PDG particle codes
have been changed for these particles, relative to what was used in
PYTHIA 6 based on previous PDG editions. Specifically their
numbers are now in the 9000000 series, and they have been replaced in the
scalar multiplet by a_0(1450) and f_0(1370).
For charm and bottom mesons the jungle of partial measurements makes
it very difficult to construct fully consistent sets of decay channels.
This part of the program has not yet been brought up to date to the
2006 RPP. Instead the LHCb decay tables (for EvtGen, but without
using the EvtGen matrix-element machinery) and the DELPHI tune for
PYTHIA 6 is being used. (This also includes a few non-c/b
hadrons that only occur in the c/b decay tables.) This has the
advantage that many tests have been made for consistency, but the
disadvantage that it is not always in agreement with the latest
measurements of some specific decay channels. The decays based
on the LHCb tables (with some modifications) are 411, 421, 431, 441,
445, 511, 521, 531, 541, 3124, 4122, 4124, 5122, 10441, 10443, 13122,
14122, 20443, 23122, 30313, 30323, 30443, 33122, 100113, 100213, 100441,
100443, 100553, 9000111, 9000211. Correspondingly the decays based on
the DELPHI tables are 415, 425, 435, 515, 525, 535, 4132, 4232, 4332,
5132, 5232 and 5332.
The data itself
Here comes the default particle data used in the program. Do not touch.
The meaning of the various properties and the format used are explained
here and the
meMode codes here.