The TAUOLA external event generator can be used to decay tau particles. The syntax for decay.dec files is

Decay tau-
<BF>   TAUOLA   <modeInt>;
Enddecay

where <BF> is the relative branching fraction (0 to 1) and modeInt is the integer value specifying what TAUOLA decay is to be generated. Note that the daughter particle types of the mode are not specified within the "Decay" command - the mode integer will generate the correct daughters for you. Alised tau particles can use the TAUOLA model, but the generated daughters cannot be aliased particles; there is no simple way to ensure that the order of any daughter particle definitions will match the order of the particles generated by TAUOLA, especially for the modes with many pions in the final state.

Allowed TAUOLA decay mode integers are (from the TAUOLA documentation):

0 All TAUOLA modes (1 to 22 below) 1 tau- -> e- anti-nu_e nu_tau 2 tau- -> mu- anti-nu_mu nu_tau 3 tau- -> pi- nu_tau 4 tau- -> rho- nu_tau 5 tau- -> a_1- nu_tau 6 tau- -> K- nu_tau 7 tau- -> K*- nu_tau 8 tau- -> 2pi- pi+ pi0 nu_tau 9 tau- -> 3pi0 pi- nu_tau 10 tau- -> 2pi- pi+ 2pi0 nu_tau 11 tau- -> 3pi- 2pi+ nu_tau 12 tau- -> 3pi- 2pi+ pi0 nu_tau 13 tau- -> 2pi- pi+ 3pi0 nu_tau 14 tau- -> K- K+ pi- nu_tau 15 tau- -> K0 anti-K0 pi- nu_tau 16 tau- -> K- anti-K0 pi0 nu_tau 17 tau- -> 2pi0 K- nu_tau 18 tau- -> pi- pi+ K- nu_tau 19 tau- -> pi- pi0 K0 nu_tau 20 tau- -> eta pi- pi0 nu_tau 21 tau- -> pi- pi0 gamma nu_tau 22 tau- -> K- K0 nu_tau

For example, to generate tau- -> 3pi0 pi- nu_tau (40%) and tau- -> mu- anti-nu_mu nu_tau (60%), use

Decay Mytau-
0.4  TAUOLA 9;
0.6  TAUOLA 2;
Enddecay
CDecay Mytau+

To ensure decay mode consistency, any tau particle decay definition (normal or alised) using TAUOLA should use it for all sub-modes, i.e do not mix TAUOLA and other decay models, such as phase space (PHSP), within the same "Decay" definition.

An example which is NOT allowed is:

Decay Mytau+
0.4  TAUOLA 9;
0.5  TAUOLA 2;
0.1  e+ nu_e anti-nu_tau PHSP;  # Decay model is not TAUOLA.
Enddecay

If this is detected, the program will stop and ask you to correct the decay definition(s). The reason for this limitation is due to the fact that TAUOLA decays any tau particles by considering the kinematics of its parent particle and the rest of the daughters, one of which is the tau particle itself and other tau particles (charged or neutrino) that are there in order to conserve tau lepton number. We let TAUOLA decay all tau particle types within the decay tree. EvtGen is then prevented from decaying the other tau particle(s), since TAUOLA has already done that. If other physics models are used for the other tau particle(s), then the correlated decay kinematics generated by TAUOLA will be lost, leading to unphysical results.

If a particle decay contains at least two tau particles, and one of these uses TAUOLA decay modes, then all other tau particles should use the same TAUOLA decay definitions to ensure consistency:

Alias MyB0 B0
Alias Myanti-B0 anti-B0
ChargeConj Myanti-B0 MyB0
Alias Mytau+ tau+
Alias Mytau- tau-
ChargeConj Mytau- Mytau+
Decay MyB0
1.0  Mytau+  Mytau-  PHSP;
Enddecay
CDecay Myanti-B0
Decay Mytau+
0.4 TAUOLA 9;
0.6 TAUOLA 2;
Enddecay
CDecay Mytau-

You should not define two or more (via aliases) tau decays with different TAUOLA sub-mode branching fractions. Also tau- and tau+ particles must have the same decay modes/branching fractions. These limitions are due to the fact that the core code of TAUOLA is FORTRAN, and changing the TAUOLA mode parameters for one tau particle will affect it for all other tau particles as well. So, EvtGen will only initialise TAUOLA modes when it counters the first tau-type particle in the "evt.pdl" file that has a TAUOLA decay mode defined for it. So, for example, if TAUOLA decays are defined for "tau-", which corresponds to the first tau particle in the evt.pdl file, then other TAUOLA decay definitions will be ignored, and the modes defined for tau- will also be used for tau+ etc.. It is important to make sure the decay file uses the TAUOLA mode definitions correctly to ensure consistency in decay tree results/information:

1) Decide what tau modes you want from TAUOLA. 2) Define the allowed decay modes for the tau- and tau+ particle, using only TAUOLA modes. It is possible to keep the decay.dec generic (non-TAUOLA) decay modes for tau- and tau+, and create aliased tau ("signal") particles such as Mytau- and Mytau+ to only use TAUOLA modes:

Alias Mytau- tau-
Alias Mytau+ tau+
ChargeConj Mytau+ Mytau-
Decay Mytau-
0.4  TAUOLA 9;
0.6  TAUOLA 2;
Enddecay
CDecay Mytau+

3) Make sure the TAUOLA modes for tau+ match those for tau- (easily done by using "CDecay"). 4) Do not create other (alised) tau particles using TAUOLA modes. These settings will be ignored.

The PHOTOS generator can be used with the TAUOLA code, although the only way at present to have this enabled is to set the "yesPhotos" global option in the decay.dec file. This means that PHOTOS will be turned on for ALL decays. More work is needed to change the decay table reading code (EvtGenBase/EvtDecayTable) to enable/disable PHOTOS for TAUOLA decays only.