Charm and bottom hadronic physics

Elastic and inelastic nuclear interactions of charm and bottom hadrons are enabled in most physics lists, in particular those of interest for HEP. (The main exception is QGSP_BIC and its derived physics lists - QGSP_BIC_HP and QGSP_BIC_AllHP - which are typically used for low-energy applications.)

For charm and bottom hadron-nuclear (elastic and inelastic) cross sections, the Glauber-Gribov approach, which is used also for all other hadrons, has been extended to cover heavy hadrons.

For final-state elastic nuclear scattering of charm and bottom hadron projectiles, a simple treatment is utilized (an improved version of Gheisha’s two-exponential momentum transfer parameterization).

For final-state inelastic nuclear scattering of charm and bottom hadron projectiles, both Geant4 string models (FTF and QGS) have been extended to deal with charm and bottom quarks and diquarks carried by the heavy projectile hadron, as well as with the possibility of creating charm-anti_charm and bottom-anti_bottom pairs from the vacuum during the string fragmentation phase.

Cascade models (Bertini, Binary, INCL) are currently not applicable for heavy hadrons and string models cannot handle them properly at very low energies. Therefore, we use FTFP down to 100 MeV, and below we use a dummy model that returns as final state the initial state unchanged.

Note that, for most applications, this is a safe simplification, giving that nearly all slowly moving charm and bottom hadrons decay before any hadronic interaction can occur.

The QGS-based physics lists use, as for other hadrons, QGSP for charm and bottom inelastic interactions above 12 GeV, whereas FTFP is used below 25 GeV. The other, FTF-based physics lists, use FTFP at all energies (indeed, as explained above, below 100 MeV we used a simplified approach).

The vast majority of charm and bottom hadrons created in high-energy collisions decay inside the beam pipe or in the inner part of the tracker detector before undergoing any nuclear interaction.

The decays of these heavy hadrons are usually pre-defined by a Monte Carlo event generator. This approach cannot, obviously, be applied for the secondary heavy hadrons, i.e. charm and bottom hadrons that are created by inelastic nuclear interactions of the primary hadrons. For these secondary heavy hadrons, a very simple approach is currently used in Geant4 to deal with their decays: one single, fully hadronic decay is defined for each “long”-lived charm and bottom hadron, with a simple multi-body phase-space treatment of the decay kinematics.