Four Generations at LHC (beta version)
In spite of rumors trying to convince the LHC community of the opposite, a fourth generation of fermions with masses below the TeV scale is by no means excluded by experimental measurements. As long as all new fermions (leptons, neutrinos, quarks) are heavier than the current LEP and Tevatron limits, a natural 10% mass splitting between up and down-type fermions allows them to perfectly well respect all limits from electroweak precision data. The question why Nature should choose precisely three generation of fermions is an obvious question to ask, and you would have to be truly brilliant to answer this question...
The tools presented on this page are based on work done together with Graham Kribs (Oregon), Tim Tait (ANL & Northwestern) and Michael Spannowsky (Karlsruhe). It originated at the Aspen workshop in the Summer of 2006, and the three old farts who spent a great summer in Aspen are very grateful for Michael joining us and getting all the work done.
Moreover, we are grateful to Heiko Lacker, who encouraged us to write a set of simple public software tools, which can be used to reproduce and expand the results presented in our original study.
The main LHC-relevant ingredients of this software package are Madevent and Bridge. Both of these codes use a set of model files in the usual Madgraph/Madevent syntax, consistently including all relevant fourth-generation masses and couplings. The input format for the new model parameters is inspired by the SUSY Les-Houches accord and can be found in Madevent's param_card.dat. This file serves not only as the input to Madevent and Bridge, but also to codes which compute Higgs-physics effects (using a hacked version of Hdecay) and the contributions to S and T.
Using G4LHC is very simple:
1. install Madevent - and type `make'
2. install G4 model directory among Madevent models
3. edit all model parameters in param_card.dat
4. type `make couplings' and run it, to create couplings_check.txt
5. install (our hacked) Bridge in Madevent directory - and type `make'
6. run Bridge to add new particles' widths to param_card.dat
7. do your usual Madevent thing
8. check your parameters using S-T tools in Models/G4/G4_EW
9. check Higgs physics in Models/G4/G4_HDECAY
10. email us your personal wish list for G4LHC...
Of course even this beta version of the G4LHC package is completely bug free. Enjoy!
Graham, Tim, Michael, and Tilman
improved beta version
bug in FCNC couplings removed (thanks to RikkertF)
Producion of G4 particles (Madevent)
Decay of G4 particles (Bridge)
S-T parameter constraints
Higgs decays (Hdecay)