== Large Extra Dimensions ==

=== Author ===

Priscila de Aquino
   * Katholieke Universiteit Leuven & Universite Catholique de Louvain - CP3 
   * priscila@itf.fys.kuleuven.be
=== Description of the model & references ===

One popular aproach to solve the Hierarchy Problem of the Standard Model is to extend space-time to higher dimensions. In this framework, the usual four-dimensional space-time is contained in a four-dimensional brane embedded in a large structure with N additional dimensions, the bulk.

Here, we shall focus on the Large Extra Dimensional theory, where gravitational interactions are the only ones propagating into the bulk, which dilutes their coupling strength and make it appear weaker inside the four-dimensional branes. As a consequence, the graviton field will be seem as a sum of N four-dimensional particles with the same quantum numbers, but increasing mass. These excitation of the gravitons are the massive Kaluza-Klein states.

In this implementation, we consider a theory with five dimensions, in which the fifth dimension is spatial and compact. We start from a generic effective Lagrangian of a unbroken gauge theory with general gauge, fermionic and scalar fields.

   * [http://prola.aps.org/abstract/PRD/v59/i10/e105006 Phys. Rev. D59 (1999) 105006]: T. Han, J. D. Lykken, and R.-J. Zhang, ''On Kaluza-Klein states from large extra dimensions''. 
   * [http://xxx.lanl.gov/abs/hep-ph/9811291  ''Nucl. Phys. B544 (1999)'' ]: G. F. Giudice, R. Rattazzi, J. D. Wells, ''Quantum gravity and extra dimensions at high-energy colliders.'' 

From this general model, we derived a realistic Large Extra Dimensional theory containing all Standard Model fields (full LED model). We only consider the lowest Kaluza-Klein massive state of the graviton.
   * [http://xxx.lanl.gov/abs/0805.2554 Eur. Phys. J. C56 (2008) 435–447]: K. Hagiwara, J. Kanzaki, Q. Li, and K. Mawatari, ''HELAS and MadGraph/MadEvent with spin-2 particles''. 

=== Model files & extensions ===
   * Main !FeynRules file for the full LED model: [https://cp3.irmp.ucl.ac.be/projects/feynrules/attachment/wiki/LED/LED.fr LED.fr]. 
   * Example of a Mathematica® notebook loading the model and the parameters: [https://cp3.irmp.ucl.ac.be/projects/feynrules/attachment/wiki/LED/LED.nb LED.nb]. 

=== Validation ===

The Feynman rules for the generic LED model were explicitly checked with those available in the literature, such as the references cited above. We found complete agreement.

Unlike for other !FeynRules implementations, we cannot use any matrix-element generator to compute cross-sections or decay-rates, because the interfaces linking !FeynRules to Monte-Carlo generators are not yet defined to work for a theory with spin-two particles. Therefore, we could extrapolate the results of the generic LED implementation to guess those of QCD and electromagnetic part of the full LED implementation. This check was performed and conclusive.