The interaction between the colliding hadrons at the LHC is described by the Standard Model (SM) of particle physics. This Standard Model covers Strong Interactions (Quantum Chromo Dynamics, QCD) as well as Electroweak Interactions.
Most of the processes at the LHC are coming from Strong Interactions: the total pp cross section as well as the so-called minimum-bias events up to events with highest transverse momentum jets. Very essential for a theoretical description of those processes is the precise knowledge and determination of the parton densities which describe the probability to find quarks and gluons inside the colliding hadrons.
At a smaller rate (compared to Strong Interaction) occur processes which involve Electroweak gauge bosons: W, Z but also the Higgs. Due to the clean signatures of W and Z bosons, they are often called Standard Candles, which are processes that can be calculated and measured very precisely.
For the proper understanding and description of measurements at the LHC, theoretical calculations and their implementation into event generators are essential. Only with those tools, a detailed comparison of the measurement with theoretical approaches are possible.
The Standard Model group in CMS/DESY is active in all these areas.