Measurement of the Jet Energy Scale in the CMS experiment ... - IIHE

48 CHAPTER 4: Reconstructing and Selecting Physics EventThe first step which is the core of the generation procedure is to produce thehard scattering process which is a result of the interaction of two high-energy partonsarising from the two colliding protons. The next step is to consider the possibility ofbranching of a parton to other partons or radiation of a gluon resulting in a cascadeof partons referred to as parton showering. After producing a list of partons carryingcolor-charges, due to the quark confinement principle, they hadronize and combinein such a way to make colorless hadrons. The produced hadrons that are not stableparticles, decay subsequently in the last step of the event generation procedure. Thefinal particles interact with the detector material and can be reconstructed as describedin Chapter 3.Some of these generation steps such as the hard scattering can be described fromfirst principles, while for others like the hadronization stage require phenomenologicalmodels to be modeled. The different generation steps are discussed in more detail inthe following sections.4.1.1 Hard Scattering EventIn a proton-proton collision, along with so many soft elastic interactions, hard inelasticscattering occurs which can include any new physics such as Higgs bosons or hints forsuper-symmetric phenomena. In order to describe the physics at high-energy hadroncolliders, the knowledge of Quantum Chromodynamics (QCD) is needed [80]. Accordingto the parton model, a proton is not an elementary particle and comprises quarksand gluons, generally called partons. When two protons collide heads on, actually thecollision happens between their constituents. Partons within the proton are describedby the parton distribution functions (pdfs) which are represented as a function of themomentum fraction carried by the parton inside the proton and also with the Q 2 energyscale at which the proton is probed. At high center of mass energies where the strongcoupling constant α s is small, perturbative calculations are feasible and the collisionbetween the two incoming protons can be described accordingly. A hard scatteringprocesses is schematically depicted in Figure 4.2.Figure 4.2: A typical hard scattering process in proton-proton collision.For the hard scattering interactions, the factorization theorem which predicts thecharacteristics of the event, is used. As a result, the total hadronic cross section can