# Analyse:Mac/Fragment 053 01

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Untersuchte Arbeit:
Seite: 53, Zeilen: 1-29
Quelle: Dobbs et al 2004
Seite(n): 25-26, Zeilen: 25:35-39.40-42; 26:1-11.13-21
The time evolution of the event goes from bottom to top in Figure 4.4. Two protons (each indicated by three solid lines to denote their valence quark content) collide and a parton is resolved at scale Q and momentum fraction x in each one. The parton density as function of $x$ and $Q^2$ is encoded in the parton distribution function which is labeled by $f(x,Q^2)$. The quark and anti-quark annihilate into an s-channel resonance denoted by a wavy line. The resonance then decays into a fermion anti-fermion pair. This part of the event is called the hard subprocess. The generator incorporates higher order QCD effects by allowing the (anti)quarks to branch into qg pairs, while the gluons may branch into q$\overline q$ or gg pairs. The resultant partons may also branch, resulting in a cascade of branchings. This part of the event is labeled parton shower in the figure (showering of or cascade of partons). The event now consists of a number of elementary particles, including quarks, antiquarks, and gluons which are not allowed to exist in isolation, as dictated by colour confinement. Next, the program groups the colored partons into colour-singlet composite hadrons using a phenomenological model referred to as hadronization. The hadronization scale is in the non-perturbative regime and the programs use fairly crude phenomenological models, which contain several non-physical parameters that have to be tuned using experimental data. After hadronization, many short-lived resonances will be present and are their decays simulated by the program. The generators also add features of the underlying event. The beam remnants are the colored remains of the proton which are left behind when the parton which participates in the hard subprocess is pulled out. The motion of the partons inside the proton results in a small (≈ 1 GeV) primordial transverse momentum, which yield a recoil energy of the beam remnants. The beam remnants are colour connected to the hard subprocess and so should be included in the same hadronization system. Multiple parton-parton interactions, wherein more than one pair of partons from the beam protons interact, are also accounted for. In a final step, pile-up from other proton-proton collisions in the same bunch crossing are added to the event [110].

[110] M. A. Dobbs et al., “Les Houches guidebook to Monte Carlo generators for hadron collider physics”. 2004. hep-ph/0403045.

[Seite 25]

The general structure of the final state of an event from an SHG is shown in Figure 5. The time evolution of the event goes from bottom to top. Two protons (each indicated by three solid lines to denote their valence quark content) collide and a parton is resolved at scale Q and momentum fraction x in each one. The phenomenology of the parton resolution is encoded in the parton distribution function $f(x,Q^2)$. [...] The quark and anti-quark annihilate into an s-channel resonance denoted by a wavy line. The resonance then decays into a fermion anti-fermion pair. This part of the event is called the hard subprocess.

[Seite 26]

As briefly outlined there, the SHG incorporates higher order QCD effects by allowing the (anti)quarks to branch into $\stackrel{(-)}{q} g$ pairs, while the gluons may branch into $q\overline q$ or $gg$ pairs. The resultant partons may also branch, resulting in a shower or cascade of partons.17 This part of the event is labelled parton shower in the figure. Showering of the initial state partons is also included in the SHG’s, but is not shown in the figure for simplicity. The event now consists of a number of elementary particles, including quarks, antiquarks, and gluons which are not allowed to exist in isolation, as dictated by colour confinement. Next, the program groups the coloured partons into colour-singlet composite hadrons using a phenomenological model referred to as hadronization. The hadronization scale is in the non-perturbative regime and the programs use fairly crude phenomenological models, which contain several non-physical parameters that are tuned using experimental data. [...] After hadronization, many short-lived resonances will be present and are decayed by the program.

The SHG’s also add in features of the underlying event. The beam remnants are the coloured remains of the proton which are left behind when the parton which participates in the hard subprocess is ‘pulled out’. The motion of the partons inside the proton results in a small (≈ 1 GeV) primordial transverse momentum, against which the beam remnants recoil. The beam remnants are colour connected to the hard subprocess and so should be included in the same hadronization system. Multiple parton-parton interactions, wherein more than one pair of partons from the beam protons interact, are also accounted for. In a final step, pile-up from other proton-proton collisions in the same bunch crossing are added to the event.

17 Though the discussion of parton showers presented here is restricted to QCD showers, an identical prescription can be applied to electromagnetic showers and is used in SHG’s to incorporate higher order QED corrections.

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