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https://github.com/vale981/bachelor_thesis
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implement falkos tipps
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10 changed files with 89 additions and 95 deletions
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@ -15,7 +15,7 @@ with the \sherpa\ event generator.
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\section*{Zusammenfassung}
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Der analytische Wirkungsquerschnitt fuer den \(\qqgg\) diphoton
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Der analytische Wirkungsquerschnitt f\"ur den \(\qqgg\) diphoton
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Prozess wird in f\"uhrender Ordnung berechnet. In Hinblick auf die
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Anwendung auf diesen Prozess werden Monte Carlo Methoden f\"ur
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Integration und zur Generierung von Stichproben aus
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@ -1,7 +1,7 @@
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\thispagestyle{empty}
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\section*{Erklärung}
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Hiermit erkläre ich, dass ich diese Arbeit im Rahmender Betreuung am
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Hiermit erkläre ich, dass ich diese Arbeit im Rahmen der Betreuung am
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Institut für Kern- und Teilchenphysik ohne unzulässige Hilfe Dritter
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verfasst und alle Quellen als solche gekennzeichnet habe.
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@ -66,8 +66,8 @@ algorithm related functionality as a module. The file
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that generates all the results of \cref{chap:mc}. The file
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\texttt{parton\_density\_function\_stuff.org} contains all the
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computations for \cref{chap:pdf}. The python code makes heavy use of
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\href{https://www.scipy.org/}{scipy}~\cite{2020Virtanen:Sc} (and of
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course \href{https://numpy.org/}{numpy}).
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\href{https://www.scipy.org/}{scipy}~\cite{2020Virtanen:Sc} and
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its component \href{https://numpy.org/}{numpy}).
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\section{Software Versions}%
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\label{sec:versions}
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@ -229,7 +229,7 @@ interval increments and applying \vegas\ to \cref{eq:crossec} with
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\result{xs/python/xs_mc_θ_vegas_K} increments yields
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\result{xs/python/xs_mc_θ_vegas} with
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\result{xs/python/xs_mc_θ_vegas_N} function evaluations (including
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\vegas\ iterations). This result is comparable with tho one obtained
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\vegas\ iterations). This result is comparable with the one obtained
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by parameter transformation in \cref{sec:naivechange}. The sample
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count \(N\) is the total number of evaluations of \(f\). The resulting
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increments and the weighted integrand \(f/\rho\) are depicted in
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@ -77,8 +77,7 @@ probability~\(f/g\), so that \(g\) cancels out. This is known as the
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%
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The thus obtained samples are then distributed according to \(f/B\)
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and the total probability of accepting a sample, also called the
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efficiency \(\mathfrak{e}\), is given by hat \cref{eq:impsampeff}
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holds.
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efficiency \(\mathfrak{e}\), is given by \cref{eq:impsampeff}.
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%
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\begin{equation}
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\label{eq:impsampeff}
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@ -93,7 +92,7 @@ Choosing \(g\) like \cref{eq:primitiveg} and looking back at
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procedure simplifies to choosing random numbers \(x\in [0,1]\) and
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accepting them with the probability \(f(x)/g(x)\). The efficiency of
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this approach is related to how much \(f\) differs from
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\(f_{\text{max}}\) which in turn related to the variance of
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\(f_{\text{max}}\) which in turn is related to the variance of
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\(f\). Minimizing variance will therefore improve sampling
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performance. The method can also be used in higher dimensions without
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modification and has again been implemented and evaluated.
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@ -135,8 +134,7 @@ upper bound are depicted in \cref{fig:distcos}.
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When transforming \(f\) to a new variable \(y=y(x)\) one arrives at
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\cref{eq:transff} and may reduce variance, analogous to
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\cref{sec:naivechange}. Transforming the distribution in a beneficial
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way in is an alternative method of performing \emph{importance
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sampling}.
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way is an alternative method of performing \emph{importance sampling}.
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%
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\begin{equation}
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\label{eq:transff}
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@ -61,7 +61,7 @@ the pseudo-rapidity one photon.
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The 4-momenta of the final state photons can be obtained by explicit
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Lorentz transformation and are listed in \cref{eq:lab-momenta-fs}
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where \(\theta\) is the polar angle of the ``first'' final state
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photon. These relation are easily transcribed to \(\eta\) dependence
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photon. These relations are easily transcribed to \(\eta\) dependence
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by using the identity \(\cos(\theta) = \tanh(\eta)\).
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%
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@ -141,8 +141,8 @@ the analysis used to produce the histograms can be found in
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\cref{fig:pdf-eta,fig:histeta} it becomes apparent, that the PDF has
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substantial influence on the resulting distribution. Also the center
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of momentum energy is not constant anymore and has a steep peak at low
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energies due to the steepness of the PDF. The convolution with the pdf
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has also smoothed out the jacobian peak seen in \cref{fig:histpt}.
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energies due to the steepness of the PDF. The convolution with the PDF
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has also smoothed out the Jacobian peak seen in \cref{fig:histpt}.
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Furthermore new observables have been introduced. The invariant mass
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of the photon pair
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@ -152,17 +152,17 @@ of mass energy of the partonic system that produces the photons (see
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fractions of the partons. \Cref{fig:pdf-inv-m} shows, that the vast
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majority of the reactions take place at a rather low c.m. energy,
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owing to the high weights of the PDF at small \(x\) values. Due to the
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\(\pt\) cuts the first bin is slightly lower then the second.
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\(\pt\) cuts the first bin is slightly lower than the second.
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The cosines of the scattering angles in the labe frame and the
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Collins-Soper (CS) frame are defined in
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\cref{eq:sangle,eq:sangle-cs}. The scattering angle is just the angle
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between one photon and the z-axis (beam axis) in the c.m. frame if
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this frame can be reached by a boost along the z-\footnote{Or me
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generally, in a z-boosted frame where the angles of the two photons
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are the same.}. Here, the partons are assumed to have no transverse
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momentum and the system is symmetric around the beam axis and
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therefore this boost is possible. When allowing transverse parton
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this frame can be reached by a boost along the z-axis\footnote{Or more
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generally, in a z-boosted frame where the polar angles of the two
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photons are the same.}. Here, the partons are assumed to have no
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transverse momentum and the system is symmetric around the beam axis
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and therefore this boost is possible. When allowing transverse parton
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momenta, as will be done in \cref{chap:pheno} this symmetry goes
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away. Defining the z-axis as one beam axis in the c.m. frame would be
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quite an arbitrary choice that disrespects the symmetry of the two
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@ -13,48 +13,3 @@ process is being described by the Feynman diagrams in
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degrees of freedom average out and will not be considered henceforth.
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Furthermore a high energy regime will be supposed and therefore masses
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will be neglected.
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%
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\begin{figure}[h]
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\centering
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\begin{subfigure}[c]{.4\textwidth}
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\centering
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\begin{tikzpicture}
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\begin{feynman}
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\diagram [small,horizontal=i2 to a] {
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i2 [particle=\(q\)] -- [fermion, momentum=\(p_2\)] a --
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[fermion, reversed momentum=\(q\)] b,
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i1 [particle=\(\bar{q}\)] -- [anti fermion, momentum'=\(p_1\)] b,
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i2 -- [opacity=0] i1,
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a -- [photon, momentum=\(p_3\)] f1 [particle=\(\gamma\)],
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b -- [photon, momentum'=\(p_4\)] f2 [particle=\(\gamma\)],
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f1 -- [opacity=0] f2,
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};
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\end{feynman}
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\end{tikzpicture}
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\subcaption{u channel}
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\end{subfigure}
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\begin{subfigure}[c]{.4\textwidth}
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\centering
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\begin{tikzpicture}
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\begin{feynman}
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\diagram [small,horizontal=i2 to a] {
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i2 [particle=\(q\)] -- [fermion, momentum=\(p_2\)] a --
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[fermion, reversed momentum'=\(q\)] b,
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i1 [particle=\(\bar{q}\)] -- [anti fermion, momentum'=\(p_1\)] b,
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i2 -- [opacity=0] i1,
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a -- [draw=none] f2 [particle=\(\gamma\)],
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b -- [draw=none] f1 [particle=\(\gamma\)],
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f1 -- [opacity=0] f2,
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};
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\diagram* {
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(a) -- [photon] (f1),
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(b) -- [photon] (f2),
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};
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\end{feynman}
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\end{tikzpicture}
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\subcaption{\label{fig:qqggfeyn2}t channel}
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\end{subfigure}
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%
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\caption{Leading order diagrams for \(\qqgg\).}%
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\label{fig:qqggfeyn}
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\end{figure}
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@ -1,6 +1,23 @@
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\section{Calculation of the Cross Section to Leading Order}%
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\label{sec:qqggcalc}
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\begin{wrapfigure}{R}{0.4\textwidth}
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\centering
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\begin{tikzpicture}
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\coordinate (origin) at (0,0);
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\draw[-Latex] (origin) -- (-2,0) node[left] {\(p_3\)};
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\draw[-Latex] (origin) -- (2,0) coordinate (p4) node[right] {\(p_4\)};
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\draw[Latex-,rotate=40] (origin) -- (2,0) coordinate (p2) node[right] {\(p_2\)};
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\draw[Latex-,rotate=40] (origin) -- (-2,0) node[left] {\(p_1\)};
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\draw[fill=black] (origin) circle (.03);
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\draw pic["$\theta$", draw=black, <->, angle eccentricity=1.2, angle radius=1cm] {angle=p4--origin--p2};
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\end{tikzpicture}
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\caption{\label{fig:qqimpulses} Momentum diagram for the process
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\(\qqgg\) in the massles limit.}
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\end{wrapfigure}
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%
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After labeling the incoming quarks and outcoming photons, as well as
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the momenta according to \cref{fig:qqggfeyn}, the Feynman rules for
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QED yield the matrix elements in \cref{eq:matel}, where \(Z\) is the
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@ -18,7 +35,52 @@ would clutter the notation. The matrix element for
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\mathcal{M}_2 &= \frac{(gZ)^2}{\qty(p_1 - p_4)^2}\vsb(1)\pses(3)(\ps_1 - \ps_3)\pses(4)\us(2)
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\end{align}
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%
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To simplify notation, some shorthands are intruduced
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\begin{figure}[hb]
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\centering
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\begin{subfigure}[c]{.4\textwidth}
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\centering
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\begin{tikzpicture}
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\begin{feynman}
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\diagram [small,horizontal=i2 to a] {
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i2 [particle=\(q\)] -- [fermion, momentum=\(p_2\)] a --
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[fermion, reversed momentum=\(q\)] b,
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i1 [particle=\(\bar{q}\)] -- [anti fermion, momentum'=\(p_1\)] b,
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i2 -- [opacity=0] i1,
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a -- [photon, momentum=\(p_3\)] f1 [particle=\(\gamma\)],
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b -- [photon, momentum'=\(p_4\)] f2 [particle=\(\gamma\)],
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f1 -- [opacity=0] f2,
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};
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\end{feynman}
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\end{tikzpicture}
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\subcaption{u channel}
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\end{subfigure}
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\begin{subfigure}[c]{.4\textwidth}
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\centering
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\begin{tikzpicture}
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\begin{feynman}
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\diagram [small,horizontal=i2 to a] {
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i2 [particle=\(q\)] -- [fermion, momentum=\(p_2\)] a --
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[fermion, reversed momentum'=\(q\)] b,
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i1 [particle=\(\bar{q}\)] -- [anti fermion, momentum'=\(p_1\)] b,
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i2 -- [opacity=0] i1,
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a -- [draw=none] f2 [particle=\(\gamma\)],
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b -- [draw=none] f1 [particle=\(\gamma\)],
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f1 -- [opacity=0] f2,
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};
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\diagram* {
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(a) -- [photon] (f1),
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(b) -- [photon] (f2),
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};
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\end{feynman}
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\end{tikzpicture}
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\subcaption{\label{fig:qqggfeyn2}t channel}
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\end{subfigure}
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%
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\caption{Leading order diagrams for \(\qqgg\).}%
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\label{fig:qqggfeyn}
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\end{figure}
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To simplify notation, some shorthands are introduced
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in \cref{eq:scshort}.
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\begin{equation}
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\label{eq:scshort}
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@ -26,23 +88,6 @@ in \cref{eq:scshort}.
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s(x) &= \sin(x) & c(x) &= \cos(x) \\ s'(x) &= \sin(\frac{x}{2}) & c'(x) &= \cos(\frac{x}{2})
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\end{split}
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\end{equation}
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\begin{wrapfigure}{R}{0.4\textwidth}
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\centering
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\begin{tikzpicture}
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\coordinate (origin) at (0,0);
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\draw[-Latex] (origin) -- (-2,0) node[left] {\(p_3\)};
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\draw[-Latex] (origin) -- (2,0) coordinate (p4) node[right] {\(p_4\)};
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\draw[Latex-,rotate=40] (origin) -- (2,0) coordinate (p2) node[right] {\(p_2\)};
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\draw[Latex-,rotate=40] (origin) -- (-2,0) node[left] {\(p_1\)};
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\draw[fill=black] (origin) circle (.03);
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\draw pic["$\theta$", draw=black, <->, angle eccentricity=1.2, angle radius=1cm] {angle=p4--origin--p2};
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\end{tikzpicture}
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\caption{\label{fig:qqimpulses} Momentum diagram for the proces
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\(\qqgg\) in the massles limit.}
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\end{wrapfigure}
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%
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All calculations are made with respect to the center of momentum
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(c.m.) frame unless stated otherwise. The momenta in the c.m. frame
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@ -118,7 +163,7 @@ form \cref{eq:gbricks}.
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\end{split}
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\end{equation}
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%
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The sum over plarisation can be simplified by utilizing the
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The sum over polarization can be simplified by utilizing the
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completeness relation for polarization vectors for \emph{real} photons
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\cref{eq:polcomp}.
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%
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@ -145,8 +190,7 @@ theorems for the gamma matrices.
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%
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The same tricks as well as the commutation relation for gamma matrices
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can be utilized for the case \(i\neq j\) and lead to \cref{eq:gij},
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albeit with more technical effort.\footnote{If I learned one thing, it
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is the importance of doing calculations with the utmost verbosity.}
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albeit with more technical effort.
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%
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\begin{equation}
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\label{eq:gij}
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@ -155,8 +199,8 @@ albeit with more technical effort.\footnote{If I learned one thing, it
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\tr[\gamma_\mu(\ps_1-\ps(a_i))\gamma^\nu\ps_2\gamma^\mu(\ps_1-\ps(a_j))\gamma_\nu\ps_1)]\\
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&= -2\tr[\ps_2\gamma^\nu(\ps_1-\ps(a_i))(\ps_1-\ps(a_j))\gamma_\nu\ps_1)]
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\\
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&=-16\qty[(p_1p_2)(2(p_ap(a_j)) - p(a_i)p(a_j)) +
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(p_1p(a_i))(p_2p(a_j)) - (p_1p(a_j))(p_2p(a_i))]
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&=-16\qty[(p_1\cdot p_2)(2(p_a\cdot p(a_j)) - p(a_i)\cdot p(a_j)) +
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(p_1\cdot p(a_i))(p_2\cdot p(a_j)) - (p_1\cdot p(a_j))(p_2\cdot p(a_i))]
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\end{split}
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\end{equation}
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%
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\(\gamma ^{\mu }\gamma ^{\nu }\gamma ^{\rho }\gamma ^{\sigma }\gamma
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_{\mu }=-2\gamma ^{\sigma }\gamma ^{\rho }\gamma ^{\nu }\).
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After multiplying out the terms in \cref{eq:averagedm} and plugging in
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\cref{eq:gii,eq:gij} there results (with some rather technical
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\cref{eq:gii,eq:gij} their results (with some rather technical
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simplifications) the averaged matrix element of
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\cref{eq:averagedm_final}. It is noteworthy that the mixing terms
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cancel out, in other terms: \(\Gamma_{12} + \Gamma_{21} = 0\). The
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@ -49,7 +49,7 @@ two identical photons in the final state.
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\cref{eq:total-crossec} of the process for a pseudo-rapidity
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integrated over \([-\eta, \eta]\).}
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\end{subfigure}
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\caption{\label{fig:xsfirst} Plots of the differntial and total cross section
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\caption{\label{fig:xsfirst} Plots of the differential and total cross section
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for \(\qqgg\).}
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\end{figure}
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%
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@ -65,11 +65,9 @@ into a form where the divergence does not occur (see
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\cref{eq:xs-eta}).
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The differential cross section clearly is symmetric around
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\(\theta=\frac{\pi}{2}\) as was to be expected\footnote{Such
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properties have been very handy and intuitive checks for
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calculations along the way.}, because the photons are
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indistinguishable. To compare the cross section to experiment and to
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simulation an interval around \(\theta=\frac{\pi}{2}\) has to be
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\(\theta=\frac{\pi}{2}\) as was to be expected, because the photons
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are indistinguishable. To compare the cross section to experiment and
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to simulation an interval around \(\theta=\frac{\pi}{2}\) has to be
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chosen, where the leading order, mass-less approximation may yield a
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physical result.
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