implement falkos tipps

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hiro98 2020-06-08 12:21:21 +02:00
parent e44a40b9eb
commit addd63b86d
10 changed files with 89 additions and 95 deletions

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@ -15,7 +15,7 @@ with the \sherpa\ event generator.
\section*{Zusammenfassung}
Der analytische Wirkungsquerschnitt fuer den \(\qqgg\) diphoton
Der analytische Wirkungsquerschnitt f\"ur den \(\qqgg\) diphoton
Prozess wird in f\"uhrender Ordnung berechnet. In Hinblick auf die
Anwendung auf diesen Prozess werden Monte Carlo Methoden f\"ur
Integration und zur Generierung von Stichproben aus

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@ -1,7 +1,7 @@
\thispagestyle{empty}
\section*{Erklärung}
Hiermit erkläre ich, dass ich diese Arbeit im Rahmender Betreuung am
Hiermit erkläre ich, dass ich diese Arbeit im Rahmen der Betreuung am
Institut für Kern- und Teilchenphysik ohne unzulässige Hilfe Dritter
verfasst und alle Quellen als solche gekennzeichnet habe.

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@ -66,8 +66,8 @@ algorithm related functionality as a module. The file
that generates all the results of \cref{chap:mc}. The file
\texttt{parton\_density\_function\_stuff.org} contains all the
computations for \cref{chap:pdf}. The python code makes heavy use of
\href{https://www.scipy.org/}{scipy}~\cite{2020Virtanen:Sc} (and of
course \href{https://numpy.org/}{numpy}).
\href{https://www.scipy.org/}{scipy}~\cite{2020Virtanen:Sc} and
its component \href{https://numpy.org/}{numpy}).
\section{Software Versions}%
\label{sec:versions}

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@ -229,7 +229,7 @@ interval increments and applying \vegas\ to \cref{eq:crossec} with
\result{xs/python/xs_mc_θ_vegas_K} increments yields
\result{xs/python/xs_mc_θ_vegas} with
\result{xs/python/xs_mc_θ_vegas_N} function evaluations (including
\vegas\ iterations). This result is comparable with tho one obtained
\vegas\ iterations). This result is comparable with the one obtained
by parameter transformation in \cref{sec:naivechange}. The sample
count \(N\) is the total number of evaluations of \(f\). The resulting
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
%
The thus obtained samples are then distributed according to \(f/B\)
and the total probability of accepting a sample, also called the
efficiency \(\mathfrak{e}\), is given by hat \cref{eq:impsampeff}
holds.
efficiency \(\mathfrak{e}\), is given by \cref{eq:impsampeff}.
%
\begin{equation}
\label{eq:impsampeff}
@ -93,7 +92,7 @@ Choosing \(g\) like \cref{eq:primitiveg} and looking back at
procedure simplifies to choosing random numbers \(x\in [0,1]\) and
accepting them with the probability \(f(x)/g(x)\). The efficiency of
this approach is related to how much \(f\) differs from
\(f_{\text{max}}\) which in turn related to the variance of
\(f_{\text{max}}\) which in turn is related to the variance of
\(f\). Minimizing variance will therefore improve sampling
performance. The method can also be used in higher dimensions without
modification and has again been implemented and evaluated.
@ -135,8 +134,7 @@ upper bound are depicted in \cref{fig:distcos}.
When transforming \(f\) to a new variable \(y=y(x)\) one arrives at
\cref{eq:transff} and may reduce variance, analogous to
\cref{sec:naivechange}. Transforming the distribution in a beneficial
way in is an alternative method of performing \emph{importance
sampling}.
way is an alternative method of performing \emph{importance sampling}.
%
\begin{equation}
\label{eq:transff}

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@ -61,7 +61,7 @@ the pseudo-rapidity one photon.
The 4-momenta of the final state photons can be obtained by explicit
Lorentz transformation and are listed in \cref{eq:lab-momenta-fs}
where \(\theta\) is the polar angle of the ``first'' final state
photon. These relation are easily transcribed to \(\eta\) dependence
photon. These relations are easily transcribed to \(\eta\) dependence
by using the identity \(\cos(\theta) = \tanh(\eta)\).
%

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@ -141,8 +141,8 @@ the analysis used to produce the histograms can be found in
\cref{fig:pdf-eta,fig:histeta} it becomes apparent, that the PDF has
substantial influence on the resulting distribution. Also the center
of momentum energy is not constant anymore and has a steep peak at low
energies due to the steepness of the PDF. The convolution with the pdf
has also smoothed out the jacobian peak seen in \cref{fig:histpt}.
energies due to the steepness of the PDF. The convolution with the PDF
has also smoothed out the Jacobian peak seen in \cref{fig:histpt}.
Furthermore new observables have been introduced. The invariant mass
of the photon pair
@ -152,17 +152,17 @@ of mass energy of the partonic system that produces the photons (see
fractions of the partons. \Cref{fig:pdf-inv-m} shows, that the vast
majority of the reactions take place at a rather low c.m. energy,
owing to the high weights of the PDF at small \(x\) values. Due to the
\(\pt\) cuts the first bin is slightly lower then the second.
\(\pt\) cuts the first bin is slightly lower than the second.
The cosines of the scattering angles in the labe frame and the
Collins-Soper (CS) frame are defined in
\cref{eq:sangle,eq:sangle-cs}. The scattering angle is just the angle
between one photon and the z-axis (beam axis) in the c.m. frame if
this frame can be reached by a boost along the z-\footnote{Or me
generally, in a z-boosted frame where the angles of the two photons
are the same.}. Here, the partons are assumed to have no transverse
momentum and the system is symmetric around the beam axis and
therefore this boost is possible. When allowing transverse parton
this frame can be reached by a boost along the z-axis\footnote{Or more
generally, in a z-boosted frame where the polar angles of the two
photons are the same.}. Here, the partons are assumed to have no
transverse momentum and the system is symmetric around the beam axis
and therefore this boost is possible. When allowing transverse parton
momenta, as will be done in \cref{chap:pheno} this symmetry goes
away. Defining the z-axis as one beam axis in the c.m. frame would be
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
degrees of freedom average out and will not be considered henceforth.
Furthermore a high energy regime will be supposed and therefore masses
will be neglected.
%
\begin{figure}[h]
\centering
\begin{subfigure}[c]{.4\textwidth}
\centering
\begin{tikzpicture}
\begin{feynman}
\diagram [small,horizontal=i2 to a] {
i2 [particle=\(q\)] -- [fermion, momentum=\(p_2\)] a --
[fermion, reversed momentum=\(q\)] b,
i1 [particle=\(\bar{q}\)] -- [anti fermion, momentum'=\(p_1\)] b,
i2 -- [opacity=0] i1,
a -- [photon, momentum=\(p_3\)] f1 [particle=\(\gamma\)],
b -- [photon, momentum'=\(p_4\)] f2 [particle=\(\gamma\)],
f1 -- [opacity=0] f2,
};
\end{feynman}
\end{tikzpicture}
\subcaption{u channel}
\end{subfigure}
\begin{subfigure}[c]{.4\textwidth}
\centering
\begin{tikzpicture}
\begin{feynman}
\diagram [small,horizontal=i2 to a] {
i2 [particle=\(q\)] -- [fermion, momentum=\(p_2\)] a --
[fermion, reversed momentum'=\(q\)] b,
i1 [particle=\(\bar{q}\)] -- [anti fermion, momentum'=\(p_1\)] b,
i2 -- [opacity=0] i1,
a -- [draw=none] f2 [particle=\(\gamma\)],
b -- [draw=none] f1 [particle=\(\gamma\)],
f1 -- [opacity=0] f2,
};
\diagram* {
(a) -- [photon] (f1),
(b) -- [photon] (f2),
};
\end{feynman}
\end{tikzpicture}
\subcaption{\label{fig:qqggfeyn2}t channel}
\end{subfigure}
%
\caption{Leading order diagrams for \(\qqgg\).}%
\label{fig:qqggfeyn}
\end{figure}

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@ -1,6 +1,23 @@
\section{Calculation of the Cross Section to Leading Order}%
\label{sec:qqggcalc}
\begin{wrapfigure}{R}{0.4\textwidth}
\centering
\begin{tikzpicture}
\coordinate (origin) at (0,0);
\draw[-Latex] (origin) -- (-2,0) node[left] {\(p_3\)};
\draw[-Latex] (origin) -- (2,0) coordinate (p4) node[right] {\(p_4\)};
\draw[Latex-,rotate=40] (origin) -- (2,0) coordinate (p2) node[right] {\(p_2\)};
\draw[Latex-,rotate=40] (origin) -- (-2,0) node[left] {\(p_1\)};
\draw[fill=black] (origin) circle (.03);
\draw pic["$\theta$", draw=black, <->, angle eccentricity=1.2, angle radius=1cm] {angle=p4--origin--p2};
\end{tikzpicture}
\caption{\label{fig:qqimpulses} Momentum diagram for the process
\(\qqgg\) in the massles limit.}
\end{wrapfigure}
%
After labeling the incoming quarks and outcoming photons, as well as
the momenta according to \cref{fig:qqggfeyn}, the Feynman rules for
QED yield the matrix elements in \cref{eq:matel}, where \(Z\) is the
@ -18,7 +35,52 @@ would clutter the notation. The matrix element for
\mathcal{M}_2 &= \frac{(gZ)^2}{\qty(p_1 - p_4)^2}\vsb(1)\pses(3)(\ps_1 - \ps_3)\pses(4)\us(2)
\end{align}
%
To simplify notation, some shorthands are intruduced
\begin{figure}[hb]
\centering
\begin{subfigure}[c]{.4\textwidth}
\centering
\begin{tikzpicture}
\begin{feynman}
\diagram [small,horizontal=i2 to a] {
i2 [particle=\(q\)] -- [fermion, momentum=\(p_2\)] a --
[fermion, reversed momentum=\(q\)] b,
i1 [particle=\(\bar{q}\)] -- [anti fermion, momentum'=\(p_1\)] b,
i2 -- [opacity=0] i1,
a -- [photon, momentum=\(p_3\)] f1 [particle=\(\gamma\)],
b -- [photon, momentum'=\(p_4\)] f2 [particle=\(\gamma\)],
f1 -- [opacity=0] f2,
};
\end{feynman}
\end{tikzpicture}
\subcaption{u channel}
\end{subfigure}
\begin{subfigure}[c]{.4\textwidth}
\centering
\begin{tikzpicture}
\begin{feynman}
\diagram [small,horizontal=i2 to a] {
i2 [particle=\(q\)] -- [fermion, momentum=\(p_2\)] a --
[fermion, reversed momentum'=\(q\)] b,
i1 [particle=\(\bar{q}\)] -- [anti fermion, momentum'=\(p_1\)] b,
i2 -- [opacity=0] i1,
a -- [draw=none] f2 [particle=\(\gamma\)],
b -- [draw=none] f1 [particle=\(\gamma\)],
f1 -- [opacity=0] f2,
};
\diagram* {
(a) -- [photon] (f1),
(b) -- [photon] (f2),
};
\end{feynman}
\end{tikzpicture}
\subcaption{\label{fig:qqggfeyn2}t channel}
\end{subfigure}
%
\caption{Leading order diagrams for \(\qqgg\).}%
\label{fig:qqggfeyn}
\end{figure}
To simplify notation, some shorthands are introduced
in \cref{eq:scshort}.
\begin{equation}
\label{eq:scshort}
@ -26,23 +88,6 @@ in \cref{eq:scshort}.
s(x) &= \sin(x) & c(x) &= \cos(x) \\ s'(x) &= \sin(\frac{x}{2}) & c'(x) &= \cos(\frac{x}{2})
\end{split}
\end{equation}
\begin{wrapfigure}{R}{0.4\textwidth}
\centering
\begin{tikzpicture}
\coordinate (origin) at (0,0);
\draw[-Latex] (origin) -- (-2,0) node[left] {\(p_3\)};
\draw[-Latex] (origin) -- (2,0) coordinate (p4) node[right] {\(p_4\)};
\draw[Latex-,rotate=40] (origin) -- (2,0) coordinate (p2) node[right] {\(p_2\)};
\draw[Latex-,rotate=40] (origin) -- (-2,0) node[left] {\(p_1\)};
\draw[fill=black] (origin) circle (.03);
\draw pic["$\theta$", draw=black, <->, angle eccentricity=1.2, angle radius=1cm] {angle=p4--origin--p2};
\end{tikzpicture}
\caption{\label{fig:qqimpulses} Momentum diagram for the proces
\(\qqgg\) in the massles limit.}
\end{wrapfigure}
%
All calculations are made with respect to the center of momentum
(c.m.) frame unless stated otherwise. The momenta in the c.m. frame
@ -118,7 +163,7 @@ form \cref{eq:gbricks}.
\end{split}
\end{equation}
%
The sum over plarisation can be simplified by utilizing the
The sum over polarization can be simplified by utilizing the
completeness relation for polarization vectors for \emph{real} photons
\cref{eq:polcomp}.
%
@ -145,8 +190,7 @@ theorems for the gamma matrices.
%
The same tricks as well as the commutation relation for gamma matrices
can be utilized for the case \(i\neq j\) and lead to \cref{eq:gij},
albeit with more technical effort.\footnote{If I learned one thing, it
is the importance of doing calculations with the utmost verbosity.}
albeit with more technical effort.
%
\begin{equation}
\label{eq:gij}
@ -155,8 +199,8 @@ albeit with more technical effort.\footnote{If I learned one thing, it
\tr[\gamma_\mu(\ps_1-\ps(a_i))\gamma^\nu\ps_2\gamma^\mu(\ps_1-\ps(a_j))\gamma_\nu\ps_1)]\\
&= -2\tr[\ps_2\gamma^\nu(\ps_1-\ps(a_i))(\ps_1-\ps(a_j))\gamma_\nu\ps_1)]
\\
&=-16\qty[(p_1p_2)(2(p_ap(a_j)) - p(a_i)p(a_j)) +
(p_1p(a_i))(p_2p(a_j)) - (p_1p(a_j))(p_2p(a_i))]
&=-16\qty[(p_1\cdot p_2)(2(p_a\cdot p(a_j)) - p(a_i)\cdot p(a_j)) +
(p_1\cdot p(a_i))(p_2\cdot p(a_j)) - (p_1\cdot p(a_j))(p_2\cdot p(a_i))]
\end{split}
\end{equation}
%
@ -164,9 +208,8 @@ The crucial step here was to sum over \(\mu\) and utilizing
\(\gamma ^{\mu }\gamma ^{\nu }\gamma ^{\rho }\gamma ^{\sigma }\gamma
_{\mu }=-2\gamma ^{\sigma }\gamma ^{\rho }\gamma ^{\nu }\).
After multiplying out the terms in \cref{eq:averagedm} and plugging in
\cref{eq:gii,eq:gij} there results (with some rather technical
\cref{eq:gii,eq:gij} their results (with some rather technical
simplifications) the averaged matrix element of
\cref{eq:averagedm_final}. It is noteworthy that the mixing terms
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.
\cref{eq:total-crossec} of the process for a pseudo-rapidity
integrated over \([-\eta, \eta]\).}
\end{subfigure}
\caption{\label{fig:xsfirst} Plots of the differntial and total cross section
\caption{\label{fig:xsfirst} Plots of the differential and total cross section
for \(\qqgg\).}
\end{figure}
%
@ -65,11 +65,9 @@ into a form where the divergence does not occur (see
\cref{eq:xs-eta}).
The differential cross section clearly is symmetric around
\(\theta=\frac{\pi}{2}\) as was to be expected\footnote{Such
properties have been very handy and intuitive checks for
calculations along the way.}, because the photons are
indistinguishable. To compare the cross section to experiment and to
simulation an interval around \(\theta=\frac{\pi}{2}\) has to be
\(\theta=\frac{\pi}{2}\) as was to be expected, because the photons
are indistinguishable. To compare the cross section to experiment and
to simulation an interval around \(\theta=\frac{\pi}{2}\) has to be
chosen, where the leading order, mass-less approximation may yield a
physical result.