[Rivet-svn] r3873 - schools/2012-CERN/handouts/day3

[blackhole at projects.hepforge.org]

Author: dgrell
Date: Thu Jul 26 08:23:36 2012
New Revision: 3873

Log:
Copied handouts

Added:
   schools/2012-CERN/handouts/day3/day3.tex
      - copied unchanged from r3869, schools/2011-Kyoto/handouts/day3/day3.tex
   schools/2012-CERN/handouts/day3/rivet-classes.pdf
      - copied unchanged from r3869, schools/2011-Kyoto/handouts/day3/rivet-classes.pdf

Copied: schools/2012-CERN/handouts/day3/day3.tex (from r3869, schools/2011-Kyoto/handouts/day3/day3.tex)
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+\documentclass{article}
+
+\usepackage[a4paper]{geometry}
+\usepackage[sc]{mathpazo}
+\usepackage{amsmath}
+\usepackage{microtype}
+\usepackage{fullpage}
+
+\def\ttbar{\ensuremath{t \bar t}}
+
+%%%%%%%%%%%%%%%%%%%%%%
+% Code
+\makeatletter
+\def\code#1{\texttt{#1}}
+
+\def\codeblock%
+  {\@verbatim\frenchspacing\@vobeyspaces
+   \@ycodeblock{$\hookrightarrow$}\@xcodeblock}
+\def\endcodeblock%
+  {\if at newlist \leavevmode\fi\endtrivlist}
+\begingroup
+  \catcode `§=13
+  \catcode `[= 1 \catcode`]=2
+  \catcode `\{=12 \catcode `\}=12
+  \catcode `|=0 \catcode`\\=12
+  |gdef|@xcodeblock#1\end{codeblock}[#1|end[codeblock]]
+  |gdef|@ycodeblock[|catcode`§=13 |def§]
+|endgroup
+\makeatother
+%%%%%%%%%%%%%%%%%%%%%%
+
+\begin{document}
+
+\title{IPMU-YITP School 2011 --\\A simple Top analysis with Rivet}
+\date{7 September 2011}
+\maketitle
+
+
+\section{Introduction}
+
+Today we will dive a bit deeper into Rivet and create a simple \ttbar{}
+mass analysis.  More explicitly you are going to look at the electron and muon
+channel in semi-leptonic \ttbar{} events, reconstructing the hadronic
+top.
+
+\subsection{Physics}
+
+The top quark is the heaviest of the six quarks, its mass is about
+173~GeV. It also has an extremely short life-time, so it doesn't have
+time to hadronise before decaying. Top quarks almost exclusively decay
+into a $W$ boson and a $b$ quark (the CKM matrix element $V_{tb}$ is
+basically 1). Therefore their decay channels can be characterised by the
+$W$ decay: If the $W$ decays into an electron, muon, or tau, and the
+associated neutrino, we talk about a leptonic $W$ decay, and if the $W$
+decays into a quark-antiquark pair, the decay is called hadronic. The
+quarks in the final state evolve into jets of hadrons.
+
+This yields three categories of final states for top pairs: In the
+dilepton channel the $W$ bosons from both top quarks decay into leptons,
+in the semi-leptonic (or lepton+jets) channel one $W$ decay is hadronic
+and the other one is leptonic, and in the alljets (or hadronic) channel
+both $W$ bosons decay into quarks.
+
+Each of the three decay modes has its own advantages and disadvantages.
+The dilepton channel features a clear signature because of its two $b$
+jets and the two high energy charged leptons, but suffers from low
+statistics --~only 10.3\,\% of the top pairs decay in this channel~--
+and kinematic ambiguities because of the momentum carried away by the
+undetected neutrinos.
+
+The semi-leptonic channel has a much higher branching ratio of 43.5\,\%
+and only one neutrino, so the statistics is higher than in the dilepton
+channel and the kinematic ambiguities are smaller. On the other hand
+this channel is affected by a large background from events with one $W$
+boson and additional jets. The signature of this channel is one high
+energy lepton, two $b$ jets, two light quark jets, and missing
+momentum from the escaping neutrino in the plane transverse to the beam
+pipe.
+
+The alljets channel finally has the largest branching ratio (46.2\,\%)
+and no neutrino at all, so there is plenty of events which are
+kinematically well constrained (but suffer from combinatoric
+ambiguities). The $W$+jets background can be neglected when the $b$ jets
+are identified, but the background from QCD multijet production is
+overwhelming. The alljets channel's signature is six high energy jets,
+two of which are $b$ jets, and no lepton.
+
+\subsection{Mass measurement in the semi-leptonic channel}
+
+To measure the mass in the semi-leptonic channel it is necessary to
+identify \ttbar{} events and then reconstruct the hadronic top to get
+its invariant mass. The signature of those events are a high momentum
+lepton from the $W$ decay, two $b$-jets from the top decay, and two
+light jets from the decay of the second $W$. All jets are coming from
+decays of heavy particles, so they are relatively hard -- harder than
+typical jets from QCD showering.
+
+After selecting events with four jets (two of which are $b$-jets) and a
+hard lepton, you might want to reconstruct the hadronic $W$ and apply a
+mass window cut around the $W$ mass. Then combine the reconstructed $W$
+with the two $b$-jets; this gives you a correct and a wrong combination,
+so your mass distribution will consist of a top mass peak and a broader
+combinatoric background.
+
+You can also run $W+\text{jets}$ Monte Carlo through your analysis to
+check how your selection is performing on background. Because it is very
+hard to get multiple additional jets together with the $W$ from a parton
+shower, and because those jets would be too soft anyhow, it's best to
+use a matrix element generator to get parton level events and let them
+evolve with a shower. We have prepared Les Houches event files with
+MadGraph for this purpose. Les Houches files are a common standard to
+exchange such data between generators.
+
+
+\section{Getting started}
+
+Before you continue, generate some event data that you can use to
+refine your analysis. Since you'll probably want to rerun the analysis
+again and again while you're developing it, we save the HepMC data to
+a file \texttt{top-signal.hepmc}.
+
+% GENERATOR-SPECIFIC
+Choose the command(s) for your generator:
+\begin{itemize}
+\item Herwig++
+\begin{codeblock}
+      Herwig++ read TopSignal.in
+      Herwig++ run TopSignal.run -N4000
+\end{codeblock}
+\item Pythia
+\begin{codeblock}
+      main32.exe topsignal.cmnd top-signal.hepmc
+\end{codeblock}
+\item Sherpa
+\begin{codeblock}
+      cd ttbar
+      Sherpa HEPMC2_GENEVENT_OUTPUT=top-signal.hepmc EVENTS=4000
+\end{codeblock}
+\end{itemize}
+
+You can interrupt the run with \texttt{CTRL-c} if it takes too
+long. While you wait for the results, please continue with looking at
+the Rivet analysis in section \ref{analysis}.
+
+We have prepared background samples for the most problematic background
+in this channel: A $W$ boson associated with two $b$-jets and two other
+jets, with the $W$ decaying into $e/\mu + \nu$. This final state has the
+same signature as our signal events. Since it is hard to get four jets
+from the parton shower, we have prepared Les Houches event files with
+MadGraph for Herwig++ and Pythia. 
+
+% GENERATOR-SPECIFIC
+You can process them like this
+\begin{itemize}
+\item Herwig++
+\begin{codeblock}
+      Herwig++ read TopBackground.in
+      Herwig++ run TopBackground.run -N4000
+\end{codeblock}
+\item Pythia
+\begin{codeblock}
+      main32.exe topbackground.cmnd top-background.hepmc
+\end{codeblock}
+\item Sherpa
+\begin{codeblock}
+      cd wbbjets
+      Sherpa HEPMC2_GENEVENT_OUTPUT=top-background.hepmc EVENTS=4000
+\end{codeblock}
+\end{itemize}
+
+Again, while you wait, please continue with looking at
+the Rivet analysis in section \ref{analysis}.
+The HepMC files will be huge, so please do not upload them! If you run
+into space problems, reduce the number of events accordingly.
+
+\subsection{Running the analysis}\label{analysis}
+
+We have prepared a Rivet analysis template which you can extend:
+\begin{codeblock}
+      cd school/day3/rivet-plugin
+\end{codeblock}
+Rivet supports plugin analyses which are compiled as a library object
+and loaded during runtime. The Makefile just contains the command
+necessary to compile this library, and the file \code{MC\_TOP.cc}
+contains the actual analysis code.
+
+The \code{MC\_TOP.cc} file has four methods:
+\begin{itemize}
+  \item The \code{MC\_TOP::init()} method is called once at the
+        beginning of the run. It is used to
+        initialise the projections used in the analysis. We will be
+        using the \code{ChargedLeptons} projection to extract the
+        leptons from the final state particles, and the \code{FastJets}
+        projection for jet reconstruction. 
+        This is also the place to book histograms.
+
+  \item The \code{MC\_TOP::analyze()} method is called for each
+        event. Here you find the main analysis code.
+
+  \item The \code{MC\_TOP::finalize()} method is called once at the
+        end of the run. Here you can for example normalise histograms.
+\end{itemize}
+
+Have a look at the files and try to understand what they are doing.
+There are plenty of comments in the source code.
+You can compile the library by calling
+\begin{codeblock}
+      make
+\end{codeblock}
+and run it as follows:
+\begin{codeblock}
+      export RIVET_ANALYSIS_PATH=$PWD
+      rivet -a MC_TOP -H ttbar.aida top-signal.hepmc 
+\end{codeblock}
+% $
+We will
+only be looking at the shapes of the distributions today,  
+the histograms are scaled by the number of events only. In
+reality at LHC energies, the cross-section for our signal process is
+much higher than the background we look at. We ignore this here to make the
+construction of a good analysis slightly more challenging. We haven't
+included other backgrounds since they are even easier to suppress. 
+
+
+%%% HERE
+\subsection{Plotting the histograms}
+
+Your signal histograms are saved in the file \code{ttbar.aida}. 
+We now run the same Rivet analysis, this time on the background dataset:
+\begin{codeblock}
+      rivet -a MC_TOP -H background.aida top-background.hepmc
+\end{codeblock}
+and compare signal and background with 
+\begin{codeblock}
+      rivet-mkhtml ttbar.aida background.aida
+\end{codeblock}
+You can look at the plots using any browser, for example:
+\begin{codeblock}
+      firefox plots/index.html
+\end{codeblock}
+
+
+\section{Improve the analysis}
+
+When you have understood what the analysis is currently doing, it's
+time to improve it. 
+
+\begin{enumerate}
+\item Can you get a better signal efficiency? 
+\item A better background discrimination? 
+\item Why does the $W$ mass window cut give such a large improvement? 
+\item Add a lepton isolation cut to the analysis, \emph{i.e.}~only
+  select jets which are well separated from the hard charged lepton.
+\item You can also adjust the jet $p_T$ cuts and the cut on the lepton $p_T$
+(but keep in mind that the jets in the background sample have been
+created with a minimum $p_T$ of 20~GeV at parton level, so better keep
+your cuts above $\sim$30~GeV). Think about how this biases the $W$ and
+the top mass.
+\item Try to add more observables. Things like $H_T = \sum_\text{jets}{E_T}$
+or Centrality $C = H_T/\sum_\text{jets} E$ might be worth looking at.
+Try just including the four light and $b$-jets you are using, or all
+jets above some $p_T$ cut. You also can include the lepton into this
+calculation. If you cut on $H_T$, how does this bias your $W$ or top
+mass distributions? 
+\item What other observables do you come up with?
+\end{enumerate}
+
+\section{Compare with other generators and groups}
+
+Share your results with students who run one of the other generators.
+By simply adding more ``.aida'' files to the \code{rivet-mkhtml}
+command, you can compare the different generators directly to each
+other.
+
+Once you are happy with your analysis, you can also
+generate more statistics than just 4000 events for
+those plots.
+
+\end{document}

Copied: schools/2012-CERN/handouts/day3/rivet-classes.pdf (from r3869, schools/2011-Kyoto/handouts/day3/rivet-classes.pdf)
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