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root/cvsroot/UserCode/Vuko/Notes/WZCSA07/samples.tex
Revision: 1.22
Committed: Fri Aug 8 00:05:00 2008 UTC (16 years, 9 months ago) by ymaravin
Content type: application/x-tex
Branch: MAIN
CVS Tags: Summer08-FinalApproved, HEAD
Changes since 1.21: +13 -11 lines
Log Message:
almost final version, the fruit of a hard work of Stephanie and YM...

File Contents

# User Rev Content
1 vuko 1.1 \section{Signal and Background Modeling}
2     \label{sec:gen}
3     \subsection{Monte Carlo generators}
4     The signal and background samples for the full detector simulation
5 vuko 1.11 are generated with the leading order (LO) event generators
6 ymaravin 1.9 {\sl PYTHIA}~\cite{Sjostrand:2003wg}, {\sl ALPGEN} and {\sl COMPHEP}.
7 ymaravin 1.16 To accommodate the next-to-leading (NLO) effects, constant $k$-factors are applied
8 ymaravin 1.17 except for the signal where a $p_T$-dependence has been taken into account.
9 ymaravin 1.16
10     The $p_T$-dependent $k$-factor for the signal is estimated using
11     the NLO cross section calculator {\sl MCFM}~\cite{Campbell:2005}.
12     We estimate the PDF uncertainty on the cross-section using
13 ymaravin 1.22 {\sl MC@NLO 3.1} NLO event generator~\cite{Frixione:2002ik}
14     together with CTEQ6M PDF set.
15 ymaravin 1.9
16     \subsection{Signal definition}
17     The goal of this analysis is to study the associative production of the on-shell
18 ymaravin 1.14 $\W$ and $\Z$ bosons that decay into three leptons and a neutrino. In the
19 ymaravin 1.9 following we refer to a lepton to as either a muon or an electron, unless
20 ymaravin 1.16 specified otherwise.
21 ymaravin 1.9
22 ymaravin 1.10 Using {\sl MCFM} we estimate the total NLO $\WZ$ cross-section to be
23 beaucero 1.5 \begin{equation}
24 ymaravin 1.10 \sigma_{NLO} ( pp \rightarrow W^+\Z; \sqrt{s}=14~{\rm TeV}) = 30.5~{\rm pb},
25 beaucero 1.5 \end{equation}
26     \begin{equation}
27 ymaravin 1.10 \sigma_{NLO} ( pp \rightarrow W^-\Z; \sqrt{s}=14~{\rm TeV}) = 19.1~{\rm pb}.
28 beaucero 1.5 \end{equation}
29    
30 ymaravin 1.10 The LO and NLO distributions of the \Z boson transverse momentum are
31 senka 1.18 shown in Fig.~\ref{fig:LOvsNLO}. The NLO/LO ratio, $k$-factor, is also presented on the figure,
32 ymaravin 1.15 and it is increasing with $p_T(\Z)$. We take into account the $p_T$ dependence
33     by re-weighting the LO Monte Carlo simulation as a function of the $p_T(\Z)$.
34     %
35     %
36     %The $p_T$ dependence of the $k$-factor
37     %becomes important when a proper NLO description of the $\Z$ boson transverse
38     %momentum must be obtained, $e.g$ to measure the strength of the $WWZ$ coupling.
39     %As the focus of this analysis is to prepare for the cross-section measurement,
40     %we take a $p_{T}$-averaged value of the $k$-factor, equal to 1.84.
41 beaucero 1.5
42     \begin{figure}[!bt]
43     \begin{center}
44 senka 1.20 \scalebox{0.8}{\includegraphics{figs/k_faktor_for_Note.eps}}
45 ymaravin 1.22 \caption{Top plot: comparison of $p_T(Z)$ distributions for NLO and LO for \WZ production
46     allowing off-shell vector bosons including photon contribution;
47     bottom plot: $k$-factor fit to a line.}
48 beaucero 1.5 \label{fig:LOvsNLO}
49     \end{center}
50     \end{figure}
51 vuko 1.1
52 vuko 1.2 %# for bbll:
53     %#CS NLO ((Z/gamma*->l+l-)bb) = 830pb = 345 pb * 2.4, where:
54     %#- 345 pb is LO CS calculated with precision of ~0.15%
55     %#- 2.4 is MCMF calculated k-factor with precision ~30% (!)
56     %# 830x0.173 (== XS x eff.) = 143.59pb
57    
58    
59 ymaravin 1.10 \subsection{Signal and background Monte Carlo samples}
60    
61     The signal Monte Carlo sample is produced using {\sl PYTHIA}
62 ymaravin 1.14 generator. The decay for the \W lepton is forced to $e\nu_e$,
63 vuko 1.11 $\mu\nu_{\mu}$ or $\tau\nu_{\tau}$ final state, while the \Z decays
64 ymaravin 1.10 into electrons or muons only.
65 beaucero 1.6
66 ymaravin 1.14 The background to the \WZ final state can be divided in physics and
67 ymaravin 1.16 instrumental. The only physics background is from $Z^0Z^0$ production
68     where one of the leptons is either mis-reconstructed or lost.
69 ymaravin 1.14
70 ymaravin 1.22 The instrumental backgrounds are all due to misidentified electron candidates
71 ymaravin 1.16 from either jets or photons. These backgrounds include production of $\W$ and $\Z$ bosons
72     with jets and $t\bar{t}$ processes and $Z^0\gamma$ process. The background from $W\gamma$
73 ymaravin 1.22 production, where the $\gamma$ converts and produces a di-electron system is neglected
74 ymaravin 1.16 due to a requirement on the $\ell^+\ell^-$ invariant mass to be consistent with the nominal \Z boson mass.
75 ymaravin 1.14
76 ymaravin 1.16 All non-negligible instrumental backgrounds are summarized below.
77 beaucero 1.6 \begin{itemize}
78 ymaravin 1.14 \item $\Z + jets$: this background is one of the dominant to the \WZ final state. Although
79     the misidentification rate for a jet to be misidentified as a lepton is quite small, the
80     $\Z+jets$ cross-section is 35 times larger than the signal one. We use the {\sl ALPGEN}
81     generated official samples of $\Z+jet$ production Monte Carlo samples for different
82     values of the jet transverse momentum.
83     \item $t\bar{t}$: each of the top quarks decay into a $\W b$ pair producing at least two
84     leptons and two $b$-quark jets. Although this process does not have a genuine $\Z$
85 ymaravin 1.22 candidate and can be suppressed by a $\Z$ candidate invariant mass requirement,
86 ymaravin 1.14 the probability for a $b$-quark jet to decay semi-leptonically and be misidentified
87     as a lepton is higher than that from a light-quark jets. The cross-section of the $t\bar{t}$
88     production is also exceed by about 15 times the cross-section of the \WZ production.
89     Thus, this background is also one of the most dominant. We use the official $t\bar{t}$
90     samples produced with {\sl ALPGEN} generator to estimate this background.
91     \item $\Z + b\bar{b}$: this process is produced by the {\sl COMPHEP}
92     generator and have a genuine $\Z$ candidate in the final state. One of the $b$-quark
93     jets are misidentified as the third lepton from the $\W$ boson.
94     \item $\W+jets$: in this process, the \W boson produces a genuine lepton,
95     while the other two leptons are misidentified jets. As the misidentification
96     probability is low, this channel does not contribute significantly to the \WZ
97     final state. The additional \Z candidate invariant mass requirement suppresses
98     this background further. We use the officially produced sample of $\W+jets$ processes
99     for different number of jets in the final state generated by the {\sl ALPGEN}
100     generator.
101 ymaravin 1.16 \item $Z^0\gamma$: this process is calculated with {\sl PYTHIA}.
102 beaucero 1.6 \end{itemize}
103 ymaravin 1.17 The background sources that have \Z bosons described above are simulated with the
104     contribution from the virtual photon.
105 beaucero 1.5
106 ymaravin 1.14 All the samples we use in this study are a part of the CSA07 production and
107     are generated using $\mathrm{CMSSW}\_1\_4_\_6$ using the full {\sl GEANT}
108     simulation of the CMS detector. The digitization and reconstruction are
109     done using a newer $\mathrm{CMSSW}\_1\_6_\_7$ release with a
110 ymaravin 1.22 misalignment/mis-calibration of the detector scenario expected
111 ymaravin 1.14 to be achieved after collection of $\sim$ 100~pb$^{-1}$ of data.
112     All {\sl ALPGEN} samples are mixed together in further referred to as to a
113 beaucero 1.7 ``Chowder soup''.
114    
115     The summary of all datasets used for signal and background is given in
116 ymaravin 1.14 Table~\ref{tab:MC}. We use the RECO production level to access to
117 beaucero 1.7 low-level detector information, such as reconstructed hits. This lets
118 ymaravin 1.14 us to use full granularity of the CMS sub-detectors to use isolation
119 beaucero 1.7 discriminants.
120    
121 ymaravin 1.14 Analysis of the samples is done using CMSSW$\_1\_6\_7$ CMS software
122     release. The information is stored in ROOT trees using a code in
123 beaucero 1.7 CVS:/UserCode/Vuko/WZAnalysis, which is based on Physics Tools candidates.
124    
125     \begin{table}[!tb]
126     %\begin{tabular}{llllll} \hline
127     %Sample & Generator & Sample name & Events & $\sigma \cdot \epsilon
128     %\cdot k$ & k-factor \\ \hline WZ & Pythia &
129     %/WZ/CMSSW\_1\_6\_7-CSA07-1195663763/RECO & 58897 & 0.585 pb & 1.92 \\
130     %$Zb\bar{b}$ & COMPHEP &
131     %/comphep-bbll/CMSSW\_1\_6\_7-CSA07-1198677426/RECO & 143.59 pb & 2.4
132     %\\ ``Chowder'' & ALPGEN &
133     %/CSA07AllEvents/CMSSW\_1\_6\_7-CSA07-Chowder-A1-PDAllEvents-ReReco-100pb/RECO
134     %& 25 M & event weights & - \\
135     \begin{tabular}{|c|c|c|c|c|} \hline
136 ymaravin 1.14 Sample & cross section, pb & Events & Dataset name \\ \hline
137     $\WZ$ & 1.12 & 59K & /WZ/CMSSW$\_1\_6\_7$-CSA07-1195663763\\ \hline
138     $\Z b\bar{b}$ & 830*0.173 (NLO) & 1.9M & /comphep-bbll/CMSSW$\_1\_6\_7$-CSA07-1198677426\\ \hline
139 ymaravin 1.22 Chowder & Event Weight & $\sim$ 25M & /CSA07AllEvents/\\ & & & CMSSW$\_1\_6\_7$-CSA07-Chowder-A1-PDAllEvents-ReReco
140 beaucero 1.7 -100pb\\ \hline
141 ymaravin 1.22 $\Z\Z$ inclusive & 16.1 (NLO) & $\sim$ 140K & /ZZ$\_$incl/CMSSW$\_1\_6\_7$-CSA07-1194964234/RECO\\ \hline
142     $\Z\gamma \rightarrow e^+e^-\gamma$ & 1.08 (NLO) & $\sim$125K &/Zeegamma/CMSSW$\_1\_6\_7$-CSA07-1198935518/RECO \\ \hline
143     $\Z\gamma \rightarrow \mu^+\mu^-\gamma$ & 1.08 (NLO) & $\sim$ 93K & /Zmumugamma/CMSSW$\_1\_6\_7$-CSA07-1194806860/RECO\\ \hline
144 vuko 1.2 \end{tabular}
145 beaucero 1.7 \label{tab:MC}
146 ymaravin 1.14 \caption{Monte Carlo samples used in this analysis using 100 pb$^{-1}$ scenario}
147 vuko 1.2 \end{table}
148    
149 vuko 1.1
150    
151