ViewVC Help
View File | Revision Log | Show Annotations | Root Listing
root/cvsroot/UserCode/claudioc/OSNote2010/datadriven.tex
Revision: 1.24
Committed: Fri Nov 19 16:57:33 2010 UTC (14 years, 5 months ago) by benhoob
Content type: application/x-tex
Branch: MAIN
CVS Tags: v2
Changes since 1.23: +48 -5 lines
Log Message:
Fix ABCD uncorrelated variables plot. Add ABCD systematics study

File Contents

# User Rev Content
1 claudioc 1.1 \section{Data Driven Background Estimation Methods}
2     \label{sec:datadriven}
3     We have developed two data-driven methods to
4     estimate the background in the signal region.
5 benhoob 1.10 The first one exploits the fact that
6 benhoob 1.22 SumJetPt and \met$/\sqrt{\rm SumJetPt}$ are nearly
7 claudioc 1.1 uncorrelated for the $t\bar{t}$ background
8     (Section~\ref{sec:abcd}); the second one
9     is based on the fact that in $t\bar{t}$ the
10     $P_T$ of the dilepton pair is on average
11     nearly the same as the $P_T$ of the pair of neutrinos
12     from $W$-decays, which is reconstructed as \met in the
13     detector.
14    
15 benhoob 1.15
16 claudioc 1.6 %{\color{red} I took these
17     %numbers from the twiki, rescaling from 11.06 to 30/pb.
18     %They seem too large...are they really right?}
19 claudioc 1.1
20    
21     \subsection{ABCD method}
22     \label{sec:abcd}
23    
24 benhoob 1.22 We find that in $t\bar{t}$ events SumJetPt and
25 benhoob 1.16 \met$/\sqrt{\rm SumJetPt}$ are nearly uncorrelated,
26     as demonstrated in Figure~\ref{fig:uncor}.
27 claudioc 1.1 Thus, we can use an ABCD method in the \met$/\sqrt{\rm SumJetPt}$ vs
28     sumJetPt plane to estimate the background in a data driven way.
29    
30 benhoob 1.24 %\begin{figure}[bht]
31     %\begin{center}
32     %\includegraphics[width=0.75\linewidth]{uncorrelated.pdf}
33     %\caption{\label{fig:uncor}\protect Distributions of SumJetPt
34     %in MC $t\bar{t}$ events for different intervals of
35     %MET$/\sqrt{\rm SumJetPt}$.}
36     %\end{center}
37     %\end{figure}
38    
39 claudioc 1.21 \begin{figure}[bht]
40 claudioc 1.1 \begin{center}
41 benhoob 1.24 \includegraphics[width=0.75\linewidth]{uncor.png}
42 claudioc 1.1 \caption{\label{fig:uncor}\protect Distributions of SumJetPt
43     in MC $t\bar{t}$ events for different intervals of
44 benhoob 1.24 MET$/\sqrt{\rm SumJetPt}$. h1, h2, and h3 refer to the MET$/\sqrt{\rm SumJetPt}$
45     intervals 4.5-6.5, 6.5-8.5 and $>$8.5, respectively.}
46 claudioc 1.1 \end{center}
47     \end{figure}
48    
49 claudioc 1.21 \begin{figure}[tb]
50 claudioc 1.1 \begin{center}
51 claudioc 1.3 \includegraphics[width=0.5\linewidth, angle=90]{abcdMC.pdf}
52 benhoob 1.22 \caption{\label{fig:abcdMC}\protect Distributions of MET$/\sqrt{\rm SumJetPt}$ vs.
53     SumJetPt for SM Monte Carlo. Here we also show our choice of ABCD regions.}
54 claudioc 1.1 \end{center}
55     \end{figure}
56    
57    
58     Our choice of ABCD regions is shown in Figure~\ref{fig:abcdMC}.
59     The signal region is region D. The expected number of events
60     in the four regions for the SM Monte Carlo, as well as the BG
61 claudioc 1.2 prediction AC/B are given in Table~\ref{tab:abcdMC} for an integrated
62 benhoob 1.24 luminosity of 35 pb$^{-1}$. The ABCD method with chosen boundaries is accurate
63     to about 20\%. As shown in Table~\ref{tab:abcdsyst}, we assess systematic uncertainties
64     by varying the boundaries by an amount consistent with the hadronic energy scale uncertainty,
65     which we take as $\pm$5\% for SumJetPt and $\pm$2.5\% for MET/$\sqrt{\rm SumJetPt}$, since the
66     uncertainty on this quantity partially cancels due to the fact that it is a ratio of correlated
67     quantities. Based on these studies we assess a correction factor $k_{ABCD} = 1.2 \pm 0.2$ to the
68     predicted yield using the ABCD method.
69    
70    
71 claudioc 1.9 %{\color{red} Avi wants some statement about stability
72     %wrt changes in regions. I am not sure that we have done it and
73     %I am not sure it is necessary (Claudio).}
74 claudioc 1.1
75 claudioc 1.21 \begin{table}[ht]
76 claudioc 1.1 \begin{center}
77     \caption{\label{tab:abcdMC} Expected SM Monte Carlo yields for
78 benhoob 1.13 35 pb$^{-1}$ in the ABCD regions, as well as the predicted yield in
79 benhoob 1.16 the signal region given by A $\times$ C / B. Here `SM other' is the sum
80 benhoob 1.13 of non-dileptonic $t\bar{t}$ decays, $W^{\pm}$+jets, $W^+W^-$,
81     $W^{\pm}Z^0$, $Z^0Z^0$ and single top.}
82 benhoob 1.16 \begin{tabular}{lccccc}
83 benhoob 1.13 \hline
84 benhoob 1.16 sample & A & B & C & D & A $\times$ C / B \\
85 benhoob 1.13 \hline
86 benhoob 1.17
87    
88     \hline
89 benhoob 1.13 $t\bar{t}\rightarrow \ell^{+}\ell^{-}$ & 7.96 & 33.07 & 4.81 & 1.20 & 1.16 \\
90 benhoob 1.17 $Z^0 \rightarrow \ell^{+}\ell^{-}$ & 0.03 & 1.47 & 0.10 & 0.10 & 0.00 \\
91 benhoob 1.13 SM other & 0.65 & 2.31 & 0.17 & 0.14 & 0.05 \\
92     \hline
93 benhoob 1.17 total SM MC & 8.63 & 36.85 & 5.07 & 1.43 & 1.19 \\
94 claudioc 1.1 \hline
95     \end{tabular}
96     \end{center}
97     \end{table}
98    
99 benhoob 1.24
100    
101     \begin{table}[ht]
102     \begin{center}
103     \caption{\label{tab:abcdsyst} Results of the systematic study of the ABCD method by varying the boundaries
104     between the ABCD regions shown in Fig.~\ref{fig:abcdMC}. Here $x_1$ is the lower SumJetPt boundary and
105     $x_2$ is the boundary separating regions A and B from C and D, their nominal values are 125 and 300~GeV,
106     respectively. $y_1$ is the lower MET/$\sqrt{\rm SumJetPt}$ boundary and
107     $y_2$ is the boundary separating regions B and C from A and D, their nominal values are 4.5 and 8.5~GeV$^{1/2}$,
108     respectively.}
109     \begin{tabular}{cccc|c}
110     \hline
111     $x_1$ & $x_2$ & $y_1$ & $y_2$ & Observed/Predicted \\
112     \hline
113     nominal & nominal & nominal & nominal & 1.20 \\
114     +5\% & +5\% & +2.5\% & +2.5\% & 1.38 \\
115     +5\% & +5\% & nominal & nominal & 1.31 \\
116     nominal & nominal & +2.5\% & +2.5\% & 1.25 \\
117     nominal & +5\% & nominal & +2.5\% & 1.32 \\
118     nominal & -5\% & nominal & -2.5\% & 1.16 \\
119     -5\% & -5\% & +2.5\% & +2.5\% & 1.21 \\
120     +5\% & +5\% & -2.5\% & -2.5\% & 1.26 \\
121     \hline
122     \end{tabular}
123     \end{center}
124     \end{table}
125    
126 claudioc 1.2 \subsection{Dilepton $P_T$ method}
127     \label{sec:victory}
128     This method is based on a suggestion by V. Pavlunin\cite{ref:victory},
129     and was investigated by our group in 2009\cite{ref:ourvictory}.
130     The idea is that in dilepton $t\bar{t}$ events the lepton and neutrinos
131     from $W$ decays have the same $P_T$ spectrum (modulo $W$ polarization
132     effects). One can then use the observed
133     $P_T(\ell\ell)$ distribution to model the sum of neutrino $P_T$'s which
134     is identified with the \met.
135    
136     Then, in order to predict the $t\bar{t} \to$ dilepton contribution to a
137     selection with \met$+$X, one applies a cut on $P_T(\ell\ell)+$X instead.
138     In practice one has to rescale the result of the $P_T(\ell\ell)+$X selection
139     to account for the fact that any dilepton selection must include a
140     moderate \met cut in order to reduce Drell Yan backgrounds. This
141     is discussed in Section 5.3 of Reference~\cite{ref:ourvictory}; for a \met
142 benhoob 1.16 cut of 50 GeV, the rescaling factor is obtained from the MC as
143 claudioc 1.2
144     \newcommand{\ptll} {\ensuremath{P_T(\ell\ell)}}
145     \begin{center}
146     $ K = \frac{\int_0^{\infty} {\cal N}(\ptll)~~d\ptll~}{\int_{50}^{\infty} {\cal N}(\ptll)~~d\ptll~}$
147     \end{center}
148    
149    
150     Monte Carlo studies give values of $K$ that are typically between 1.5 and 1.6,
151 benhoob 1.10 depending on selection details.
152     %%%TO BE REPLACED
153     %Given the integrated luminosity of the
154     %present dataset, the determination of $K$ in data is severely statistics
155     %limited. Thus, we take $K$ from $t\bar{t}$ Monte Carlo as
156    
157     %\begin{center}
158     %$ K_{MC} = \frac{\int_0^{\infty} {\cal N}(\met)~~d\met~}{\int_{50}^{\infty} {\cal N}(\met)~~d\met~}$
159     %\end{center}
160 claudioc 1.9
161 benhoob 1.10 %\noindent {\color{red} For the 11 pb result we have used $K$ from data.}
162 claudioc 1.2
163     There are several effects that spoil the correspondance between \met and
164     $P_T(\ell\ell)$:
165     \begin{itemize}
166     \item $Ws$ in top events are polarized. Neutrinos are emitted preferentially
167 benhoob 1.22 parallel to the $W$ velocity while charged leptons are emitted prefertially
168     anti-parallel. Thus the $P_T(\nu\nu)$ distribution is harder
169 claudioc 1.2 than the $P_T(\ell\ell)$ distribution for top dilepton events.
170     \item The lepton selections results in $P_T$ and $\eta$ cuts on the individual
171     leptons that have no simple correspondance to the neutrino requirements.
172     \item Similarly, the \met$>$50 GeV cut introduces an asymmetry between leptons and
173     neutrinos which is only partially compensated by the $K$ factor above.
174     \item The \met resolution is much worse than the dilepton $P_T$ resolution.
175 benhoob 1.16 When convoluted with a falling spectrum in the tails of \met, this results
176 claudioc 1.2 in a harder spectrum for \met than the original $P_T(\nu\nu)$.
177     \item The \met response in CMS is not exactly 1. This causes a distortion
178     in the \met distribution that is not present in the $P_T(\ell\ell)$ distribution.
179     \item The $t\bar{t} \to$ dilepton signal includes contributions from
180     $W \to \tau \to \ell$. For these events the arguments about the equivalence
181     of $P_T(\ell\ell)$ and $P_T(\nu\nu)$ do not apply.
182     \item A dilepton selection will include SM events from non $t\bar{t}$
183     sources. These events can affect the background prediction. Particularly
184     dangerous are high $P_T$ Drell Yan events that barely pass the \met$>$ 50
185     GeV selection. They will tend to push the data-driven background prediction up.
186 benhoob 1.16 Therefore we estimate the number of DY events entering the background prediction
187     using the $R_{out/in}$ method as described in Sec.~\ref{sec:othBG}.
188 claudioc 1.2 \end{itemize}
189    
190     We have studied these effects in SM Monte Carlo, using a mixture of generator and
191     reconstruction level studies, putting the various effects in one at a time.
192     For each configuration, we apply the data-driven method and report as figure
193     of merit the ratio of observed and predicted events in the signal region.
194     The results are summarized in Table~\ref{tab:victorybad}.
195    
196     \begin{table}[htb]
197     \begin{center}
198     \caption{\label{tab:victorybad} Test of the data driven method in Monte Carlo
199     under different assumptions. See text for details.}
200 claudioc 1.6 \begin{tabular}{|l|c|c|c|c|c|c|c|c|}
201 claudioc 1.2 \hline
202 claudioc 1.6 & True $t\bar{t}$ dilepton & $t\to W\to\tau$& other SM & GEN or & Lepton $P_T$ & Z veto & \met $>$ 50& obs/pred \\
203     & included & included & included & RECOSIM & and $\eta$ cuts & & & \\ \hline
204     1&Y & N & N & GEN & N & N & N & 1.90 \\
205     2&Y & N & N & GEN & Y & N & N & 1.64 \\
206     3&Y & N & N & GEN & Y & Y & N & 1.59 \\
207     4&Y & N & N & GEN & Y & Y & Y & 1.55 \\
208     5&Y & N & N & RECOSIM & Y & Y & Y & 1.51 \\
209     6&Y & Y & N & RECOSIM & Y & Y & Y & 1.58 \\
210 benhoob 1.17 7&Y & Y & Y & RECOSIM & Y & Y & Y & 1.38 \\
211 benhoob 1.16 %%%NOTE: updated value 1.18 -> 1.46 since 2/3 DY events have been removed by updated analysis selections,
212     %%%dpt/pt cut and general lepton veto
213 claudioc 1.2 \hline
214     \end{tabular}
215     \end{center}
216     \end{table}
217    
218    
219     The largest discrepancy between prediction and observation occurs on the first
220     line of Table~\ref{tab:victorybad}, {\em i.e.}, at the generator level with no
221     cuts. We have verified that this effect is due to the polarization of
222     the $W$ (we remove the polarization by reweighting the events and we get
223     good agreement between prediction and observation). The kinematical
224 claudioc 1.6 requirements (lines 2,3,4) compensate somewhat for the effect of W polarization.
225     Going from GEN to RECOSIM, the change in observed/predicted is small.
226     % We have tracked this down to the fact that tcMET underestimates the true \met
227     % by $\approx 4\%$\footnote{We find that observed/predicted changes by roughly 0.1
228     %for each 1.5\% change in \met response.}.
229     Finally, contamination from non $t\bar{t}$
230 benhoob 1.16 events can have a significant impact on the BG prediction.
231     %The changes between
232     %lines 6 and 7 of Table~\ref{tab:victorybad} is driven by 3
233     %Drell Yan events that pass the \met selection in Monte Carlo (thus the effect
234     %is statistically not well quantified).
235 claudioc 1.2
236     An additional source of concern is that the CMS Madgraph $t\bar{t}$ MC does
237     not include effects of spin correlations between the two top quarks.
238     We have studied this effect at the generator level using Alpgen. We find
239 claudioc 1.7 that the bias is at the few percent level.
240 claudioc 1.2
241 benhoob 1.10 %%%TO BE REPLACED
242     %Based on the results of Table~\ref{tab:victorybad}, we conclude that the
243     %naive data driven background estimate based on $P_T{(\ell\ell)}$ needs to
244     %be corrected by a factor of {\color{red} $ K_{\rm{fudge}} =1.2 \pm 0.3$
245     %(We still need to settle on thie exact value of this.
246     %For the 11 pb analysis it is taken as =1.)} . The quoted
247     %uncertainty is based on the stability of the Monte Carlo tests under
248     %variations of event selections, choices of \met algorithm, etc.
249     %For example, we find that observed/predicted changes by roughly 0.1
250     %for each 1.5\% change in the average \met response.
251    
252 claudioc 1.2 Based on the results of Table~\ref{tab:victorybad}, we conclude that the
253 claudioc 1.6 naive data driven background estimate based on $P_T{(\ell\ell)}$ needs to
254 benhoob 1.11 be corrected by a factor of $ K_C = X \pm Y$.
255 benhoob 1.10 The value of this correction factor as well as the systematic uncertainty
256     will be assessed using 38X ttbar madgraph MC. In the following we use
257 benhoob 1.11 $K_C = 1$ for simplicity. Based on previous MC studies we foresee a correction
258 benhoob 1.14 factor of $K_C \approx 1.2 - 1.5$, and we will assess an uncertainty
259 benhoob 1.10 based on the stability of the Monte Carlo tests under
260 claudioc 1.2 variations of event selections, choices of \met algorithm, etc.
261 claudioc 1.8 For example, we find that observed/predicted changes by roughly 0.1
262 benhoob 1.10 for each 1.5\% change in the average \met response.
263 claudioc 1.2
264    
265 claudioc 1.6
266 claudioc 1.2 \subsection{Signal Contamination}
267     \label{sec:sigcont}
268    
269 claudioc 1.6 All data-driven methods are in principle subject to signal contaminations
270 claudioc 1.2 in the control regions, and the methods described in
271     Sections~\ref{sec:abcd} and~\ref{sec:victory} are not exceptions.
272     Signal contamination tends to dilute the significance of a signal
273     present in the data by inflating the background prediction.
274    
275     It is hard to quantify how important these effects are because we
276     do not know what signal may be hiding in the data. Having two
277     independent methods (in addition to Monte Carlo ``dead-reckoning'')
278     adds redundancy because signal contamination can have different effects
279     in the different control regions for the two methods.
280     For example, in the extreme case of a
281     new physics signal
282 claudioc 1.6 with $P_T(\ell \ell) = \met$, an excess of events would be seen
283 claudioc 1.2 in the ABCD method but not in the $P_T(\ell \ell)$ method.
284    
285 claudioc 1.4
286 claudioc 1.2 The LM points are benchmarks for SUSY analyses at CMS. The effects
287     of signal contaminations for a couple such points are summarized
288 benhoob 1.14 in Table~\ref{tab:sigcont}. Signal contamination is definitely an important
289 claudioc 1.2 effect for these two LM points, but it does not totally hide the
290     presence of the signal.
291 claudioc 1.1
292    
293 claudioc 1.2 \begin{table}[htb]
294     \begin{center}
295 benhoob 1.14 \caption{\label{tab:sigcont} Effects of signal contamination
296     for the two data-driven background estimates. The three columns give
297     the expected yield in the signal region and the background estimates
298 benhoob 1.20 using the ABCD and $P_T(\ell \ell)$ methods. Results are normalized to 35~pb$^{-1}$.}
299 benhoob 1.14 \begin{tabular}{lccc}
300 claudioc 1.2 \hline
301 benhoob 1.14 & Yield & ABCD & $P_T(\ell \ell)$ \\
302     \hline
303 benhoob 1.17 SM only & 1.43 & 1.19 & 1.03 \\
304 benhoob 1.19 SM + LM0 & 7.90 & 4.23 & 2.35 \\
305     SM + LM1 & 4.00 & 1.53 & 1.51 \\
306 claudioc 1.2 \hline
307     \end{tabular}
308     \end{center}
309     \end{table}
310    
311 benhoob 1.14
312    
313     %\begin{table}[htb]
314     %\begin{center}
315     %\caption{\label{tab:sigcontABCD} Effects of signal contamination
316     %for the background predictions of the ABCD method including LM0 or
317     %LM1. Results
318     %are normalized to 30 pb$^{-1}$.}
319     %\begin{tabular}{|c|c||c|c||c|c|}
320     %\hline
321     %SM & BG Prediction & SM$+$LM0 & BG Prediction & SM$+$LM1 & BG Prediction \\
322     %Background & SM Only & Contribution & Including LM0 & Contribution & Including LM1 \\ \hline
323     %1.2 & 1.0 & 6.8 & 3.7 & 3.4 & 1.3 \\
324     %\hline
325     %\end{tabular}
326     %\end{center}
327     %\end{table}
328    
329     %\begin{table}[htb]
330     %\begin{center}
331     %\caption{\label{tab:sigcontPT} Effects of signal contamination
332     %for the background predictions of the $P_T(\ell\ell)$ method including LM0 or
333     %LM1. Results
334     %are normalized to 30 pb$^{-1}$.}
335     %\begin{tabular}{|c|c||c|c||c|c|}
336     %\hline
337     %SM & BG Prediction & SM$+$LM0 & BG Prediction & SM$+$LM1 & BG Prediction \\
338     %Background & SM Only & Contribution & Including LM0 & Contribution & Including LM1 \\ \hline
339     %1.2 & 1.0 & 6.8 & 2.2 & 3.4 & 1.5 \\
340     %\hline
341     %\end{tabular}
342     %\end{center}
343     %\end{table}
344 claudioc 1.1