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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 The first one exploits the fact that
6 SumJetPt and \met$/\sqrt{\rm SumJetPt}$ are nearly
7 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
16 %{\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
20
21 \subsection{ABCD method}
22 \label{sec:abcd}
23
24 We find that in $t\bar{t}$ events SumJetPt and
25 \met$/\sqrt{\rm SumJetPt}$ are nearly uncorrelated,
26 as demonstrated in Fig.~\ref{fig:uncor}.
27 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 %\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 \begin{figure}[bht]
40 \begin{center}
41 \includegraphics[width=0.75\linewidth]{uncor.png}
42 \caption{\label{fig:uncor}\protect Distributions of SumJetPt
43 in MC $t\bar{t}$ events for different intervals of
44 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 $>$8.5, respectively. }
46 \end{center}
47 \end{figure}
48
49 \begin{figure}[tb]
50 \begin{center}
51 \includegraphics[width=0.75\linewidth]{ttdil_uncor_38X.png}
52 \caption{\label{fig:abcdMC}\protect Distributions of MET$/\sqrt{\rm SumJetPt}$ vs.
53 SumJetPt for SM Monte Carlo. Here we also show our choice of ABCD regions. The correlation coefficient
54 ${\rm corr_{XY}}$ is computed for events falling in the ABCD regions.}
55 \end{center}
56 \end{figure}
57
58
59 Our choice of ABCD regions is shown in Figure~\ref{fig:abcdMC}.
60 The signal region is region D. The expected number of events
61 in the four regions for the SM Monte Carlo, as well as the background
62 prediction A $\times$ C / B are given in Table~\ref{tab:abcdMC} for an integrated
63 luminosity of 34.0 pb$^{-1}$. In Table~\ref{tab:abcdsyst}, we test the stability of
64 observed/predicted with respect to variations in the ABCD boundaries.
65 Based on the results in Tables~\ref{tab:abcdMC} and~\ref{tab:abcdsyst}, we assess
66 a systematic uncertainty of 20\% on the prediction of the ABCD method.
67
68 %As shown in Table~\ref{tab:abcdsyst}, we assess systematic uncertainties
69 %by varying the boundaries by an amount consistent with the hadronic energy scale uncertainty,
70 %which we take as $\pm$5\% for SumJetPt and $\pm$2.5\% for MET/$\sqrt{\rm SumJetPt}$, since the
71 %uncertainty on this quantity partially cancels due to the fact that it is a ratio of correlated
72 %quantities. Based on these studies we assess a correction factor $k_{ABCD} = 1.2 \pm 0.2$ to the
73 %predicted yield using the ABCD method.
74
75
76 %{\color{red} Avi wants some statement about stability
77 %wrt changes in regions. I am not sure that we have done it and
78 %I am not sure it is necessary (Claudio).}
79
80 \begin{table}[ht]
81 \begin{center}
82 \caption{\label{tab:abcdMC} Expected SM Monte Carlo yields for
83 34.0~pb$^{-1}$ in the ABCD regions, as well as the predicted yield in
84 the signal region given by A $\times$ C / B. Here `SM other' is the sum
85 of non-dileptonic $t\bar{t}$ decays, $W^{\pm}$+jets, $W^+W^-$,
86 $W^{\pm}Z^0$, $Z^0Z^0$ and single top.}
87 \begin{tabular}{lccccc}
88 %%%official json v3, 38X MC (D6T ttbar and DY)
89 \hline
90 sample & A & B & C & D & PRED \\
91 \hline
92 $t\bar{t}\rightarrow \ell^{+}\ell^{-}$ & 8.44 $\pm$ 0.18 & 32.83 $\pm$ 0.35 & 4.78 $\pm$ 0.14 & 1.07 $\pm$ 0.06 & 1.23 $\pm$ 0.05 \\
93 $Z^0 \rightarrow \ell^{+}\ell^{-}$ & 0.17 $\pm$ 0.08 & 1.18 $\pm$ 0.22 & 0.04 $\pm$ 0.04 & 0.12 $\pm$ 0.07 & 0.01 $\pm$ 0.01 \\
94 SM other & 0.53 $\pm$ 0.03 & 2.26 $\pm$ 0.11 & 0.23 $\pm$ 0.03 & 0.07 $\pm$ 0.01 & 0.05 $\pm$ 0.01 \\
95 \hline
96 total SM MC & 9.14 $\pm$ 0.20 & 36.26 $\pm$ 0.43 & 5.05 $\pm$ 0.14 & 1.27 $\pm$ 0.10 & 1.27 $\pm$ 0.05 \\
97 \hline
98 \end{tabular}
99 \end{center}
100 \end{table}
101
102
103
104 \begin{table}[ht]
105 \begin{center}
106 \caption{\label{tab:abcdsyst}
107 Results of the systematic study of the ABCD method by varying the boundaries
108 between the ABCD regions shown in Fig.~\ref{fig:abcdMC}. Here $x_1$ is the lower SumJetPt boundary and
109 $x_2$ is the boundary separating regions A and B from C and D, their nominal values are 125 and 300~GeV,
110 respectively. $y_1$ is the lower MET/$\sqrt{\rm SumJetPt}$ boundary and
111 $y_2$ is the boundary separating regions B and C from A and D, their nominal values are 4.5 and 8.5~GeV$^{1/2}$,
112 respectively.}
113 \begin{tabular}{cccc|c}
114 \hline
115 $x_1$ & $x_2$ & $y_1$ & $y_2$ & Observed/Predicted \\
116 \hline
117
118 nominal & nominal & nominal & nominal & $1.00 \pm 0.08$ \\
119
120 +5\% & +5\% & +2.5\% & +2.5\% & $1.08 \pm 0.11$ \\
121
122 +5\% & +5\% & nominal & nominal & $1.04 \pm 0.10$ \\
123
124 nominal & nominal & +2.5\% & +2.5\% & $1.03 \pm 0.09$ \\
125
126 nominal & +5\% & nominal & +2.5\% & $1.05 \pm 0.10$ \\
127
128 nominal & -5\% & nominal & -2.5\% & $0.95 \pm 0.07$ \\
129
130 -5\% & -5\% & +2.5\% & +2.5\% & $1.00 \pm 0.08$ \\
131
132 +5\% & +5\% & -2.5\% & -2.5\% & $0.98 \pm 0.09$ \\
133 \hline
134 \end{tabular}
135 \end{center}
136 \end{table}
137
138 \subsection{Dilepton $P_T$ method}
139 \label{sec:victory}
140 This method is based on a suggestion by V. Pavlunin\cite{ref:victory},
141 and was investigated by our group in 2009\cite{ref:ourvictory}.
142 The idea is that in dilepton $t\bar{t}$ events the lepton and neutrinos
143 from $W$ decays have the same $P_T$ spectrum (modulo $W$ polarization
144 effects). One can then use the observed
145 $P_T(\ell\ell)$ distribution to model the sum of neutrino $P_T$'s which
146 is identified with the \met.
147
148 Then, in order to predict the $t\bar{t} \to$ dilepton contribution to a
149 selection with \met$+$X, one applies a cut on $P_T(\ell\ell)+$X instead.
150 In practice one has to rescale the result of the $P_T(\ell\ell)+$X selection
151 to account for the fact that any dilepton selection must include a
152 moderate \met cut in order to reduce Drell Yan backgrounds. This
153 is discussed in Section 5.3 of Reference~\cite{ref:ourvictory}; for a \met
154 cut of 50 GeV, the rescaling factor is obtained from the MC as
155
156 \newcommand{\ptll} {\ensuremath{P_T(\ell\ell)}}
157 \begin{center}
158 $ K = \frac{\int_0^{\infty} {\cal N}(\ptll)~~d\ptll~}{\int_{50}^{\infty} {\cal N}(\ptll)~~d\ptll~} = 1.5$
159 \end{center}
160
161
162 %%%TO BE REPLACED
163 %Given the integrated luminosity of the
164 %present dataset, the determination of $K$ in data is severely statistics
165 %limited. Thus, we take $K$ from $t\bar{t}$ Monte Carlo as
166
167 %\begin{center}
168 %$ K_{MC} = \frac{\int_0^{\infty} {\cal N}(\met)~~d\met~}{\int_{50}^{\infty} {\cal N}(\met)~~d\met~}$
169 %\end{center}
170
171 %\noindent {\color{red} For the 11 pb result we have used $K$ from data.}
172
173 There are several effects that spoil the correspondance between \met and
174 $P_T(\ell\ell)$:
175 \begin{itemize}
176 \item $Ws$ in top events are polarized. Neutrinos are emitted preferentially
177 parallel to the $W$ velocity while charged leptons are emitted prefertially
178 anti-parallel. Thus the $P_T(\nu\nu)$ distribution is harder
179 than the $P_T(\ell\ell)$ distribution for top dilepton events.
180 \item The lepton selections results in $P_T$ and $\eta$ cuts on the individual
181 leptons that have no simple correspondance to the neutrino requirements.
182 \item Similarly, the \met$>$50 GeV cut introduces an asymmetry between leptons and
183 neutrinos which is only partially compensated by the $K$ factor above.
184 \item The \met resolution is much worse than the dilepton $P_T$ resolution.
185 When convoluted with a falling spectrum in the tails of \met, this results
186 in a harder spectrum for \met than the original $P_T(\nu\nu)$.
187 \item The \met response in CMS is not exactly 1. This causes a distortion
188 in the \met distribution that is not present in the $P_T(\ell\ell)$ distribution.
189 \item The $t\bar{t} \to$ dilepton signal includes contributions from
190 $W \to \tau \to \ell$. For these events the arguments about the equivalence
191 of $P_T(\ell\ell)$ and $P_T(\nu\nu)$ do not apply.
192 \item A dilepton selection will include SM events from non $t\bar{t}$
193 sources. These events can affect the background prediction. Particularly
194 dangerous are high $P_T$ Drell Yan events that barely pass the \met$>$ 50
195 GeV selection. They will tend to push the data-driven background prediction up.
196 Therefore we estimate the number of DY events entering the background prediction
197 using the $R_{out/in}$ method as described in Sec.~\ref{sec:othBG}.
198 \end{itemize}
199
200 We have studied these effects in SM Monte Carlo, using a mixture of generator and
201 reconstruction level studies, putting the various effects in one at a time.
202 For each configuration, we apply the data-driven method and report as figure
203 of merit the ratio of observed and predicted events in the signal region.
204 The results are summarized in Table~\ref{tab:victorybad}.
205
206 \begin{table}[htb]
207 \begin{center}
208 \caption{\label{tab:victorybad}
209 Test of the data driven method in Monte Carlo
210 under different assumptions, evaluated using Spring10 MC. See text for details.}
211 \begin{tabular}{|l|c|c|c|c|c|c|c|c|}
212 \hline
213 & True $t\bar{t}$ dilepton & $t\to W\to\tau$& other SM & GEN or & Lepton $P_T$ & Z veto & \met $>$ 50& obs/pred \\
214 & included & included & included & RECOSIM & and $\eta$ cuts & & & \\ \hline
215 1&Y & N & N & GEN & N & N & N & 1.90 \\
216 2&Y & N & N & GEN & Y & N & N & 1.64 \\
217 3&Y & N & N & GEN & Y & Y & N & 1.59 \\
218 4&Y & N & N & GEN & Y & Y & Y & 1.55 \\
219 5&Y & N & N & RECOSIM & Y & Y & Y & 1.51 \\
220 6&Y & Y & N & RECOSIM & Y & Y & Y & 1.58 \\
221 7&Y & Y & Y & RECOSIM & Y & Y & Y & 1.38 \\
222 \hline
223 \end{tabular}
224 \end{center}
225 \end{table}
226
227
228 \begin{table}[htb]
229 \begin{center}
230 \caption{\label{tab:victorysyst}
231 Summary of variations in $K_C$ due to the MET scale and resolution uncertainty, and to backgrounds other than $t\bar{t} \to$~dilepton.
232 In the first table, `up' and `down' refer to shifting the hadronic energy scale up and down by 5\%. In the second table, the quoted value
233 refers to the amount of additional smearing of the MET, as discussed in the text. In the third table, the normalization of all backgrounds
234 other than $t\bar{t} \to$~dilepton is varied.{\bf \color{red} UPDATE } }
235 \begin{tabular}{ lcccc }
236 \hline
237 MET scale & Predicted & Observed & Obs/pred \\
238 \hline
239 nominal & 0.92 $ \pm $ 0.09 & 1.27 $ \pm $ 0.10 & 1.39 $ \pm $ 0.18 \\
240 up & 0.90 $ \pm $ 0.09 & 1.58 $ \pm $ 0.10 & 1.75 $ \pm $ 0.21 \\
241 down & 0.70 $ \pm $ 0.06 & 0.96 $ \pm $ 0.09 & 1.37 $ \pm $ 0.18 \\
242 \hline
243 MET smearing & Predicted & Observed & Obs/pred \\
244 \hline
245 nominal & 0.92 $ \pm $ 0.09 & 1.27 $ \pm $ 0.10 & 1.39 $ \pm $ 0.18 \\
246 10\% & 0.88 $ \pm $ 0.09 & 1.28 $ \pm $ 0.10 & 1.47 $ \pm $ 0.19 \\
247 20\% & 0.87 $ \pm $ 0.09 & 1.26 $ \pm $ 0.10 & 1.44 $ \pm $ 0.19 \\
248 30\% & 1.03 $ \pm $ 0.17 & 1.33 $ \pm $ 0.10 & 1.29 $ \pm $ 0.23 \\
249 40\% & 0.88 $ \pm $ 0.09 & 1.36 $ \pm $ 0.10 & 1.55 $ \pm $ 0.20 \\
250 50\% & 0.80 $ \pm $ 0.07 & 1.39 $ \pm $ 0.10 & 1.73 $ \pm $ 0.19 \\
251 \hline
252 non-$t\bar{t} \to$~dilepton bkg & Predicted & Observed & Obs/pred \\
253 \hline
254 ttdil only & 0.79 $ \pm $ 0.07 & 1.07 $ \pm $ 0.06 & 1.36 $ \pm $ 0.14 \\
255 nominal & 0.92 $ \pm $ 0.09 & 1.27 $ \pm $ 0.10 & 1.39 $ \pm $ 0.18 \\
256 double non-ttdil yield & 1.04 $ \pm $ 0.15 & 1.47 $ \pm $ 0.16 & 1.40 $ \pm $ 0.25 \\
257 \hline
258 \end{tabular}
259 \end{center}
260 \end{table}
261
262 The largest discrepancy between prediction and observation occurs on the first
263 line of Table~\ref{tab:victorybad}, {\em i.e.}, at the generator level with no
264 cuts. We have verified that this effect is due to the polarization of
265 the $W$ (we remove the polarization by reweighting the events and we get
266 good agreement between prediction and observation). The kinematical
267 requirements (lines 2,3,4) compensate somewhat for the effect of W polarization.
268 Going from GEN to RECOSIM, the change in observed/predicted is small.
269 % We have tracked this down to the fact that tcMET underestimates the true \met
270 % by $\approx 4\%$\footnote{We find that observed/predicted changes by roughly 0.1
271 %for each 1.5\% change in \met response.}.
272 Finally, contamination from non $t\bar{t}$
273 events can have a significant impact on the BG prediction.
274 %The changes between
275 %lines 6 and 7 of Table~\ref{tab:victorybad} is driven by 3
276 %Drell Yan events that pass the \met selection in Monte Carlo (thus the effect
277 %is statistically not well quantified).
278
279 An additional source of concern is that the CMS Madgraph $t\bar{t}$ MC does
280 not include effects of spin correlations between the two top quarks.
281 We have studied this effect at the generator level using Alpgen. We find
282 that the bias is at the few percent level.
283
284 Based on the results of Table~\ref{tab:victorysyst}, we conclude that the
285 naive data-driven background estimate based on $P_T{(\ell\ell)}$ needs to
286 be corrected by a factor of $ K_C = 1.4 \pm 0.2({\rm stat})$.
287
288 The dominant sources of systematic uncertainty in $K_C$ are due to non-$t\bar{t} \to$~dilepton backgrounds,
289 and the MET scale and resolution uncertainties, as summarized in Table~\ref{tab:victorysyst}.
290 The impact of non-$t\bar{t}$-dilepton background is assessed
291 by varying the yield of all backgrounds except for $t\bar{t} \to$~dilepton.
292 The uncertainty is assessed as the larger of the differences between the nominal $K_C$ value and the values
293 obtained using only $t\bar{t} \to$~dilepton MC and obtained by doubling the non $t\bar{t} \to$~dilepton component,
294 giving an uncertainty of $0.03$.
295
296 The uncertainty in $K_C$ due to the MET scale uncertainty is assessed by varying the hadronic energy scale using
297 the same method as in~\cite{ref:top}, giving an uncertainty of 0.36.
298 We also assess the impact of the MET resolution
299 uncertainty on $K_C$ by applying a random smearing to the MET. For each event, we determine the expected MET resolution
300 based on the sumJetPt, and smear the MET to simulate an increase in the resolution of 10\%, 20\%, 30\%, 40\% and 50\%.
301 The results show that $K_C$ does not depend strongly on the MET resolution and we therefore do not assess any uncertainty.
302
303 Incorporating all the statistical and systematic uncertainties we find $K_C = 1.4 \pm 0.4$.
304
305 \subsection{Signal Contamination}
306 \label{sec:sigcont}
307
308 All data-driven methods are in principle subject to signal contaminations
309 in the control regions, and the methods described in
310 Sections~\ref{sec:abcd} and~\ref{sec:victory} are not exceptions.
311 Signal contamination tends to dilute the significance of a signal
312 present in the data by inflating the background prediction.
313
314 It is hard to quantify how important these effects are because we
315 do not know what signal may be hiding in the data. Having two
316 independent methods (in addition to Monte Carlo ``dead-reckoning'')
317 adds redundancy because signal contamination can have different effects
318 in the different control regions for the two methods.
319 For example, in the extreme case of a
320 new physics signal
321 with $P_T(\ell \ell) = \met$, an excess of events would be seen
322 in the ABCD method but not in the $P_T(\ell \ell)$ method.
323
324
325 The LM points are benchmarks for SUSY analyses at CMS. The effects
326 of signal contaminations for a couple such points are summarized
327 in Table~\ref{tab:sigcont}. Signal contamination is definitely an important
328 effect for these two LM points, but it does not totally hide the
329 presence of the signal.
330
331
332 \begin{table}[htb]
333 \begin{center}
334 \caption{\label{tab:sigcont} Effects of signal contamination
335 for the two data-driven background estimates. The three columns give
336 the expected yield in the signal region and the background estimates
337 using the ABCD and $P_T(\ell \ell)$ methods. Results are normalized to 34.0~pb$^{-1}$.}
338 \begin{tabular}{lccc}
339 \hline
340 & Yield & ABCD & $P_T(\ell \ell)$ \\
341 \hline
342 SM only & 1.3 & 1.3 & 0.9 \\
343 SM + LM0 & 7.4 & 4.4 & 1.9 \\
344 SM + LM1 & 3.8 & 1.6 & 1.4 \\
345 %SM only & 1.27 & 1.27 & 0.92 \\
346 %SM + LM0 & 7.39 & 4.38 & 1.93 \\
347 %SM + LM1 & 3.77 & 1.62 & 1.41 \\
348 \hline
349 \end{tabular}
350 \end{center}
351 \end{table}
352