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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 35 pb$^{-1}$. The ABCD method with chosen boundaries is accurate
64 to about 20\%, and we assess a corresponding systematic uncertainty
65 {\bf \color{red} More detail needed here???}
66
67 %As shown in Table~\ref{tab:abcdsyst}, we assess systematic uncertainties
68 %by varying the boundaries by an amount consistent with the hadronic energy scale uncertainty,
69 %which we take as $\pm$5\% for SumJetPt and $\pm$2.5\% for MET/$\sqrt{\rm SumJetPt}$, since the
70 %uncertainty on this quantity partially cancels due to the fact that it is a ratio of correlated
71 %quantities. Based on these studies we assess a correction factor $k_{ABCD} = 1.2 \pm 0.2$ to the
72 %predicted yield using the ABCD method.
73
74
75 %{\color{red} Avi wants some statement about stability
76 %wrt changes in regions. I am not sure that we have done it and
77 %I am not sure it is necessary (Claudio).}
78
79 \begin{table}[ht]
80 \begin{center}
81 \caption{\label{tab:abcdMC} Expected SM Monte Carlo yields for
82 35 pb$^{-1}$ in the ABCD regions, as well as the predicted yield in
83 the signal region given by A $\times$ C / B. Here `SM other' is the sum
84 of non-dileptonic $t\bar{t}$ decays, $W^{\pm}$+jets, $W^+W^-$,
85 $W^{\pm}Z^0$, $Z^0Z^0$ and single top.}
86 \begin{tabular}{lccccc}
87 \hline
88 sample & A & B & C & D & A $\times$ C / B \\
89 \hline
90 $t\bar{t}\rightarrow \ell^{+}\ell^{-}$ & 8.27 $\pm$ 0.18 & 32.16 $\pm$ 0.35 & 4.69 $\pm$ 0.13 & 1.05 $\pm$ 0.06 & 1.21 $\pm$ 0.04 \\
91 $Z^0 \rightarrow \ell^{+}\ell^{-}$ & 0.22 $\pm$ 0.11 & 1.54 $\pm$ 0.29 & 0.05 $\pm$ 0.05 & 0.16 $\pm$ 0.09 & 0.01 $\pm$ 0.01 \\
92 SM other & 0.54 $\pm$ 0.03 & 2.28 $\pm$ 0.12 & 0.23 $\pm$ 0.03 & 0.07 $\pm$ 0.01 & 0.05 $\pm$ 0.01 \\
93 \hline
94 total SM MC & 9.03 $\pm$ 0.21 & 35.97 $\pm$ 0.46 & 4.97 $\pm$ 0.15 & 1.29 $\pm$ 0.11 & 1.25 $\pm$ 0.05 \\
95 \hline
96 \end{tabular}
97 \end{center}
98 \end{table}
99
100
101
102 \begin{table}[ht]
103 \begin{center}
104 \caption{\label{tab:abcdsyst}
105 {\bf \color{red} Do we need this study at all? Observed/predicted is consistent within stat uncertainties as the boundaries are varied- is it enough to simply state this fact in the text??? }
106 Results of the systematic study of the ABCD method by varying the boundaries
107 between the ABCD regions shown in Fig.~\ref{fig:abcdMC}. Here $x_1$ is the lower SumJetPt boundary and
108 $x_2$ is the boundary separating regions A and B from C and D, their nominal values are 125 and 300~GeV,
109 respectively. $y_1$ is the lower MET/$\sqrt{\rm SumJetPt}$ boundary and
110 $y_2$ is the boundary separating regions B and C from A and D, their nominal values are 4.5 and 8.5~GeV$^{1/2}$,
111 respectively.}
112 \begin{tabular}{cccc|c}
113 \hline
114 $x_1$ & $x_2$ & $y_1$ & $y_2$ & Observed/Predicted \\
115 \hline
116 nominal & nominal & nominal & nominal & $1.20 \pm 0.12$ \\
117 +5\% & +5\% & +2.5\% & +2.5\% & $1.38 \pm 0.15$ \\
118 +5\% & +5\% & nominal & nominal & $1.31 \pm 0.14$ \\
119 nominal & nominal & +2.5\% & +2.5\% & $1.25 \pm 0.13$ \\
120 nominal & +5\% & nominal & +2.5\% & $1.32 \pm 0.14$ \\
121 nominal & -5\% & nominal & -2.5\% & $1.16 \pm 0.09$ \\
122 -5\% & -5\% & +2.5\% & +2.5\% & $1.21 \pm 0.11$ \\
123 +5\% & +5\% & -2.5\% & -2.5\% & $1.26 \pm 0.12$ \\
124 \hline
125 \end{tabular}
126 \end{center}
127 \end{table}
128
129 \subsection{Dilepton $P_T$ method}
130 \label{sec:victory}
131 This method is based on a suggestion by V. Pavlunin\cite{ref:victory},
132 and was investigated by our group in 2009\cite{ref:ourvictory}.
133 The idea is that in dilepton $t\bar{t}$ events the lepton and neutrinos
134 from $W$ decays have the same $P_T$ spectrum (modulo $W$ polarization
135 effects). One can then use the observed
136 $P_T(\ell\ell)$ distribution to model the sum of neutrino $P_T$'s which
137 is identified with the \met.
138
139 Then, in order to predict the $t\bar{t} \to$ dilepton contribution to a
140 selection with \met$+$X, one applies a cut on $P_T(\ell\ell)+$X instead.
141 In practice one has to rescale the result of the $P_T(\ell\ell)+$X selection
142 to account for the fact that any dilepton selection must include a
143 moderate \met cut in order to reduce Drell Yan backgrounds. This
144 is discussed in Section 5.3 of Reference~\cite{ref:ourvictory}; for a \met
145 cut of 50 GeV, the rescaling factor is obtained from the MC as
146
147 \newcommand{\ptll} {\ensuremath{P_T(\ell\ell)}}
148 \begin{center}
149 $ K = \frac{\int_0^{\infty} {\cal N}(\ptll)~~d\ptll~}{\int_{50}^{\infty} {\cal N}(\ptll)~~d\ptll~} = 1.52$
150 \end{center}
151
152
153 %%%TO BE REPLACED
154 %Given the integrated luminosity of the
155 %present dataset, the determination of $K$ in data is severely statistics
156 %limited. Thus, we take $K$ from $t\bar{t}$ Monte Carlo as
157
158 %\begin{center}
159 %$ K_{MC} = \frac{\int_0^{\infty} {\cal N}(\met)~~d\met~}{\int_{50}^{\infty} {\cal N}(\met)~~d\met~}$
160 %\end{center}
161
162 %\noindent {\color{red} For the 11 pb result we have used $K$ from data.}
163
164 There are several effects that spoil the correspondance between \met and
165 $P_T(\ell\ell)$:
166 \begin{itemize}
167 \item $Ws$ in top events are polarized. Neutrinos are emitted preferentially
168 parallel to the $W$ velocity while charged leptons are emitted prefertially
169 anti-parallel. Thus the $P_T(\nu\nu)$ distribution is harder
170 than the $P_T(\ell\ell)$ distribution for top dilepton events.
171 \item The lepton selections results in $P_T$ and $\eta$ cuts on the individual
172 leptons that have no simple correspondance to the neutrino requirements.
173 \item Similarly, the \met$>$50 GeV cut introduces an asymmetry between leptons and
174 neutrinos which is only partially compensated by the $K$ factor above.
175 \item The \met resolution is much worse than the dilepton $P_T$ resolution.
176 When convoluted with a falling spectrum in the tails of \met, this results
177 in a harder spectrum for \met than the original $P_T(\nu\nu)$.
178 \item The \met response in CMS is not exactly 1. This causes a distortion
179 in the \met distribution that is not present in the $P_T(\ell\ell)$ distribution.
180 \item The $t\bar{t} \to$ dilepton signal includes contributions from
181 $W \to \tau \to \ell$. For these events the arguments about the equivalence
182 of $P_T(\ell\ell)$ and $P_T(\nu\nu)$ do not apply.
183 \item A dilepton selection will include SM events from non $t\bar{t}$
184 sources. These events can affect the background prediction. Particularly
185 dangerous are high $P_T$ Drell Yan events that barely pass the \met$>$ 50
186 GeV selection. They will tend to push the data-driven background prediction up.
187 Therefore we estimate the number of DY events entering the background prediction
188 using the $R_{out/in}$ method as described in Sec.~\ref{sec:othBG}.
189 \end{itemize}
190
191 We have studied these effects in SM Monte Carlo, using a mixture of generator and
192 reconstruction level studies, putting the various effects in one at a time.
193 For each configuration, we apply the data-driven method and report as figure
194 of merit the ratio of observed and predicted events in the signal region.
195 The results are summarized in Table~\ref{tab:victorybad}.
196
197 \begin{table}[htb]
198 \begin{center}
199 \caption{\label{tab:victorybad}
200 {\bf \color{red} Need to either update this with 38X MC or remove it }
201 Test of the data driven method in Monte Carlo
202 under different assumptions. See text for details.}
203 \begin{tabular}{|l|c|c|c|c|c|c|c|c|}
204 \hline
205 & True $t\bar{t}$ dilepton & $t\to W\to\tau$& other SM & GEN or & Lepton $P_T$ & Z veto & \met $>$ 50& obs/pred \\
206 & included & included & included & RECOSIM & and $\eta$ cuts & & & \\ \hline
207 1&Y & N & N & GEN & N & N & N & 1.90 \\
208 2&Y & N & N & GEN & Y & N & N & 1.64 \\
209 3&Y & N & N & GEN & Y & Y & N & 1.59 \\
210 4&Y & N & N & GEN & Y & Y & Y & 1.55 \\
211 5&Y & N & N & RECOSIM & Y & Y & Y & 1.51 \\
212 6&Y & Y & N & RECOSIM & Y & Y & Y & 1.58 \\
213 7&Y & Y & Y & RECOSIM & Y & Y & Y & 1.38 \\
214 \hline
215 \end{tabular}
216 \end{center}
217 \end{table}
218
219
220 \begin{table}[htb]
221 \begin{center}
222 \caption{\label{tab:victorysyst}
223 Summary of uncertainties in $K_C$ due to the MET scale and resolution uncertainty, and to backgrounds other than $t\bar{t} \to$~dilepton.
224 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
225 refers to the amount of additional smearing of the MET, as discussed in the text. In the third table, the normalization of all backgrounds
226 other than $t\bar{t} \to$~dilepton is varied.
227 {\bf \color{red} Should I remove `observed' and `predicted' and show only the ratio? }}
228
229 \begin{tabular}{ lcccc }
230 \hline
231 MET scale & Predicted & Observed & Obs/pred \\
232 \hline
233 nominal & 0.92 $ \pm $ 0.11 & 1.29 $ \pm $ 0.11 & 1.40 $ \pm $ 0.20 \\
234 up & 0.92 $ \pm $ 0.11 & 1.53 $ \pm $ 0.12 & 1.66 $ \pm $ 0.23 \\
235 down & 0.81 $ \pm $ 0.07 & 1.08 $ \pm $ 0.11 & 1.32 $ \pm $ 0.17 \\
236 \hline
237 MET smearing & Predicted & Observed & Obs/pred \\
238 \hline
239 nominal & 0.92 $ \pm $ 0.11 & 1.29 $ \pm $ 0.11 & 1.40 $ \pm $ 0.20 \\
240 10\% & 0.90 $ \pm $ 0.11 & 1.30 $ \pm $ 0.11 & 1.44 $ \pm $ 0.21 \\
241 20\% & 0.84 $ \pm $ 0.07 & 1.36 $ \pm $ 0.11 & 1.61 $ \pm $ 0.19 \\
242 30\% & 1.05 $ \pm $ 0.18 & 1.32 $ \pm $ 0.11 & 1.27 $ \pm $ 0.24 \\
243 40\% & 0.85 $ \pm $ 0.07 & 1.37 $ \pm $ 0.11 & 1.61 $ \pm $ 0.19 \\
244 50\% & 1.08 $ \pm $ 0.18 & 1.36 $ \pm $ 0.11 & 1.26 $ \pm $ 0.24 \\
245 \hline
246 non-$t\bar{t} \to$~dilepton scale factor & Predicted & Observed & Obs/pred \\
247 \hline
248 ttdil only & 0.77 $ \pm $ 0.07 & 1.05 $ \pm $ 0.06 & 1.36 $ \pm $ 0.14 \\
249 nominal & 0.92 $ \pm $ 0.11 & 1.29 $ \pm $ 0.11 & 1.40 $ \pm $ 0.20 \\
250 double non-ttdil yield & 1.06 $ \pm $ 0.18 & 1.52 $ \pm $ 0.20 & 1.43 $ \pm $ 0.30 \\
251 \hline
252 \end{tabular}
253 \end{center}
254 \end{table}
255
256
257
258 The largest discrepancy between prediction and observation occurs on the first
259 line of Table~\ref{tab:victorybad}, {\em i.e.}, at the generator level with no
260 cuts. We have verified that this effect is due to the polarization of
261 the $W$ (we remove the polarization by reweighting the events and we get
262 good agreement between prediction and observation). The kinematical
263 requirements (lines 2,3,4) compensate somewhat for the effect of W polarization.
264 Going from GEN to RECOSIM, the change in observed/predicted is small.
265 % We have tracked this down to the fact that tcMET underestimates the true \met
266 % by $\approx 4\%$\footnote{We find that observed/predicted changes by roughly 0.1
267 %for each 1.5\% change in \met response.}.
268 Finally, contamination from non $t\bar{t}$
269 events can have a significant impact on the BG prediction.
270 %The changes between
271 %lines 6 and 7 of Table~\ref{tab:victorybad} is driven by 3
272 %Drell Yan events that pass the \met selection in Monte Carlo (thus the effect
273 %is statistically not well quantified).
274
275 An additional source of concern is that the CMS Madgraph $t\bar{t}$ MC does
276 not include effects of spin correlations between the two top quarks.
277 We have studied this effect at the generator level using Alpgen. We find
278 that the bias is at the few percent level.
279
280 Based on the results of Table~\ref{tab:victorysyst}, we conclude that the
281 naive data-driven background estimate based on $P_T{(\ell\ell)}$ needs to
282 be corrected by a factor of $ K_C = 1.4 \pm 0.2({\rm stat})$.
283
284 The dominant sources of systematic uncertainty in $K_C$ are due to non-$t\bar{t} \to$~dilepton backgrounds,
285 and the MET scale and resolution uncertainties, as summarized in Table~\ref{tab:victorysyst}.
286 The impact of non-$t\bar{t}$-dilepton background is assessed
287 by varying the yield of all backgrounds except for $t\bar{t} \to$~dilepton.
288 The uncertainty is assessed as the larger of the differences between the nominal $K_C$ value and the values
289 obtained using only $t\bar{t} \to$~dilepton MC and obtained by doubling the non $t\bar{t} \to$~dilepton component,
290 giving an uncertainty of $0.04$.
291
292 The uncertainty in $K_C$ due to the MET scale uncertainty is assessed by varying the hadronic energy scale using
293 the same method as in~\ref{ref:top}, giving an uncertainty of 0.3. We also assess the impact of the MET resolution
294 uncertainty on $K_C$ by applying a random smearing to the MET. For each event, we determine the expected MET resolution
295 based on the sumJetPt, and smear the MET to simulate an increase in the resolution of 10\%, 20\%, 30\%, 40\% and 50\%.
296 The results show that $K_C$ does not depend strongly on the MET resolution and we therefore do not assess any uncertainty.
297
298 Incorporating all the statistical and systematic uncertainties we find $K_C = 1.4 \pm 0.4$.
299
300 \subsection{Signal Contamination}
301 \label{sec:sigcont}
302
303 All data-driven methods are in principle subject to signal contaminations
304 in the control regions, and the methods described in
305 Sections~\ref{sec:abcd} and~\ref{sec:victory} are not exceptions.
306 Signal contamination tends to dilute the significance of a signal
307 present in the data by inflating the background prediction.
308
309 It is hard to quantify how important these effects are because we
310 do not know what signal may be hiding in the data. Having two
311 independent methods (in addition to Monte Carlo ``dead-reckoning'')
312 adds redundancy because signal contamination can have different effects
313 in the different control regions for the two methods.
314 For example, in the extreme case of a
315 new physics signal
316 with $P_T(\ell \ell) = \met$, an excess of events would be seen
317 in the ABCD method but not in the $P_T(\ell \ell)$ method.
318
319
320 The LM points are benchmarks for SUSY analyses at CMS. The effects
321 of signal contaminations for a couple such points are summarized
322 in Table~\ref{tab:sigcont}. Signal contamination is definitely an important
323 effect for these two LM points, but it does not totally hide the
324 presence of the signal.
325
326
327 \begin{table}[htb]
328 \begin{center}
329 \caption{\label{tab:sigcont} Effects of signal contamination
330 for the two data-driven background estimates. The three columns give
331 the expected yield in the signal region and the background estimates
332 using the ABCD and $P_T(\ell \ell)$ methods. Results are normalized to 35~pb$^{-1}$.}
333 \begin{tabular}{lccc}
334 \hline
335 & Yield & ABCD & $P_T(\ell \ell)$ \\
336 \hline
337 SM only & 1.29 & 1.25 & 0.92 \\
338 SM + LM0 & 7.57 & 4.44 & 1.96 \\
339 SM + LM1 & 3.85 & 1.60 & 1.43 \\
340 \hline
341 \end{tabular}
342 \end{center}
343 \end{table}
344