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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 Figure~\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 $>$8.5, respectively.}
46 \end{center}
47 \end{figure}
48
49 \begin{figure}[tb]
50 \begin{center}
51 \includegraphics[width=0.5\linewidth, angle=90]{abcdMC.pdf}
52 \caption{\label{fig:abcdMC}\protect Distributions of MET$/\sqrt{\rm SumJetPt}$ vs.
53 SumJetPt for SM Monte Carlo. Here we also show our choice of ABCD regions.}
54 \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 prediction AC/B are given in Table~\ref{tab:abcdMC} for an integrated
62 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 %{\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
75 \begin{table}[ht]
76 \begin{center}
77 \caption{\label{tab:abcdMC} Expected SM Monte Carlo yields for
78 35 pb$^{-1}$ in the ABCD regions, as well as the predicted yield in
79 the signal region given by A $\times$ C / B. Here `SM other' is the sum
80 of non-dileptonic $t\bar{t}$ decays, $W^{\pm}$+jets, $W^+W^-$,
81 $W^{\pm}Z^0$, $Z^0Z^0$ and single top.}
82 \begin{tabular}{lccccc}
83 \hline
84 sample & A & B & C & D & A $\times$ C / B \\
85 \hline
86 $t\bar{t}\rightarrow \ell^{+}\ell^{-}$ & 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 \\
87 $Z^0 \rightarrow \ell^{+}\ell^{-}$ & 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 \\
88 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 \\
89 \hline
90 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 \\
91 \hline
92 \end{tabular}
93 \end{center}
94 \end{table}
95
96
97
98 \begin{table}[ht]
99 \begin{center}
100 \caption{\label{tab:abcdsyst}
101 {\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??? }
102 Results of the systematic study of the ABCD method by varying the boundaries
103 between the ABCD regions shown in Fig.~\ref{fig:abcdMC}. Here $x_1$ is the lower SumJetPt boundary and
104 $x_2$ is the boundary separating regions A and B from C and D, their nominal values are 125 and 300~GeV,
105 respectively. $y_1$ is the lower MET/$\sqrt{\rm SumJetPt}$ boundary and
106 $y_2$ is the boundary separating regions B and C from A and D, their nominal values are 4.5 and 8.5~GeV$^{1/2}$,
107 respectively.}
108 \begin{tabular}{cccc|c}
109 \hline
110 $x_1$ & $x_2$ & $y_1$ & $y_2$ & Observed/Predicted \\
111 \hline
112 nominal & nominal & nominal & nominal & $1.20 \pm 0.12$ \\
113 +5\% & +5\% & +2.5\% & +2.5\% & $1.38 \pm 0.15$ \\
114 +5\% & +5\% & nominal & nominal & $1.31 \pm 0.14$ \\
115 nominal & nominal & +2.5\% & +2.5\% & $1.25 \pm 0.13$ \\
116 nominal & +5\% & nominal & +2.5\% & $1.32 \pm 0.14$ \\
117 nominal & -5\% & nominal & -2.5\% & $1.16 \pm 0.09$ \\
118 -5\% & -5\% & +2.5\% & +2.5\% & $1.21 \pm 0.11$ \\
119 +5\% & +5\% & -2.5\% & -2.5\% & $1.26 \pm 0.12$ \\
120 \hline
121 \end{tabular}
122 \end{center}
123 \end{table}
124
125 \subsection{Dilepton $P_T$ method}
126 \label{sec:victory}
127 This method is based on a suggestion by V. Pavlunin\cite{ref:victory},
128 and was investigated by our group in 2009\cite{ref:ourvictory}.
129 The idea is that in dilepton $t\bar{t}$ events the lepton and neutrinos
130 from $W$ decays have the same $P_T$ spectrum (modulo $W$ polarization
131 effects). One can then use the observed
132 $P_T(\ell\ell)$ distribution to model the sum of neutrino $P_T$'s which
133 is identified with the \met.
134
135 Then, in order to predict the $t\bar{t} \to$ dilepton contribution to a
136 selection with \met$+$X, one applies a cut on $P_T(\ell\ell)+$X instead.
137 In practice one has to rescale the result of the $P_T(\ell\ell)+$X selection
138 to account for the fact that any dilepton selection must include a
139 moderate \met cut in order to reduce Drell Yan backgrounds. This
140 is discussed in Section 5.3 of Reference~\cite{ref:ourvictory}; for a \met
141 cut of 50 GeV, the rescaling factor is obtained from the MC as
142
143 \newcommand{\ptll} {\ensuremath{P_T(\ell\ell)}}
144 \begin{center}
145 $ K = \frac{\int_0^{\infty} {\cal N}(\ptll)~~d\ptll~}{\int_{50}^{\infty} {\cal N}(\ptll)~~d\ptll~}$
146 \end{center}
147
148
149 Monte Carlo studies give values of $K$ that are typically between 1.5 and 1.6,
150 depending on selection details.
151 %%%TO BE REPLACED
152 %Given the integrated luminosity of the
153 %present dataset, the determination of $K$ in data is severely statistics
154 %limited. Thus, we take $K$ from $t\bar{t}$ Monte Carlo as
155
156 %\begin{center}
157 %$ K_{MC} = \frac{\int_0^{\infty} {\cal N}(\met)~~d\met~}{\int_{50}^{\infty} {\cal N}(\met)~~d\met~}$
158 %\end{center}
159
160 %\noindent {\color{red} For the 11 pb result we have used $K$ from data.}
161
162 There are several effects that spoil the correspondance between \met and
163 $P_T(\ell\ell)$:
164 \begin{itemize}
165 \item $Ws$ in top events are polarized. Neutrinos are emitted preferentially
166 parallel to the $W$ velocity while charged leptons are emitted prefertially
167 anti-parallel. Thus the $P_T(\nu\nu)$ distribution is harder
168 than the $P_T(\ell\ell)$ distribution for top dilepton events.
169 \item The lepton selections results in $P_T$ and $\eta$ cuts on the individual
170 leptons that have no simple correspondance to the neutrino requirements.
171 \item Similarly, the \met$>$50 GeV cut introduces an asymmetry between leptons and
172 neutrinos which is only partially compensated by the $K$ factor above.
173 \item The \met resolution is much worse than the dilepton $P_T$ resolution.
174 When convoluted with a falling spectrum in the tails of \met, this results
175 in a harder spectrum for \met than the original $P_T(\nu\nu)$.
176 \item The \met response in CMS is not exactly 1. This causes a distortion
177 in the \met distribution that is not present in the $P_T(\ell\ell)$ distribution.
178 \item The $t\bar{t} \to$ dilepton signal includes contributions from
179 $W \to \tau \to \ell$. For these events the arguments about the equivalence
180 of $P_T(\ell\ell)$ and $P_T(\nu\nu)$ do not apply.
181 \item A dilepton selection will include SM events from non $t\bar{t}$
182 sources. These events can affect the background prediction. Particularly
183 dangerous are high $P_T$ Drell Yan events that barely pass the \met$>$ 50
184 GeV selection. They will tend to push the data-driven background prediction up.
185 Therefore we estimate the number of DY events entering the background prediction
186 using the $R_{out/in}$ method as described in Sec.~\ref{sec:othBG}.
187 \end{itemize}
188
189 We have studied these effects in SM Monte Carlo, using a mixture of generator and
190 reconstruction level studies, putting the various effects in one at a time.
191 For each configuration, we apply the data-driven method and report as figure
192 of merit the ratio of observed and predicted events in the signal region.
193 The results are summarized in Table~\ref{tab:victorybad}.
194
195 \begin{table}[htb]
196 \begin{center}
197 \caption{\label{tab:victorybad}
198 {\bf \color{red} Need to either update this with 38X MC or remove it }
199 Test of the data driven method in Monte Carlo
200 under different assumptions. See text for details.}
201 \begin{tabular}{|l|c|c|c|c|c|c|c|c|}
202 \hline
203 & True $t\bar{t}$ dilepton & $t\to W\to\tau$& other SM & GEN or & Lepton $P_T$ & Z veto & \met $>$ 50& obs/pred \\
204 & included & included & included & RECOSIM & and $\eta$ cuts & & & \\ \hline
205 1&Y & N & N & GEN & N & N & N & 1.90 \\
206 2&Y & N & N & GEN & Y & N & N & 1.64 \\
207 3&Y & N & N & GEN & Y & Y & N & 1.59 \\
208 4&Y & N & N & GEN & Y & Y & Y & 1.55 \\
209 5&Y & N & N & RECOSIM & Y & Y & Y & 1.51 \\
210 6&Y & Y & N & RECOSIM & Y & Y & Y & 1.58 \\
211 7&Y & Y & Y & RECOSIM & Y & Y & Y & 1.38 \\
212 \hline
213 \end{tabular}
214 \end{center}
215 \end{table}
216
217
218 \begin{table}[htb]
219 \begin{center}
220 \caption{\label{tab:victorysyst}
221 {Summary of uncertainties in $K_C$ due to the MET scale and resolution uncertainty, and to backgrounds other than $t\bar{t} \to$~dilepton.
222 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
223 refers to the amount of additional smearing of the MET, as discussed in the text. In the third table, the normalization of all backgrounds
224 other than $t\bar{t} \to$~dilepton is varied.
225 {\bf \color{ref} Should I remove `observed' and `predicted' and show only the ratio? }}
226
227 \begin{tabular}{ lcccc }
228 \hline
229 MET scale & Predicted & Observed & Obs/pred \\
230 \hline
231 nominal & 0.92 $ \pm $ 0.11 & 1.29 $ \pm $ 0.11 & 1.40 $ \pm $ 0.20 \\
232 up & 0.92 $ \pm $ 0.11 & 1.53 $ \pm $ 0.12 & 1.66 $ \pm $ 0.23 \\
233 down & 0.81 $ \pm $ 0.07 & 1.08 $ \pm $ 0.11 & 1.32 $ \pm $ 0.17 \\
234 \hline
235
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
247 \hline
248 non-$t\bar{t} \to$~dilepton scale factor & Predicted & Observed & Obs/pred \\
249 \hline
250 ttdil only & 0.77 $ \pm $ 0.07 & 1.05 $ \pm $ 0.06 & 1.36 $ \pm $ 0.14 \\
251 nominal & 0.92 $ \pm $ 0.11 & 1.29 $ \pm $ 0.11 & 1.40 $ \pm $ 0.20 \\
252 double non-ttdil yield & 1.06 $ \pm $ 0.18 & 1.52 $ \pm $ 0.20 & 1.43 $ \pm $ 0.30 \\
253 \hline
254 \end{tabular}
255 \end{center}
256 \end{table}
257
258
259
260 The largest discrepancy between prediction and observation occurs on the first
261 line of Table~\ref{tab:victorybad}, {\em i.e.}, at the generator level with no
262 cuts. We have verified that this effect is due to the polarization of
263 the $W$ (we remove the polarization by reweighting the events and we get
264 good agreement between prediction and observation). The kinematical
265 requirements (lines 2,3,4) compensate somewhat for the effect of W polarization.
266 Going from GEN to RECOSIM, the change in observed/predicted is small.
267 % We have tracked this down to the fact that tcMET underestimates the true \met
268 % by $\approx 4\%$\footnote{We find that observed/predicted changes by roughly 0.1
269 %for each 1.5\% change in \met response.}.
270 Finally, contamination from non $t\bar{t}$
271 events can have a significant impact on the BG prediction.
272 %The changes between
273 %lines 6 and 7 of Table~\ref{tab:victorybad} is driven by 3
274 %Drell Yan events that pass the \met selection in Monte Carlo (thus the effect
275 %is statistically not well quantified).
276
277 An additional source of concern is that the CMS Madgraph $t\bar{t}$ MC does
278 not include effects of spin correlations between the two top quarks.
279 We have studied this effect at the generator level using Alpgen. We find
280 that the bias is at the few percent level.
281
282 Based on the results of Table~\ref{tab:victorybad}, we conclude that the
283 naive data-driven background estimate based on $P_T{(\ell\ell)}$ needs to
284 be corrected by a factor of $ K_C = 1.4 \pm 0.2(stat)$.
285
286 The 2 dominant sources of systematic uncertainty in $K_C$ are due to non-$t\bar{t} \to$~dilepton backgrounds,
287 and the MET scale and resolution uncertainties. The impact of non-$t\bar{t}$-dilepton background is assessed
288 by varying the yield of all backgrounds except for $t\bar{t} \to$~dilepton, as shown in Table~\ref{table_kc}.
289 The systematic is assessed as the larger of the differences between the nominal $K_C$ value and the values
290 obtained using only $t\bar{t} \to$~dilepton MC and obtained by doubling the non $t\bar{t} \to$~dilepton component,
291 giving an uncertainty of $0.04$.
292
293 The uncertainty in $K_C$ due to the MET scale uncertainty is assessed by varying the hadronic energy scale using
294 the same method as in~\ref{} and checking how much $K_C$ changes, as summarized in Table~\ref{tab:victorysyst}.
295 This gives an uncertainty of 0.3. We also assess the impact of the MET resolution uncertainty on $K_C$ by applying
296 a random smearing to the MET. For each event, we determine the expected MET resolution based on the sumJetPt, and
297 smear the MET to simulate an increase in the resolution of 10\%, 20\%, 30\%, 40\% and 50\%. The results show that
298 $K_C$ does not depend strongly on the MET resolution and we therefore do not assess any uncertainty.
299
300 Incorporating all the statistical and systematic uncertainties we find $K_C = 1.4 \pm 0.4$.
301
302 \subsection{Signal Contamination}
303 \label{sec:sigcont}
304
305 All data-driven methods are in principle subject to signal contaminations
306 in the control regions, and the methods described in
307 Sections~\ref{sec:abcd} and~\ref{sec:victory} are not exceptions.
308 Signal contamination tends to dilute the significance of a signal
309 present in the data by inflating the background prediction.
310
311 It is hard to quantify how important these effects are because we
312 do not know what signal may be hiding in the data. Having two
313 independent methods (in addition to Monte Carlo ``dead-reckoning'')
314 adds redundancy because signal contamination can have different effects
315 in the different control regions for the two methods.
316 For example, in the extreme case of a
317 new physics signal
318 with $P_T(\ell \ell) = \met$, an excess of events would be seen
319 in the ABCD method but not in the $P_T(\ell \ell)$ method.
320
321
322 The LM points are benchmarks for SUSY analyses at CMS. The effects
323 of signal contaminations for a couple such points are summarized
324 in Table~\ref{tab:sigcont}. Signal contamination is definitely an important
325 effect for these two LM points, but it does not totally hide the
326 presence of the signal.
327
328
329 \begin{table}[htb]
330 \begin{center}
331 \caption{\label{tab:sigcont} Effects of signal contamination
332 for the two data-driven background estimates. The three columns give
333 the expected yield in the signal region and the background estimates
334 using the ABCD and $P_T(\ell \ell)$ methods. Results are normalized to 35~pb$^{-1}$.}
335 \begin{tabular}{lccc}
336 \hline
337 & Yield & ABCD & $P_T(\ell \ell)$ \\
338 \hline
339 SM only & 1.29 & 1.25 & 0.92 \\
340 SM + LM0 & 7.57 & 4.44 & 1.96 \\
341 SM + LM1 & 3.85 & 1.60 & 1.43 \\
342 \hline
343 \end{tabular}
344 \end{center}
345 \end{table}
346