| 1 |
benhoob |
1.1 |
\section{Same-flavour Dilepton Search}
|
| 2 |
|
|
\label{sec:HT}
|
| 3 |
|
|
|
| 4 |
|
|
The result of Section~\ref{sec:results} is cross-checked in a similar kinematic region with an
|
| 5 |
|
|
independent
|
| 6 |
|
|
search relying on a different trigger path, different methods for ``physics object'' reconstruction, and a
|
| 7 |
|
|
different background estimation method.
|
| 8 |
|
|
This search is directed at BSM scenarios in which decay chains of a pair of new heavy particles
|
| 9 |
|
|
produce an excess of same-flavour ($e^{+}e^{-}$ and $\mu^{+}\mu^{-}$) events over opposite-flavour ($e^{\pm}\mu^{\mp}$) events.
|
| 10 |
|
|
For example, in the context of the CMSSM, this excess may be caused by decays of neutralinos and Z bosons to same-flavour lepton pairs.
|
| 11 |
|
|
For the benchmark scenario LM0 (LM1), the fraction of same-flavour events in the signal region discussed
|
| 12 |
|
|
below is 0.67 (0.86).
|
| 13 |
|
|
|
| 14 |
|
|
The dominant background in this search is also dilepton $t\bar{t}$, for which such an excess does not exist
|
| 15 |
|
|
because the flavours of the two leptons are uncorrelated.
|
| 16 |
|
|
Therefore, the rate of $t\bar{t}$ decays with two same-flavour leptons
|
| 17 |
|
|
may be estimated from the number of opposite-flavour events,
|
| 18 |
|
|
after correcting for the
|
| 19 |
|
|
ratio of muon to electron selection efficiencies, $r_{\mu{}e}$.
|
| 20 |
|
|
This method actually estimates the contribution of any uncorrelated pair of leptons, including
|
| 21 |
|
|
e.g.\ $Z\to\tau\tau$ events where the two $\tau$ leptons decay leptonically.
|
| 22 |
|
|
This method will also subtract any BSM signal producing lepton pairs of uncorrelated flavour.
|
| 23 |
|
|
|
| 24 |
|
|
Events with two leptons with $\pt>10\GeVc$ are selected. Because the lepton triggers are not fully
|
| 25 |
|
|
efficient for events with two leptons of $\pt>10\GeVc$,
|
| 26 |
|
|
the data sample for this analysis is selected with hadronic triggers based on the
|
| 27 |
|
|
scalar sum of the transverse energies of all jets reconstructed from calorimeter signals with $\pt>20\GeVc$.
|
| 28 |
|
|
The event is required to pass at least one of a set of hadronic triggers with transverse energy thresholds
|
| 29 |
|
|
ranging from 100 to 150~GeV. The efficiency of this set of triggers with respect to the analysis selection is
|
| 30 |
|
|
greater than 99\%.
|
| 31 |
|
|
In addition to the trigger, we require $\HT>350\GeV$,
|
| 32 |
|
|
where \HT\ in this analysis is defined as the scalar sum of
|
| 33 |
|
|
the transverse energies of all selected jets with $\pt>30\GeVc$
|
| 34 |
|
|
and within an increased pseudorapidity range $|\eta|<3$, in line with the trigger requirement.
|
| 35 |
|
|
The jets, \MET, and leptons are reconstructed with the Particle Flow technique~\cite{CMS-PAS-PFT-10-002}.
|
| 36 |
|
|
The resulting performance of the selection of leptons and jets does not differ
|
| 37 |
|
|
significantly from the selection discussed in Section~\ref{sec:eventSel}.
|
| 38 |
|
|
|
| 39 |
|
|
The signal region is defined by additionally requiring $\MET>150\GeV$.
|
| 40 |
|
|
This signal region is chosen such that approximately one SM event is expected in our
|
| 41 |
|
|
current data sample.
|
| 42 |
|
|
|
| 43 |
|
|
The lepton selection efficiencies are measured using the $Z$
|
| 44 |
|
|
resonance. As discussed in Section~\ref{sec:systematics}, these
|
| 45 |
|
|
efficiencies are known with a systematic uncertainty of $2\%$. The
|
| 46 |
|
|
selection efficiencies of isolated leptons are different in the $t\bar{t}$
|
| 47 |
|
|
and $Z+\textrm{jets}$ samples. The ratio of muon to electron
|
| 48 |
|
|
efficiencies $r_{\mu{}e}$, however, is found to differ by less than
|
| 49 |
|
|
5\% in the MC simulations, and a corresponding systematic
|
| 50 |
|
|
uncertainty is assigned to this ratio. This procedure gives $r_{\mu{}e} = 1.07 \pm
|
| 51 |
|
|
0.06$.
|
| 52 |
|
|
|
| 53 |
|
|
The $W+\textrm{jets}$ and QCD multijet contributions, where at least one
|
| 54 |
|
|
of the two leptons is a secondary lepton from a heavy flavour decay or
|
| 55 |
|
|
a jet misidentified as a lepton (non-$W/Z$ leptons) are estimated from
|
| 56 |
|
|
a fit to the lepton isolation distribution, after relaxing
|
| 57 |
|
|
the isolation requirement on the leptons.
|
| 58 |
|
|
%The estimated purity is then applied to the number of observed events in the signal region to
|
| 59 |
|
|
%infer the number of non-$W/Z$ leptons therein.
|
| 60 |
|
|
Contributions from
|
| 61 |
|
|
other SM backgrounds, such as DY or processes with two gauge bosons,
|
| 62 |
|
|
are strongly suppressed by the \MET\ requirement and are expected to
|
| 63 |
|
|
be negligible.
|
| 64 |
|
|
|
| 65 |
|
|
We first estimate the number of SM events in a \ttbar-dominated region
|
| 66 |
|
|
with $100 < \HT < 350\GeV$ and $\MET>80\GeV$. In order to cope with the lower \HT\ requirement,
|
| 67 |
|
|
we use the same high-\pt lepton trigger sample as described in Section~\ref{sec:eventSel}.
|
| 68 |
|
|
In this region we observe $26$ opposite-flavour candidates and predict $1.0\pm0.5$
|
| 69 |
|
|
non-$W/Z$ lepton events from the fit to the lepton isolation distribution. This results in an
|
| 70 |
|
|
estimate of $25.0 \pm 5.0$ \ttbar\ events in the $e\mu$ channel. Using the efficiency
|
| 71 |
|
|
ratio $r_{\mu{}e}$ this estimate is then converted into a prediction for the number
|
| 72 |
|
|
of same-flavour events in the $ee$ and $\mu\mu$ channels.
|
| 73 |
|
|
|
| 74 |
|
|
\begin{table}[hbt]
|
| 75 |
|
|
\begin{center}
|
| 76 |
|
|
\caption{\label{tab:CRresults}
|
| 77 |
|
|
Number of predicted and observed $ee$ and $\mu\mu$
|
| 78 |
|
|
events in the control region, defined as $100 < \HT < 350\GeV$ and $\MET > 80\GeV$.
|
| 79 |
|
|
``SM MC'' indicates the sum of all MC samples ($t\bar{t}$, DY, $W+\textrm{jets}$, and $WW/WZ/ZZ$)
|
| 80 |
|
|
and includes statistical uncertainties only.
|
| 81 |
|
|
}
|
| 82 |
|
|
\vspace{2 mm}
|
| 83 |
|
|
\begin{tabular}{l|cc}
|
| 84 |
|
|
\hline
|
| 85 |
|
|
& \multicolumn{2}{c}{Control region} \\
|
| 86 |
|
|
\hline
|
| 87 |
|
|
Process & $ee$ & $\mu\mu$ \\
|
| 88 |
|
|
\hline
|
| 89 |
|
|
$t\bar{t}$ predicted from $e\mu$ & $11.7\pm 2.4$ & $13.4\pm 2.8$ \\
|
| 90 |
|
|
Non-$W/Z$ leptons & $0.5\pm 0.3$ & $0.4\pm0.2$ \\
|
| 91 |
|
|
\hline
|
| 92 |
|
|
Total predicted & $12.2\pm 2.4$ & $13.8 \pm 2.8$ \\
|
| 93 |
|
|
\hline\hline
|
| 94 |
|
|
Total observed & $10$ & $15$ \\
|
| 95 |
|
|
\hline \hline
|
| 96 |
|
|
SM MC & $8.4\pm 0.2$ & $10.5 \pm 0.3$ \\
|
| 97 |
|
|
%LM0 & $3.7\pm0.2$ & $4.2\pm0.2$ \\
|
| 98 |
|
|
%LM1 & $0.5\pm0.1$ & $0.7\pm0.1$ \\
|
| 99 |
|
|
|
| 100 |
|
|
\hline
|
| 101 |
|
|
\end{tabular}
|
| 102 |
|
|
\end{center}
|
| 103 |
|
|
\end{table}
|
| 104 |
|
|
|
| 105 |
|
|
Table~\ref{tab:CRresults} shows the number of expected SM background same-flavour events in the control region for the MC,
|
| 106 |
|
|
as well as the prediction from the background estimation techniques based on data. There are a total of 25 same-flavour
|
| 107 |
|
|
events, in good agreement with the prediction of $25.9 \pm 5.2$ events.
|
| 108 |
|
|
We thus proceed to the signal region selection.
|
| 109 |
|
|
%It is worth noting that in this control region, we expect $7.9 \pm 1.4$ and $1.2 \pm 0.2$ from LM0 and LM1 respectively.
|
| 110 |
|
|
|
| 111 |
|
|
The SM background predictions in the signal region from the opposite-flavour and non-$W/Z$ lepton methods
|
| 112 |
|
|
are summarized in Table~\ref{tab:results}.
|
| 113 |
|
|
We find one event in the signal region in the $e\mu$ channel with a prediction of non-$W/Z$ leptons
|
| 114 |
|
|
of $0.1\pm0.1$, and thus predict $0.9 {}_{-0.8}^{+2.2}$ same-flavour events using Poisson statistical uncertainties.
|
| 115 |
|
|
In the data we find no same-flavour events, in agreement with the prediction, in contrast with $7.3\pm1.6$
|
| 116 |
|
|
and $3.6\pm0.7$ expected events for the benchmark points LM0 and LM1, respectively.
|
| 117 |
|
|
%With zero events observed, the non-$W/Z$ lepton prediction is also zero.
|
| 118 |
|
|
The predicted background from non-$W/Z$ leptons is negligible.
|
| 119 |
|
|
|
| 120 |
|
|
\begin{table}[hbt]
|
| 121 |
|
|
\begin{center}
|
| 122 |
|
|
\caption{\label{tab:results}
|
| 123 |
|
|
Number of predicted and observed events in the signal region, defined as $\HT > 350\GeV$ and $\MET> 150\GeV$.
|
| 124 |
|
|
``SM MC'' indicates the sum of all MC samples ($t\bar{t}$, DY, $W+\textrm{jets}$, and $WW/WZ/ZZ$)
|
| 125 |
|
|
and includes statistical uncertainties only.
|
| 126 |
|
|
}
|
| 127 |
|
|
\vspace{2 mm}
|
| 128 |
|
|
\begin{tabular}{l|cc}
|
| 129 |
|
|
\hline
|
| 130 |
|
|
& \multicolumn{2}{c}{Signal region} \\
|
| 131 |
|
|
\hline
|
| 132 |
|
|
Process & $ee$ & $\mu\mu$ \\
|
| 133 |
|
|
\hline
|
| 134 |
|
|
$t\bar{t}$ predicted from $e\mu$ & $0.4 {}_{-0.4}^{+1.0}$ & $0.5 {}_{-0.4}^{+1.2}$ \\
|
| 135 |
|
|
Non-$W/Z$ & 0 & 0 \\
|
| 136 |
|
|
\hline
|
| 137 |
|
|
Total predicted & $0.4 {}_{-0.4}^{+1.0}$ & $0.5 {}_{-0.4}^{+1.2}$ \\
|
| 138 |
|
|
\hline\hline
|
| 139 |
|
|
Total observed & $0$ & $0$ \\
|
| 140 |
|
|
\hline \hline
|
| 141 |
|
|
SM MC & $0.38\pm 0.08$ & $0.56 \pm 0.07$ \\
|
| 142 |
|
|
LM0 & $3.4\pm0.2$ & $3.9\pm0.2$ \\
|
| 143 |
|
|
LM1 & $1.6\pm0.1$ & $2.0\pm0.1$ \\
|
| 144 |
|
|
|
| 145 |
|
|
\hline
|
| 146 |
|
|
\end{tabular}
|
| 147 |
|
|
\end{center}
|
| 148 |
|
|
\end{table}
|
| 149 |
|
|
|
| 150 |
|
|
Table~\ref{tab:results} demonstrates the sensitivity of this approach.
|
| 151 |
|
|
We observe comparable yields of the same benchmark points as for the high-\pt\
|
| 152 |
|
|
lepton trigger search, where 35--60\% of the events are common to both
|
| 153 |
|
|
searches for LM0 and LM1.
|
| 154 |
|
|
Either approach would have given an excess in the presence of a signal.
|
