| 22 |
|
(i) $t\bar{t} \to \ell $+ jets and (ii) $t\bar{t} \to \ell^+ \ell^-$ where one of the two |
| 23 |
|
leptons is not found by the second-lepton-veto (here the second lepton can be a hadronically |
| 24 |
|
decaying $\tau$). |
| 25 |
< |
For a reasonable $M_T$ cut, say $M_T >$ 150 GeV, the dilepton background is of order 80\% of |
| 25 |
> |
For a reasonable $M_T$ cut, say $M_T >$ 150 GeV, the dilepton background is approximately 80\% of |
| 26 |
|
the total. This is because in dileptons there are two neutrinos from $W$ decay, thus $M_T$ |
| 27 |
|
is not bounded by $M_W$. This is a very important point: while it is true that we are looking in |
| 28 |
|
the tail of $M_T$, the bulk of the background events end up there not because of some exotic |
| 67 |
|
there is a significant contribution to the $M_T$ tail from very off-shell |
| 68 |
|
$W$. |
| 69 |
|
This contribution is much smaller in top events because $M(\ell \nu)$ |
| 70 |
< |
cannot excees $M_{top}-M_b$. |
| 70 |
> |
cannot excees $M_{top}-M_b$. Therefore the large \mt\ tail in |
| 71 |
> |
$t\bar{t}$/single top is dominated by jet resolution effects, |
| 72 |
> |
while for \wjets\ events the large \mt\ tail is dominated by off-shell W production. |
| 73 |
> |
|
| 74 |
> |
|
| 75 |
|
|
| 76 |
|
For $W +$ jets the ability of the Monte Carlo to model this ratio |
| 77 |
|
($R_{wjet}$) is tested in a sample of $\ell +$ jets enriched in |
| 93 |
|
\subsection{Dilepton background} |
| 94 |
|
\label{sec:dil-general} |
| 95 |
|
|
| 96 |
< |
To suppress dilepton backgrounds, we veto events with an isolated track of \pt $>$ 10 GeV. |
| 96 |
> |
To suppress dilepton backgrounds, we veto events with an isolated track of \pt $>$ 10 GeV (see Sec.~\ref{sec:tkveto} for details). |
| 97 |
|
Being the common feature for electron, muon, and one-prong |
| 98 |
|
tau decays, this veto is highly efficient for rejecting |
| 99 |
|
$t\bar{t}$ to dilepton events. The remaining dilepton background can be classified into the following categories: |
| 120 |
|
%Monte Carlo studies indicate that there is no dominant contribution: it is ``a little bit of this, |
| 121 |
|
%and a little bit of that''. |
| 122 |
|
|
| 123 |
< |
The last category includes 3-prong tau decays as well as electrons and muons from W decay that fail the isolation requirement. |
| 124 |
< |
Monte Carlo studies indicate that these three components populate the $M_T$ tail in the proportions of roughly 6\%, 47\%, 47\%. |
| 123 |
> |
The last category includes 1-prong and 3-prong hadronic tau decays, as well as electrons and muons either from direct W decay or via W$\to\tau\to\ell$ decay |
| 124 |
> |
that fail the isolation requirement. |
| 125 |
> |
% HOOBERMAN: commenting out for now |
| 126 |
> |
%Monte Carlo studies indicate that these three components populate the $M_T$ tail in the proportions of roughly 6\%, 47\%, 47\%. |
| 127 |
|
We note that at present we do not attempt to veto 3-prong tau decays as they are only 16\% of the total dilepton background according to the MC. |
| 128 |
|
|
| 129 |
|
The high $M_T$ dilepton backgrounds come from MC, but their rate is normalized to the |
| 156 |
|
|
| 157 |
|
The main instrumental effect is associated with the efficiency of the isolated track veto. |
| 158 |
|
We use tag-and-probe to compare the isolated track veto performance in $Z + 4$ jet data and |
| 159 |
< |
MC, and we extract corrections if necessary. Note that the performance of the isolated track veto |
| 159 |
> |
MC. Note that the performance of the isolated track veto |
| 160 |
|
is not exactly the same on $e/\mu$ and on one prong hadronic tau decays. This is because |
| 161 |
|
the pions from one-prong taus are often accompanied by $\pi^0$'s that can then result in extra |
| 162 |
< |
tracks due to phton conversions. We let the simulation take care of that. |
| 162 |
> |
tracks due to photon conversions. We let the simulation take care of that. |
| 163 |
|
Note that JES uncertainties are effectively ``calibrated away'' by the $N_{jet}$ rescaling described above. |
| 164 |
|
|
| 165 |
|
%Similarly, at the moment |
| 173 |
|
\label{sec:other-general} |
| 174 |
|
Other backgrounds are $tW$, $ttV$, dibosons, tribosons, Drell Yan. |
| 175 |
|
These are small. They are taken from MC with appropriate scale |
| 176 |
< |
factors |
| 171 |
< |
for trigger efficiency, etc. |
| 176 |
> |
factors for trigger efficiency, and reweighting to match the distribution of reconstructed primary vertices in data. |
| 177 |
|
|
| 178 |
|
|
| 179 |
|
\subsection{Future improvements} |
