Background estimation

The W boson, in the electron-neutrino decay channel, is identified by the presence of a well isolated and high energy electron plus a large amount of missing transverse energy. Although this signature is particularly clean, some fake events can pass the selection and contribute as background to W events. The processes which emulate the W signal can be classified into two categories: QCD and W-like events. The latter is represented by electroweak bosons and top production which manifest themselves as real electrons or missing $E_T$ in the final state. The former is mainly coming from dijet production, where a jet escapes through a crack in the calorimeter and another fakes an electron leaving a track in the COT associated to an electromagnetic energy deposit. There is another kind of background which is related to the jet counting. It might happen that, due to extra interactions in the same bunch crossing that produces the $W$ event, one or more jets in an event are not related to the hard process studied in this analysis. This effect results in an event migration between different W + jet multiplicity samples.

QCD is the largest source of background to W + n jets and the most important one, because it contributes differently to each jet multiplicity sample.

The other sources, except top background in the higher jet multiplicity events, are never larger than a few percent of the total W candidate sample. They also have a very similar jet structure so they don't distort the shape of kinematic distribution. We could imagine that the decay of the electroweak boson factorizes from the whole event leaving the QCD production of jets unchanged.

Table 3: The table shows all the background contributions as absolute number of events. In the last line is also reported the total background fraction normalized to the signal. (.ps)

Sample	$\ge$ 0 jet	$\ge$ 1 jet	$\ge$ 2 jet	$\ge$ 3 jet	$\ge$ 4 jet
Top	$42^{+13}_{-10}$	$42^{+13}_{-10}$	$41^{+13}_{-10}$	$35^{+11}_{-9}$	$24^{+8}_{-7}$
QCD	794$\pm$238	398$\pm$119	150$\pm$50	64$\pm$ 19	20$\pm$6
$W\to \tau \nu$	918$\pm$30	182$\pm$13	64$\pm$8	14 $\pm$3.7	3.1 $\pm$1.7
$Z\to e^+e^-$	136$\pm$11	20$\pm$4.4	6.9$\pm$2.6	1.7$\pm$1.3	0.5$\pm$0.7
Promotion	$-$	298$\pm$149	83$\pm$41	22$\pm$11	5.0$\pm$2.5
Total	$1869^{+232}_{-232}$	$951^{+193}_{-192}$	$349^{+63}_{-62}$	$134^{+24}_{-23}$	$52^{+10}_{-9}$
Fraction	0.034	0.082	0.13	0.23	0.38

Figure 3: The plot shows the relative fraction of backgrounds events compared to signal, highlighting with different colours the contribution to the total background fraction coming from each source.