Tag jets

Table: The efficiency of requiring tag jets in the forward regions for the signal H $\rightarrow$ W⁺W^- $\rightarrow$ l $\nu$ jj and for the background samples t $\bar{t}$ and jW. Efficiencies are relative to the amount of events left after the identification cuts on the central jets. Effects of pile-up are not taken into account.
E_T cut Efficiency 1 tag jet (%) Efficiency 2 tag jets (%)

(GeV) H t $\bar{t}$ jW H t $\bar{t}$ jW

15 85.1 $\pm$ 0.6 35.8 $\pm$ 0.4 29.0 $\pm$ 0.5 36.4 $\pm$ 0.3 4.0 $\pm$ 0.1 2.4 $\pm$ 0.1

20 81.3 $\pm$ 0.6 30.2 $\pm$ 0.4 23.8 $\pm$ 0.4 31.6 $\pm$ 0.3 2.8 $\pm$ 0.1 1.6 $\pm$ 0.1

30 71.4 $\pm$ 0.6 21.2 $\pm$ 0.3 16.1 $\pm$ 0.3 21.0 $\pm$ 0.3 1.3 $\pm$ 0.1 0.7 $\pm$ 0.1

50 49.9 $\pm$ 0.3 12.1 $\pm$ 0.2 8.8 $\pm$ 0.2 8.5 $\pm$ 0.0 0.4 $\pm$ 0.0 0.2 $\pm$ 0.0

**Table:** The efficiency of requiring tag jets in the forward regions for the signal **H $\rightarrow$ W⁺W^- $\rightarrow$ l $\nu$ jj** and for the background samples **t $\bar{t}$** and jW. Efficiencies are relative to the amount of events left after the identification cuts on the central jets. Effects of pile-up are not taken into account.
E_T cut	Efficiency 1 tag jet (%)			Efficiency 2 tag jets (%)
(GeV)	H	t $\bar{t}$	jW	H	t $\bar{t}$	jW
15	85.1 $\pm$ 0.6	35.8 $\pm$ 0.4	29.0 $\pm$ 0.5	36.4 $\pm$ 0.3	4.0 $\pm$ 0.1	2.4 $\pm$ 0.1
20	81.3 $\pm$ 0.6	30.2 $\pm$ 0.4	23.8 $\pm$ 0.4	31.6 $\pm$ 0.3	2.8 $\pm$ 0.1	1.6 $\pm$ 0.1
30	71.4 $\pm$ 0.6	21.2 $\pm$ 0.3	16.1 $\pm$ 0.3	21.0 $\pm$ 0.3	1.3 $\pm$ 0.1	0.7 $\pm$ 0.1
50	49.9 $\pm$ 0.3	12.1 $\pm$ 0.2	8.8 $\pm$ 0.2	8.5 $\pm$ 0.0	0.4 $\pm$ 0.0	0.2 $\pm$ 0.0

Rejecting background through a requirement of tag jets in the forward region sensitive to the pile-up creating both real and fake jets in the forward regions. With a full GEANT simulation of the Higgs events and pile-up at high luminosity is was in [77] shown that the rate of jets originating from the pile-up with a transverse energy above 30 GeV is below 2% in one hemisphere. This means that a background event with no tag jets has a 4% probability (two hemispheres) to pick up a tag jet from the pile-up while a background event with one tag jet has a 2% probability to pick up a second jet from the pile-up (pile-up jet has to be in the opposite hemisphere of the real tag jet). With the numbers for the t $\bar{t}$ and jW background from table 8.5 with a cut on the transverse energy of 30 GeV it can be seen that this seriously affects the rejection using two tag jets; the t $\bar{t}$ efficiency rise from 1.3% to 1.8% and the jW efficiency rise from 0.7% to 1.1%.

In principle a clean discovery within one year at high luminosity should be possible with the rejection achieved against the reducible background, but the signal to background ratio is a problem. The background cannot be accurately calibrated as the signal is not a clear peak on top of a flat background. With the above numbers an uncertainty in the background level of 30% will be able to fake a discovery. Some calibration can be done by specifically looking at the events not surviving the cut confining the jet mass to the W mass. For the Higgs signal and the t $\bar{t}$ and jW backgrounds the mass spectrums for the central jet with the highest E_T are shown in fig. 8.7. All other central identification cuts and the requirement on two tag jets have been applied. It can be seen that both the t $\bar{t}$ and the JW backgrounds are peaked as the signal around the W mass and hence a calibration looking at the side-bands to the W mass peak will have a limited value.

**Figure:** The mass spectrum of the central jets in events surviving the central identification cuts and have two tag jets. In (a) the Higgs signal and in (b) the **t $\bar{t}$** (crosses) and jW (boxes) backgrounds.