Figure 5.8: Invariant mass of all tagged b-jets for different standard model processes with the cuts in table 5.2 applied and a b-tagging efficiency of 60%.
5.7 b-jets
Since our channel for discovering Higgs is the decay into a b-quark pair, the presence of at least two b-jets in the event is important. In figure 5.9 and 5.10 (taken from [1]) the energy resolution and the light jet rejection are shown as a function of energy and pT of the jets, respectively. These plots show that both the energy resolution and the light jet rejection get worse at low E/pT. The energy resolution steadily improves at high energies, whereas the light jet rejection increases with pT, reaches a maximum at pT ∼ 150 GeV and decreases for higher pT values. This should be kept in mind when we pick out the b-jets.
The histograms in figure 5.11 illustrates that the hardest b-jets coming from Higgs most often have pT > 50 GeV, while the second hardest could be softer. This is confirmed by table 5.3, showing the effect of two different pT requirements on the two b-jets. The table 5.3 illustrates what is expected from the histograms in figure 5.11, that we lose much of the b-pairs from Higgs when requiring pT > 50 GeV on both of the b-jets. A better cut will be to require pT > 50 GeV for the hardest b-jet and the second hardest with pT > 20 GeV. The cuts presented in table 5.4 are further used, for which the significance is better for all samples. The invariant mass distributions after the cuts in table 5.4 are
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Figure 5.9: The energy resolution for cone-tower jets with R=0.7 and R=0.4, in two re-gions of |η|, as a function of Etruthjet. The higher the jet energy, the better resolution.
Plot taken from [1]
Figure 5.10: The rejection of light jets (not c-jets) as a function of the transverse momen-tum for two different b-tagging algorithm.
Both are showed with a b-tagging efficiency of 60%. The filled dot is the algorithm which is used in this analysis. Sometimes a require-ment of the maximum pT of the b-jets is also used, since the light jet rejection decreases at high pT. Plot taken from [1]
(1) hardest b-jet (2) second hardest b-jet
Figure 5.11: The pT of the hardest (1) and second hardest (2) b-jet in the different categories of signal after the cuts in table 5.2 are applied.
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Table 5.3: The significance for signal and combinatorial background+SUSY background. This is after requiring at least 2 b-jets with (left): p1T > 50GeV and p2T > 50GeV or (right): p1T > 50 and p2T > 20GeV.
In both tables the cuts in table 5.2 are used together with a b-tagging efficiency of 60%
CUT # VALUE
CUT 1 6ET > 100 GeV CUT 2 2 jets p1,2T > 100, 50 GeV CUT 3 2 b-jets with p1,2T > 50, 20 GeV
Table 5.4: Cuts used in the analysis.
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applied is shown in figure 5.12 and 5.13 for the signal samples and SM background respectively. The plots in 5.12 do not differ considerably from the plots in 5.7, illustrating that CUT 3 does not remove many of the events with b-jets. Although it removes ∼ 80% of the SUSY background events, since they often do not contain b-jets at all.
From the discussion in section 4.4.1-4.4.7 in chapter 4 it is clear that b-jets are often produced
(1) SU 1 (2) SU 2 (3) SU 6
(4) SU 8.2 (5) SU 9
Figure 5.12: Invariant mass of all tagged jets in the different signal scenarios after the cuts in table 5.4 are applied and a b-tagging efficiency of 60%. These plots are very similar to the ones in 5.7, which illustrate that CUT 3 does not remove much of the signal or the other event categories. Although this cut removes a factor of ∼ 80% of the SUSY background, since this background contain many events without b-jets at all (but this will not be clear from these plots, since they only show events with at least two b-jets).
from decays of sbottom or stop in SUSY cascades. These b-jets originate from an early stage in the cascade and have therefor usually higher pT. These b-jets do not come from Higgs, and will contribute to wrong combinations in the invariant mass calculation. Figure 5.14 shows for SU 9 where the different b-jets come from according to their pT. It is clear from this that the hardest (second hardest) b-jet is mostly from other sources than h if there are three (more than three) b-jets in the event. This is also reflected in table 5.5 which shows an increase in the significance when this constraint is applied.
The cuts presented in table 5.6 are further used in the analysis. The invariant mass distribution for the various signals before (figure 5.12) and after (figure 5.15) the cuts in table 5.6 are applied is shown.
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Figure 5.13: Invariant mass of all tagged b-jets for different Standard Model processes with the cuts in table 5.4 applied and a b-tagging efficiency of 60%. For the Standard Model one see a reduction in the number of events compared with 5.8.
(1) event with 3 b-jets (2) event with 4 b-jets
Figure 5.14: Number of b-jets coming from the decay of h compared to that of other b-jets in events with (1): three or (2): four b-jets for the SU 9 signal sample. At the x-axis p1T,p2T,... correspond to the hardest, second hardest,... b-jet in the event. The y-axis shows how many of the hardest, second hardest... b-jet in each event that comes from Higgs and from other sources.
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SU 1 SU 2 SU 6
CUT S B s±25 S B s±25 S B s±25
use all b-jets 250 5116 3.5 20 1191 0.58 130 5525 1.75 omit b-jet #1 (#2) 250 2792 4.7 20 629 0.79 130 2743 2.48
SU 8.2 SU 9
CUT S B s±25 S B s±25
use all b-jets 167 6245 2.11 980 4197 15.1 omit b-jet #1 (#2) 167 4321 2.54 980 2250 20.7
Table 5.5: The significance and the number of signal (S) and background (B) events in a ±25GeV mass window around the Higgs mass for the two different methods. Upper row: Use all b-jets in the event in the Mbbcalculation. Second row: the hardest (two hardest) b-jet(s) are omitted from the invariant mass calculation in case of three (four) b-jets in the event. This is after the cuts in table 5.4 are applied and for only SUSY signal and background.L = 10fb−1.
CUT # VALUE
CUT 1 6ET > 100 GeV
CUT 2 2 jets p1,2T > 100, 50 GeV CUT 3 2 b-jets with p1,2T > 50, 20 CUT 4 omit the hardest (sec.
hardest) b-jet in events with 3 (> 3) b-jets in the Mbbcalculation
Table 5.6: The cuts used further in the analysis.