Analysis of b−tagged Sample

6.2 Analysis of b−tagged Sample

The b−tagged sample is defined as the preselected muon+tau+jets sample with an additional requirement of the presence of one or more jets which are identified as a b−jet. In data events, we apply the b−tagging algorithm into every jet in the event. For each jet, the algorithm returns a binary value whether the jet is a b−jet or not. In Monte Carlo events, we first assign a probability to each jet in the event to be a b−jet. We then compute the probability for at least one jet in the

Table 6.1: Efficiency of muon+tau+jets selection cuts on t¯ t → dilepton and t¯t → µτ h b¯b generated by ALPGEN Monte Carlo.

t¯ t → µτ h b¯b At least one jet with p T > 20 GeV

t¯ t → dilepton inclusive

0.9909 ± 0.0012 Leading jet p T > 30 GeV

0.9753 ± 0.0012 No tight electron in central calorimeter

0.9737 ± 0.0012 One or more loosely isolated muons with p T > 20 GeV

0.4392 ± 0.0015 Exactly one tightly isolated muon with p T > 20 GeV

0.4237 ± 0.0015 Veto second tightly isolated muon with p T > 15 GeV

0.4237 ± 0.0015 Veto on Z−window

0.4232 ± 0.0015 Primary vertex (|z PV | < 60 cm, number of tracks ≥ 3

0.4177 ± 0.0015 Muon comes from primary vertex, |∆z(µ, P V )| < 1 cm

0.4172 ± 0.0015 MET cut, 15 < E/ T < 200 GeV

0.3962 ± 0.0015 Trigger Efficiency

0.3255 ± 0.0012 Muon ID correction factors

0.2961 ± 0.0012 Loose tau (without NN cut)

0.2673 ± 0.0011 Tight tau (NN cut)

0.0257 ± 0.0002 0.0931 ± 0.00076 Opposite sign muon-tau pair

0.0227 ± 0.0002 0.0868 ± 0.00074 Number of jets not matched to the tau ≥ 1

0.0225 ± 0.0002 0.0861 ± 0.00074 Leading jet p T > 30 GeV

0.0222 ± 0.0002 0.0846 ± 0.00073 Two or more jets not matched to the tau

0.0175 ± 0.0001 0.0663 ± 0.00065 At least one b−tagged jet

event to be a b−jet. The probability can be

P (at least one tag) = 1.0 − P (zero tag)

all jets

= 1.0 − Y P

j (not tagged)

Multijet background contributions in the tagged sample are estimated using the same method discussed in Section 5.5.1, with the addition of the b−jet requirement. Table 6.4 lists the yield

of W +jets and t¯ t lepton+jets events in the same-sign (SS) data events, as well as the observed number of events in SS data. Nine events are found in the sS data sample.

Finally, in Table 6.5 we list the expected yields from the signal and background processes in the tagged sample. We find that the multijet process is the dominant background in the tagged sample, both in terms of magnitude and error. The principal source of error is the small size of the same-sign sample. As expected, the requirement of at least one b−jet in the sample reduced the contributions from W + N lp and Z + N lp significantly. The final selected, b−tagged sample now

Table 6.2: Efficiency of muon+tau+jets selection cuts on t¯ t → lepton+jets signal generated by ALPGEN Monte Carlo.

Cumulative efficiency At least one jet with p T > 20 GeV

Selection criteria

0.9958 ± 0.0005 Leading jet p T > 30 GeV

0.9943 ± 0.0005 No tight electron in central calorimeter

0.8162 ± 0.0006 One or more loosely isolated muons with p T > 20 GeV

0.1705 ± 0.0005 Exactly one tightly isolated muon with p T > 20 GeV

0.1619 ± 0.0005 Veto second tightly isolated muon with p T > 15 GeV

0.1619 ± 0.0005 Veto on Z−window

0.1618 ± 0.0005 Primary vertex (|z PV | < 60 cm, number of tracks ≥ 3

0.1595 ± 0.0005 Muon comes from primary vertex, |∆z(µ, P V )| < 1 cm

0.1594 ± 0.0005 MET cut, 15 < E/ T < 200 GeV

0.1498 ± 0.0005 Trigger Efficiency

0.1300 ± 0.0004 Muon ID correction factors

0.1183 ± 0.0004 Loose tau (without NN cut)

0.1116 ± 0.0004 Tight tau (NN cut)

0.00836 ± 0.00011 Opposite sign muon-tau pair

0.00449 ± 0.00008 Number of jets not matched to the tau ≥ 1

0.00448 ± 0.00008 Leading jet p T > 30 GeV

0.00448 ± 0.00008 Two or more jets not matched to the tau

0.00414 ± 0.00008 At least one b−tagged jet

Table 6.3: Efficiencies of the muon+tau+jets selection for t¯ t dilepton and t¯ tlepton+jets sample, and the total efficiencies for t¯ t → inclusive sample.

Processes

ǫ × BR t¯ t → dilepton

Efficiency ǫ Branching ratio BR

0.001304 ± 0.000015 t¯ t → lepton+jets 0.00274 ± 0.0001

0.001247 ± 0.000027 t¯ t → inclusive

0.002551 ± 0.000031 is dominated by top quark events.

Figure 6.1 shows some distributions in the tagged sample. Due to the poor statistics, we chose to check kinematics of individual physics object and variables which describes t¯ t events, such as H T (the sum of transverse energy of all physics object in the event). Appendix A has a more exhaustive set of distributions in the b−tagged sample.

Table 6.4: Observed same-sign (SS) events in data at the tagged level, and the expected number of W +jets and t¯ t lepton+jets events to be subtracted from SS data to get estimation of multijet background in the opposite sign (OS) sample. Notice the large statistical error on the SS data sample due to small statistics.

All type SS data

9 ± 3.00 SS t¯ t → dilepton

−0.16 ± 0.01 SS t¯ t → lepton+jets

−3.09 ± 0.07 SS W b¯b

−0.48 ± 0.07 SS W N lp

SS W c¯ c −0.02 ± 0.01