The final state containing four jets and one charged lepton is not unique to the tt¯pair decay in the lepton + jets channel. Several other physics processes contain a similar signature. This signature can either occur due to four jets and a real lepton or as the result of a misidentification of a lepton. The following processes are separated by either real charged lepton or a misidentified lepton.
2.6.1. Real Charged Lepton
The largest physical background in the t¯t decay to lepton + jets channel is from W + jets events. This is a process which contains a real charged lepton + neutrino and jets. With larger energies, this physics process occurs significantly in the four jet inclusive signature2. The significant contribution is the result of the production of aW through the weak force and at least four jets due to QCD multijet production from the strong force.
In some occasions, though only with a small probability, a gluon can produce a b¯b pair similar to the two b quarks in the t¯t decay3. Therefore b-tagging, the process via which a jet is determined to have come from a b quark, can significantly help reduce this large background, however cannot completely eliminate it. The background process is realized in Figure 2.12.
In addition to W b¯b + jets heavy quark production, it is possible to produce c¯c pairs which are also treated differently from the light quark pairs, since they are also heavy with respect to the light pairs. The final heavy quark production can also be the result of W c + jets. In this scenario, however, one of the quarks from theW must be improperly reconstructed as a lepton. This process is considered, but does not have a significant contribution to the total W + jets background.
W
¯ q q
g
¯b b
¯ νl
l
W
¯ q q
g
¯ νl l
Figure 2.12.: W + jets background processes. (Left): W b¯b+ jets event where a gluon splits into a b¯b and the W decays into a charged lepton and neutrino.
There is no misidentification in this process as it contains the same final state as a single lepton t¯t decay with two b quarks. (Right): W + jets event where a light quark is improperly tagged as a bjet.
The second process which contains a real lepton is in theZ + jets channel. Similar to the W + jets, the Z + jets channel contains a Z produced from the weak interaction of
2Four or more jets in the final state.
3Heavy quark production is denoted byW b¯b+ jets.
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2.6. Four Jet Inclusive Final State
quarks. The extra jets are produced via QCD. In this process, a gluon splitting into a b¯b pair can produce real heavy jets in this signature. This process is denoted by Zb¯b+ jets.
The same is true for Zc¯c+ jets, similar to theW + jets process.
In theZ + jets signature, theZ decays into al¯l pair, in which one of the leptons is not reconstructed. The result is a real lepton, missing energy and jets. TheZ + jets Feynman diagram is found in Figure 2.13.
Z
Figure 2.13.: Z + jets background signature. One of the leptons from theZ →l¯ldecay is not reconstructed. The Z is produced via the weak interaction whereas the addition jets arise via the nature of the strong force.
The third process which may result in a four jet inclusive final state with a real charged lepton comes from single top decay, described in Section 2.4. Single top’s most dominant decay channel at the LHC is the t-channel, where a space-like W and b result in a top quark. Since this final state is top mass dependent, it is taken as a signal contribution for the analysis. This leaves the background mass independent. The three possible decay signatures for single top at the LHC are shown in Figure 2.14. In all three possible scenarios, a real charged lepton and neutrino result from a t → W b decay. Due to the strong force, QCD multijets appear as the other jets in the signature.
W
Figure 2.14.: (Left): t-channel, (middle): Wt associate production channel, and (right):
s-channel single top production along with the decay signature. In all three contributions, a real charged lepton and neutrino appear in the final state. The resulting extra quarks are produced by the strong interaction.
Some of these quarks may be b quarks. This signature is also top mass dependent.
2. Physics
The last background process described is the diboson production. In this case, two bosons, either: W W, W Z, or ZZ are produced simultaneously. The resulting decay produces a real lepton. The W W channel will have a resulting charged lepton and jets similar to the two W bosons decaying in the lepton + jets decay. The same signature is true in theW Z diboson scenario, where theW boson decays leptonically and theZ boson into two quarks. In theZZchannel, two quarks and two charged leptons are produced, one of the leptons is not be reconstructed. The decay signature of the three diboson processes are found in Figure 2.15.
W
Figure 2.15.: Diboson production and decay signature. (Left): W W decay into qq¯pair and a charged lepton and neutrino. (Middle): W Z decay similar to the W W decay signature and (right): ZZ decay where one Z decays into a qq¯pair and the otherZ boson decays into two leptons, where one of the leptons is missed.
2.6.2. Misidentified Lepton
The following set of background events arise from a misidentification of the charged lepton. This misidentification occurs in both the e + jets and µ + jets channels as a result of several different reasons. In the e + jets channel, a jet can be misidentified as an electron. This occurs as a result of the signature in the detector which are similar for jets and electrons. In the µ + jets channel, a bjet may decay semi-leptonically, resulting in a µ. This µ is expected to be reconstructed inside the jet4, in this case however it is reconstructed outside the jet and is therefore taken as a signal lepton from aW decay and not from a bjet.
QCD multijets is a background in the four jet inclusive channel as a result of a misiden-tified lepton. QCD multijets production occurs via the strong force. In this scenario, jets are produced due to the nature of the strong interaction. As a result, in a significant portion of events, the event will only contain jets. Due to misidentifications of the jets, it is possible that a jet is improperly reconstructed as a lepton in both channels. The resulting QCD multijets production will then appear as t¯tbackground. The signature of this process in both channels is found in Figure 2.16. This background is significantly reduced through the identification of a b jet.
4Aµreconstructed within the jet is denoted asnon-isolated; aµoutside of a jet: isolated.
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