5.2 The light-flavoured Squarks Benchmark Process
5.2.2 Background Processes
The event signature of the signal process with two jets and missing energy is very generic and can be generated by a couple of standard model and SUSY processes as well. In the following a list of these processes and their relevance to the benchmark process will be discussed. The list of the processes along with their respective cross sections can also be seen in Table 5.3.
γ/Z0 e qr e qr e χ0 1 e χ0 1 q q e− e+
Figure 5.2: Feynman diagram for the production and the predominant decay of two right handed scalar quarks at the compact linear collider. Here, eqr denotes first and second generation squarks, but not the stopet and sbottomeb as their mass eigenstates is an admixture of left and right handed squarks.
process cross section σ[fb] σ/σsignal
signal e+e−→qe rqer →qqχe01χe01 (q =udsc) 1.47 1 SM,Emiss e+e−→qqνν ∼1500 ∼1.0·103 e+e−→qqe±ν ∼5300 ∼3.6·103 e+e− →τ−τ+νν ∼130 ∼8.8·102 SM, noEmiss e+e−→qq ∼3000 ∼2.0·103 e+e− →qqe+e− ∼3300 ∼2.2·103 SUSY e+e−→qqννχe0 1χe01 ∼1.0 ∼0.7 e+e− →qq`±ν e χ0 1χe01 ∼8.5 ∼5.8 e+e−→`+`−νν e χ0 1χe01 ∼0.6 ∼0.4
Table 5.3: List of signal and background processes with their corresponding cross section at the compact linear collider at 3 TeV. The signal process only includes right handed squarks of the first and second generation.
Standard Model Processes with Missing Energy
Within the standard model missing energy is created only through neutrinos. The number of processes with neutrinos and an event signature of two jets is limited. Jets are typically created by high energetic quarks. So the processes to look at creates a
q-q pair alongside with at least one neutrino. Due to lepton number conservation the neutrino is created together with another lepton.
A neutrino as second lepton is closest to the event signature in question. This results in the first considered background process: e+e−
→qqνν, where q can be any of the six quarks and ν can be any of the three neutrinos, withq and ν being the corresponding anti particles.
A second choice for the remaining lepton is to be an electron (positron). If this electron is hidden in one of the jets, this process, too, has the required event shape. This results in our second background process e+e−
→ qqeν. This is short hand for the two groups of flavour changing charged current processes e+e−
→ ude+ν e and
e+e− →ude−ν
e, with u standing for all up type quarks (uct), and d representing all
down type quarks (dsb).
The case where the second lepton is a muon or tau are not considered here, as they have a significantly lower cross section and thus have negligible impact. In addition, taus create an electromagnetic jet of many particles and thus will not meet the criterion for escaping the interaction point undetected.
Although most jets are created by strong interacting particles there is an exception: The lifetime of taus is short enough for it to decay in flight, creating a number of particles alongside. There are different decay modes, which include both pure leptonic decays and a mix of lepton and hadronic decays, both of which create a mix of particles resembling a jet. Together with two neutrinos accounting for the missing energy, the third considered background process is e+e−
→τ−τ+νν.
As one can see in the right most column in Table 5.3, the cross section of these processes is significantly above the signal one. Especially the first two processes are three orders of magnitudes higher. This requires a sophisticated and reliable background rejection.
Standard Model Processes without Real Missing Energy
Events with missing energy can be created by processes without real missing energy. This can be due to insensitive detector regions, inefficiencies in the reconstruction or the creation of neutrinos at some point within the event.
The first background process considered here is the pair production of two quarks in a s-channel process e+e− → qq. This will create two high energetic jets with an
1 + cos2θ distributed flight direction. Thus part of the particles may hit the insensitive
detector regions such as the beam pipe and fake missing energy.
The second background process is a scattering process in which two quarks are generated in addition to the electron and the positron: e+e− → qqe+e−. There are
several possible Feynman diagrams generating these final state particles, but the dominating contribution is a t-channel scattering process in which both the electron and the positron will only emit a single photon which is used to generate the pair of quarks. In this process the flight direction of the electron (positron) is not altered significantly and peaks in the forward direction (|cosθ|.1). Consequently the electron (positron) has a non negligible chance of escaping detection through the beampipe, faking missing energy Emiss.
Compared to the signal process the cross sections of these two processes are three orders of magnitude higher (see Table 5.3). So although the probability of generating fake missing energy events is low, one could expect that these two processes interfere with the measurement of the signal process. However, as we will see in subsection 5.3.2 their contribution is negligible and consequently these processes were not considered in the analysis.
SUSY Processes with Missing Energy
In supersymmetry in addition to the standard model neutrino, the neutralino creates missing energy. So, the relevant supersymmetric background processes are similar to the standard model ones discussed before, but with an added pair of neutralinos. The process e+e−
→qqe−e+ however, is excluded here, as the two electrons in addition to
the two jets do not fulfill the required two jets and missing energy event shape. The result are the three processes e+e− → qqννχe0
1χe01, e+e− → qq`±νχe01χe01 and
e+e−
→`+`−νν
e
χ0
1χe01. They all have a final state of six particles, requiring at least six
vertices in the Feynman diagram. Hence, as shown in Table 5.3, their cross section is very small and it is expected that they do not contribute significantly to the background spectrum.
As all these processes have six particles in the final state, the integration and consequently the event generation takes significantly longer than for processes with two or four final state particles. This time was too high for any kind of large scale event generation. Consequently, none of the discussed processes were considered during the analysis as of now, but will be at a later time. However, as the cross section of the processes is low it is expected that they can easily be rejected using the techniques discussed in this thesis.