5.3 Comparison between k T and Cone algorithm
6.1.1 Choice of the subprocess
ALPGEN provides the generation of processes with N jets in the final state with up to two light quark pairs [47]. This operation was used for simulating the 4-vectors of the QCD multijet background events needed for this thesis. The shower evolution and jet reconstruction for these events was carried out as an independent step in PYTHIA [13]
(Section 6.2) succeeding the generation of unweighted parton-level events [47]. The available hard 2
2 processes for the N Jets code, to which up to four partons are added, are listed in Table 6.1. The other processes available in ALPGEN are described in [47]. process gg gg q ¯q gg gq qg qg qg gg q ¯q process qq qq qq qq q ¯q q ¯q q ¯q q ¯q q ¯q q q¯
Table 6.1: Hard 2 2 processes available in the ALPGEN N Jets option. Up to N 2 4
final state partons are added to these light quark pairs.
In the initial and final state only the quark types u d c and s are considered [47], thus, no b-quarks are included in the QCD multijet background events. However, b- quarks constitute a significant feature of the QCD events. For this reason b-quarks were additionally included in the 4-jet, 5-jet and 6-jet events1 by replacing a q ¯q pair
with a b ¯b pair in the ALPGEN 4-vectors while keeping the original kinematics. This was only done if the q ¯q pair did not originate from the proton structure. Thus, only the u-, d-, s-, c-quark pairs which were additionally added to the light quark pairs from
the hard 2
2 reaction have been transformed into b-quark pairs. The b-quark events were added to the original background 4-vectors. This procedure is possible as QCD is invariant in terms of the quark flavour and electric charge.
[GeV] T,jets p 0 50 100 150 200 250 300 350 400 entries -1 10 1 10 2 10 3 10 own b-quarks ALPGEN b-quarks jets T # k 0 2 4 6 8 10 12 14 16 18 20 entries -1 10 1 10 2 10 own b-quarks ALPGEN b-quarks
Figure 6.1: Comparison between b-quark events generated by replacing light quark pairs in
existing QCD multijet samples (”own” b-quarks, illustrated in green) and b-quark events gen- erated with the ALPGEN Q ¯Q 4 jets mode (red). The distributions are normalized to the same
cross section. Left: Transverse momentum of the jets in the b-quark events. Right: Number of reconstructed kTjets.
The Q ¯Q 4 jets mode of ALPGEN, which is also available in version 2.03, effectively provides the same kind of b-quark events apart from the fraction of b-quarks stem- 1The fraction of b-quarks in the 3-jet QCD samples has not been considered, as the b-quark density in the proton is tiny.
6.1. ALPGEN 41
ming from the heavy flavour structure of the proton or from the gg
b ¯b processes. A
comparison between the pT distributions of the b-quarks produced with the ALPGEN
Q ¯Q 4 jets mode and b-quarks generated by replacing the light quark pairs – which did not originate from the structure of the proton – as described above is shown in Figure 6.1. There is a difference between the two distributions regarding small and especially very high jet-pT.
The pT distribution for own b-quark events divided by the distribution for the ALP-
GEN Q ¯Q 4 jets mode b-quark events is illustrated in Figure 6.2. As the replacement of the light quark pairs by b ¯b pairs produces more soft b-quarks than the ALPGEN
Q ¯Q 4 jets mode, there is a difference at small transverse momenta of the jets.
[GeV] T,jets p 0 50 100 150 200 250 300 350 400 ratio 0 0.5 1 1.5 2
Figure 6.2: pTdistribution for own b-quark events divided by the distribution for the ALPGEN
Q ¯Q 4 jets mode b-quark events for estimating the differences between the different b-quark
samples.
The number of reconstructed jets for the two b-quark samples is presented in Figure 6.1 right and shows a slight tendency to a larger number of jets in maximum for the own b-quarks.
The comparison shows that our sample of b-quarks approximately represents – apart from the cross section normalization – the b ¯b+4 jets sample of ALPGEN. Therefore, it can be used to estimate the background contributions due to b-quarks. Thus, for statistics and time reasons the previous described method of generating b-quarks from the existing ALPGEN QCD multijet 4-vectors has been used for the estimation of the
b-quarks in the QCD events.
An additional ALPGEN setting is the choice of one of the three different factorization renormalization scales2Q20available for the N Jets code, which are shown in Table 6.2 and picked up again in Table 6.3. For the generation of QCD multijet events iqopt=1 was chosen, since the∑jetsp2T is the only quantity which is experimentally accessible.
iqopt 0 1 9
Q20 1 ∑jetsp2T ˆs
Table 6.2: Choice of factorization and renormalization scale for the N Jets code. The switch iqopt can either be set to 0, 1 or 9 corresponding to a specific value of Q2
0, respectively. ˆs is the
centre-of-mass energy of the hard interaction.