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9.1. Discussion of Results
Measurements of the production cross section of a top quark pair in association with a W -or Z-boson using 20.3 fb−1 of data collected at √
s = 8 TeV by the ATLAS detector have been presented in this thesis. The combined results, with final states with two or three charged leptons, were obtained in various scenarios. All measurements are consistent with the NLO QCD theoretical calculations. Evidence of botht¯tZ andt¯tW processes was obtained when measuring each process individually, and, most importantly, when measuring both processes simultaneously.
Furthermore, this is the first time, that evidence of thettW¯ production mechanism is obtained.
A summary of the most precise cross section measurements from processes predicted by the SM, performed by ATLAS up to July 2014, can be seen in Figure 9.1. The last two entries on the right side of the plot correspond to the combinedttZ¯ and t¯tW production cross section measurements presented in this thesis. The figure highlights, as well, how rare thet¯tV processes are, compared to, for example, thet¯tproduction; 1 t¯tZ/W event is expected for 1000t¯tevents at√
s= 8 TeV.
9.1.1. Comparison with CMS ttV¯ Measurements
It is interesting to compare the combinedt¯tV production cross section measurements presented in this thesis to the results obtained by CMS at √
s = 8 TeV, presented in Section 2.4.1. In order to understand the differences in sensitivity to t¯tZ and t¯tW between the ATLAS and CMS measurements, a summary of the multi-leptonic channels considered by each experiment is shown in Table 9.1.
The most sensitive channel to thet¯tZproduction is the trilepton 3`Z region, whereas the most sensitive channel tot¯tW production is the 2µSS region. Both experiments include those channels in their combination results. However, ATLAS split the 3`Z region into three fit regions, based on the jet andb-jet multiplicities, in order to separate regions with different S/B; CMS, on the contrary, includes only one of those fit regions in the combination. This aspect, together with the different treatment of the backgrounds, leads to a higher expected significance of the t¯tZ measurement in ATLAS than in CMS, as shown in Table 9.2. Concerning the sensitivity to the t¯tW production, although CMS performs a finer splitting of the 2µSS region and includes all
9. Conclusionst¯tV Results Standard Model Total Production Cross Section Measurements Status: July 2014
ATLAS Preliminary Run 1 √
s= 7, 8 TeV
Figure 9.1.: Summary of several total production cross section measurements from processes predicted by the SM, performed by ATLAS at√
s= 7 and 8 TeV up to July 2014.
The measurements are compared to the corresponding theoretical expectations, which were calculated at NLO or higher order in QCD. The combined measure-ments oft¯tZ andttW¯ production cross sections at √
s= 8 TeV presented in this thesis are shown in the last two bins of the plot [207].
lepton flavours, ATLAS treats the 2µSS and 3`Zveto regions separately in the fit, and performs separate optimisation in each region. As a result, the expected sensitivity tot¯tW is comparable between ATLAS and CMS.
To understand the observed significances shown in Table 9.2, Figure 9.2 and Figure 9.3 high-light the pre-fit normalisation in the trilepton and SS dilepton regions, for ATLAS and CMS, respectively. A slight deficit (excess) is seen in the expected pre-fit event yields with respect to data in the two first bins (last bin) in Figure 9.2, corresponding to the regions sensitive to t¯tZ (t¯tW). This explains the differences between expected and observed significances for the t¯tZ and t¯tW production processes in the ATLAS analysis. On the other hand, the pre-fit event yields in the CMS analysis in the trilepton and SS dilepton channels are similar between data and prediction, and therefore the observed significances are close to the expected ones.
The OS dilepton channel (ATLAS) and the four-lepton channel (CMS) add to the respective combinations an expected sensitivity to thet¯tZprocess of 1.5 and 2.0, respectively. Furthermore, the OS dilepton channel improves slightly the sensitivity tot¯tW in the ATLAS combined result.
Both ATLAS and CMS perform a combination of the channels and measure ttZ¯ , ttW¯ and
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9.1. Discussion of Results
Trilepton Same-sign Four Opposite-sign
dilepton lepton dilepton
Channels 3`Z 3`Zveto 2µSS 4` 2`OSZveto 2`OSZ
Signal t¯tZ t¯tW t¯tW t¯tZ ttZ¯ andt¯tW t¯tZ Main Bkgds tZ, W Z t¯tZ,ttH¯ charge misID (CMS) ZZ t¯t+jets Z+jets
lepton misID Fit regions:
(3j,≥2b) (3j + 2j,≥2b) 1 region: (3j,1b + 2b) (3j,2b)
ATLAS (≥4j,1b) (µµ) in - (4j,1b + 2b) (4j,2b)
(≥4j,≥2b) (≥2j,≥2b) (≥5j,1b + 2b) (≥5j,2b)
(≥4j,≥2b) [included 6 regions: (1b)
CMS in2µSS] (e±e±),(µ±µ±), (2b) -
-(e±µ±) in (≥3j,≥1b)
Table 9.1.: Overview of final state channels used in the ATLAS and CMSt¯tV production cross section measurements at √
s = 8 TeV. The suffices “Z” and “Zveto” refer to the lepton invariant mass requirement,|m``−mZ|<10 GeV and|m``−mZ|>10 GeV, respectively, where in the trilepton case, them`` is the invariant mass of the same-flavour opposite-sign lepton pair. The fit regions are split into jet multiplicity, denoted by “j”, andb-jet multiplicity, denoted by “b”. Regions are exclusive unless given with a “≥”.
Sensitivity trilepton + SS dilepton
t¯tZ t¯tW
Observed σ Expectedσ Observed σ Expectedσ
ATLAS 2.8 3.4 3.0 2.3
CMS 2.3 2.4 1.6 2.0
Table 9.2.: The observed and expected significance for t¯tZ and t¯tW production when per-forming the fit with the trilepton and SS dilepton channels, as defined by ATLAS and CMS. For ATLAS, only the SS dimuon channel is considered [185, 84].
t¯tV production cross sections, performing a profile likelihood fit in each case. The simultaneous measurements for both experiments indicate a small correlation betweent¯tZandt¯tW production cross sections. All combined cross section measurements are statistically limited.
9. Conclusionst¯tV Results
SRZb1j4 SRZb2j3 SRZb2j4 SRWL3 SR2µSS
Data/Pred 0.5
1 1.5
2
Figure 9.2.: Summary of the pre-fit observed and expected numbers of events in the fit regions considered in the trilepton and same-sign dilepton channels in the ATLAS analysis at√
s= 8 TeV. The three first bins correspond to the 3`Z regions, sensitive to the t¯tZ production, whereas the last two bins correspond to the 3`Zveto and 2µSS regions, sensitive to the ttW¯ production. The shaded bands include all pre-fit systematic uncertainties [185].
Figure 9.3.: Pre-fit observed and expected event yields in the (left): trilepton, and (right): SS dilepton channels, split into the lepton flavours, in the CMS analysis at √
s = 8 TeV. While for the trilepton channel, the “Total” corresponds to one fit region, each lepton flavour channel in the SS dilepton region is included in the fit as separate regions [84].
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