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4 | Signal and background processes

4.3 SM background processes

mixing between the two stop decay modes (i.e. x “0.5), as this results in the highest contribution of asymmetric decays, but the results for mixing withx“0.25 or x“0.75 are evaluated as well.

4.3 SM background processes

We briefly mention the SM backgrounds relevant for the stop search and some corre-sponding experimental results. Since the theoretical stop production cross-section is on the order of a fewpbor lower (depending on the assumed stop mass), also SM processes with a low cross-section need to be considered.

However, these measurements are not used directly for the stop search, with the exception of the t¯t differential cross-section measurement (see section 4.4.3). Instead, all SM background processes are modelled using simulation and normalised using the theoretical cross-sections shown in table 4.3.

4.3.1 Dominant SM backgrounds: top-antitop andW+jets production W+jets andt¯tevents are the most relevant background processes for the stop search, as they have relatively high production cross-sections and because they share some of the event characteristics of stop pair production events, for example the final state objects (one lepton, four or more jets, and high missing transverse momentum), or traces of the (intermediate) presence of top quarks andW bosons, which can also be found in events with stop decays.

The cross-section for the production of aW boson and additional jets is on the order of 100 nb for pp collisions at ?

s “ 7–8 TeV, sufficiently high to allow a measurement already usingLint“33pb´1 of collision data recorded with ATLAS in 2010 [122]. About one third of the W bosons decay leptonically (32.4%˘0.3% [2]), possibly resulting in significant missing transverse momentum due to the neutrino. Due to the additional jets, the event may satisfy event selection criteria used in a stop search. These jets originate from extra partons that were radiated either before or after the hard process (initial or final state radiation, respectively) or from the underlying event. The majority of additional jets come from gluons or light quarks (u,d,s), but c- and b-quark initiated jets (heavy flavour jets) do occur (approximately5% and 1% of the W+jets events, cf.

table 4.3). The fiducial cross-section for W Ñ`ν ` ě4 jets (` “eor µ; p`T ą20 GeV, pνT ą25 GeV, pjetT ą30 GeV; |η`| ă2.5, |ηjet| ă4.4), has been measured to be20 pb at

?s“7 TeV [123].

Top quarks are assumed to always decay into a bottom quark and aW boson, andtt¯ events are then classified by the decay modes of the twoW bosons. From the leptonicW branching fraction one finds that fully leptonict¯tdecays1occur least often (10.5%˘0.1%), and fully hadronic t¯t decays most often (45.7%˘0.3%). The remaining t¯t decays are referred to assemi-leptonic (43.8%˘0.4%) .

The production cross-section oft¯tevents inpp-collisions at?

s“7TeV was also mea-sured using the 2010 ATLAS dataset [124], with about 3000 selected semi-leptonic events.

The reason this channel was used initially, is that it is almost as frequent as the fully hadronic channel, while the requirement of one lepton rejects the majority of multi-jet events. Meanwhile, the production cross-sectionσt¯thas been measured by ATLAS in all decay channels [125, 126]. It is a function of the centre-of-mass energy of theppcollisions:

σ7 TeVt¯t “ p173˘10q pb (LHC combined result), and σt8 TeV¯t “ p242˘10q pb (ATLAS, [118]). Other important results include differential cross-section measurements (σtt¯ as a function of jet multiplicity or jet momentum [127] or the momentum of the tt¯ sys-tem [128]), or measurements of the top massmt“ p174.5˘2.4qGeV [129]; for the latter,

1Because of their short life-time, 87.11µm, decays involvingτ leptons sometimes need special attention when classifying events; only 35% of the time, aτ decays into an electron or muon (plus the corresponding neutrino and antineutrino required to conserve lepton family numbers). For computing the branching ratios, we do not make this distinction.

the precision has been improved through a combination of LHC and Tevatron results to mt “ p173.34˘0.76q GeV [130]; the simulated t¯t samples used in this thesis have been produced assumingmt“172.5 GeV, a common choice in the existing literature.

For the stop analysis with one lepton final states, the dominant background initially stems from semi-leptonict¯tevents, since their final state objects differ from a stop pair event only through the absence of two neutralinos. Fully hadronic t¯t decays are a neg-ligible background process: besides the absence of a lepton (which will occasionally be imitated by a jet), these events have no invisible particles and therefore no genuineETmiss. Dileptonict¯tevents will be selected if additional jets are present and one of the leptons is not identified or out of acceptance (geometrically or kinematically). This case is referred to as an incomplete dileptonic tt¯event, and it is difficult to identify. Top-pair produc-tion events where one top quark decays leptonically, and the other decays to a τ which then decays hadronically, may have an enhancedEmissT due to additional neutrinos, and therefore also present an important category oft¯tdecays.

4.3.2 Other SM backgrounds

There are several additional processes that may satisfy typical stop event selection cri-teria. Pairs of electroweak bosons (W W,W Z,ZZ) [131] can be radiated from a quark-antiquark pair or through anomalous triple gauge coupling (figure 4.6). An event with two W bosons can be mistaken for a tt¯or stop pair-production event if additional jets are present.

q

¯ q

W/Z

W/Z q

¯ q

W/Z

W/Z q

¯

q W/Z

W/Z

TGC

Figure 4.6: Feynman diagrams contributing to diboson production. The rightmost plot involves a triple gauge coupling (TGC) vertex.

Single top quarks [132] can be produced through the three channels shown in fig-ure 4.7. For pp-collisions at the LHC, t-channel production (exchange of a W boson in the t-channel) is dominant (28.4 pb at ?

s “ 8 TeV), while s-channel production is suppressed (1.82pb) due to the small antiquark contribution to proton PDFs. At leading order,W t-channel production proceeds through a single top-quark being produced from ab-quark radiating a W´ boson. There is an interference betweent¯tand W tdiagrams:

the production of a top-antitop pair and the decay of one of the top quarks also results in a single-top final state (at NLO). This is treated by removingW tdiagrams that also represent top-antitop production at amplitude level (diagram removal scheme). Events with a top quark and an additional jet (t- or s-channel) or an additional W boson (W t

production) may occasionally also pass t¯t requirements, especially when only one b-tag is required; hence they also constitute a relevant background in a stop search.

q

¯ q

W

¯b

t

b

g

W

t t

q

b

q

t W

a) b) c)

Figure 4.7: Feynman diagrams contributing to single-top production: a) the t-channel production is dominant in pp-collisions at the LHC; b) due to its low cross-section, single-top s-channel production has not yet been observed by the LHC experiments; c) tree-level diagram of W tproduction; at NLO, the interference witht¯tproduction needs to be taken into account.

Events withZ boson production [133] are not an important background to a search requiring exactly one lepton in the final state, asZ bosons decay either into two leptons, into two quarks, or invisibly. QCD multi-jet production events are negligible, as they will not satisfy ETmiss requirements and do not have a lepton.

The associated production of a top-antitop pair and an electroweak boson [134] is a relevant background, especiallyt¯t`Z followed by an invisible Z decay (Z Ñνν), since¯ this final state is identical to the considered stop decays. Measurements at?

s“7 TeV constrain the cross-section fort¯t`Zproduction to below0.71pb [135, 136], the theoretical production cross-section is115fb at8TeV, and the branching ratio for invisibleZ decays is p20.00˘0.06q% [2], making this process a relevant background at high stop masses (σ˜t1˜t˚1 “7fb for m`

˜t1

˘“700GeV).

The theoretical cross-section oft¯twith associated Higgs-boson production inpp colli-sions at?

s“7TeV is on the order of100fb [137]. Invisible Higgs decay modes can safely be neglected for a SM Higgs boson, but might pose a sizeable background otherwise.