Top-quark pair cross-section measurement in the lepton+jets channel

First of all it allows a direct comparison with theoretical calculations providing a precision test of the predictions of perturbative QCD. Additionally t t production is an important background in many searches for physics beyond the Standard Model, and new physics may also give rise to additional t t production mechanisms or modifications of the top quark decay channels. Finally, this is one of the first precision measurements implying the reconstruction of final states including jets, electrons (e), muons (µ) and missing transverse energy (E miss T ), and since many models of physics beyond the Standard Model predict events with similar signatures it provides an essential stepping stone toward the identification of new physics.

In the Standard Model (SM) the t t production cross-section in pp collisions is calculated to be 165 +11

-16 pb at a centre of mass energy of √ s =7 TeV assuming a top mass of 172.5 GeV 1 . Top quarks are predicted to decay into a W boson and a b-quark (t → W b) nearly 100% of the time. Depending on the decays of the two W bosons into a pair of quarks (W → q q′ ) or a lepton-neutrino pair (W → ℓν), events with a t t pair can be classified as:

• dilepton: when both W s decay leptonically;

• single-lepton: when one of the W decays leptonically and the second one hadronically;

• all-hadronic: when both W s decay into quarks.

For the analysis reported here single-lepton t t events are selected, considering only events with exactly one electron (e+jets channel) and exactly one muon (µ+jets channel) and without using any b-tagging information. A more detailed description of the analysis can be found in 2 .

Other complementary analyses are performed in ATLAS to extract the t t production crosssection in dilepton 3 and all-hadronic 4 channels as well as in the single-lepton channel making use of the b-tagging information 5 .

To select t t events in the single lepton final state, the following event selections are applied:

• the appropriate single-electron or single-muon trigger has fired;

• the event contains exactly one reconstructed lepton (electron or muon) with p T >20 GeV, matching the corresponding high-level trigger object;

• if an electron is reconstructed, E miss T >35 GeV and mT (W ) >25 GeV are required;

• the event is required to have ≥ 3 jets with p T >25 GeV and |η| <2.5.

Depending on the flavour of the lepton (e or µ) and on the number of reconstructed jets (exactly three or at least four) the events are classified as e+3-jets, µ+3-jets, e+≥4-jets or µ+≥4-jets, giving rise to four statistically independent channels.

The most important backgrounds after the event selections described above are:

• the production of a W boson in association with jets (W +jets),

• the production of QCD multi-jet events in which a fake or non-prompt lepton is reconstructed as a real prompt electron or muon,

• other minor backgrounds including single top electro-weak production, Z+jets and diboson (W W ,W Z and ZZ) events.

The number of events observed in data and predicted by simulation or by data-driven estimates in each of the four channels are given in Table 1.

The different backgrounds are treated in different ways to determine the shape and the normalization of the kinematical distributions used to build the likelihood discriminant to extract the cross-section measurement. For the W +jets background the shapes are taken from Monte Carlo (MC) simulation, while the normalization is extracted from the fit (see Section 5). For the QCD multi-jet background both the shapes and the normalization are extracted with datadriven methods. For the other backgrounds, both the shapes and the normalization are taken from MC simulation. Table 2: List of the main sources of uncertainty affecting the final measurement. For each of the listed sources the relative effect on the measured t t crosssection expressed as relative uncertainty is reported.

The t t production cross-section is extracted by exploiting the different properties of t t events with respect to the dominant W +jets background. Three variables were selected for their discriminant power, for the small correlation between them and by considering the effect of the jet energy scale uncertainty. These variables are:

• the pseudorapidity of the lepton η lepton , which exploits the fact that t t events produce more central leptons than W +jet events;

• the charge of the lepton q lepton , which uses the fact that t t events produce charge-symmetric leptons while W +jet events produce an excess of positively charged leptons;

• the exponential of the aplanarity (exp(8 × A)), b which exploits the fact that t t events are more isotropic than W +jets.

A likelihood discriminant is built from these input variables following the projective likelihood approach defined in the TMVA pa