Top Physics

Ever since its discovery at the Tevatron collider in 1995, the top quark has been a major research target for particle physicists. Due to its unique properties, in particular its mass (it is by far the heaviest quark known to-date), it serves as an essential testing ground of the Standard Model (SM) of particle physics and plays a key role in the measurement of Higgs boson properties. Moreover, many BSM scenarios expect the top quark to couple to as yet undiscovered particles whose existence might be revealed through measurements of top quark production.

Due to the high center-of-mass energy of the LHC it proved to be a true top quark factory. The large data samples collected by the ATLAS experiment have provided a means to measure the top quark production process with sufficient precision to challenge the accuracy of the state-of-the-art SM theoretical predictions with the LHC Run1 results.

Due in part to the high energy of the LHC, half of the top quark pair events are produced with additional high-momentum jets, which usually arise from radiation processes calculable in high-order perturbative QCD. These additional jets cannot be unambiguously associated with the top or initial-state quark from which they originate. This leads to measurement uncertainties that often limit the measurement precision of many quantities of interest, for example, of the top quark mass. Moreover, events with top quark pairs and additional jets give the largest contribution to the background plaguing the measurement of ttH. While predictions of the inclusive top pair production cross sections have reached very high precision, the prediction of additional QCD radiation is difficult and significantly less precise. Several theoretical approaches exist that show significant differences in their predictions of the QCD radiation. The ATLAS DESY group has measured the additional jet production in top events at 7 TeV[1] and at 13 TeV [2] as shown in this plot.

 

The largest background for ttH are events where one of the hard radiated gluons splits into a b-quark pair. These processes are very difficult to calculate due to the rather high b-quark mass. A direct measurement of this process is therefore important and the DESY ATLAS group performed it in the di-lepton decay channel using LHC Run 1 data [3] as shown in the figure.

 

With the high rate expected in LHC Run2, the measurement of the very rare process a Higgs produced in association with a top quark pair (ttH) will be possible. This process is of particular interest since it allows for the direct determination of the strength of the coupling of the Higgs boson to the top quark. Since the SM confidently predicts that the Higgs boson couples much more strongly to the top quark than to the other, lighter quarks, the measurement is a sensitive indicator of whether or not the boson discovered at the LHC is indeed the Higgs boson predicted by the SM or something even more interesting. As indicated in the figure, this process could not be measured precisely in LHC Run1 due to its very low cross section and high background of top pairs produced in association with a b-quark pair. However, it is on the top of the attention for LHC Run2 and the DESY ATLAS group contributes to this effort.↵



[1] "Measurement of jets produced in top quark events using the di-lepton final state with 2 bb-tagged jets in p p collisions at s√s = 13 TeV with the ATLAS detector" , ATLAS Collaboration, ATLAS-CONF-2015-065
[2]"Measurement of the top--anti-top production cross-section as a function of jet multiplicity and jet transverse momentum produced in 7 TeV proton--proton collisions with the ATLAS detector", ATLAS Collaboration, JHEP01(2015)020.
[3] "Measurements of fiducial cross-sections for tt¯ production with one or two additional b-jets in pp collisions at s√=8 TeV using the ATLAS detector", ATLAS Collaboration, Eur. Phys. J. C (2016) 76:11.


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