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Figure 1: Event display of one tZ signal candidate event collected in 2016. This event contains one electron, two muons and two jets. Muons tracks are shown in red, while the energy clusters in the calorimeters, used to reconstruct electrons and jets, are shown in yellow. (Image: ATLAS Collaboration/CERN)

Observing rare productions of heavy elementary particles can provide fresh insight into the Standard Model of particle physics. In a new result, the ATLAS Experiment presents strong evidence for the production of a single top quark in association with a Z boson.

The top quark, in its many combinations

The high proton–proton collision energy and rate of the Large Hadron Collider (LHC) makes it possible to produce the heaviest elementary particles, such as the top quark — which is about 175 times heavier than a proton — in many different combinations. The production of a single top quark through the weak interaction was observed already in the second year of LHC data-taking. A few years later, the production of a W boson in association with a top quark (tW) was observed. The next in line, the production of a Z boson in association with a top quark (tZ), is much harder to observe as its production rate is about one tenth that of tW.

Because this process is so rare, separating it from other sources (background events) is very hard. Finding the most promising event topology, that is which decays of the top quark and Z boson to consider, for the search is a challenge in itself. We first considered events with one, two or three leptons before deciding that the three-lepton topology was the most promising for a first observation.

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Figure 2: ANN output (ONN) distribution for the events selected by the analysis. Data is shown in black, the simulated signal is shown in pink and backgrounds are shown in other colours. The high part of the ONN spectrum is dominated by signal events. (Image: ATLAS Collaboration/CERN)

The events we are looking for contain leptons (electrons or muons), jets (sprays of hadrons) and transverse momentum imbalance. A key input to the analysis is to combine and condense all the information from the measured particles into one “discriminator” trained to separate signal from background events. The output of this discriminator tells us for a data event whether it is more background or signal-like.

This is done using an artificial neural network (ANN). We teach the ANN what signal and background events look like using simulated data. It is thus necessary to carefully check that the simulated events match well with the actual data. This is achieved with the use of validation regions defined to be similar to but not equal to the signal region, which allow us to test the simulation while not exposing the signal.

Finding the needle in the haystack

The latest preliminary ATLAS result looks at data collected 2015 and 2016. During this time, roughly 700 million collisions per second occurred at a proton–proton centre-of-mass energy of 13 TeV… very much looking for a needle in a haystack! The result after the event selection finds 25 signal events, where a Z boson was produced in association with a top quark, together with 125 background events. Applying the ANN allowed us to further separate these two categories leading to a significance for signal of 4.2 standard deviations. This constitutes strong evidence that the associated production of a single top quark and a Z boson has been seen, and the observed production rate agrees with that predicted by the Standard Model.

With the additional data to be collected over the next years, ATLAS will be able to study tZ production in more detail, and improve its searches for the even rarer and more elusive production of a top quark in association with a (single) Higgs boson.