Producing four top quarks at once to explore the unknown

November 8, 2018, ATLAS Experiment
Rate of the four-top-quark production in units of the Standard Model prediction measured by the ATLAS Experiment, for the different analysed final states and their combination. Credit: ATLAS Collaboration/CERN

For several decades, particle physicists have been trying to better understand nature at the smallest distances by colliding particles at the highest energies. While the Standard Model of particle physics has successfully explained most of the results produced by experiments, many phenomena remain baffling. Thus, new particles, forces or more general concepts must exist and – if the history of particle physics is any indication – they could well be revealed at the high-energy frontier.

A promising testbed for such new physics is "four-top- production," an elusive Standard Model process that has not yet been observed experimentally. In this production, two pairs of top quarks – the heaviest-known elementary – are simultaneously created in a collision, thus concentrating a huge amount of energy in a single point. This is so rare that, in the dataset of 30 million top-quark pairs analysed by the ATLAS experiment at CERN for this study, only around 350 collisions are expected to have produced four top quarks.

The ATLAS collaboration just released its latest results on the search for four-top-quark production based on proton-proton collision data collected in 2015 and 2016 at the Large Hadron Collider (LHC). As a top quark decays, it gives rise to "final states" with either three (lighter) quarks or a quark, a neutrino and a charged lepton. Hence, events where four top quarks are produced simultaneously can have very different final state topologies depending on the combination of these decays. ATLAS physicists analysed these topologies individually before combining them for the final result.

All of these final states are characterised by the presence of many highly energetic particles. While this makes the four-top signal signatures easier to distinguish from background processes, it also makes it harder to predict how many background events are misidentified as four-top-quark-production events. ATLAS teams thus implemented sophisticated new analysis techniques to estimate the amount of background in these "busy" environments. In combination with the excellent detector performance, a result with unprecedented sensitivity was achieved, excluding a signal with a production rate larger than 2.1 times the rate predicted by the Standard Model (to be compared with a factor of 11.6 for the previous most sensitive search).

The data analysis resulted in a small, not yet significant four-top signal of 2.8 standard deviations giving an observed upper limit of 5.3 times the Standard Model rate. Could this be a hint or simply a statistical fluctuation? Only an updated result using the larger available dataset and an even smarter analysis can tell.

Explore further: The hunt for leptoquarks is on

More information: ATLAS Collaboration. Search for four-top-quark production in the single-lepton and opposite-sign dilepton final states in proton-proton collisions at 13 TeV with the ATLAS detector: arXiv:1811.02305 [hep-ex].

Related Stories

The hunt for leptoquarks is on

September 19, 2018

Matter is made of elementary particles, and the Standard Model of particle physics states that these particles occur in two families: leptons (such as electrons and neutrinos) and quarks (which make up protons and neutrons). ...

ATLAS experiment studies fragments of the top quark

October 9, 2017

Top quarks in the Large Hadron Collider (LHC) proton-proton collisions are predominantly produced in pairs, with one top quark and one top antiquark. In order to measure the production rates of top quark pairs, the ATLAS ...

Long-sought decay of Higgs boson observed

August 28, 2018

Six years after its discovery, the Higgs boson has at last been observed decaying to fundamental particles known as bottom quarks. The finding, presented today at CERN by the ATLAS and CMS collaborations at the Large Hadron ...

Hunting for dark quarks

August 31, 2018

Quarks are the smallest particles that we know of. In fact, according to the Standard Model of particle physics, which describes all known particles and their interactions, quarks should be infinitely small. If that's not ...

Recommended for you

Topological quantities flow

January 14, 2019

Topology is an emerging field within many scientific disciplines, even leading to a Nobel Physics Prize in 2016. Leiden physicist Marcello Caio and his colleagues have now discovered the existence of topological currents ...


Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.