For several decades, particle physicists having 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 that have arisen from 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.

The study of hadrons – particles that combine together quarks to form mesons or baryons – is a vital part of the ATLAS physics programme. Their analysis has not only perfected our understanding of the Standard Model, it has also provided excellent opportunities for discovery. On 20 September 2018, at the International Workshop on the CKM Unitarity Triangle (CKM 2018), ATLAS revealed the most stringent experimental constraint of the very rare decay of the B0 meson into two muons (μ). 

The Brout-Englert-Higgs (BEH) mechanism is at the core of the Standard Model, the theory that describes the fundamental constituents of matter and their interactions. It introduces a new field, the Higgs field, through which the weak bosons (W and Z) become massive while the photon remains massless. The excitation of this field is a physical particle, the Higgs boson, which was discovered by the ATLAS and CMS collaborations in 2012.

While the Standard Model has proven tremendously successful, much experimental evidence points to it not being a complete description of our universe. The search for “new physics” is therefore an important component of the ATLAS experimental programme, where a number of analyses are looking for signs of new heavy particles decaying to different final states. Though these searches have not yet found a significant signal, they have allowed physicists to place stringent constraints on different new physics scenarios. These can be further tightened by combining different analysis channels and approaches.

While the discovery of the Higgs boson at the Large Hadron Collider (LHC) in 2012 confirmed many Standard Model predictions, it has raised as many questions as it has answered. For example, interactions at the quantum level between the Higgs boson and the top quark ought to lead to a huge Higgs boson mass, possibly as large as the Planck mass (>10¹⁸ GeV). So why is it only 125 GeV? Is there a mechanism at play to cancel these large quantum corrections caused by the top quark (t)? Finding a way to explain the lightness of the Higgs boson is one of the top (no pun intended) questions in particle physics.

Today, at the 2018 International Conference on High Energy Physics in Seoul, the ATLAS experiment reported a preliminary result establishing the observation of the Higgs boson decaying into pairs of b quarks, furthermore at a rate consistent with the Standard Model prediction. 

Higgs boson couplings manifest themselves in the rate of production of the Higgs boson at the LHC, and its decay branching ratios into various final states. These rates have been precisely measured by the ATLAS experiment, using up to 80 fb^–1 of data collected at a proton-proton collision energy of 13 TeV from 2015 to 2017. Measurements were performed in all of the main decay channels of the Higgs boson: to pairs of photons, W and Z bosons, bottom quarks, taus, and muons. The overall production rate of the Higgs boson was measured to be in agreement with Standard Model predictions, with an uncertainty of 8%. The uncertainty is reduced from 11% in the previous combined measurements released last year.

The top quark is a unique particle due to its phenomenally high mass. It decays in less than 10-24 seconds, that is, before it had time to interact with any other particles. Therefore many of its quantum numbers, such as its spin, are transferred to its decay particles. When created in matter-antimatter pairs, the spins of the top quark and the antitop quark are expected to be correlated to some degree.

Two among the rarest processes probed so far at the LHC, the scattering between W and Z bosons emitted by quarks in proton-proton collisions, have been established by the ATLAS experiment at CERN.

A decisive property of the Higgs boson is its affinity to mass. The heavier a particle is, the stronger the Higgs boson will couple to it. While physicists have firmly established this property for heavy W and Z bosons (force carriers), more data are needed to measure the Higgs boson coupling to the heavy fermions (matter particles). These interactions, known as Yukawa couplings, are very interesting as they proceed through a quite different mechanism than the coupling to force-carrying bosons in the Standard Model.