The discovery of a Higgs Boson in 2012 by the ATLAS and CMS experiments marked a key milestone in the history of particle physics. It confirmed a long-standing prediction of the Standard Model, the theory that underlines our present understanding of elementary particles and their interactions.
The ATLAS experiment has released results confirming that the Higgs boson has spin 0 (it is a so-called “scalar”) and positive parity as predicted by the Standard Model, making it the only elementary scalar particle to be observed in nature.
In proton-proton collisions, several processes can lead to the production of a Higgs boson. The most “frequent” process (which is about one collision in four billion!) is the fusion of two gluons, contained in the initial protons, into a Higgs boson through a “top-quark loop”. Least frequent is a mode where the Higgs boson is produced in association with a pair of top-quarks.
The search continues for dark matter, a new kind of matter that doesn’t emit or absorb light. It is assumed to account for the missing amount of mass in our Universe. The total mass in our Universe can be inferred from the observation of gravitational effects of stars in galaxies, and galaxies in clusters of galaxies. However the amount of mass calculated from the observed distribution of light is much less. It is proposed that dark matter makes up the discrepancy as it does not emit light.
If all the experimental evidence supports a theory, why should anyone want to dream up additional particles? Yet exactly this situation arose in the late 1960s. At that time, when the complete table of the known hadrons could be explained with just three quarks, theorists were already proposing a fourth, which they whimsically called “charm”.
The Large Hadron Collider is known to collide protons, but for one month a year, beams of lead ions are circulated in the 27-km tunnel and made to collide in the centre of the experiments. The ATLAS experiment has made new precise measurements of the suppression of jets as they blast through the dense matter created by the lead ion collisions.
The Standard Model makes many different predictions regarding the production and decay properties of the Higgs boson, most of which can be tested at the Large Hadron Collider (LHC). Since the discovery, experimentalists from the ATLAS collaboration have analysed the complete dataset recorded in 2011 and 2012, have improved the calibration of the detector, and have increased substantially the sensitivity of their analyses.
Theories, such as supersymmetry, propose the existence of new types of particles to explain important questions about the universe, such as the nature of dark matter. ATLAS has performed a search for one such type – exotic heavy particles that have lifetimes long enough that they travel partway through the detector before decaying, at what is called a displaced vertex.
The Standard Model of particle physics has been extremely successful in predicting a vast variety of phenomena – so successful, that it is easy to forget that some of its predictions have not yet been verified. A very important one, related intimately to electroweak symmetry breaking, is that the gauge bosons (γ, W and Z) can interact with each other through quartic interactions.
The fusion of two weak bosons is an important process that can be used to probe the electroweak sector of the Standard Model. Measurements of Higgs production via weak-boson fusion are crucial for precise extraction of the Higgs-boson couplings and have the potential to help pin down the charge conjugation and parity of the Higgs boson. A similar process, weak-boson scattering, is sensitive to alternative electroweak symmetry-breaking models and to anomalous weak-boson gauge couplings. These processes are extremely rare and the experimental observation of the production of heavy bosons via weak-boson fusion has become possible only recently with the large centre-of-mass energy and luminosity provided by the LHC. Extracting the signals from the huge backgrounds in the high pile-up conditions at the LHC is a major challenge.