Particle Physics at the LHC era
Since the start of its operations, the CERN Large Hadron Collider (LHC) has been central to high-energy physics research, delivering groundbreaking data. By colliding particles at extremely high energies, the LHC allows us to probe matter at the smallest distance scales and study its fundamental constituents. In this sense, particle accelerators act as powerful microscopes: the higher the energy of the probe, the smaller the distances we can explore and the more detailed our understanding of the structure of matter becomes.
The most prominent milestone of the LHC programme so far was the 2012 discovery of the Higgs boson, which completed the Standard Model — the theory describing matter and the fundamental forces acting upon it — and earned François Englert and Peter Higgs the 2013 Nobel Prize in Physics.
Today, LHC experiments continue to deepen our understanding of matter and forces, providing increasingly precise measurements of the Higgs boson and other Standard Model processes, which so far remain in remarkable agreement with theoretical predictions. At the same time, the physics programme at the LHC is far from complete. The upcoming High-Luminosity LHC (HL-LHC) upgrade will deliver roughly five times more data than has been collected so far, significantly extending the sensitivity to rare processes. Moreover, the detailed scrutiny of the existing datasets — in particular those recorded since the start of Run-3 in 2022 — is only now fully beginning. This means that a large discovery potential remains, both in the data already recorded and in the much larger datasets that are still to come.
The quest for new physics
Despite its success in describing particle interactions and properties, the Standard Model leaves many fundamental questions unanswered. It does not explain the hierarchy of particle masses, nor why gravity is so much weaker than the other fundamental forces. In addition, it provides no viable candidate for dark matter, which is required by astrophysical and cosmological observations.
Physics beyond the Standard Model is therefore crucially required for our understanding of nature to advance. Many theories and models have been proposed by the theoretical community in attempts to tackle the Standard Model open questions, and many of these can be probed with the LHC data. The discovery of new phenomena is one of the reasons the LHC was built, and many searches for these have been performed at the experiments. Regardless of the variety and ingenious methodology of these searches, no indication of new physics has emerged yet. Instead, there has been a broad exclusion of theories, and the survivors have had to evolve.
One of the best motivated extensions of the Standard Model, accessible at the LHC, is SUperSYmmetry (SUSY); it makes the hierarchy of mass scales more natural, provides an origin for the dark matter (with the Lightest Supersymmetric Particle, LSP, being a dark matter candidate) and facilitates the unification of the forces.
Despite all these motivations, direct SUSY searches at the LHC have so far not been conclusive, but have imposed tight constraints on the predicted SUSY particle masses, e.g. the SUSY partner of the gluon, the gluino, is excluded up to about 2 TeV in models with a light LSP.
Although SUSY, as well as other theories beyond the Standard Model, remain elusive so far, the open questions of the Standard Model are so pressing that it is certain there is something beyond. With the increasing amount of available LHC data, we are on the right track to find what that is.
We are faced with a unique chance to make a unique discovery at the LHC, and history has taught us it may be unexpected. At the LHC experiments, we are looking forward to it.
New Frontiers in Exploration
In addition to the traditional experiments at the LHC, new experiments are emerging to explore previously inaccessible domains of particle physics. One such experiment is FASER (Forward Search Experiment), which aims to detect light, weakly interacting particles that could hold the key to understanding dark matter and other mysteries of the universe. FASER, positioned 480 meters downstream of the ATLAS interaction point, is uniquely designed to search for particles produced at very small angles to the beamline, which are often missed by the larger LHC detectors. By exploring this new frontier, FASER is offering fresh insights into the nature of matter, potentially unlocking further understanding of dark matter, neutrinos, and other elusive phenomena.
With the ongoing efforts of both existing and new experiments, the LHC is providing us with unprecedented opportunities to push the boundaries of our knowledge and uncover the fundamental nature of the universe. We look forward with anticipation, knowing that the next major discovery could be just around the corner.