Large Hadron Collider is back to change our understanding of the universe ... again

The Large Hadron Collider (LHC) has just begun smashing particles together at higher energies than ever before. This marks the start of the second run of the world’s largest physics experiment, the huge particle accelerator that sits beneath the Alps and in 2012 was used to prove the existence of the Higgs boson.

Now, after more than two years' work upgrading the accelerator systems and the particle detectors (and more years of preparation before that), the team at research group CERN are ready to start using the LHC to answer more questions about how the universe works.

The goal is to explain the missing pieces in our understanding of fundamental physics. One example is the nature of the so-called dark matter that scientists say we can’t see directly but that dominates the universe. Another is the imbalance between matter and antimatter in the present-day universe. Our current theories suggest there would have been almost exactly equal amounts of matter and antimatter in the early universe. But somehow the antimatter decayed, allowing the universe that we know made entirely of matter to emerge.

Physicists have proposed a range of theories, such as “supersymmetry”, to answer these questions and that also predict the existence of new particles and subtle changes to the behaviour of known particles. By colliding particles at energies measured at 13 teraelectronvolts, researchers may also find evidence of the hidden extra dimensions that feature in many theories. Or it could show that the Higgs boson, the particle associated with giving mass to the other particles that make up matter, is one of a whole family of related particles.

Energy levels upM Brice/CERN

The significance of almost doubling the energy at which particles are fired around the LHC is that the resulting collisions should produce new particles that were inaccessible before. Rarer processes should also become more frequent and so easier to distinguish from the approximately 600m “ordinary” collisions that occur in each experiment each second. And the rate at which Higgs bosons are produced should increase, allowing researchers to determine their true nature.

There are several different experiments scheduled for the higher-energy LHC. My team at the University of Lancaster is part of the ATLAS experiment and we will be looking studying how the Higgs boson decays into a particle called the tau, a heavier version of the electron. We will be seeing if the decay exhibits what is called CP violation, a process that distinguishes between matter and antimatter and might help explain the matter-antimatter imbalance.

The improvements to the ATLAS detector for measuring the paths of the particles produced by collisions and the points where they decay mean we in Lancaster will be able to make really precise measurements of CP violation and particle lifetimes in more conventional particles. The extremely large samples of the relevant decays will also contribute to the high precision required to see the influence of any new physics effects such as supersymmetry.

Smashing jobCMS/CERN

We will also be looking for other new particles, particularly those that decay into two “jets” of ordinary particles. This is really important for understanding how often you get double collisions between the particles inside the protons. The energy signature from these double collisions can mimic some of the effects predicted by new theories. So we need to understand the collisions before we can claim them as evidence for those theories.

The two year period during which the LHC was offline was an intensely busy time for the accelerator and detector teams. But the work will now intensify at major analysis centres such as Lancaster to extract the relevant results from the large volumes of data the LHC is producing. For the young physicists doing their PhD studies or in their first research positions and the older hands directing them, this is the most exciting time when the work all comes together.

What will be found is unknown – and an unexpected finding could transform our whole programme of work. Whatever nature reveals, it will be interesting and potentially could profoundly change our view of the fundamental workings of the universe.

Roger Jones receives funding from STFC.