The Conversation: Probing greatest mysteries of physics

The Large Hadron Collider is the most powerful particle accelorator in the world but the proposed new collider would dwarf it.

The Large Hadron Collider (LHC) at Cern is the most powerful particle accelerator in the world. During its 10 years of operations it has led to remarkable discoveries, including the long sought-after Higgs boson. On January 15, an international team of physicists unveiled the concept design for a new particle accelerator that would dwarf the LHC.

The Future Circular Collider is conceived as a successor to the LHC, and — if given the green light — it would allow physicists to seek answers to some of greatest mysteries in physics. This includes finding out what most of the universe is made of or discovering entirely new physics.

The proposal envisages a new 100km circumference tunnel encircling the city of Geneva and the surrounding countryside. The 27km LHC would feed particles into the the new collider — like a motorway slipway. This would ultimately allow it to collide particles with energies around seven times higher than the LHC can manage, pushing particle physics deep into an unexplored tiny realm.

Portal to a dark world

The Future Circular Collider is really several projects in one. The first phase imagines a machine that collides electrons with their so-called "antimatter versions", positrons. All particles are thought to have an antimatter companion, virtually identical to itself but with opposite charge. When a matter and an antimatter particle meet, they completely annihilate each other, with all their energy converted into new particles.

The collision energy of such a collider could be very precisely controlled. Also, collisions would be very "clean" compared with the LHC, which collides protons — haphazard bags of smaller particles including quarks and gluons. When protons collide, their innards get sprayed all over the place, making it much harder to spot new particles.

The primary goal of the electron-positron collider would be to study the Higgs boson, the particle implicated in the origin of the masses of the other fundamental particles. It would create millions of Higgs bosons and measure their properties in unprecedented detail.

The Higgs could act as portal connecting our world of ordinary atomic matter with a hidden world of particles that are otherwise undetectable. Some 85 per cent of all the matter in the universe is "dark", made up of particles we have never been able to see. We only know it exists because of the gravitational pull it has on surrounding matter. In the second phase, the collider would be replaced by a far more powerful proton-proton collider — reaching collision energies of 100 trillion electron volts. It would be a discovery machine, capable of creating a huge range of new particles physicists suspect may lie beyond the reach of the LHC.

It would almost completely explore the energy range where most forms of dark matter are likely to be found. It would also be able to probe the conditions that existed a trillionth of a second after the Big Bang. This moment in the universe's history is crucial as it is when the Higgs field — an all-pervading energy field that the Higgs boson is a little ripple in — collapsed into its current state, generating the masses of the fundamental particles.

Understanding how the Higgs field acquired its current energy is one of the greatest outstanding problems in physics, as it appears to be unbelievably finely tuned to allow atoms — and thus stars, planets and people — to exist.

I hope this new collider could eventually also help us work out why the universe is made almost entirely from matter and not antimatter.

Hefty price tag

The first phase of the new collider would come online in the 2040s, after the final run of the upgraded LHC. The more powerful proton-proton collider would be installed in the 2050s. Both projects come with a hefty price tag: €9 billion ($14.9b) for the electron-positron machine and €15b for the proton-proton collider.

Senior physicist John Womersley says that beyond the value of fundamental knowledge in its own right, there will be other significant short-term benefits.

"The world wide web, Wi-Fi and superconducting magnets in MRI machines were all developed to meet the needs of fundamental physics."

The challenges are huge.

• Harry Cliff is a particle physicist at the University of Cambridge and part of the LHCb Experiment at Cern.

- The Conversation