Australasian Science Magazine

By Nitesh Soni

The Large Hadron Collider has temporarily shut down, but will return stronger than ever.

The lord of the particle accelerator, CERN’s Large Hadron Collider (LHC), went out of particle collision business for almost two years as of late last week. For particle physicists, Valentine’s Day 2013 will be remembered for the successful completion of phase 1 of the LHC’s operations.

But the two-year shutdown won’t involve much relaxing for the tens of thousands of scientists, engineers and technicians involved in one of world’s largest scientific projects. During this period – known as the Long Shutdown 1 (LS1) – the LHC will undergo a significant upgrade and consolidation.

It will be an exciting and intense period. One part of the CERN team will be preparing the LHC to be operational again after its planned upgrade by 2015, while others will be finalising their results on the vast amount of data that has been accumulated in the last three years.

Under the hood

The collider is housed in a 27km ring-shaped tunnel that is approximately 100m underground between the Swiss-French border at the CERN laboratory. It contains a large assembly of different kinds of superconducting magnets, including dipoles, quadrupoles and sextupoles.

These special types of magnets can conduct larger electric currents than ordinary wire and are required to create intense magnetic fields – 8.4 Tesla at the LHC is more than 100,000 times more powerful than Earth’s magnetic field – to focus and accelerate the particles to high energies.

The magnetic fields are connected electronically to a carrying current between different magnets. During the course of the overhaul of the LHC, more than 10,000 of these high current splices will be consolidated.

In the coming days and weeks, the engineers and scientific teams at the LHC will open up 1,695 interconnections of the main magnets' cryostats (the devices used to maintain the LHC temperature at -271.3°C degrees Celsius – colder than the temperature of outer space, at -270.5°C).

The LHC’s tunnel ventilation system will also be replaced. It is worth noting that, before entering into the LHC, protons (or lead ions) undergo several acceleration steps with the combination of linear and other small circular accelerators.

CERN’s Proton Synchrotron (PS) and the Super Proton Synchrotron (SPS) are small accelerators compared to the LHC, and use the technique of accelerating the particles through time-dependant magnetic fields.

During the LS1, renovation work for the accelerator magnetic chains in both of these accelerators has also been planned.

In addition to above, other facilities at the LHC such as the ventilation system at the PS tunnel (629m in circumference) will also be replaced. At the SPS, the instrumentation and control systems cables (approximately 100km in length) will be either removed or replaced.

The particles in the LHC are made to collide at the four different locations and at each of the locations the outcome of the collisions is recorded by the detectors (essentially the giant cameras are taking pictures millions of times a second).

Four locations constitute four different experiments at the LHC, with the strength of the scientific teams ranging from 500 to 3,500 for different experiments.

Two experiments – namely ATLAS and CMS – are general-purpose discovery experiments. The other two – ALICE and LHCb – are for more specific physics studies. These experiments will also undergo maintenance and upgrade to meet the machine’s new environment in 2015.

Australia is part of the ATLAS Collaboration (with around 3,500 scientists from 178 institutes, it’s one of the largest collaborations among all LHC experiments) and play a leading role in the experiment. Australian physicists are very busy in preparing the results with the LHC data in addition to making a core contribution in the planned ATLAS upgrade work during the LS1.

Until December last year, the LHC operated at the energy 8 Trillion electron Volt (TeV). In 2015, when the LHC springs back to life, it will operate at the energy 13 TeV at the start, approximately by a factor of 1.6 higher compared to last proton-proton collisions and then will ramp up to 14 TeV to reach the physics goals.

Major physics breakthroughs

The LHC is a discovery machine and has been designed to shed light on the unanswered questions in science. It is now best known for the discovery of a particle last July, the properties of which are quite similar to a long-sought Higgs boson particle.

With about a 2.5-times-larger data-set compared to last year, researchers are currently trying to pin down the property of this particle.

In addition to this, a tremendous amount of effort is being made to study every possible physics theory emerging from the LHC data.

One of the elegant theories that is like a “falling apple” of particle physics is called the Standard Model (SM). So far, any prediction made by the SM has been tested and verified successfully by the experiments, and any deviation in the experimental results from SM predictions would constitute “new physics”.

In 2013, researchers at the LHC have also collided protons with lead ions, which are absolutely crucial in understanding the behaviour of matter just after the Big Bang.

When the LHC is switched back on for the next phase of operations, scientists are hoping to uncover deep explanations regarding the other mysteries of nature, such as yet-undiscovered super-symmetric partners, the evidence of dark matter candidate and the existence of extra-dimensions of space.

With the existing data-set analysis, researchers have not found any evidence of answers to these questions yet. But with the higher energies, scientists are hoping to get some new physics surprises beyond the Standard Model.

This will be an intense and exciting period.