Update 25.03.21015: CERN announced that the LHC restart was postponed due to a short circuit. Current indications suggest a delay of between a few days and several weeks. Further details are available here.
World’s largest and most powerful particle accelerator, CERN’s Large Hadron Collider (LHC), is gradually restarting after two years of maintenance and upgrades costing €124m. LHC is now ready to explore new realms of particle physics over the next three years at a collision energy of 13 TeV[1], a significant increase over the initial three-year LHC run, which began with a collision energy of 7 TeV, rising to 8 TeV. The first circulating beams of protons in the LHC are planned for the week beginning 23 March, and first 13 TeV collisions are expected in late May to early June.
In early 2013, after three years of running, the Large Hadron Collider (LHC) shut down and hundreds of engineers and technicians spent about two years preparing the accelerator to run at 13 TeV collision energy, nearly double the collision energy of the LHC’s first run. At first, the engineers wanted to go as high as 14 TeV, but eventually decided to limit themselves at 13 TeV aiming to reduce the probability of technical defects and failures. Furthermore, the facility will be much more reliable and stable at 13 TeV collision energy.
Since its initial launch in 2008, CERN researchers in Geneva have been using the 27-kilometer-long particle accelerator to smash protons together at just under the speed of light. Directly after the collision, the particles break up into their smallest building blocks.
One of particles that can be generated at these energies is the Higgs-Boson, a fundamental particle that has been predicted theoretically since 1964, but its existence has been established three years ago.
To make collisions more effective, the CERN physicists are currently testing a new way of launching protons through the pipe. The number of particles in each “packet” flying through the LHC will be reduced; however, the rate of collisions of these groups of particles will be increased. The distance between the particle packets will now be extremely short, only 25 nanoseconds. This will make it easier for the detectors to analyse the data obtained from the collisions.
Even after the restart in late March, the LHC will accelerate protons at energy lower than the nominal energy. What will follow are tests and trials of the system, before the first collisions will be recorded in early June, when scientists will start collecting data for particle physics. Before that, the provided data will be mostly interesting for people tasked with running the accelerator.
Improvements
Besides the superior collision energy, the “luminosity” and intensity of the beams will be also higher, producing more collisions per second. The combination of increased energy and luminosity will mean a huge increase in the amount of data produced, giving more statistical power to the physics analyses. In addition, the higher-energy collisions could produce new particles and phenomena that have never been seen before.
Engineers have also used the two-year break to enhance the stability of the accelerator. To cope with higher potential energy, the upgraded electric contacts have improved the cooling systems for the supraconducting of electric magnets and new detectors have been installed.
Furthermore, the mechanisms for selecting the relevant events for the physics of particle collisions at high energies have been fine-tuned. It is practically impossible to record and process all of the billions of particle crashes that take place at the LHC. Without some pre-selection, the computers would be quickly overloaded. However, the amount is still impressive: before, the LHC processed about 20 million collisions per second between the packages of accelerated protons. Now the rate will increase to 40 million per second.
To prepare the accelerator for this new energy frontier, 18 of the LHC’s 1.232 superconducting dipole magnets, which steer particle beams around the accelerator, were replaced due to wear and tear. More than 10.000 electrical interconnections between dipole magnets were fitted with shunts – pieces of metal that act as an alternative path for the 11.000 amp current, protecting the interconnection if there is a fault. The machine will operate at a higher voltage to run the higher energy beams, and has been fitted with new sets of radiation-resistant electronics. The vacuum system that keeps the beam pipe clear of stray molecules has been upgraded and the cryogenics system for the LHC’s superconducting dipole magnets has been refurbished.
Bunches of protons in the accelerator will be separated in time by 25 nanoseconds compared to 50 nanoseconds, the characteristic value of the previous cycle. The LHC will thus deliver more particles per unit time, as well as more collisions, to the experiments. To prepare for the new challenges, the LHC experiments, including ALICE, ATLAS, CMS and LHCb, underwent full consolidation and maintenance programmes, including upgrades to their subdetectors and data-acquisition systems.
The CERN IT department purchased and installed almost 60.000 new cores and over 100 petabytes of additional disk storage to cope with the increased amount of data that is expected from the experiments during run 2. Significant upgrades have also been made to the networking infrastructure, including the installation of new uninterruptible power supplies.
Future plans
In addition to studying the Higgs boson with much greater precision, physicists will be looking for evidence of new physics beyond the “Standard Model,” which describes all the known subatomic particles. Some theories, such as “Supersymmetry”, predict the existence of exotic particles that should appear in high-energy collisions at the LHC. Another possibility is the discovery of extra dimensions beyond the familiar four dimensions of space-time. Physicists could also identify the mysterious dark matter that makes up most of the mass in the universe. Dark matter candidates arise frequently in theories of physics beyond the “Standard Model”, including supersymmetric particles and extra dimensions.
The international LHC collaboration involves about 10 000 scientists from 113 countries. The Institute of Space Science is involved in the ALICE experiment.
[1] tera electron volt