After two years of slumber, the world’s biggest particle accelerator has come back to life. This marks the official beginning of Run 2 of the LHC, which will collide protons at nearly twice the energies achieve in Run 1. Results from this data were already presented at the recently concluded European Physical Society (EPS) Conference on High Energy Physics. And after achieving fame in 2012 through observation of the Higgs boson, it’s no surprise that the scientific community is waiting with bated breath to see what the LHC will do next.
The first official 13 TeV stable beam physics data arrived on June 5th. One of the first events recorded by the CMS detector is shown in Figure 2. But as it turns out, you can’t just walk up to the LHC, plug it back into the wall, and press the on switch (crazy, I know.) It takes an immense amount of work, planning, and coordination to even get the thing running.
The machine testing begins with the magnets. Since the LHC dipole magnets are superconducting, they need to be cooled to about 1.9K in order to function, which can take weeks. Each dipole circuit then must be tested to ensure functionality of the quench protection circuit, which will dump the beam in the event of sudden superconductivity loss. This process occurred between July and December of 2014.
Once the magnets are set, it’s time to start actually making beam. Immediately before entering the LHC, protons are circling around the Super Proton Synchroton, which acts as a pre-accelerator. Getting beam from the SPS to the LHC requires synchronization, a functional injection system, beam dump procedure, and a whole lot of other processes that are re-awoken and carefully tested. By April, beam commissioning was officially underway, meaning that protons were injected and circulating, and a mere 8 weeks later there were successful collisions at the safe energy of 6.5 TeV. As of right now, the CMS detector is reporting 84 pb-1 total integrated luminosity; a day-by-day breakdown can be seen in Figure 3.
But just having collisions does not mean that the LHC is up and fully functional. Sometimes things go wrong right when you least expect it. For example, the CMS magnet has been off to a bit of a rough start—there was an issue with its cooling system that kept the magnetic field off, meaning that charged particles would not bend. The LHC has also been taking the occasional week off for “scrubbing”, in which lots of protons are circulated to burn off electron clouds in the beam pipes.
This is all leading up to the next technical stop, when the CERN engineers get to go fix things that have broken and improve things that don’t work perfectly. So it’s a slow process, sure. But all the caution and extra steps and procedures are what make the LHC a one-of-a-kind experiment that has big sights set for the rest of Run 2. More posts to follow when more physics results arrive!
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