More celebrations were to follow as the machine smashed proton beams together and then, on 29 November, its beams set a new energy record as they exceeded one TeV, making it the world’s highest-energy particle accelerator.
But what’s it like to be part of such a huge experiment?
To find out I talked to two of the young Oxford University scientists who have been working, living and breathing LHC science.
When particles collide
Hugo Beauchemin and Caterina Doglioni from Oxford’s Department of Physics are both involved in the ATLAS experiment, one of the four detectors which will examine the particles produced from collisions inside the LHC.
‘The LHC is built to look for answers to questions such as why the fundamental particles have mass, or whether the visible matter we see is only a small part of the matter that fills the universe, or why we are made of matter and we see no antimatter in our universe,’ Caterina tells me.
Yet her own research is less about exotic physics and more about seeing where the physics we think we know matches up to real experimental results.
‘I am working on the 'rediscovery' of the standard model we use so far for describing particle physics,’ she explains. ‘First of all we need to be sure the ATLAS experiment works correctly if we want to claim we have an answer to any of the questions above.’
‘And if we are sure everything works, and we still see discrepancies with the current model... then there will be quite a bit of excitement in the physics community!’
Hugo is investigating the extension of that model to see where gravity fits in with the three other fundamental forces of nature: ‘weak nuclear force’ [weak interaction], strong interaction, and electromagnetism.
He tells me: ‘Having done my PhD on the theoretical aspect of the structure of space-time, I’m now leading investigations for LHC evidence of new dimensions of space. In terms of detector operations, I have been centrally involved in studying trigger performance and operation.’
These ‘trigger systems’ are vital to the science going on at the LHC – with so much happening when particles collide, and only a limited amount of storage space for the huge amounts of data generated, automated systems need to decide very quickly what’s ‘important’ or ‘interesting’ and record it.
Hugo feels that, in some senses, the year’s delay wasn’t a complete loss:
‘Despite the disappointment of having to wait an extra year before getting data, I don’t feel that we lost a year… during this last year, a lot of developments on the trigger and data acquisition systems have been made. We are now ready to take good data and to properly store them, which, in my opinion, was not exactly the case last year.’
From frustration to elation
After last year’s frustrations the clock was quickly ticking again as all those involved counted down to the restart.
‘You could feel the excitement growing, everything needed to be ready and working and every possibility had to be explored: which for me means more work coming from a few different directions, not just from my supervisor for physics analysis!’ Caterina comments.
‘I’ve been involved in testing the reconstruction algorithms that allow physicists to decode what the detector registers, and I've written quite a bit of computer software for that so far. It might seem disconnected to the LHC restart, but I knew that I was one of those working towards a common goal, and in my day to day tasks I was also contributing to make all of this work.’
Finally, on Friday 20 November, the LHC was switched back on and successfully circulated two stable proton beams in opposite directions.
So what it was like being there when this giant machine came back to life?
‘Being at CERN is the best place for working as a PhD in particle physics, it feels like we are living at the centre of the world, where everybody converges. And being here at the start-up is like a dream come true!’ Caterina tells me.
‘Even though physicists who work on commissioning the detector with first data from the LHC do not expect a Nobel Prize, seeing a peak that means the rediscovery of a particle that has been known from the 1940s is an amazing experience and it pays back all the work that's been done to prepare for this moment!’
Hugo thinks it may take longer for the sheer magnitude of the occasion to sink in: ‘I was extremely excited, like most of my colleagues, but, to be honest, I didn’t have much time to celebrate. There are so many things need to be checked, studied, monitored, etc. This period is frantic, exciting, but exhausting. I believe that it’s only during the Christmas break that I will have time to realise all that has been accomplished.’
Looking to the future
And for those working on the LHC, exciting as it is, the restart is just the beginning of many years of hard work:
As Hugo points out, despite setting a new energy record, it will take many months to gradually ramp up the energy levels of the LHC to the point where it is likely to make genuinely new discoveries.
He comments: ‘So far it doesn’t provide a sufficient number of collisions for thorough studies of new phenomena, and is even still far from reaching the sensitivity of the Tevatron [a proton vs anti-proton machine, where the LHC is a proton on proton collider]. So this restart gives hope and excitement, but there is still a lot of work to do before being able to achieve anything really new.’
So how do Hugo and Caterina think they will look on this period of their life, dominated by the giant silver machine?
Hugo tells me: ‘Hopefully with a smile: it is a stressful and quite uncertain period. Expectations are high, all eyes are turned toward us, but we don’t know what will be the outcome of this experiment. I hope that in 10 years from now, the LHC will have revealed a large spectrum of unknown phenomena and mysteries to be investigated.’
Caterina adds: ‘I think I will be look back at this time with happiness and pride. I am at CERN, working together with amazingly talented people on something that is an incredible achievement, even because of its start-up alone. There's nowhere else I'd rather be!’