At 6 a.m. on 17 December, operators ended the LHC’s first three-year-long run for proton physics with a new performance milestone. In the preceding days, the space between proton bunches had been successfully halved to the design specification of 25 ns rather than the 50 ns used so far.
Halving the bunch spacing allowed the number of bunches in the machine to be doubled, resulting in a record number of 2748 bunches in each beam; previously the LHC had been running with around 1380 bunches per beam. This gave a record beam intensity of 2.7 × 1014 protons in both beams at the injection energy of 450 GeV.
The LHC operations team then performed a number of ramps taking 25 ns beams from 450 GeV to 4 TeV, increased the total number of bunches at each step to a maximum of 804 per beam. The stepwise approach is needed to monitor the effects of additional electron cloud produced when synchrotron radiation emitted by the protons strikes the vacuum chamber – the synchrotron-radiation photon flux increases significantly as the energy of the protons is increased.
Electron cloud is strongly enhanced by the reduced spacing between bunches and is one of the main limitations for 25 ns operation. It has negative effects on the beam (increasing beam size and losses), the cryogenics (in the heat load on the beam pipe) and the vacuum (pressure rise). As a result, a period of beam-pipe conditioning known as “scrubbing” was needed before ramping the beams. During this period, the machine was operated in a controlled way with beams of increasingly high intensity. This helps to improve the surface characteristics of the beam pipe and reduces the density of the electron cloud. Once each beam had been ramped to 4 TeV, a pilot physics run of several hours took place with up to 396 bunches, spaced at 25 ns, in each beam. Although the tests were successful, significantly more scrubbing will be required before the full 25 ns beam can be used operationally.
While these tests were taking place, on 13 December representatives of the LHC and five of its experiments delivered a round-up report to CERN Council. All of the collaborations congratulated the LHC team on the machine’s exemplary performance over the first three full years of running. In 2012, not only did the collision energy increase from 7 TeV to 8 TeV but the instantaneous luminosity reached 7.7 × 1033 cm–2 s–1, more than twice the maximum value obtained in 2011 (3.5 × 1033 cm–2 s–1). News from the experiments included LHCb’s measurement of the decay of the Bs meson into two muons (Bs → μμ seen after being sought for decades), ALICE’s detailed studies of the quark–gluon plasma and TOTEM’s insights on the structure of the proton.
ATLAS and CMS gave updates on the Higgs-like particle first announced in July, with each experiment now observing the new particle with a significance close to 7σ, well beyond the 5σ required for a discovery. So far, the particle’s properties seem consistent with those of a Standard Model Higgs boson. The two collaborations are, however, careful to say that further analysis of the data – and a probable combination of both experiments’ data next year – will be required before some key properties of the new particle, such as its spin, can be determined conclusively. The focus of the analysis has now moved from discovery to measurement of the new particle in its individual decay channels.
With December 2012 marking the end of the first LHC proton physics running period, 2013 sees a four-week run from mid-January to mid-February for proton–lead collisions before going into a long shut-down for consolidation and maintenance until the end of 2014. Running will resume in 2015 at an increased collision energy of 13 TeV.