During the last weekend of October, particles once again entered the LHC after the one-year interruption following the incident of September 2008, travelling through one sector in each direction – clockwise and anticlockwise. ALICE and LHCb, the two experiments sitting along the portion of the beam lines in question, were able to observe the effects of beams in the machine. A week later, at around 8 p.m. on 7 November, protons travelling anticlockwise arrived at the doorstep of the CMS experiment, thus completing half of the journey around the LHC.
On 23 October, a first beam of ions entered the clockwise beam pipe of the LHC. Previous tests, on 25–26 September, had involved injecting lead-ion beams through the whole injection chain right up to the threshold of the LHC. This time, the lead ions entered the LHC just before point 2, where the ALICE experiment is installed, and were dumped before point 3. These tests enabled the machine experts to test the operation of the whole injection chain and an entire sector (sector 1-2) of the LHC.
Several sub-detectors of the ALICE experiment were switched on and saw their first beam. This helped them synchronize with the LHC clock and test the capability of the sub-detectors to measure high particle multiplicities.
During the afternoon on the following day, the first proton beam made its way through the TI8 transfer line up to the anticlockwise beam pipe of the LHC. Protons passed through the LHCb experiment and were dumped just before point 7.
Most of the LHCb sub-detectors were switched off to keep the experiment safe during these delicate operations. Only the beam and background monitors remained switched on, allowing an opportunity for commissioning of the beam-monitoring software. A highlight of the weekend was the switching on of the LHCb magnet, with operators able to measure its effect on the LHC beam and adjust the magnetic compensators around LHCb accordingly to bring the beam back into orbit.
The first weekend of November saw protons complete their journey anticlockwise through three octants before being dumped in collimators just prior to entry to the cavern of the CMS experiment. The particles produced by the impact of the protons on the tertiary collimators (used to stop the beam) left their tracks in the calorimeters and the muon chambers of the experiment. The more delicate inner detectors remained switched off for protection reasons.
During the same weekend, bunches of protons were also sent in the clockwise direction, passing through the ALICE detector before being dumped at point 3.
Hardware commissioning and magnet-powering tests have also continued in the LHC. By the first week in November, six of the eight sectors had been commissioned up to 2 kA, sufficient to guide a beam at an energy of about 1.2 TeV. Furthermore, the qualification of the new quench-protection system is progressing well, with the measured values complying with the stringent standards.
• CERN publishes regular updates on the LHC in its internal Bulletin, available at www.cern.ch/bulletin, as well as on the main site www.cern.ch, via twitter at www.twitter.com/cern and on YouTube at www.youtube.com/cern.