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First ions for ALICE and rings for LHCb

30 October 2009

Injection tests on 25–29 September delivered heavy ions for the first time to the threshold of the LHC. Particles were extracted from the Super Proton Synchrotron (SPS) and transported along the TI2 and TI8 transfer lines towards the LHC, before being dumped on beam stoppers. These crucial tests not only showed that the whole injection chain performs well but they were also interesting for the ALICE collaboration because they included bunches of lead ions. By using a dedicated “beam injection” trigger, the ALICE detector registered bursts of particles emerging from the beam stopper at the end of the TI2 transfer line, some 300 m upstream of the detector, shedding light on the timing of the trigger.

While the LHC has undergone repairs and consolidation work since the incident that brought commissioning to an abrupt end in September 2008, the ALICE collaboration has been busy with important installation work, which has included the first modules of the electromagnetic calorimeter (CERN Courier May 2009 p7). This allowed the start in August of a full detector run with cosmic rays, which was scheduled to last until the end of October. In addition to trigger information from the silicon pixel and ACORDE detectors (the latter built specially for triggering on cosmic muons) ALICE is now making extensive use of the trigger provided by its time-of-flight array (TOF). The high granularity and the low noise (0.1 Hz/cm2) of the multigap resistive-plate chambers of the TOF, combined with the large coverage (around 150 m2), offers a range of trigger combinations.

More than 100 million cosmic events had been accumulated in the central detectors by early October, both with and without magnetic field. Even the forward muon system – oriented parallel to the LHC beam – has collected several tens of thousands of the very rare quasi-horizontal cosmic rays, which traverse the full length of the spectrometer at a rate of one particle every couple of minutes.

Near-horizontal cosmic rays are also valuable for checking out the LHCb detector, which is aligned along the LHC beam line, and they recently allowed observation of the first rings from the one of the two ring-imaging Cherenkov detectors, RICH1 (CERN Courier July/August 2007 p30). There are two types of radiating material in RICH1: aerogel for lowest momentum particles (around a few GeV/c) and perfluoro-n-butane (C4F10) to cover momenta from 10 GeV/c to around 65 GeV/c. This is the first time that the RICH detector has seen a particle as it will once the LHC re-starts.

The shutdown of the LHC has also provided the opportunity for the LHCb collaboration to finish the detector completely, with the installation of the fifth and final plane of muon chambers. Other improvements include modifications to reduce noise in the electromagnetic calorimeter to a negligible level and network upgrades. During a recent commissioning week, in preparation for the LHC re-start, the LHCb team managed to read out the full detector at a rate of almost 1 MHz. Data packets were sent at 100 kHz through to the LHCb computer farm and each sub-detector was tested to ensure that the system could handle data at this rate.

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