LS1 report: ALICE ups the ante
SPS up and running... LHC almost cold... CCC Operators back at their desks... all telltale signs of the start of Run 2! For the experiments, that means there are just a few short months left for them to prepare for beams.
It has been a long road for the ALICE LS1 team. From major improvements to the 19 sub-detectors to a full re-cabling and replacement of LEP-era electrical infrastructure, no part of the ALICE cavern has gone untouched. With the experiment set to close in December, the teams are making finishing touches before turning their focus towards re-commissioning and calibration.
"Earlier last month we installed the last two modules of the di-jet calorimeter," explains Werner Riegler, ALICE technical coordinator. "These are the final parts of a 60 degree calorimeter extension that is installed opposite the present calorimeter, covering 120 degrees of polar angle. We also installed the last transition radiation detectors; these were the final pieces to go into the detector before closing the magnet doors."
For the next run, ALICE will be doubling the rate at which they write data to disk - a decision that has meant significant upgrades to its DAQ system. "Our readout rate has been increased from a few hundred Hz to 1 kHz," says Riegler. "With the lead-lead (Pb-Pb) collision rate in Run 2 foreseen at around 10 kHz, we will be saving one in ten events for analysis."
Along with this higher readout rate come customisations to ALICE's trigger parameters: "Selective triggers pick out events with specific characteristics, but in heavy ion physics, there is a lot that can be studied in the 'untriggerable' events," explains Riegler. "By increasing the readout rate, we will be looking at more minimum bias data and so can extend our physics reach. By Run 3, where the Pb-Pb collision rate of the LHC should be 50 kHz, we plan to be writing all of that data to disk! This way ALICE can fully exploit all Pb-Pb collisions, specifically the production of short-lived mesons and baryons at low transverse momentum."
With operational efficiency a priority for Run 2, the detector and the online systems teams have worked to improve the software as well as the hardware. "After three years of operation, we came to know our detector very well," says Federico Ronchetti, ALICE run coordinator. "We have been redefining virtually all of the operating procedures to eliminate inefficiencies. For example, our DAQ system can now be engaged much faster than before and is able to perform automatic detector recovery in the event of an electronics hiccup or high voltage trip. In practice, the need to stop data acquisition to fix common errors is now much reduced."
ALICE will also be benefiting from improvements to the LHC's long straight sections. In order to improve the quality of the vacuum before and after the interaction point, several sections of the vacuum pipe were NEG coated and the injection protection collimators (TDI) were refurbished during the shutdown. Better vacuum means fewer unwanted beam-gas interactions, thus reducing potential background in ALICE.
However, at times events not arising from a beam-beam collision can prove very useful. In the beginning of December, during testing of the SPS-LHC transfer line, secondary muons leaving the SPS beam dump arrived at ALICE. These "splash" events, being in sync with the LHC clock, were very useful to check the time alignment of the ALICE trigger detectors!
With installation completed, the task falls to the run coordinator to bring the systems in line: "We are presently carrying out technical runs, adding each subsystem as it comes online," says Ronchetti. "We expect a full global run by the end of the year."