Exit protons, enter ions
The 2015 LHC proton run, the first after the long shutdown 1, ended last week with a good performance of the LHC given the challenges of running at the new collision energy of 13 TeV. The new energy regime brought up several issues such as increased electron-cloud effects at high beam intensities, and Unidentified Falling Objects (UFO), particles of dust inside the beam pipe causing premature beam dumps. To resolve these issues, the LHC had to patiently ramp up the beam intensity over a period of weeks using small numbers of bunches in various configurations until it was finally running with 2244 bunches of protons at 25 ns separation delivering roughly 4 fb-1 of data to the high luminosity experiments. During the proton collision phase ALICE accumulated more then 4 pb-1 of di-muon data, 600 M minimum bias events, 150 M and 130 M high multiplicity V0 and SPD triggers, respectively. In addition, roughly 400 M triggers were accumulated for the calibration calorimeters EMCAL and the newly installed DCAL. Despite the large beam currents present in our interaction point (more then 1700 bunches colliding at IP2), ALICE has operated smoothly at very large beam separation from the very beginning of each fill, thanks to the improved vacuum conditions in the beam transfer line collimation system.
The data taking period was necessarily interleaved with progressive commissioning of several systems since many features were introduced which had to be validated with beam conditions. The ALICE High Level Trigger online compression for the TPC clusters was validated and is now fully active allowing data taking with proton collisions at an instantaneous luminosity of 5 Hz/ub with a stunning readout rate of 11 GB/s (trimmed down at the event building lever by the HLT at 2 GB/s). The HLT ITS (Inner Tracking System) reconstruction was also recently validated and deployed to allow for efficient online data reconstruction for QA purposes. The Central Trigger Processor (CTP) LM functionality was deployed and put in production. The LM (Level Minus One) is an ultra fast (425 ns at the CTP) sub-Level-Zero minimum bias trigger generated by the T0 and V0 detectors to wake up the TRD electronics early enough to collect the full transition radiation signature of electrons. Also, the CTP readout into DAQ was recently upgraded to allow for the storage of additional interaction records in view of the HI run..
Optimization of the CTP/ECS (Experiment Control System) now allows loading trigger configurations and excluding and including trigger detector inputs on the fly without the need of stopping the runs. The commissioning of the new fast End-of-Run sequences is at a closing chapter and will allow much more flexible reconfiguration of the ALICE detector during the data taking. The DAQ system Event Building capability was augmented to cope with data rates of 8 GB/s sustained, 11 GB/s peak. On the detector front, extensive tests at high luminosities were successfully done with the central barrel detectors (ITS, TPC, TRD, TOF) and the Muon Arm at 800 kHz and 2.4 MHz of interaction rates respectively, to assess the response to RUN3-like conditions.
ALICE running at design luminosity of 1E27 Hz/cm2, 16 GB/s readout, 6 GB/s on disk.
The ALICE calorimeter system (EMCAL/DCAL/PHOS) readout was commissioned as the L0 triggering while the L1 gamma and jet trigger are being finalized in these days. The ALICE TPC was being operated with a new gas mixture of Ar:CO2 (instead of the Ne:CO2 used in RUN1 which was more prone to discharge) allowing for a very stable response at high particle fluxes.
At the time of this writing the LHC is entering the last week of technical stop for 2015, needed to prepare to collide lead ions, for a physics run due to start in mid November. This pause in the accelerator activity is used also by the experiments – including ALICE – to perform last minute maintenance and optimizations before resuming the quest for the Quark-Gluon Plasma, a state of matter thought to have existed just after the Big Bang. After the LHC was confidently colliding protons at 13 TeV, it's now time to switch to lead, the so-called "heavy ions" collisions.
The average centre-of-mass energy of pairs of colliding nucleons, is now 5.02 TeV which the machine is able to deliver without extra commission costs with respect to the 6.5 TeV equivalent energy of 5.1 TeV. The 5.02 TeV figure matches the energy of the pPb run of 2013 for reference purposes.
Also, the 2015 HI run will be preceded by the so called proton reference run where protons will be in collision once again at the same equivalent energy (2.51 TeV). After this short phase, when ALICE will collect mostly minimum bias data, the real lead run will start with a short ramp up phase until the machine will be filled with an intensity of roughly 400 bunches per ring organized in doublets spaced by 225 ns. Within each doublet the bunches will have a spacing of 100 ns. In order to precisely control potential bias arising from systematic effects ALICE will reverse the polarity of its two magnets halfway through the run in correspondence with the refill of the lead source of the injectors. Contrary to the proton run, where the source is a simple bottle of gas of molecular hydrogen, the lead source is solid and the metal has to be vaporized at very high temperatures and stripped of its electrons in subsequent steps, before the naked ions are accumulated into the LEIR in a continuous stream which is bunched before being injected into the PS.
During the foreseen three weeks of ion collisions, ALICE will constantly run at an instantaneous luminosity of 1E27 Hz/cm2, finally a figure that matches its design value, with an optimized mixture between rare and minimum bias triggers to best exploit our detector readout capabilities before its upgrade.
The upcoming heavy ion run is a milestone for ALICE and for the entire CERN heavy ion program since the last data of this kind (at 2.76 TeV) date back to 2011. After this run, the next lead-lead collisions will happen at the end of 2018, and for this reason ALICE is frantically preparing to fully exploit this beam time of paramount importance!