First collisions of 2017 in ALICE: ready to go
In the last weeks, since the restart of LHC, ALICE has performed a number of tests and fine-tuning exercises and is now ready to take data for physics.
After the long winter shutdown, on May 23 the first proton-proton collisions with stable beams of 2017 were delivered by LHC and detected by the four experiments. The ALICE detector was fully operative and took great snapshots of these collisions (such as the event display in the picture).
The accelerator is now in the intensity ramp-up phase: it started injecting only few bunches and is gradually increasing the number in subsequent fills. It is expected to reach the nominal working conditions towards the beginning of July.
In reality, the very first collisions were delivered a week before the official date, but they were not in optimal conditions, since the beams were not stable. During this phase, which is called ‘quiet beam', the experts of LHC perform tests and make adjustments to the various components of the accelerator.
ALICE used the quiet beam collisions to perform some performance tests, in particular on the forward detectors (AD, V0) and the Electromagnetic Calorimeter (EMCAL), but only a minor part of the whole apparatus was switched on. This is because the quiet beam is not totally safe for the instrumentation: when the experts of LHC change the settings of the machine and make adjustments, there is the risk of beam losses hitting directly the detectors, and thus damaging them. In particular, the parts that need to stay off are those closest to the beam line, such as parts of the inner tracking system and the gaseous detectors.
When collisions with stable beams were delivered, ALICE started its data-taking programme. The LHC ramp-up plan started with three circulating bunches per beam, and moved on to about 12, 75, 300, 600.
Even if at the beginning the collision rate was very low, a number of operations could be performed, such as a trigger alignment scan for the pixel detector and a high-voltage scan for the V0 and AD sub-detectors to find the optimal working voltage.
In order to have precise information on the alignment of the central barrel detectors, data were taken with different polarities of the dipole and the solenoid (specifically, minus-minus, plus-plus and no magnetic fields). This information will be used to reconstruct the data that will be collected along the whole year.
Following the requirements of some physics group, data taking at low rate – with the whole detector on – was also performed, as well as at high interaction rate (150 kHz, the nominal one) with the Time Projection Chamber (TPC). This was particularly important, since during the shutdown the gas mixture filling the TPC was changed from Ar-CO2 to Ne- CO2-N2. In this test, the detector showed high performance, as expected.
Finally, during the 300-bunches fill ALICE took data with a reduced (halved) magnetic field of the solenoid, since these conditions are recommended to study the low mass di-muon spectrum.
“The restart has been great,” comments Grazia Luparello, run coordinator of ALICE, “in just a few fills of the accelerator we managed to perform all the tests and the special data taking included in our programme; we are satisfied and ready to go for physics”.