by Siegfried Förtsch. Published: 06 December 2016

One month of p-Pb collisions - with three different configurations of the beams, long fills and fantastic luminosity - allowed ALICE to take a huge quantity of data and meet the preset goals.

Event from first lead-proton run at sNN=8.16 TeV registered by ALICE on 26 November 2016.


In the morning of 10 November, after a delay of one day due to operational problems at LINAC2, the p-Pb run of 2016 got underway. An extremely ambitious and challenging schedule was launched: the proton-nucleus physics programme of 2016, spanning one month, foresees three different modes of beams.

After carefully scrutinizing all the physics programmes of the four LHC experiments, it was agreed to start this proton-nucleus physics run at √sNN=5.02 TeV with beam1 being the proton and beam2 the lead ion beam, respectively.  This specific run was mainly dedicated to ALICE to collect a large sample of minimum-bias events, in order to measure reference p-A data at the same energy as Pb-Pb data from previous runs. Seven days of nearly uninterrupted operation at a luminosity of 0.8 1028 Hz/cm2, which was made possible by the outstanding availability of the beams from the LHC, allowed for a total of 660 million minimum bias events to be collected, increasing the data set of 2013 of this kind of events by a factor of six. Thanks to the long lifetime of the beams, during this period the LHC managed to produce very long fills; among these, the longest fill ever, lasting for almost 38 hours.

Just one day after the completion of the data taking at 5.02 TeV, on 18 November, the LHC delivered beams for the p-Pb  run at √sNN=8.16 TeV, the highest energy ever produced by a collider for such an asymmetric system. At this energy the main focus of ALICE was on rare triggers. During a six-day data taking campaign at delivered peak luminosities varying between 1.0 - 1.5 1029 Hz/cm2, ALICE collected muon triggers resulting from a recorded integrated luminosity of 8.68 nb-1.  An event display registered during this period is shown in the figure below.

Event from first lead-proton run at √sNN=8.16 TeV registered by ALICE on 18 November 2016, produced in the fill with 10 colliding bunches with the proton beam going towards the Muon Arm. Tracks triggered by a coincidence between the calorimeters (PHOS) and the muon arm in the  forward direction are shown. In green the reconstructed muon track can be seen while the yellow (PHOS) towers reflect the energy registered in the calorimeters in the central barrel.


A further smooth operational transition was carried out by the LHC over less than two days by reversing the beams. Thus, this second stage of run at √sNN=8.16 TeV has beam1 of lead ions and beam2 of protons. This run produced muon triggers corresponding to a recorded luminosity of 12.4 nb-1 which were collected under very similar running conditions as in the reversed system.   

Since the 8.16 TeV run could be completed ahead of schedule, the LHC was switched back to 5.02 TeV for 20 hours of p-Pb data taking yielding a further 105 million minimum bias events. Data taking in 2016 ended on Monday morning, 5 December at 6 am. Thanks to the excellent and commendable availability of the LHC throughout 2016, all major goals in terms of data taking both in pp as well as p-Pb have been met, making it an extremely successful and productive year for ALICE!