CERN Accelerating science

An ambitious p-Pb data taking campaign just came to an end

by Virginia Greco


These asymmetric collisions were originally meant as a benchmark to control for background effects unrelated to the production of a quark-gluon plasma, expected in Pb-Pb collisions simply due to the use of Pb nuclei instead of protons in the collisions. Distinguishing the two classes of effects in Pb-Pb collisions is difficult, whereas the study of p-Pb collisions allows physicists to isolate them one from the other.

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

The fist time LHC physicists collided a beam of lead ions with one of protons was September 2012. Analysing the data collected in this run, the researchers were surprised to see, in a fraction of the collisions, the signs of a collective expansion of the system, a sort of mini-Big Bang which is a characteristic hallmark of lead-lead collisions, and is commonly associated with the quark-gluon plasma properties. A full month run of p-Pb collisions with much greater luminosity then took place in early 2013, confirming and extending those first observations.

This year 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. During this period, LHC produced 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.

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 will be met, making it an extremely successful and productive year for ALICE!“. We’re very excited by the possibility in this run of studying how strongly interacting matter behaves in the simpler p-Pb system, because this could actually hold the key to understanding how the quark-gluon plasma if formed” explains Federico Antinori, spokesperson-elect for ALICE.


Lead ions have 82 times the charge and are 206.4 times more massive than protons. Colliding these asymmetric beams, with very different properties and lifetimes, leads to many challenges for the LHC accelerator physicists and operators.  

An exciting period for ALICE’s researchers lies ahead. 


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