The period of the year when ALICE plays the leading role at LHC has finally arrived: after smashing protons for months, the machine will start accelerating lead ions on November 7th and continue until the winter shut-down. After ramp up and adjustment, the accelerator will deliver stable beams and proton-lead collisions, first at 5 TeV and then at 8 TeV of energy per colliding nucleon pair.
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.
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’s run is divided in two phases. In the first, proton and lead beams will be collided at 5 TeV. This energy is equivalent to that of the Pb-Pb collisions in 2015, and of a special pp reference data sample which is being collected through Run 2, thus researchers will be able to make direct comparisons between all three at the same energy. In this year’s 5 TeV p-Pb run, ALICE scientists aim at collecting a much larger statistics of events than in 2013, in view of a campaign of high-precision measurements on the p-Pb system.
In the second part of the run, the maximum energy per colliding nucleon pair (8 TeV) allowed by the present machine will be reached. It will be thus possible to study the dependence on the collision energy of the observed phenomena, in particular for quarkonia (flavourless mesons whose constituents are a quark and its own antiquark), including J/ψ meson and its first excitation ψ’.
“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. A great deal of preparatory engineering work was done in last week’s technical stop, including special modifications to the LHC’s beam instrumentation and the systems which inject the beam.
An exciting period for ALICE’s researchers lies ahead.
Interview of Federico Antinori, ALICE spokesperson-elect, on the experiment's objectives for this year's collisions at the LHC between lead ions and protons. (Video: Paola Catapano & Maximilien Brice/CERN)