First proton-nucleus collisions in the LHC: the p-Pb pilot physics

During the night of 12-13 September 2012 the LHC collided protons with lead nuclei for the first time, demonstrating the feasibility of hybrid collisions despite the basic two-in-one magnet design. The centre-of-mass energy was 5 TeV per colliding nucleon pair, "Stable Beams" were declared 9 hours after the first injection of Pb beams in 2012. The integrated luminosity delivered to the four large LHC experiments was sufficient to yield new physics results. Within the same fill, stable beams were declared twice more, with the collision points displaced longitudinally by ±0.5 m from their usual locations. We provide a general overview of this p-Pb pilot physics fill before focusing on beam data at injection energy and at flat-top, before stable beams for physics were declared. We monitored the beam parameters throughout the fill and present an analysis of their evolution based on a simulation of intra-beam scattering (IBS), synchrotron radiation and the consumption of the beam intensity by collisions ("luminosity burn-off"). We also present some considerations on beam-beam effects with unequal beam sizes and the pilot run is compared, in this respect, to expectations for the forthcoming physics run in January. This pilot run was a major step in the preparation of the physics run. However it was not possible to perform an additional feasibility test designed to clarify the limits to the intensity of two beams injected and ramped with unequal revolution frequencies. We describe the plan for this test and discuss the reasons why it could not be carried out.