Start of the 2012 Proton-Proton Run

Giacomo Contin

Following the 2011/2012 Winter Shutdown, during which new sectors of two ALICE detectors (the Transition Radiation Detector (TRD) and Electromagnetic Calorimeter (EMCAL)) were installed and the cooling problem of the Silicon Pixel Detector (SPD) was fixed, we at ALICE worked on the integration and commissioning of the whole detector.

In March 2012 the LHC started the commissioning of the beam and found, on April 5th, its first proton-proton collisions at the energy of 8 TeV with stable beams. In April, the LHC carried out the intensity ramp-up, achieving 1380 bunches per beam and establishing the bunch intensity at 1.46 * 1011 protons per bunch. During this period ALICE took data with minimum bias trigger, adjusting its detector configuration and operations to the new beam conditions experienced at each intensity step, in order to optimise the data taking efficiency. The filling scheme prepared by the machine experts for this year allows ALICE to record the collisions between main bunches and satellite bunches, in order to keep the luminosity at optimal levels, even with such beam intensity conditions and without separating the beams.

Starting from April 9th, ALICE used the intensity ramp-up step with 624 bunches to take data with the L3 and Dipole magnetic fields off: these data will be used to study the material distribution in the Calorimeters, to align the Muon Chambers, and to align and calibrate the Central Barrel. In the following days, ALICE profited from special fills for the time alignment of the trigger inputs and for setting up the time gates of the V0 detector that, once applied, will allow an efficient background rejection.

Afterwards, on April 17th, the LHC provided a special fill to perform the Van Der Meer scan for ALICE, this is a luminosity calibration experiment, essential for absolute cross section determinations of physics processes. The measurements performed during the Van Der Meer scan will serve as a reference for the rest of the proton-proton data taking.

In the last part of the month, the LHC devoted two days to machine development activities and then reserved five days for the Technical Stop, in that special week without beams, both the machine and experiment experts had the possibility to access the tunnel and the experimental sites in order to perform the periodic maintenance of the systems and fix the problems that appeared during the run. ALICE profited from this week to upgrade the software of the central systems, to replace malfunctioning components of the detectors and to verify the system stability through stress tests: these operations will allow optimisation of ALICE performance during the next weeks, which will be devoted to the continuous data taking of proton-proton collisions.

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