Charged particle multiplicities in pp interactions at \sqrt{s} = 2.36$ TeV measured with the ATLAS detector at the LHC

In December 2009 data at the centre-of-mass energy of sqrt(s) = 2.36 TeV were recorded with the ATLAS detector. This was the first time the LHC had been operated at this beam energy and stable beam conditions were not declared. Therefore, to ensure detector safety, the silicon strip detector was in standby mode with reduced sensor bias voltage, which makes track reconstruction more difficult. Two complementary methods were developed to measure the charged particle multiplicity distributions and, in particular, estimate the track reconstruction efficiency under these challenging conditions. The first uses the full Inner Detector information and corrects the efficiency from the simulation using a data-driven technique. The second uses tracks reconstructed from pixel detector information only. The charged particle multiplicity and its dependence on transverse momentum and pseudorapidity are measured for events with at least one charged particle in the kinematic range |eta|<2.5 and pT > 500 MeV. The average charged particle multiplicity at central pseudorapidity is measured to be 1.739 +- 0.019 (stat) +- 0.058 (syst) at sqrt(s) = 2.36 TeV.