Charged-particle multiplicities in pp interactions at root s=900 GeV measured with the ATLAS detector at the LHC ATLAS Collaboration RID F-6345-2010 RID E-5642-2011 RID G-8816-2011 RID A-4857-2011 RID D-3706-2011 RID A-9201-2012 RID F-7696-2011 RID B-1016-2012 RID A-2675-2009 RID G-6301-2011 RID A-6738-2012 RID C-5889-2009 RID A-9192-2012 RID D-2615-2009 RID C-1161-2011 RID H-5578-2011 RID G-6305-2011 RID G-4666-2010 RID B-2261-2010 RID F-3085-2011 RID F-6478-2010 RID G-1279-2010 RID B-4633-2010 RID D-8604-2011 RID G-1087-2011 RID A-3097-2011 RID F-1014-2011 RID B-6717-2011 RID B-5662-2011 RID C-9883-2009

The first measurements from proton-proton collisions recorded with the ATLAS detector at the LHC are presented. Data were collected in December 2009 using a minimum-bias trigger during collisions at a centre-of-mass energy of 900 GeV. The charged-particle multiplicity, its dependence on transverse momentum and pseudorapidity, and the relationship between mean transverse momentum and charged-particle multiplicity are measured for events with at least one charged particle in the kinematic range vertical bar eta vertical bar < 2.5 and p(T) > 500 MeV. The measurements are compared to Monte Carlo models of proton-proton collisions and to results from other experiments at the same centre-of-mass energy. The charged-particle multiplicity per event and unit of pseudorapidity eta = 0 is measured to be 1.333 +/- 0.003(stat.) +/- 0.040(syst.), which is 5-15% higher than the Monte Carlo models predict. 2010 Published by Elsevier B.V.