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. RI Gauzzi, Paolo/D-2615-2009; Moraes, Arthur/F-6478-2010; Perrino, Roberto/B-4633-2010; Gutierrez, Phillip/C-1161-2011; Stoicea, Gabriel/B-6717-2011; Alexa, Calin/F-6345-2010; Lin, SC/G-4666-2010; Buttar, Craig/D-3706-2011; de Groot, Nicolo/A-2675-2009; Pouzada, Antonio/D-8451-2011; Rescia, Sergio/D-8604-2011; Doyle, Anthony/C-5889-2009; Andreazza, Attilio/E-5642-2011; Marti-Garcia, Salvador/F-3085-2011; turton, miranda/F-4682-2011; Conde Muino, Patricia/F-7696-2011; Robson, Aidan/G-1087-2011; Di Domenico, Antonio/G-6301-2011; Kladiva, Eduard/G-6305-2011; Losada, Marta/B-2261-2010; Bauer, Florian/G-8816-2011; crosetti, nanni/H-3040-2011; Jones, Roger/H-5578-2011; Ferrando, James/A-9192-2012; collins-tooth, christopher/A-9201-2012; De Cecco, Sandro/B-1016-2012; branchini, paolo/A-4857-2011; Wolter, Marcin/A-7412-2012; Rotaru, Marina/A-3097-2011; O'Shea, Val/G-1279-2010; Szczygiel, Robert/B-5662-2011; Pina, Joao /C-4391-2012; Takai, Helio/C-3301-2012; Britton, David/F-2602-2010; Li, Xuefei/C-3861-2012; Smirnova, Lidia/D-8089-2012; Smirnov, Sergei/F-1014-2011; Gladilin, Leonid/B-5226-2011; Kramarenko, Victor/E-1781-2012; Prokoshin, Fedor/E-2795-2012; Livan, Michele/D-7531-2012; Petrucci, Fabrizio/G-8348-2012; Wemans, Andre/A-6738-2012; Fabbri, Laura/H-3442-2012; Kurashige, Hisaya/H-4916-2012; Villa, Mauro/C-9883-2009; Kuzhir, Polina/H-8653-2012; Delmastro, Marco/I-5599-2012

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.

Charged-particle multiplicities in pp interactions at root s=900 GeV measured with the ATLAS detector at the LHC ATLAS Collaboration

CAPUA, Marcella;Crosetti G.;