Transverse sphericity of primary charged particles in minimum bias proton-proton collisions at $\sqrt{s}=0.9$, 2.76 and 7 TeV

Abelev, Betty; Adam, Jaroslav; Adamova, Dagmar; Adare, Andrew Marshall; Aggarwal, Madan; Aglieri Rinella, Gianluca; Agocs, Andras Gabor; Agostinelli, Andrea; Aguilar Salazar, Saul; Ahammed, Zubayer

doi:10.1140/epjc/s10052-012-2124-9

Transverse sphericity of primary charged particles in minimum bias proton-proton collisions at $\sqrt{s}=0.9$ , 2.76 and 7 TeV

ALICE

Collaboration

May, 2012

21 pages

Published in:

Eur.Phys.J.C 72 (2012) 2124

Published: Sep 18, 2012

e-Print:

1205.3963 [hep-ex]

DOI:

10.1140/epjc/s10052-012-2124-9

Report number:

CERN-PH-EP-2012-136

Experiments:

CERN-LHC-ALICE

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Abstract: (arXiv)

Measurements of the sphericity of primary charged particles in minimum bias proton--proton collisions at

\sqrt{s}=0.9

, 2.76 and 7 TeV with the ALICE detector at the LHC are presented. The observable is linearized to be collinear safe and is measured in the plane perpendicular to the beam direction using primary charged tracks with

p_{\rm T}\geq0.5

GeV/c in

|\eta|\leq0.8

. The mean sphericity as a function of the charged particle multiplicity at mid-rapidity (

N_{\rm ch}

) is reported for events with different

p_{\rm T}

scales ("soft" and "hard") defined by the transverse momentum of the leading particle. In addition, the mean charged particle transverse momentum versus multiplicity is presented for the different event classes, and the sphericity distributions in bins of multiplicity are presented. The data are compared with calculations of standard Monte Carlo event generators. The transverse sphericity is found to grow with multiplicity at all collision energies, with a steeper rise at low

N_{\rm ch}

, whereas the event generators show the opposite tendency. The combined study of the sphericity and the mean

p_{\rm T}

with multiplicity indicates that most of the tested event generators produce events with higher multiplicity by generating more back-to-back jets resulting in decreased sphericity (and isotropy). The PYTHIA6 generator with tune PERUGIA-2011 exhibits a noticeable improvement in describing the data, compared to the other tested generators.

Note:

Long author list - awaiting processing •
21 pages, 9 captioned figures, 3 tables, authors from page 16, published version, figures from http://aliceinfo.cern.ch/ArtSubmission/node/3080

sphericity
charged particle: multiplicity
charged particle: transverse momentum
p p: scattering
Monte Carlo
PYTHIA
leading particle: transverse momentum
ALICE
rapidity: central region
event shape analysis

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