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Measurements of the sphericity of primary charged particles in minimum biasproton--proton collisions at $\sqrt{s}=0.9$, 2.76 and 7 TeV with the ALICEdetector at the LHC are presented. The observable is linearized to be collinearsafe and is measured in the plane perpendicular to the beam direction usingprimary charged tracks with $p_{\rm T}\geq0.5$ GeV/c in $|\eta|\leq0.8$. Themean sphericity as a function of the charged particle multiplicity atmid-rapidity ($N_{\rm ch}$) is reported for events with different $p_{\rm T}$scales ("soft" and "hard") defined by the transverse momentum of the leadingparticle. In addition, the mean charged particle transverse momentum versusmultiplicity is presented for the different event classes, and the sphericitydistributions in bins of multiplicity are presented. The data are compared withcalculations of standard Monte Carlo event generators. The transversesphericity is found to grow with multiplicity at all collision energies, with asteeper rise at low $N_{\rm ch}$, whereas the event generators show theopposite tendency. The combined study of the sphericity and the mean $p_{\rmT}$ with multiplicity indicates that most of the tested event generatorsproduce events with higher multiplicity by generating more back-to-back jetsresulting in decreased sphericity (and isotropy). The PYTHIA6 generator withtune PERUGIA-2011 exhibits a noticeable improvement in describing the data,compared to the other tested generators.

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