Measurement of flow harmonics with multi-particle cumulants in Pb+Pb collisions at sNN=2.76 Â TeV with the ATLAS detector

ATLAS measurements of the azimuthal anisotropy in lead–lead collisions at <math><mrow><msqrt><msub><mi>s</mi><mi mathvariant="normal">NN</mi></msub></msqrt><mo>=</mo><mn>2.76</mn></mrow></math>  TeV are shown using a dataset of approximately 7 <math><mi mathvariant="normal">μ</mi></math> b <math><msup><mrow/><mrow><mo>-</mo><mn>1</mn></mrow></msup></math> collected at the LHC in 2010. The measurements are performed for charged particles with transverse momenta <math><mrow><mn>0.5</mn><mo>&lt;</mo><msub><mi>p</mi><mi mathvariant="normal">T</mi></msub><mo>&lt;</mo><mn>20</mn></mrow></math>  GeV and in the pseudorapidity range <math><mrow><mo stretchy="false">|</mo><mi mathvariant="italic">η</mi><mo stretchy="false">|</mo><mo>&lt;</mo><mn>2.5</mn></mrow></math> . The anisotropy is characterized by the Fourier coefficients, <math><msub><mi mathvariant="normal">v</mi><mi>n</mi></msub></math> , of the charged-particle azimuthal angle distribution for <math><mrow><mi>n</mi><mo>=</mo><mn>2</mn></mrow></math> –4. The Fourier coefficients are evaluated using multi-particle cumulants calculated with the generating function method. Results on the transverse momentum, pseudorapidity and centrality dependence of the <math><msub><mi mathvariant="normal">v</mi><mi>n</mi></msub></math> coefficients are presented. The elliptic flow, <math><msub><mi mathvariant="normal">v</mi><mn>2</mn></msub></math> , is obtained from the two-, four-, six- and eight-particle cumulants while higher-order coefficients, <math><msub><mi mathvariant="normal">v</mi><mn>3</mn></msub></math> and <math><msub><mi mathvariant="normal">v</mi><mn>4</mn></msub></math> , are determined with two- and four-particle cumulants. Flow harmonics <math><msub><mi mathvariant="normal">v</mi><mi>n</mi></msub></math> measured with four-particle cumulants are significantly reduced compared to the measurement involving two-particle cumulants. A comparison to <math><msub><mi mathvariant="normal">v</mi><mi>n</mi></msub></math> measurements obtained using different analysis methods and previously reported by the LHC experiments is also shown. Results of measurements of flow fluctuations evaluated with multi-particle cumulants are shown as a function of transverse momentum and the collision centrality. Models of the initial spatial geometry and its fluctuations fail to describe the flow fluctuations measurements.