Measurement of the azimuthal anisotropy for charged particle production insNN=2.76TeV lead-lead collisions with the ATLAS detector

Abstract

Differential measurements of charged particle azimuthal anisotropy are presented for lead-lead collisions at $\sqrt{{s}_{NN}}=2.76$ TeV with the ATLAS detector at the LHC, based on an integrated luminosity of approximately 8 $\ensuremath{\mu}{\mathrm{b}}^{\ensuremath{-}1}$. This anisotropy is characterized via a Fourier expansion of the distribution of charged particles in azimuthal angle relative to the reaction plane, with the coefficients ${v}_{n}$ denoting the magnitude of the anisotropy. Significant ${v}_{2}$--${v}_{6}$ values are obtained as a function of transverse momentum ($0.5<{p}_{\mathrm{T}}<20$ GeV), pseudorapidity ($|\ensuremath{\eta}|<2.5$), and centrality using an event plane method. The ${v}_{n}$ values for $n\ensuremath{\ge}3$ are found to vary weakly with both $\ensuremath{\eta}$ and centrality, and their ${p}_{\mathrm{T}}$ dependencies are found to follow an approximate scaling relation, ${v}_{n}^{1/n}({p}_{\mathrm{T}})\ensuremath{\propto}{v}_{2}^{1/2}({p}_{\mathrm{T}})$, except in the top 5$%$ most central collisions. A Fourier analysis of the charged particle pair distribution in relative azimuthal angle ($\ensuremath{\Delta}\ensuremath{\varphi}={\ensuremath{\varphi}}_{\mathrm{a}}\ensuremath{-}{\ensuremath{\varphi}}_{\mathrm{b}}$) is performed to extract the coefficients ${v}_{n,n}=\ensuremath{\langle}\mathrm{cos}n\ensuremath{\Delta}\ensuremath{\varphi}\ensuremath{\rangle}$. For pairs of charged particles with a large pseudorapidity gap ($|\ensuremath{\Delta}\ensuremath{\eta}={\ensuremath{\eta}}_{\mathrm{a}}\ensuremath{-}{\ensuremath{\eta}}_{\mathrm{b}}|>2$) and one particle with ${p}_{\mathrm{T}}<3$ GeV, the ${v}_{2,2}$--${v}_{6,6}$ values are found to factorize as ${v}_{n,n}({p}_{\mathrm{T}}^{\mathrm{a}},{p}_{\mathrm{T}}^{\mathrm{b}})\ensuremath{\approx}{v}_{n}({p}_{\mathrm{T}}^{\mathrm{a}}){v}_{n}({p}_{\mathrm{T}}^{\mathrm{b}})$ in central and midcentral events. Such factorization suggests that these values of ${v}_{2,2}$--${v}_{6,6}$ are primarily attributable to the response of the created matter to the fluctuations in the geometry of the initial state. A detailed study shows that the ${v}_{1,1}({p}_{\mathrm{T}}^{\mathrm{a}},{p}_{\mathrm{T}}^{\mathrm{b}})$ data are consistent with the combined contributions from a rapidity-even ${v}_{1}$ and global momentum conservation. A two-component fit is used to extract the ${v}_{1}$ contribution. The extracted ${v}_{1}$ is observed to cross zero at ${p}_{\mathrm{T}}\ensuremath{\approx}1.0$ GeV, reaches a maximum at 4--5 GeV with a value comparable to that for ${v}_{3}$, and decreases at higher ${p}_{\mathrm{T}}$.