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

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 ($|ensuremath{eta}|2$) 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 ${p}_{mathrm{T}}$.