Azimuth Quadrupole Spectra derived from 2.76 TeV Pb-Pb PID Differential $v_2(p_t)$ Data

$v_2(p_t)$ data are intended to estimate the amplitude of an azimuth component of particle spectra interpreted as representing elliptic flow of a dense QCD medium. As defined, $ v_2(p_t)$ is a ratio with a single-particle spectrum appearing in its denominator. Its numerator represents a spectrum Fourier component arising from a boosted particle source. The Cooper-Frye (CF) formalism may be used to describe emission from a boosted source. CF analysis reveals that the $v_2(p_t)$ numerator includes a factor $ p_t$ in the boost frame with major consequences for data interpretation. A unique quadrupole $p_t$ spectrum may be isolated from $v_2(p_t)$ data and compared directly with the single-particle spectrum in the $v_2$ denominator and with hydro theory. A monopole boost (aka radial flow) value may be estimated from $v_2(p_t)$ data. Several novel results emerge via the Cooper-Frye analysis.

💡 Research Summary

The paper re‑examines the conventional interpretation of the differential elliptic‑flow observable v₂(pₜ) in heavy‑ion collisions by applying the Cooper–Frye (CF) formalism to isolate the true quadrupole component of the particle spectra. The authors start from the standard definition of v₂(pₜ) as a ratio of a Fourier coefficient extracted from two‑dimensional (η, φ) angular correlations (the numerator) to the inclusive single‑particle spectrum (the denominator). By modeling the particle source as a boosted thermal emitter with a rapidity boost field Δyₜ(φᵣ)=Δyₜ₀+Δyₜ₂ cos 2φᵣ, they derive an analytic expression for the quadrupole term V₂ᵢ that contains an explicit factor of the transverse momentum in the boost frame, p′ₜ. This factor survives the ratio and leads to the key relation

 v₂ᵢ(pₜ) ≈