Anisotropic flow of charged hadrons, pions and (anti-)protons measured at high transverse momentum in Pb-Pb collisions at $\sqrt{s_{\rm NN}}=2.76$ TeV

Abstract: The elliptic, $v_2$, triangular, $v_3$, and quadrangular, $v_4$, azimuthal anisotropic flow coefficients are measured for unidentified charged particles, pions and (anti-)protons in Pb-Pb collisions at $\sqrt{s_{\rm NN}} = 2.76$ TeV with the ALICE detector at the Large Hadron Collider. Results obtained with the event plane and four-particle cumulant methods are reported for the pseudo-rapidity range $|\eta|<0.8$ at different collision centralities and as a function of transverse momentum, $p_{\rm T}$, out to $p_{\rm T}=20$ GeV/$c$. The observed non-zero elliptic and triangular flow depends only weakly on transverse momentum for $p_{\rm T}>8$ GeV/$c$. The small $p_{\rm T}$ dependence of the difference between elliptic flow results obtained from the event plane and four-particle cumulant methods suggests a common origin of flow fluctuations up to $p_{\rm T}=8$ GeV/$c$. The magnitude of the (anti-)proton elliptic and triangular flow is larger than that of pions out to at least $p_{\rm T}=8$ GeV/$c$ indicating that the particle type dependence persists out to high $p_{\rm T}$.

• The paper shows that elliptic flow (v2) remains nearly constant beyond 8 GeV/c, signaling continued collective behavior at high transverse momentum.
• It employs both event-plane and four-particle cumulant methods to distinguish genuine flow signals from non-flow effects and capture initial geometric fluctuations.
• Enhanced anisotropy in (anti-)protons compared to pions supports jet quenching models and deepens insights into quark-gluon plasma dynamics.

Azimuthal Anisotropic Flow of Charged Particles in High Energy Pb-Pb Collisions

The research presents a detailed study of anisotropic flow in Pb-Pb collisions at a center-of-mass energy of 2.76 TeV per nucleon pair, utilizing the ALICE detector at the LHC. Anisotropic flow, quantified by Fourier coefficients $v_n$ (where $n$ indicates the harmonic), characterizes the azimuthal distribution of produced particles and is a pivotal observable in understanding the properties of the quark-gluon plasma (QGP).

Key Measurements and Methodology

The study focuses on the elliptic ($v_2$), triangular ($v_3$), and quadrangular flow ($v_4$) coefficients for unidentified charged particles, pions, and (anti-)protons. Measurements extend up to high transverse momentum ($p_T$) regions (up to 20 GeV/c), which is significant given that earlier studies predominantly focused on low and intermediate $p_T$ spectra.

Two primary methods are employed to analyze the data: the event plane method and the four-particle cumulant method. The event plane method is sensitive to flow fluctuations, whereas the cumulant method helps separate genuine flow from non-flow correlations.

Principal Findings

1. Elliptic Flow: Elliptic flow $v_2$ persists up to high $p_T$ with a relatively weak dependency on $p_T$ beyond 8 GeV/c. This behavior indicates that collective flow effects extend into high $p_T$ regions typically associated with jet fragmentation.
2. Triangular Flow: The triangular flow $v_3$ also shows non-zero values at high $p_T$, albeit with a magnitude smaller than $v_2$. This suggests the existence of initial geometric fluctuations persisting at high transverse momentum.
3. Particle Type Dependence: The study highlights a notable particle-type dependence, with (anti-)proton flow coefficients surpassing those of pions up to at least 8 GeV/c, suggesting differences in the underlying mechanism of particle production across different species.
4. Comparison with Models: The $v_2$ data at high $p_T$ are in reasonable agreement with jet quenching models such as WHDG, which incorporate partonic energy loss mechanisms.

Implications and Future Directions

The persistence of anisotropic flow signals at high $p_T$ challenges simple models of parton energy loss without medium interaction and corroborates the idea that the medium created in heavy-ion collisions affects particles across a wider energy range than previously thought. These results contribute to the understanding of the QGP and its evolution, informing theoretical models concerning energy loss and collective behavior in high-temperature QCD matter.

Moreover, these observations provide a baseline for future studies at even higher energies and different collision systems, such as smaller nuclei or proton-ion collisions, where initial geometric fluctuations might be less pronounced. Continued exploration into the high $p_T$ regime and further analysis of additional harmonic coefficients will likely yield more insights into the nature of the QGP and the fundamental properties of high-energy nuclear matter.