Quadrupole Anisotropy in Dihadron Azimuthal Correlations in Central $d$$+$Au Collisions at $\sqrt{s_{_{NN}}}$=200 GeV (1303.1794v2)

Abstract: The PHENIX collaboration at the Relativistic Heavy Ion Collider (RHIC) reports measurements of azimuthal dihadron correlations near midrapidity in $d$$+$Au collisions at $\sqrt{s_{{NN}}}$=200 GeV. These measurements complement recent analyses by experiments at the Large Hadron Collider (LHC) involving central $p$$+$Pb collisions at $\sqrt{s{_{NN}}}$=5.02 TeV, which have indicated strong anisotropic long-range correlations in angular distributions of hadron pairs. The origin of these anisotropies is currently unknown. Various competing explanations include parton saturation and hydrodynamic flow. We observe qualitatively similar, but larger, anisotropies in $d$$+$Au collisions compared to those seen in $p$$+$Pb collisions at the LHC. The larger extracted $v_2$ values in $d$$+$Au collisions at RHIC are consistent with expectations from hydrodynamic calculations owing to the larger expected initial-state eccentricity compared with that from $p$$+$Pb collisions. When both are divided by an estimate of the initial-state eccentricity the scaled anisotropies follow a common trend with multiplicity that may extend to heavy ion data at RHIC and the LHC, where the anisotropies are widely thought to arise from hydrodynamic flow.

• The paper reveals significant v2 anisotropy in central d+Au collisions that links to higher initial-state eccentricity compared to p+Pb observations.
• It employs 1.56 billion minimum-bias events and mixed-event corrections to isolate anisotropic signals and validate hydrodynamic interpretations in small systems.
• The study demonstrates a scaling relationship between v2 and initial eccentricity, underlining the pivotal role of geometry in driving collective flow behavior.

Analysis of Quadrupole Anisotropy in Dihadron Azimuthal Correlations in Central $d$+Au Collisions at $\sqrt{s_{_{NN}}=200~GeV$

The paper under consideration presents a detailed examination of azimuthal dihadron correlations in central $d$+$Au collisions at a center-of-mass energy of$\sqrt{s_{_{NN}}=200~GeV$$, as measured by the PHENIX collaboration at the Relativistic Heavy Ion Collider (RHIC). These findings are evaluated in the context of similar studies conducted at the Large Hadron Collider (LHC), where analogous anisotropic phenomena were reported in$$p$+$Pb collisions.

Key Observations and Results

1. Anisotropy in $d$+$$Au Collisions</strong>: The study reports strong azimuthal anisotropies, quantified through the second Fourier harmonic coefficient$$v_2$, in$d$+$Au collisions, which exceed those observed in $p$+$Pb collisions at higher energies. The observed$v_2$$values align with expectations from hydrodynamic models that suggest larger initial-state eccentricity in$$d$+$Au collisions.
2. Event Selection and Methodology: The analysis capitalizes on data from 1.56 billion minimum-bias $d$+$$Au events. Yield distributions are corrected using mixed-event techniques, and the contributions from central and peripheral events are distinguished to isolate anisotropic components.</li> <li><strong>Geometrical Scaling</strong>: A noteworthy conclusion from the paper is the scaling relationship between the measured anisotropy$$v_{2}$and the calculated initial-state eccentricity$\varepsilon_{2}$$, derived from Monte Carlo-Glauber models. This correspondence suggests that both$$d$+$Au at RHIC and $p$+$$Pb at the LHC might be influenced by similar initial geometric conditions.</li> <li><strong>Absence of$$v_3$$</strong>: The analysis also scrutinizes potential contributions from the third-order harmonic$$c_3$. No significant indication of a substantial$v_3$$<a href="https://www.emergentmind.com/topics/wireless-agents-was" title="" rel="nofollow" data-turbo="false" class="assistant-link" x-data x-tooltip.raw="">was</a> detected within the measured range, which diverges from certain theoretical predictions that suggest multipole expansions beyond the quadrupole might be relevant.</li> </ol> <h3 class='paper-heading' id='implications-and-theoretical-considerations'>Implications and Theoretical Considerations</h3> <ul> <li><strong>Hydrodynamic Flow in Small Systems</strong>: The paper contributes to the ongoing debate regarding the existence and nature of collective flow in small collision systems like$$d$+$Au and $p$+$Pb. Results suggest that dynamic collective behaviors, typically associated with larger nuclei collisions (e.g., heavy ion collisions), may also manifest effectively in smaller systems.
3. Initial State Effects: The study reinforces the relevance of initial-state configurations in determining anisotropic flow patterns, indicating that even in small systems, initial spatial anisotropies play a pivotal role in final-state momentum distributions.
4. Future Directions: The findings necessitate further theoretical work to reconcile the observed results with various models (e.g., gluon saturation, hydrodynamic expansion, etc.), especially considering the substantial differences in energy scales between RHIC and LHC experiments. Additionally, a broader pseudorapidity acceptance in future experiments might provide deeper insights into the longitudinal structure of these correlations.

In summary, this research underscores the complexity of dihadron correlations in small collision systems and challenges traditional boundaries regarding the applicability of hydrodynamic interpretations to relativistic collision environments. The study paves the way for refined investigations into the interplay of initial geometry and final-state interactions across different collisional regimes.