[Paper Review] Correlations of $μμ$, $eμ$, and $ee$ pairs in $p$+$p$ collisions at $\sqrt{s}=200$ GeV and implications for $c\bar{c}$ and $b\bar{b}$ production mechanisms

C. Aidala, Y. Akiba+292 more|arXiv (Cornell University)|May 10, 2018

Particle physics theoretical and experimental studies4 citations

TL;DR

This study analyzes angular correlations of muon, electron-muon, and electron pairs from $c\bar{c}$ and $b\bar{b}$ quark pairs in $p+p$ collisions at $\sqrt{s} = 200$ GeV using PHENIX data. It finds that pair creation (PC) dominates $b\bar{b}$ production (76%±19%), while flavor excitation (FE) dominates $c\bar{c}$ production, with data best described by PYTHIA Tune A, not POWHEG.

ABSTRACT

PHENIX has measured the azimuthal correlations of muon pairs from charm and bottom semi-leptonic decays in $p$+$p$ collisions at $\sqrt{s}=200$ GeV, using a novel analysis technique utilizing both unlike- and like-sign muon pairs to separate charm, bottom and Drell-Yan contributions. The dimuon measurements combined with the previous electron-muon and dielectron measurements span a wide range in rapidity, and are well described by PYTHIA Tune A. Through a Bayesian analysis based on PYTHIA Tune A, we show that leading order pair creation is the dominant $(76\%\pm^{14}_{19}\%)$ contribution for $b\bar{b}$ production, whereas the data favor the scenario in which next-to-leading-order processes dominate $c\bar{c}$ production. The small contribution of next-to-leading-order processes in $b\bar{b}$ production at the collision energies of the Relativistic Heavy Ion Collider contrasts with the case at Large-Hadron-Collider energies, where next-to-leading-order processes are expected to dominate.

Motivation & Objective

• To determine the relative contributions of pair creation (PC), flavor excitation (FE), and gluon splitting (GS) to $c\bar{c}$ and $b\bar{b}$ production in $p+p$ collisions at $\sqrt{s} = 200$ GeV.
• To test the validity of leading-order and next-to-leading-order mechanisms in heavy quark production using angular correlation data.
• To assess whether PYTHIA Tune A or POWHEG models better describe the observed angular correlations of $\mu\mu$, $e\mu$, and $ee$ pairs.
• To constrain the fraction of $c\bar{c}$ and $b\bar{b}$ pairs produced via each mechanism using Bayesian shape analysis.
• To provide a foundation for interpreting heavy quark production in A+A and $p+A$ collisions by establishing baseline mechanisms in $p+p$.

Proposed method

• Measures angular correlations of $\mu\mu$, $e\mu$, and $ee$ pairs in three rapidity regions: forward-forward, mid-forward, and mid-mid.
• Performs a Bayesian shape analysis on combined angular correlation data to extract relative fractions of PC, FE, and GS contributions.
• Compares data distributions to Monte Carlo simulations generated by PYTHIA Tune A and POWHEG to assess model consistency.
• Uses a hierarchical model to assign probabilities to different production mechanisms, incorporating $95\%$ credible intervals for uncertainty.
• Applies priors based on theoretical expectations and kinematic constraints to refine the analysis of relative contributions.
• Analyzes the energy dependence of mechanisms by comparing results to existing data at other energies and models.

Experimental results

Research questions

• RQ1 What is the relative contribution of pair creation (PC), flavor excitation (FE), and gluon splitting (GS) to $b\bar{b}$ production in $p+p$ collisions at $\sqrt{s} = 200$ GeV?
• RQ2 What is the relative contribution of PC, FE, and GS to $c\bar{c}$ production in the same kinematic regime?
• RQ3 Which event generator—PYTHIA Tune A or POWHEG—better describes the observed angular correlations of $\mu\mu$, $e\mu$, and $ee$ pairs?
• RQ4 How do the relative contributions of PC, FE, and GS differ between $c\bar{c}$ and $b\bar{b}$ systems, and what explains the reversal in hierarchy?
• RQ5 What are the upper limits on the GS contribution to $c\bar{c}$ and $b\bar{b}$ production at the $95\%$ credible interval?

Key findings

• The dominant production mechanism for $b\bar{b}$ pairs is pair creation (PC), with a fraction of $76\% \pm^{14}_{19}\%$.
• For $c\bar{c}$ pairs, flavor excitation (FE) is the dominant mechanism, with $F_{\rm FE} > F_{\rm PC} > F_{\rm GS}$, consistent with PYTHIA Tune A.
• The hierarchy of contributions for $b\bar{b}$ is reversed compared to $c\bar{c}$, with $F_{\rm PC} > F_{\rm FE} > F_{\rm GS}$, due to the larger $b$ quark mass increasing kinematic suppression of NLO processes.
• The upper limit on the gluon splitting (GS) contribution to $c\bar{c}$ production at $95\%$ credible interval is $52\%$, and for $b\bar{b}$ it is $31\%$.
• Angular correlations from data are best described by PYTHIA Tune A, while those from POWHEG are broader and inconsistent with the measured data.
• The analysis supports a scenario in which flavor excitation dominates $c\bar{c}$ production and pair creation dominates $b\bar{b}$ production, with implications for understanding initial-state dynamics in heavy-ion collisions.