[Paper Review] New results on collectivity with LHCb

R. Kopecna|arXiv (Cornell University)|Sep 28, 2017

Particle physics theoretical and experimental studies22 references3 citations

TL;DR

This paper presents the first measurement of two-particle angular correlations in $pp$ collisions at $\sqrt{s} = 13$ TeV using the LHCb detector in the very forward pseudorapidity region ($2 < \eta < 5$). By analyzing correlations in $\Delta\eta$ and $\Delta\phi$ across different $p_T$ and activity classes, it observes a near-side ridge structure at high activity and medium $p_T$, suggesting collectivity-like effects in small systems, with results complementary to other LHC experiments and sensitive to low-$x$ physics and Color-Glass Condensate models.

ABSTRACT

Two-particle angular correlations are studied in $pp$ collisions at $\sqrt{s}=\,13\,$TeV, collected with the LHCb detector at the LHC. The LHCb detector provides measurement in the very forward region, $2 &lt; η&lt; 5$. This region is complementary to other experiments and allows to explore low Bjorken-$x$ region. The correlations are studied as a function of difference in pseudorapidity $\left(Δη ight)$ and azimuthal angle $\left(Δϕ ight)$ in several $p_T$ and activity classes. Proton-proton collisions are studied using two datasets corresponding to minimum-bias sample and a sample obtained via a dedicated trigger to study the highest-activity events.

Motivation & Objective

• To study two-particle angular correlations in $pp$ collisions at $\sqrt{s} = 13$ TeV using the LHCb detector’s unique forward coverage.
• To probe collectivity and jet-like correlations in the very forward region ($2 < \eta < 5$), which accesses the low Bjorken-$x$ regime.
• To investigate the presence of near-side ridge and away-side ridge structures in different $p_T$ and event activity classes.
• To provide complementary data to other LHC experiments on low-multiplicity $pp$ and $p$Pb collisions, testing models like Color-Glass Condensate and jet-medium interactions.
• To assess the robustness of correlation measurements using mixed-event normalization to correct for detector acceptance and reconstruction effects.

Proposed method

• Two-particle correlation functions $C(\Delta\eta, \Delta\phi)$ are measured using charged prompt particles in $pp$ collisions at $\sqrt{s} = 13$ TeV.
• The signal distribution $S(\Delta\eta, \Delta\phi)$ is defined as the normalized pair yield per trigger particle in the same event.
• Background $B(\Delta\eta, \Delta\phi)$ is constructed from mixed events, combining trigger particles from one event with tracks from similar events in the same $p_T$ and activity bin.
• The correlation function is computed as $B(0,0) \times S(\Delta\eta, \Delta\phi)/B(\Delta\eta, \Delta\phi)$, normalizing by the zeroth bin to correct for acceptance and efficiency.
• Event activity is quantified via $N_{\text{hit}}^{\text{VELO}}$, the hit multiplicity in the VELO detector, to define low, medium, and high activity classes.
• Corrections are applied in four dimensions ($\eta$, $p_T$, $\phi$, $N_{\text{hit}}^{\text{VELO}}$) to account for tracking efficiency, fake tracks, and non-prompt contributions.

Experimental results

Research questions

• RQ1 Does a near-side ridge structure appear in $pp$ collisions at $\sqrt{s} = 13$ TeV in the forward region ($2 < \eta < 5$)?
• RQ2 How do the near-side and away-side ridge features vary with transverse momentum ($p_T$) and event activity?
• RQ3 Is there evidence of collectivity-like effects in low-multiplicity $pp$ collisions, as suggested by the presence of a near-side ridge?
• RQ4 How does the LHCb forward coverage in the $2 < \eta < 5$ region complement existing measurements from ALICE, ATLAS, and CMS?
• RQ5 To what extent do resonance decays and jet fragmentation shape the correlation function in different $p_T$ and activity regimes?

Key findings

• A clear Gaussian ridge is observed at low $p_T$ due to isotropic resonance decays, stretching across all $\Delta\phi$ at $\Delta\eta \approx 0$.
• The near-side peak becomes narrower with increasing $p_T$, consistent with more collimated jet fragmentation at higher momentum.
• The away-side ridge is most prominent at low $p_T$, attributed to back-to-back jet pairs and resonance decays.
• At high activity and medium $p_T$, a hint of a near-side ridge emerges, suggesting possible collectivity or medium effects in high-multiplicity $pp$ events.
• The Gaussian ridge disappears at high $p_T$, where jet-like correlations dominate and resonance contributions are suppressed.
• The mixed-event normalization method effectively corrects for detector acceptance and reconstruction inefficiencies, yielding robust correlation functions across all $p_T$ and activity classes.