[Paper Review] Toward an effective centrality trigger in pp collisions at LHC

H. J. Drescher, M. Strikman|ArXiv.org|Dec 19, 2007

High-Energy Particle Collisions Research3 citations

TL;DR

This paper proposes centrality triggers for pp collisions at the LHC to study high gluon density effects similar to those in heavy-ion collisions. By vetoing leading baryons with xF > 0.1, especially via single- or double-sided triggers, the method selects events at small impact parameters (⟨b⟩ ≈ 0.6 fm), where saturation-scale gluon fields induce strong transverse momentum broadening and enhanced energy flow to central rapidities, enabling access to black disk regime dynamics in pp collisions.

ABSTRACT

We investigate the impact of very strong small x gluon fields in colliding nucleons at LHC energies on the interaction of valence quarks. We find that in the range of small impact parameters, which contribute significantly to the production of heavy new particles, several of the valence quarks receive large transverse momenta, exceeding 1 GeV/c. This results in a suppression of leading baryon production and consequently in an additional energy flow to smaller rapidities. We suggest several triggers for centrality in pp collisions which allow one to study the propagation of partons through gluon fields of a strength comparable to the ones encountered in heavy ion collisions at the LHC.

Motivation & Objective

• To develop effective triggers that select central pp collisions at the LHC with impact parameters comparable to those in heavy-ion collisions.
• To enable the study of strong small-x gluon fields in pp collisions, analogous to those in heavy-ion collisions, where saturation effects are expected.
• To quantify how high gluon densities suppress leading baryon production and enhance energy flow to central rapidities.
• To assess the feasibility of using baryon veto triggers (especially xF > 0.1) to isolate events with high gluon density and saturation-scale dynamics.
• To evaluate the performance of combined dijet and veto triggers in achieving the narrowest impact parameter distributions for central pp events.

Proposed method

• Uses a color glass condensate (CGC) framework to model the saturation scale Qs and gluon density at small x in nucleons, assuming exponential transverse profile.
• Models valence quark positions in the projectile nucleon via uncorrelated Gaussian distributions with ⟨ρ²⟩ ≈ 0.3 fm², matched to axial nucleon form factor.
• Estimates local gluon density at each transverse distance ρ from the target nucleon, computing the probability of hard interactions via the saturation scale Qs(ρ).
• Applies rejection sampling to select events based on xB and ρ distributions, with Qs² as a key input parameter.
• Proposes centrality triggers based on vetoing leading baryons with xF > 0.1 in one or both fragmentation regions to select small-b collisions.
• Combines dijet triggers with baryon veto triggers to further narrow the impact parameter distribution and enhance centrality selection.

Experimental results

Research questions

• RQ1 Can baryon veto triggers at xF > 0.1 effectively select pp collisions at small impact parameters comparable to central heavy-ion collisions?
• RQ2 What is the expected suppression of leading baryon production in central pp collisions due to strong small-x gluon fields?
• RQ3 How does energy flow to central rapidities change in central pp collisions compared to generic inelastic events due to gluon field effects?
• RQ4 To what extent do combined dijet and veto triggers improve centrality resolution in pp collisions?
• RQ5 What is the average gluon density encountered by partons in veto-selected central pp events, and how does it compare to RHIC and LHC heavy-ion collisions?

Key findings

• A single-sided veto for baryons with xF > 0.1 achieves a centrality distribution similar to the standard dijet trigger, selecting events with ⟨b⟩ ≈ 0.6 fm.
• A double-sided veto for baryons with xF > 0.1 produces the narrowest impact parameter distribution, significantly improving centrality resolution.
• The combination of dijet and baryon veto triggers achieves the narrowest ⟨b⟩ distribution, approaching the theoretical limit of ⟨ρtr⟩ ≈ ⟨ρ⟩, the smallest possible average impact parameter for pp or DIS.
• In 16% of dijet-triggered events, no leading baryons with xF ≥ 0.1 are produced in either fragmentation region, compared to 8% in generic inelastic events, indicating enhanced energy flow to central rapidities.
• The average gluon density in veto-selected central pp events exceeds that in central pA collisions at RHIC (where BDR holds for pt ≤ 1.5 GeV/c) and reaches about half the average density in LHC heavy-ion collisions.
• The model predicts strong transverse momentum broadening of partons and significant multiplicity fluctuations in central pp events due to high gluon fields, with observable effects in Z-boson pT distributions and elliptic flow.