Emittance increase caused by core depletion in collisions

TL;DR

This paper introduces the core depletion effect in circular colliders, showing how preferential particle removal from the beam core increases emittance and reduces luminosity, with implications for collider operation and data analysis.

Contribution

The paper presents the first analytic model and simulation validation of core depletion, highlighting its impact on emittance growth and luminosity in colliders like the LHC.

Findings

Luminosity drops by 3-4% due to core depletion.

Core depletion increases transverse emittance during collisions.

Effect is more pronounced with active experiments.

Abstract

A new effect is presented, which changes the emittance during colliding-beam operation in circular colliders. If the initial transverse distribution is Gaussian, the collision probability is much higher for particles in the core of the beam than in the tails. When small-amplitude particles are removed, the remaining ones therefore have a larger transverse emittance. This effect, called core depletion, may cause a decrease in luminosity. An approximate analytic model is developed to study the effect and benchmarked against a multiparticle tracking simulation. Finally, the time evolution of the intensity and emittances of a Pb bunch in the Large Hadron Collider (LHC) at CERN is calculated, taking into account also other processes than collisions. The results show that integrated luminosity drops by 3--4% if core depletion is taken into account. It is also found that core depletion causes the transverse emittance to be larger when more experiments are active. This observation could be checked against experimental data once the LHC is operational.