Abstract
Many observables measured at the Relativistic Heavy Ion Collider and the Large Hadron Collider show a smooth transition between proton-proton and protonnucleus collisions (small systems), and nucleus-nucleus collisions (large systems), when represented versus some variable like the multiplicity in the event. In this contribution I review some of the physics mechanisms, named cold nuclear matter effects, that may lead to a collective-like behaviour in small systems beyond the macroscopic description provided by relativistic hydrodynamics. I focus on the nuclear modification of parton densities, single inclusive particle production and correlations.
Highlights
In recent years much attention has been devoted in the high-energy physics community to the experimental findings that indicate a smooth transition between "small" collisions systems and "large" collision systems
These findings at both the Large Hadron Collider (LHC) and the Relativistic Heavy Ion Collider (RHIC), are striking as some of the involved observables have been traditionally considered as signatures of the creation of deconfined partonic matter close to thermodynamical equilibrium - Quark Gluon Plasma - in high-energy AA collisions
The aim of this contribution is to review effects that may cause or contribute to the collectivity found in small systems, whose origin lies in the initial state or the very initial stages of the collision
Summary
In recent years much attention has been devoted in the high-energy physics community to the experimental findings that indicate a smooth transition between "small" collisions systems (proton-proton, pp, and proton-nucleus, pA) and "large" collision systems (nucleus-nucleus, AA) These findings at both the Large Hadron Collider (LHC) and the Relativistic Heavy Ion Collider (RHIC), are striking as some of the involved observables have been traditionally considered as signatures of the creation of deconfined partonic matter close to thermodynamical equilibrium - Quark Gluon Plasma - in high-energy AA collisions. The aim of this contribution is to review effects that may cause or contribute to the collectivity found in small systems, whose origin lies in the initial state or the very initial stages of the collision. I apologise in advance to those who may find their work underrepresented
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