Abstract
The observation in small size collision systems, pp and pA, of strong correlations with long range in rapidity and a characteristic structure in azimuth, the ridge phenomenon, is one of the most interesting results obtained at the large hadron collider. Earlier observations of these correlations in heavy ion collisions at the relativistic heavy ion collider are standardly attributed to collective flow due to strong final state interactions, described in the framework of viscous relativistic hydrodynamics. Even though data for small size systems are well described in this framework, the applicability of hydrodynamics is less well grounded and initial state based mechanisms have been suggested to explain the ridge. In this review, we discuss particle correlations from the initial state point of view, with focus on the most recent theoretical developments.
Highlights
While the focus of the physics programme at the large hadron collider (LHC) is the discovery and understanding of the properties of the previously missing piece in the Standard Model – the Higgs boson – and the search for its eventual failure, it has shown very surprising and unexpected aspects of quantum chromodynamics (QCD), in small collisions systems, pp and pA
15 There are attempts to describe the near side ridge as a consequence of the momentum kick given by the leading parton to medium constituents [108], with a medium already present in pp collisions, or to minijets [109]
We have focused on the standard eikonal treatment within the glasma graph approximation which is valid for collisions between dilute objects – pp
Summary
While the focus of the physics programme at the large hadron collider (LHC) is the discovery and understanding of the properties of the previously missing piece in the Standard Model – the Higgs boson – and the search for its eventual failure, it has shown very surprising and unexpected aspects of quantum chromodynamics (QCD), in small collisions systems, pp and pA. One of the most exciting observations made in high multiplicity pp collisions by the CMS collaboration during the first LHC run is the discovery of the correlations between produced particles over large intervals of rapidity, peaking at zero relative azimuthal angle [1]. A (2020) 56:215 final state interactions and hadronisation – that we will assume in the following, may lead to azimuthal asymmetries as observed in data We start by those based on the weak coupling but non-perturbative description of dense partonic systems offered by the the Color Glass Condensate (CGC) effective theory, see the reviews [36–38] and the book [39]. This will be the subject of Sects. We will comment briefly on the status of the comparison to experimental data in the summary
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