Particle correlations from the initial state
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Review
Particle correlations from the initial state Tolga Altinoluk1,a , Néstor Armesto2 1 2
National Centre for Nuclear Research, 02-093 Warsaw, Poland Instituto Galego de Física de Altas Enerxías IGFAE, Universidade de Santiago de Compostela, 15782 Santiago de Compostela, Galicia, Spain
Received: 18 May 2020 / Accepted: 12 August 2020 © The Author(s) 2020 Communicated by Laura Tolos
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, withh focus on the most recent theoretical developments.
1 Introduction 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 also shown very surprising and unexpected aspects of quantum chromodynamics (QCD), particularly 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]. This phenomenon was dubbed ridge due its shape in the azimuthal angle-rapidity plot, and constitute one of the key findings at the LHC (see Fig. 1). Later on, this structure was found by other collaborations and for smaller multiplicities [2–5] and in association with Z boson production [6]. A similar ridge structure was also a e-mail:
observed in pPb collisions at the LHC by the four large collaborations [7–10]. A maximum in the correlations also appears at azimuthal angle π , called the away side ridge in contrast to the near side ridge peaked at zero azimuthal angle. They have also been observed in PbPb collisions, see e.g. [11–13] for PbPb results and a comparison with those in pPb. Similar correlations were observed in AuAu, dAu and 3 HeAu collisions at the relativistic heavy ion collider (RHIC) [14– 19]. They have also been observed in photoproduction on Pb in ultraperipheral collisions (UPCs) at the LHC [20]. Their existence in smaller systems like e+ e− collisions [21] at the large electron-positron collider and deep inelastic scattering (DIS) events in ep at the Hadron-Elektron-Ringanlage [22] has be
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