Small system studies: A theory overview

Abstract

There is now a substantial body of evidence that a deconfined quark-gluon plasma is created in ultrarelativistic collisions of heavy nuclei. Some key observables which are used to gauge the production of the quark-gluon plasma are the hadronic spectra and multiplicities across many species, azimuthal anisotropies in the production of various hadrons, jet quenching, photon/dilepton production, and heavy quarkonium suppression. A key question in the study of the quark-gluon plasma is what happens to these observables as one changes the system size. In particular, it is very interesting to study collisions of `small systems' such as pp, dA, pA, and $e^{+}{e}^{-}$ since naively one does not expect to produce a QGP in these cases. One of the surprises from such studies was the existence of sizable azimuthal anisotropies in the hadron spectra in the highest multiplicity classes, which led to speculations that one can generate a QGP in such relatively rare events. This interpretation, however, is complicated by the fact that there can be multiple sources of azimuthal anisotropy. I will discuss our current understanding of the different sources of azimuthal anisotropies and the multiplicity windows in which each mechanism is expected to dominate. I will also provide a critical discussion of the reliability of hydrodynamic models applied to small systems. Finally, I will briefly discuss jet quenching and heavy quarkonium suppression in small systems and discuss whether or not these observables show any indication of the production of a small QGP droplet.

7 pages, 1 figure; Proceedings of Quark Matter 2018, Venice, Italy, May 2018; v2 - typo fixes, version accepted for publication in NPA

References in corpus (32)

• The equation of state in (2+1)-flavor QCD
• Observation of long-range near-side angular correlations in proton-lead collisions at the LHC
• The ridge in proton-proton collisions at the LHC
• Evidence for collective multi-particle correlations in pPb collisions
• Matching the Nonequilibrium Initial Stage of Heavy Ion Collisions to Hydrodynamics with QCD Kinetic Theory
• Elliptic and triangular flow in p+Pb and peripheral Pb+Pb collisions from parton scatterings
• Imprints of fluctuating proton shapes on flow in proton-lead collisions at the LHC
• The anisotropic non-equilibrium hydrodynamic attractor
• Color Screening and Regeneration of Bottomonia in High-Energy Heavy-Ion Collisions
• Angular Correlations in Gluon Production at High Energy
• Collectivity and electromagnetic radiation in small systems
• Mass ordering of spectra from fragmentation of saturated gluon states in high multiplicity proton-proton collisions
• Relativistic anisotropic hydrodynamics
• Measurement of quarkonium production in proton--lead and proton--proton collisions at $5.02$ $\mathrm{TeV}$ with the ATLAS detector
• Hierarchy of azimuthal anisotropy harmonics in collisions of small systems from the Color Glass Condensate
• Long Range Forward-Backward Correlations and the Color Glass Condensate
• Coupling Relativistic Viscous Hydrodynamics to Boltzmann Descriptions
• Bottomonium suppression using a lattice QCD vetted potential
• Constraints on jet quenching in p-Pb collisions at $\sqrt{{\mathbf{s}}_{\mathbf{NN}}}$ = 5.02 TeV measured by the event-activity dependence of semi-inclusive hadron-jet distributions
• Is bottomonium suppression in proton-nucleus and nucleus-nucleus collisions at LHC energies due to the same effects?
• Importance of initial and final state effects for azimuthal correlations in p+Pb collisions
• Relativistic Fluid Dynamics In and Out of Equilibrium -- Ten Years of Progress in Theory and Numerical Simulations of Nuclear Collisions
• Parton model description of multiparticle azimuthal correlations in $pA$ collisions
• Systematics of azimuthal anisotropy harmonics in proton-nucleus collisions at the LHC from the Color Glass Condensate
• Azimuthal Anisotropies at High Momentum from Purely Non-Hydrodynamic Transport
• Non-Abelian Bremsstrahlung and Azimuthal Asymmetries in High Energy p+A Reactions
• When gluons go odd: how classical gluon fields generate odd azimuthal harmonics for the two-gluon correlation function in high-energy collisions
• How large is the Knudsen number reached in fluid dynamical simulations of ultrarelativistic heavy ion collisions?
• Anisotropic hydrodynamic modeling of 200 GeV Au-Au collisions
• Symmetric cumulants as a probe of the proton substructure at LHC energies
• Exploring the applicability of dissipative fluid dynamics to small systems by comparison to the Boltzmann equation
• Constraints for proton structure fluctuations from exclusive scattering

Cited by in corpus (18)

• Non-equilibrium attractor in high-temperature QCD plasmas
• Measurement of $B^{+}$, $B^0$ and ${\Lambda }_b^0$ production in $p\mathrm{Pb}$ collisions at $\sqrt{s_{\mathrm{NN}}}=8.16\mathrm{TeV}$
• Jet quenching test of the QCD matter created at RHIC and the LHC needs opacity-resummed medium induced radiation
• Progress and Challenges in Small Systems
• Bayesian quantification of strongly-interacting matter with color glass condensate initial conditions
• The Dynamics of Particle-Particle Correlations and the Ridge Effect in Proton-Proton Collisions
• Multi-Parton Interactions in pp collisions from Machine Learning-based regression
• Emergence of long-range angular correlations in low-multiplicity proton-proton collisions
• Long- and short-range correlations and their event-scale dependence in high-multiplicity pp collisions at $\sqrt{\text{s}}=13$ TeV
• Long-range near-side correlation in $e^{+}{e}^{-}$ Collisions at 183-209 GeV with ALEPH Archived Data
• Thermal and hard scales in transverse momentum distributions, fluctuations and entanglement
• Hydrodynamic generators in relativistic kinetic theory
• Collectivity in large and small systems formed in ultrarelativistic collisions
• Multiplicity and event-scale dependent flow and jet fragmentation in pp collisions at $\sqrt{s}$ = 13 TeV and in p$-$Pb collisions at $\sqrt{s_{\mathrm{NN}}}$ = 5.02 TeV
• Investigating the soft and hard limits in transverse momentum spectra in pp collisions
• Particle production and flow-like effects in small systems
• Pseudothermalization of the quark-gluon plasma
• Revisiting the soft-hard separation in the transverse momentum spectra of $pp$ collisions