Perfect QCD – a new Universal approach to soft QCD


  • Peter Christiansen Lund University





The ideas presented in this proceeding aims to be a first step towards a description of hadronic collisions where all soft processes are fundamentally strongly coupled and the same Universal strongly coupled physics drives both initial and final-state interactions. As it is not currently possible to derive such a picture from first principles, instead, an attempt to generalize the perfect liquid observation to a “perfect QCD” guiding principle is presented, focusing on implications for particle production in small systems. The first steps towards a microscopic model is taken by arguing that “perfect QCD” suggests that the screening in the initial state is so large that multi-parton interactions are of little or no importance. Instead, a target and projectile remnant is coherently excited and particle production is mainly driven by radiation in a qualitative similar manner as e+ e ̅  \rightarrow qq̅. Finally, some of the possible implications of this “excited remnant model” are presented. It is argued that the time ordering of soft and hard physics can explain the absence of jet quenching in small systems and that the coherence scale of the projectile and target provides insights into what small systems will exhibit flow.


T. Sjöstrand, et al., An Introduction to PYTHIA 8.2, Comput. Phys. Commun. 191 (2015) 159, 10.1016/j.cpc.2015.01.024

T. Sjöstrand, S. Mrenna, and P. Z. Skands, PYTHIA 6.4 Physics and Manual, JHEP 05 (2006) 026, 10.1088/1126-6708/2006/05/026

C. Loizides, Experimental overview on small collision systems at the LHC, Nucl. Phys. A 956 (2016) 200, 10.1016/j.nuclphysa.2016.04.022

J. L. Nagle and W. A. Zajc, Small System Collectivity in Relativistic Hadronic and Nuclear Collisions, Ann. Rev. Nucl. Part. Sci. 68 (2018) 211, 10.1146/annurev-nucl-101916-123209

A. Kurkela and A. Mazeliauskas, Chemical Equilibration in Hadronic Collisions, Phys. Rev. Lett. 122 (2019) 142301, 10.1103/PhysRevLett.122.142301

B. Andersson, et al., Parton Fragmentation and String Dynamics, Phys. Rept. 97 (1983) 31, 10.1016/0370-1573(83)90080-7

F. Gelis, et al., The Color Glass Condensate, Ann. Rev. Nucl. Part. Sci. 60 (2010) 463, 10.1146/annurev.nucl.010909.083629

I. Arsene et al., Quark gluon plasma and color glass condensate at RHIC? The Perspective from the BRAHMS experiment, Nucl. Phys. A 757 (2005) 1, 10.1016/j.nuclphysa.2005.02.130

K. Adcox et al., Formation of dense partonic matter in relativistic nucleus-nucleus collisions at RHIC: Experimental evaluation by the PHENIX collaboration, Nucl. Phys. A 757 (2005) 184, 10.1016/j.nuclphysa.2005.03.086

B. Back et al., The PHOBOS perspective on discoveries at RHIC, Nucl. Phys. A 757 (2005) 28, 10.1016/j.nuclphysa.2005.03.084

J. Adams et al., Experimental and theoretical challenges in the search for the quark gluon plasma: The STAR Collaboration’s critical assessment of the evidence from RHIC collisions, Nucl. Phys. A 757 (2005) 102, 10.1016/j.nuclphysa.2005.03.085

K. Aamodt et al., Elliptic flow of charged particles in Pb-Pb collisions at 2.76 TeV, Phys. Rev. Lett. 105 (2010) 252302, 10.1103/PhysRevLett.105.252302

G. Aad et al., Measurement of the pseudorapidity and transverse momentum dependence of the elliptic flow of charged particles in lead-lead collisions at √ sNN = 2.76 TeV with the ATLAS detector, Phys. Lett. B 707 (2012) 330, 10.1016/j.physletb.2011.12.056

S. Chatrchyan et al., Measurement of the elliptic anisotropy of charged particles produced in PbPb collisions at √ sNN =2.76 TeV, Phys. Rev. C 87 (2013) 014902, 10.1103/PhysRevC.87.014902

P. Kovtun, D. T. Son, and A. O. Starinets, Viscosity in strongly interacting quantum field theories from black hole physics, Phys. Rev. Lett. 94 (2005) 111601, 10.1103/PhysRevLett.94.111601

B. Alver et al., Importance of correlations and fluctuations on the initial source eccentricity in high-energy nucleus-nucleus collisions, Phys. Rev. C 77 (2008) 014906, 10.1103/PhysRevC.77.014906

B. Alver and G. Roland, Collision geometry fluctuations and triangular flow in heavy-ion collisions, Phys. Rev. C 81 (2010) 054905, 10.1103/PhysRevC.82.039903

V. Khachatryan et al., Evidence for Collective Multiparticle Correlations in p-Pb Collisions, Phys. Rev. Lett. 115 (2015) 012301, 10.1103/PhysRevLett.115.012301

M. Aaboud et al., Measurement of long-range multiparticle azimuthal correlations with the subevent cumulant method in pp and p + P b collisions with the ATLAS detector at the CERN Large Hadron Collider, Phys. Rev. C 97 (2018) 024904, 10.1103/PhysRevC.97.024904

S. J. Brodsky, J. Gunion, and J. H. Kuhn, Hadron Production in Nuclear Collisions: A New Parton Model Approach, Phys. Rev. Lett. 39 (1977) 1120, 10.1103/PhysRevLett.39.1120

B. Back et al., Centrality and energy dependence of chargedparticle multiplicities in heavy ion collisions in the context of elementary reactions, Phys. Rev. C 74 (2006) 021902, 10.1103/PhysRevC.74.021902

A. Capella, et al., Dual parton model, Phys. Rept. 236 (1994) 225, 10.1016/0370-1573(94)90064-7

G. J. Alner et al., Scaling of Pseudorapidity Distributions at c.m. Energies Up to 0.9-TeV, Z. Phys. C 33 (1986) 1, 10.1007/BF01410446

S. Acharya et al., Constraints on jet quenching in p-Pb collisions at √ sNN = 5.02 TeV measured by the event-activity dependence of semi-inclusive hadron-jet distributions, Phys. Lett. B 783 (2018) 95, 10.1016/j.physletb.2018.05.059

A. Zichichi, The ISR’s totally unexpected results, CERN Cour. 51N6 (2011) 39 26. A. Badea, et al., Measurements of two-particle correlations in e +e − collisions at 91 GeV with ALEPH archived data, Phys. Rev. Lett. 123 (2019) 212002, 10.1103/PhysRevLett.123.212002

G. Aad et al., Two-particle azimuthal correlations in photonuclear ultraperipheral Pb+Pb collisions at 5.02 TeV with ATLAS (2021)

Search for elliptic azimuthal anisotropies in γp interactions within ultra-peripheral pPb collisions at √ sNN = 8.16 TeV (2020)

I. Abt et al., Two-particle azimuthal correlations as a probe of collective behaviour in deep inelastic ep scattering at HERA, JHEP 04 (2020) 070, 10.1007/JHEP04(2020)070

I. Abt et al., Azimuthal correlations in photoproduction and deep inelastic ep scattering at HERA, JHEP 12 (2021) 102, 10.1007/JHEP12(2021)102

Search for collectivity in e-p collisions with H1 (2020)




How to Cite

Christiansen P. Perfect QCD – a new Universal approach to soft QCD. Supl. Rev. Mex. Fis. [Internet]. 2022 Dec. 10 [cited 2023 Feb. 6];3(4):040901 1-6. Available from: