Constituent quarks in a nucleon are the essential elements in the standard “quark model” associated with the electric charge, spin, mass, and baryon number of a nucleon. Quantum chromodynamics (QCD) describes nucleon as a composite object containing current quarks (valence quarks and sea (anti-)quarks) and gluons. These subatomic elements and their interactions are known to contribute in complex ways to the overall nucleon spin and mass. In the early development of QCD theory in the 1970s, an alternative hypothesis postulated that the baryon number might manifest itself through a non-perturbative configuration of gluon fields forming a Y-shaped topology known as the gluon junction. In this work, we propose to test such hypothesis by measuring (i) the Regge intercept of the net-baryon distributions for +( )Au collisions, (ii) baryon and charge transport in the isobaric ratio between +Ru and +Zr collisions, and (iii) target flavor dependence of proton and antiproton yields at large rapidity, transported from the hydrogen and deuterium targets in $$e+p$$ (d) collisions. Our study indicates that these measurements at the EIC can help determine what carries the baryon number.

United States

Niseem Magdy

Abhay Deshpande

Roy Lacey

Wenliang Li

Prithwish Tribedy

Zhangbu Xu

Search for baryon junctions in e+A collisions at the electron ion collider

In the constituent quark model, each quark inside a baryon carries 1/3 unit of the baryon number. An alternative picture exists where the center of a Y-shaped topology of gluon fields, called the baryon junction, carries a unit baryon number. Studying baryon transport over a large rapidity gap (δy) in nuclear collisions provides a possible tool to distinguish these two pictures. A recent analysis of global data on net-proton yield at mid-rapidity in Au+Au collisions showed an exponential dependence on δy and the exponential slope does not vary with event centrality, favoring the baryon junction picture. Since junctions are flavor blind, hyperons – baryons containing valence strange quarks – are expected to exhibit a similar behavior as the proton. This study aims to test this prediction by analyzing hyperon yields in Au+Au collisions at various energies. We observe that net-hyperon yields, after correcting for the strangeness production suppression, adhere to the expected exponential form. The extracted slope parameters for net-Λ, net-Ξ and net-Ω are consistent with each other and with those of net-proton within uncertainties, and exhibit no centrality dependence. Various implementations of the PYTHIA event generator, primarily based on valence quarks for baryon transport, are unable to simultaneously describe the slope parameters for all baryons.

Chun Yuen Tsang

Rongrong Ma

Beam energy dependence of net-hyperon yield and its implication on baryon transport mechanism

During the early development of quantum chromodynamics, it was proposed that baryon number could be carried by a non-perturbative Y-shaped topology of gluon fields, called the gluon junction, rather than by the valence quarks as in the QCD standard model. A puzzling feature of ultra-relativistic nucleus-nucleus collisions is the apparent substantial baryon excess in the mid-rapidity region that could not be adequately accounted for in most conventional models of quark and diquark transport. The transport of baryonic gluon junctions is predicted to lead to a characteristic exponential distribution of net-baryon density with rapidity and could resolve the puzzle. In this context we point out that the rapidity density of net-baryons near mid-rapidity indeed follows an exponential distribution with a slope of  $$-0.61\pm 0.03$$  <math> <mrow> <mo>-</mo> <mn>0.61</mn> <mo>±</mo> <mn>0.03</mn> </mrow> </math>   as a function of beam rapidity in the existing global data from A+A collisions at AGS, SPS and RHIC energies. To further test if quarks or gluon junctions carry the baryon quantum number, we propose to study the absolute magnitude of the baryon vs. charge stopping in isobar collisions at RHIC. We also argue that semi-inclusive photon-induced processes ( $$\gamma +p$$  <math> <mrow> <mi>γ</mi> <mo>+</mo> <mi>p</mi> </mrow> </math>  /A) at RHIC kinematics provide an opportunity to search for the signatures of the baryon junction and to shed light onto the mechanisms of observed baryon excess in the mid-rapidity region in ultra-relativistic nucleus-nucleus collisions. Such measurements can be further validated in A+A collisions at the LHC and  $$e+p$$  <math> <mrow> <mi>e</mi> <mo>+</mo> <mi>p</mi> </mrow> </math>  /A collisions at the EIC.

Nicole Lewis

China

Wendi Lv

Mason Ross

Chun Tsang

James Brandenburg

Zi-Wei Lin

Zebo Tang

Search for baryon junctions in photonuclear processes and isobar collisions at RHIC

Multi-particle correlation observables in the Relativistic Heavy Ion Collider small system scan are computed in a framework that contains both initial state momentum anisotropies from the Color Glass Condensate effective theory and final state hydrodynamic evolution. The initial state is computed using the IP-Glasma model and coupled to viscous relativistic hydrodynamic simulations, which are followed by microscopic hadronic transport. All parameters of the calculation were previously constrained using experimental data on Au+Au collisions at the same center of mass energy. We find that the qualitative features of the experimental data, such as the system and centrality dependence of the charged hadron momentum anisotropy, can only be reproduced when final state interactions are present. On the other hand, we also demonstrate that the details of the initial state are crucially important for the quantitative description of observables in the studied small systems, as neglecting the initial transverse flow profile or the initial shear stress tensor, which contain information on the momentum anisotropy from the Color Glass Condensate, has dramatic effects on the produced final state anisotropy. We further show that the initial state momentum anisotropy is correlated with the observed elliptic flow in all small systems, with the effect increasing with decreasing multiplicity. We identify the precise measurement of <math><msub><mrow><mi>v</mi></mrow><mrow><mn>2</mn></mrow></msub></math> in d+Au and Au+Au collisions at RHIC energy at the same multiplicity as a means to reveal effects of the initial state momentum anisotropy.

Björn Schenke

Chun Shen

Hybrid Color Glass Condensate and hydrodynamic description of the Relativistic Heavy Ion Collider small system scan

Finland

Mark Mace

Vladimir V. Skokov

Raju Venugopalan

Corrigendum to “Systematics of azimuthal anisotropy harmonics in proton–nucleus collisions at the LHC from the Color Glass Condensate” [Phys. Lett. B 788 (2019) 161–165]

Simple power counting arguments in the dilute-dense framework of the Color Glass Condensate (CGC) Effective Field Theory predict that even and odd azimuthal anisotropy harmonics of two-particle correlations in proton–nucleus collisions at the LHC will respectively satisfy <math><msubsup><mrow><mi>v</mi></mrow><mrow><mn>2</mn><mi>n</mi></mrow><mrow><mn>2</mn></mrow></msubsup><mo>{</mo><mn>2</mn><mo>}</mo><mo>∝</mo><msubsup><mrow><mi>N</mi></mrow><mrow><mi>ch</mi></mrow><mrow><mn>0</mn></mrow></msubsup></math> and <math><msubsup><mrow><mi>v</mi></mrow><mrow><mn>2</mn><mi>n</mi><mo>+</mo><mn>1</mn></mrow><mrow><mn>2</mn></mrow></msubsup><mo>{</mo><mn>2</mn><mo>}</mo><mo>∝</mo><msub><mrow><mi>N</mi></mrow><mrow><mi>ch</mi></mrow></msub></math>, where <math><msub><mrow><mi>N</mi></mrow><mrow><mi>ch</mi></mrow></msub></math> denotes the number of charged particles. We show that these expectations are borne out qualitatively, and even quantitatively, within systematic uncertainties, for <math><msub><mrow><mi>v</mi></mrow><mrow><mn>2</mn></mrow></msub></math> and <math><msub><mrow><mi>v</mi></mrow><mrow><mn>4</mn></mrow></msub></math> in comparisons with data from the ATLAS collaboration. We also observe that ATLAS data for the <math><msub><mrow><mi>v</mi></mrow><mrow><mn>3</mn></mrow></msub></math> azimuthal harmonic are in excellent agreement with our qualitative expectation; quantitative comparisons are numerically challenging at present. The lessons from this study fully complement those gained by the recent comparison of the CGC dilute-dense framework [1] to data from the PHENIX collaboration on small system collisions at RHIC.

Systematics of azimuthal anisotropy harmonics in proton–nucleus collisions at the LHC from the Color Glass Condensate

Heavy flavor measurements in high multiplicity proton-proton and proton-nucleus collisions at collider energies enable unique insights into their production and hadronization mechanism because experimental and theoretical uncertainties cancel in ratios of their cross sections relative to minimum bias events. We explore such event engineering using the color glass condensate (CGC) effective field theory to compute short-distance charmonium cross sections. The CGC is combined with heavy-quark fragmentation functions to compute <math><mrow><mi>D</mi></mrow></math>-meson cross sections; for the <math><mi>J</mi><mo>/</mo><mi>ψ</mi></math>, hadronization is described employing nonrelativistic QCD (NRQCD) and an improved color evaporation model. Excellent agreement is found between the CGC computations and the LHC heavy flavor data in high multiplicity events. Event engineering in this <math><mrow><mi>CGC</mi><mo>+</mo><mi>NRQCD</mi></mrow></math> framework reveals a very rapid growth in the fragmentation of the <math><mrow><mmultiscripts><mrow><msubsup><mrow><mi>S</mi></mrow><mrow><mn>1</mn></mrow><mrow><mo>[</mo><mn>8</mn><mo>]</mo></mrow></msubsup></mrow><mprescripts></mprescripts><none></none><mrow><mn>3</mn></mrow></mmultiscripts></mrow></math> state in rare events relative to minimum bias events.

Yan-Qing Ma

Kazuhiro Watanabe

Event engineering studies for heavy flavor production and hadronization in high multiplicity hadron-hadron and hadron-nucleus collisions

We demonstrate that the striking systematics of two-particle azimuthal Fourier harmonics <math><mrow><msub><mrow><mi>v</mi></mrow><mrow><mn>2</mn></mrow></msub></mrow></math> and <math><msub><mi>v</mi><mn>3</mn></msub></math> in ultrarelativistic collisions of protons, deuterons, and helium-3 ions off gold nuclei measured by the PHENIX Collaboration at the Relativistic Heavy Ion Collider (RHIC) is reproduced in the color glass condensate effective field theory. This contradicts the claim in C. Aidala et al. (PHENIX Collaboration), arXiv:1805.02973, that their data rule out initial state-based explanations. The underlying systematics of the effect, as discussed previously in K. Dusling, M. Mace, R. Venugopalan, Phys. Rev. D 97, 016014 (2018); Phys. Rev. Lett. 120, 042002 (2018); Proc. Sci., QCDEV2017 (2018) 039, arise from the differing structure of strong color correlations between gluon domains of size <math><mn>1</mn><mo>/</mo><msub><mi>Q</mi><mi>S</mi></msub></math> at fine (<math><mrow><msub><mrow><mi>p</mi></mrow><mrow><mo>⊥</mo></mrow></msub><mo>⪆</mo><msub><mrow><mi>Q</mi></mrow><mrow><mi>S</mi></mrow></msub></mrow></math>) or coarser (<math><mrow><msub><mrow><mi>p</mi></mrow><mrow><mo>⊥</mo></mrow></msub><mo>⪅</mo><msub><mrow><mi>Q</mi></mrow><mrow><mi>S</mi></mrow></msub></mrow></math>) transverse momentum resolution. Further tests of the limits of validity of this framework can be carried out in light-heavy ion collisions at both RHIC and the Large Hadron Collider. Such measurements also offer novel opportunities for further exploration of the role of the surprisingly large short-range nuclear correlations measured at Jefferson Lab.

Hierarchy of Azimuthal Anisotropy Harmonics in Collisions of Small Systems from the Color Glass Condensate

Results for particle production in s=5.02TeV p + Pb collisions at the Large Hadron Collider within a combined classical Yang–Mills and relativistic viscous hydrodynamic calculation are presented. We emphasize the importance of sub-nucleon scale fluctuations in the proton projectile to describe the experimentally observed azimuthal harmonic coefficients vn , demonstrating their sensitivity to the proton shape. We stress that the proton shape and its fluctuations are not free parameters in our calculations. Instead, they have been constrained using experimental data from HERA on exclusive vector meson production. Including temperature dependent shear and bulk viscosities, as well as UrQMD for the low temperature regime, we present results for mean transverse momenta, harmonic flow coefficients for charged hadrons and identified particles, as well as Hanbury–Brown–Twiss radii.

Heikki Mäntysaari

Imprints of fluctuating proton shapes on flow in proton-lead collisions at the LHC

Measurements of multiparticle correlations in the collisions of small systems such as p+p, p/d/3He+A show striking similarity to the observations in heavy-ion collisions. A number of observables measured in the high-multiplicity events of these systems resemble features that are attributed to collectivity driven by hydrodynamics. However, alternative explanations based on initial-state dynamics are able to describe many characteristic features of these measurements. In this brief review, we highlight some of the recent developments and outstanding issues in this direction.

Sören Schlichting

Collectivity in Small Collision Systems: An Initial-State Perspective