Going against the flow : Revealing the QCD degrees of freedom in hadronic collisions
Bierlich, Christian LU ; Christiansen, Peter LU ; Gustafson, Gösta LU ; Lönnblad, Leif LU
; Törnkvist, Robin
LU
and Zapp, Korinna
LU
(2026)
In Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics
874.
- Abstract
In collisions between heavy nuclei, such as those at the Large Hadron Collider (LHC) at CERN, hydrodynamic models have successfully related measured azimuthal momentum anisotropies to the transverse shape of the collision region. For an elliptically shaped interaction area, the hydrodynamic pressure gradient is greater along the minor axis, resulting in increased particle momentum in that direction — a phenomenon known as positive elliptic flow. In this paper, we demonstrate that in smaller systems, such as proton–proton and peripheral ion–ion collisions, microscopic models for final state interactions, can produce anisotropies where the elliptic flow is negative — that is, the momentum is largest along the major axis, contrary to... (More)
In collisions between heavy nuclei, such as those at the Large Hadron Collider (LHC) at CERN, hydrodynamic models have successfully related measured azimuthal momentum anisotropies to the transverse shape of the collision region. For an elliptically shaped interaction area, the hydrodynamic pressure gradient is greater along the minor axis, resulting in increased particle momentum in that direction — a phenomenon known as positive elliptic flow. In this paper, we demonstrate that in smaller systems, such as proton–proton and peripheral ion–ion collisions, microscopic models for final state interactions, can produce anisotropies where the elliptic flow is negative — that is, the momentum is largest along the major axis, contrary to hydrodynamic predictions. We present results from two distinct microscopic models: one based on repulsion between string-like fields and another based on effective kinetic theory. Negative elliptic flow is a solid prediction of the string interaction model while in the model based on kinetic theory it is linked to a finite interaction range. Consequently, an experimental determination of the sign of elliptic flow, will provide novel insights into the degrees of freedom governing strong nuclear interactions in high-energy collisions and the way in which they interact.
(Less)
- author
- Bierlich, Christian
LU
; Christiansen, Peter
LU
; Gustafson, Gösta
LU
; Lönnblad, Leif
LU
; Törnkvist, Robin
LU
and Zapp, Korinna
LU
- organization
- publishing date
- 2026-03
- type
- Contribution to journal
- publication status
- published
- subject
- keywords
- Elliptic flow, Kinetic theory, Lund strings, Quantum chromodynamics, Small systems
- in
- Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics
- volume
- 874
- article number
- 140216
- publisher
- Elsevier
- external identifiers
-
- scopus:105029251274
- ISSN
- 0370-2693
- DOI
- 10.1016/j.physletb.2026.140216
- language
- English
- LU publication?
- yes
- id
- 05025fa0-9e67-4425-b9e8-29f0c50de0eb
- date added to LUP
- 2026-02-18 11:55:37
- date last changed
- 2026-02-18 11:56:56
@article{05025fa0-9e67-4425-b9e8-29f0c50de0eb,
abstract = {{<p>In collisions between heavy nuclei, such as those at the Large Hadron Collider (LHC) at CERN, hydrodynamic models have successfully related measured azimuthal momentum anisotropies to the transverse shape of the collision region. For an elliptically shaped interaction area, the hydrodynamic pressure gradient is greater along the minor axis, resulting in increased particle momentum in that direction — a phenomenon known as positive elliptic flow. In this paper, we demonstrate that in smaller systems, such as proton–proton and peripheral ion–ion collisions, microscopic models for final state interactions, can produce anisotropies where the elliptic flow is negative — that is, the momentum is largest along the major axis, contrary to hydrodynamic predictions. We present results from two distinct microscopic models: one based on repulsion between string-like fields and another based on effective kinetic theory. Negative elliptic flow is a solid prediction of the string interaction model while in the model based on kinetic theory it is linked to a finite interaction range. Consequently, an experimental determination of the sign of elliptic flow, will provide novel insights into the degrees of freedom governing strong nuclear interactions in high-energy collisions and the way in which they interact.</p>}},
author = {{Bierlich, Christian and Christiansen, Peter and Gustafson, Gösta and Lönnblad, Leif and Törnkvist, Robin and Zapp, Korinna}},
issn = {{0370-2693}},
keywords = {{Elliptic flow; Kinetic theory; Lund strings; Quantum chromodynamics; Small systems}},
language = {{eng}},
publisher = {{Elsevier}},
series = {{Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics}},
title = {{Going against the flow : Revealing the QCD degrees of freedom in hadronic collisions}},
url = {{http://dx.doi.org/10.1016/j.physletb.2026.140216}},
doi = {{10.1016/j.physletb.2026.140216}},
volume = {{874}},
year = {{2026}},
}