arXiv subjects

Aleksi Kurkela

Publications and source records attributed to Aleksi Kurkela.

At least 55 records · Page 3Linked to original sources

arXiv2022

How perturbative QCD constrains the Equation of State at Neutron-Star densities

We demonstrate in a general and analytic way how high-density information about the equation of state (EoS) of strongly interacting matter obtained using perturbative Quantum Chromodynamics (pQCD) constrains the same EoS at densities reachable in physical neutron stars. Our approach is based on utilizing the full information of the thermodynamic potentials at the high-density limit together with thermodynamic stability and causality. This requires considering the pressure as a function of chemical potential $p(μ)$ instead of the commonly used pressure as a function of energy density $p(ε)$. The results can be used to propagate the pQCD calculations reliable around 40$n_s$ to lower densities in the most conservative way possible. We constrain the EoS starting from only few times the nuclear saturation density $n \gtrsim 2.2 n_s$ and at $n = 5 n_s$ we exclude at least 65% of otherwise allowed area in the $(ε- p)$-plane. This provides information complementary to astrophysical observations that should be taken into account in any complete statistical inference study of the EoS. These purely theoretical results are independent of astrophysical neutron-star input, and hence, they can also be used to test theories of modified gravity and BSM physics in neutron stars.

nucl-th↗

arXiv2022

Degenerate fermionic matter at N$^3$LO: Quantum Electrodynamics

We determine the pressure of a cold and dense electron gas to a nearly complete next-to-next-to-next-to-leading order (N$^3$LO) in the fine-structure constant $α_e$, utilizing a new result for the two-loop photon self-energy from a companion paper. Our result contains all infrared-sensitive contributions to the pressure at this order, including the coefficient of the $O(α_e^3 \ln α_e^{ })$ term, and leaves only a single coefficient associated with the contributions of unresummed hard momenta undetermined. Moreover, we explicitly demonstrate the complete cancellation of infrared divergences according to the effective field theory paradigm by determining part of the hard contributions at this order. Our calculation provides the first improvement to a 45-year-old milestone result and demonstrates the feasibility of the corresponding N$^3$LO calculation for cold and dense quark matter.

hep-ph↗

arXiv2022

Soft photon propagation in a hot and dense medium to next-to-leading order

We present the first complete calculation of the soft photon self-energy to the next-to-leading order in a hot and/or dense ultrarelativistic plasma in Quantum Electrodynamics (QED). The calculation is performed within the real-time formalism utilizing dimensional regularization in $4-2ε$ dimensions, while the result is reported including explicit $O(ε)$ terms in the zero-temperature limit. This information is required to extend the perturbative calculation of the pressure of cold and dense QED matter to partial next-to-next-to-next-to-leading order in a weak-coupling expansion, reported in a companion paper. These results pave the way for a similar future calculation in Quantum Chromodynamics.

hep-ph↗

arXiv2021

Collective flow in single-hit QCD kinetic theory

Motivated by recent interest in collectivity in small systems, we calculate the harmonic flow response to initial geometry deformations within weakly coupled QCD kinetic theory using the first correction to the free-streaming background. We derive a parametric scaling formula that relates harmonic flow in systems of different sizes and different generic initial gluon distributions. We comment on similarities and differences between the full QCD effective kinetic theory and the toy models used previously. Finally we calculate the centrality dependence of the integrated elliptic flow $v_2$ in oxygen-oxygen, proton-lead and proton-proton collision systems.

hep-ph↗

arXiv2021

Thermalization of non-abelian gauge theories at next-to-leading order

We provide the first next-to-leading-order (NLO) weak-coupling description of the thermalization process of far-from-equilibrium systems in non-abelian gauge theory. We study isotropic systems starting from either over- or under-occupied initial conditions and follow their time evolution towards thermal equilibrium by numerically solving the QCD effective kinetic theory at NLO accuracy. We find that the NLO corrections remain well under control for a wide range of couplings and that the overall effect of NLO corrections is to reduce the time needed to reach thermal equilibrium in the systems considered.

hep-ph↗

arXiv2021

Evidence for quark-matter cores in massive neutron stars

The theory governing the strong nuclear force, Quantum Chromodynamics, predicts that at sufficiently high energy densities hadronic nuclear matter undergoes a deconfinement transition to a new phase of quarks and gluons. Although this has been observed in ultrarelativistic heavy-ion collisions, it is currently an open question whether quark matter exists inside neutron stars. By combining astrophysical observations and theoretical ab-initio calculations in a model-independent way, we find that the inferred properties of matter in the cores of neutron stars with mass corresponding to 1.4 solar masses are compatible with nuclear model calculations. However, the matter in the interior of maximally massive, stable neutron stars exhibits characteristics of the deconfined phase, which we interpret as evidence for the presence of quark-matter cores. For the heaviest reliably observed neutron stars with masses of about two solar masses, the presence of quark matter is found to be linked to the behaviour of the speed of sound c_s in strongly interacting matter. If the conformal bound (c_s)^2 < 1/3 is not strongly violated, massive neutron stars are predicted to have sizable quark-matter cores. This finding has important implications for the phenomenology of neutron stars, and affects the dynamics of neutron star mergers with at least one sufficiently massive participant.

astro-ph.HE↗

arXiv2021

Predicting parton energy loss in small collision systems

Medium induced parton energy loss is not conclusively established neither in very peripheral heavy-ion collisions nor in proton-ion collisions. However, the standard interpretation of azimuthal momentum anisotropies in theses systems implies some partonic rescattering. The upcoming light-ion runs at the LHC provide a unique opportunity to search for parton energy loss in different systems of similar size. Here, we make predictions for the expected parton energy loss signal in the charged hadron spectra in a system size scan at LHC. We test a large set of model assumptions against the transverse momentum and centrality dependence of the charged hadron nuclear modification factor in lead-lead and xenon-xenon collisions at the LHC. We then attempt to make a model agnostic prediction for the charged hadron nuclear modification factor in oxygen-oxygen collisions.

hep-ph↗

arXiv2021

Discovering partonic rescattering in light nucleus collisions

We demonstrate that oxygen-oxygen (OO) collisions at the LHC provide unprecedented sensitivity to parton energy loss in a system whose size is comparable to those created in very peripheral heavy-ion collisions. With leading and next-to-leading order calculations of nuclear modification factors, we show that the baseline in the absence of partonic rescattering is known with up to 2% theoretical accuracy in inclusive OO collisions. Surprisingly, a $Z$-boson normalized nuclear modification factor does not lead to higher theoretical accuracy within current uncertainties of nuclear parton distribution functions. We study a broad range of parton energy loss models and we find that the expected signal of partonic rescattering can be disentangled from the baseline by measuring charged hadron spectra in the range $20\,\text{GeV}<p_T<100\,\text{GeV}$

hep-ph↗

arXiv2020

Perturbative Thermal QCD: Formalism and Applications

In this review article, we discuss the current status and future prospects of perturbation theory as a means of studying the equilibrium thermodynamic and near-equilibrium transport properties of deconfined QCD matter. We begin with a brief introduction to the general topic, after which we review in some detail the foundations and modern techniques of the real- and imaginary-time formalisms of thermal field theory, covering e.g. the different bases used in the real-time formalism and the resummations required to deal with soft and collinear contributions. After this, we discuss the current status of applications of these techniques, including topics such as electromagnetic rates, transport coefficients, jet quenching, heavy quarks and quarkonia, and the Equations of State of hot quark-gluon plasma as well as cold and dense quark matter. Finally, we conclude with our view of the future directions of the field, i.e. how we anticipate perturbative calculations to contribute to our collective understanding of strongly interacting matter in the coming years.

hep-ph↗

arXiv2020

Hydrodynamization in systems with detailed transverse profiles

The observation of fluid-like behavior in nucleus-nucleus, proton-nucleus and high-multiplicity proton-proton collisions motivates systematic studies of how different measurements approach their fluid-dynamic limit. We have developed numerical methods to solve the ultra-relativistic Boltzmann equation for systems of arbitrary size and transverse geometry. Here, we apply these techniques for the first time to the study of azimuthal flow coefficients $v_n$ including non-linear mode-mode coupling and to an initial condition with realistic event-by-event fluctuations. We show how both linear and non-linear response coefficients extracted from $v_n$ develop as a function of opacity from free streaming to perfect fluidity. We note in particular that away from the fluid-dynamic limit, the signal strength of linear and non-linear response coefficients does not reduce uniformly, but that their hierarchy and relative size shows characteristic differences.

hep-ph↗

arXiv2020

Non-equilibrium attractor in high-temperature QCD plasmas

We establish the existence of a far-from-equilibrium attractor in weakly-coupled gauge theory undergoing one-dimensional Bjorken expansion. We demonstrate that the resulting far-from-equilibrium evolution is insensitive to certain features of the initial condition, including both the initial momentum-space anisotropy and initial occupancy. We find that this insensitivity extends beyond the energy-momentum tensor to the detailed form of the one-particle distribution function. Based on our results, we assess different procedures for reconstructing the full one-particle distribution function from the energy-momentum tensor along the attractor and discuss implications for the freeze-out procedure used in the phenomenological analysis of ultra-relativistic nuclear collisions.

hep-ph↗

arXiv2019

What attracts to attractors?

Whether, how, and to what extent solutions of Bjorken-expanding systems become insensitive to aspects of their initial conditions is of importance for heavy-ion collisions. Here we study 1+1D and phenomenologically relevant boost-invariant 3+1D systems in which initial conditions approach a universal attractor solution. In Israel-Stewart theory (IS) and kinetic theory where the universal attractor extends to arbitrarily early times, we show that all initial conditions approach the attractor at early times by a power-law while their approach is exponential at late times. In these theories, the physical mechanisms of hydrodynamization operational at late times do not drive the approach to the attractor at early times, and the early-time attractor is reached prior to hydrodynamization. In marked contrast, the attractor in strongly coupled systems is realized concurrent with hydrodynamization. This qualitative difference may offer a basis for discriminating weakly and strongly coupled scenarios of heavy-ion collisions.

hep-ph↗

arXiv2019

Chemical equilibration of QGP in hadronic collisions

We performed state-of-the-art QCD effective kinetic theory simulations of chemically equilibrating QGP in longitudinally expanding systems. We find that chemical equilibration takes place after hydrodynamization, but well before local thermalization. By relating the transport properties of QGP and the system size we estimate that hadronic collisions with final state multiplicities $dN_\text{ch}/dη> 10^2$ live long enough to reach approximate chemical equilibrium for all collision systems. Therefore we expect the saturation of strangeness enhancement to occur at the same multiplicity in proton-proton, proton-nucleus and nucleus-nucleus collisions.

hep-ph↗

arXiv2019

Opacity dependence of elliptic flow in kinetic theory

The observation of large azimuthal anisotropies $v_n$ in the particle spectra of proton-proton (pp) and proton-nucleus (pA) collisions challenges fluid dynamic interpretations of $v_n$, as it remains unclear how small collision systems can hydrodynamize and to what extent hydrodynamization is needed to build up $v_n$. Here, we study in a simple kinetic theory how the same physics that leads to hydrodynamization in large systems represents itself in small systems. We observe that one third to one half of the elliptic flow signal seen in fully hydrodynamized systems can be built up in collisions that extend over only one mean free path $l_{\rm mfp}$ and that do not hydrodynamize. This is qualitatively in line with observing a sizeable $v_2$ in $pp$ collisions for which other characteristics of soft multi-particle production seem well-described in a free-streaming picture. We further expose a significant system size dependence in the accuracy of hybrid approaches that match kinetic theory to viscous fluid dynamics. The implications of these findings for a reliable extraction of shear viscosity are discussed.

hep-ph↗

arXiv2019

Dynamical Friction in Interacting Relativistic Systems

We study dynamical friction in interacting relativistic systems with arbitrary mean free paths and medium constituent masses. Our novel framework recovers the known limits of ideal gas and ideal fluid when the mean free path goes to infinity or zero, respectively, and allows for a smooth interpolation between these limits. We find that in an infinite system the drag force can be expressed as a sum of ideal-gas-like and ideal-fluid-like contributions leading to a finite friction even at subsonic velocities. This simple picture receives corrections in any finite system and the corrections become especially significant for a projectile moving at a velocity $v$ close to the speed of sound $v\approx c_s$. These corrections smoothen the ideal fluid discontinuity around the speed of sound and render the drag force a continuous function of velocity. We show that these corrections can be computed to a good approximation within effective theory of viscous fluid dynamics.

hep-ph↗

arXiv2019

Flow in AA and pA as an interplay of fluid-like and non-fluid like excitations

To study the microscopic structure of quark-gluon plasma, data from hadronic collisions must be confronted with models that go beyond fluid dynamics. Here, we study a simple kinetic theory model that encompasses fluid dynamics but contains also particle-like excitations in a boost invariant setting with no symmetries in the transverse plane and with large initial momentum asymmetries. We determine the relative weight of fluid dynamical and particle like excitations as a function of system size and energy density by comparing kinetic transport to results from the 0th, 1st and 2nd order gradient expansion of viscous fluid dynamics. We then confront this kinetic theory with data on azimuthal flow coefficients over a wide centrality range in PbPb collisions at the LHC, in AuAu collisions at RHIC, and in pPb collisions at the LHC. Evidence is presented that non-hydrodynamic excitations make the dominant contribution to collective flow signals in pPb collisions at the LHC and contribute significantly to flow in peripheral nucleus-nucleus collisions, while fluid-like excitations dominate collectivity in central nucleus-nucleus collisions at collider energies.

hep-ph↗

arXiv2019

Chemical Equilibration in Weakly Coupled QCD

We study thermalization, hydrodynamization, and chemical equilibration in out-of-equilibrium Quark-Gluon Plasma starting from various initial conditions using QCD effective kinetic theory, valid at weak coupling. In non-expanding systems gauge bosons rapidly lose information of the initial state and achieve kinetic equilibrium among themselves, while fermions approach the equilibrium distribution only at later time. In systems undergoing rapid longitudinal expansion, both gluons and quarks are kept away from equilibrium by the expansion, but the evolution is well described by fluid dynamics even before local thermal equilibrium is reached. For realistic couplings we determine the ordering between the separate hydrodynamization, chemical equilibration and thermalization time scales to be $τ_\text{hydro}<τ_\text{chem}<τ_\text{therm}$.

hep-ph↗

arXiv2019

Matching the Nonequilibrium Initial Stage of Heavy Ion Collisions to Hydrodynamics with QCD Kinetic Theory

High-energy nuclear collisions produce a nonequilibrium plasma of quarks and gluons which thermalizes and exhibits hydrodynamic flow. There are currently no practical frameworks to connect the early particle production in classical field simulations to the subsequent hydrodynamic evolution. We build such a framework using nonequilibrium Green's functions, calculated in QCD kinetic theory, to propagate the initial energy-momentum tensor to the hydrodynamic phase. We demonstrate that this approach can be easily incorporated into existing hydrodynamic simulations, leading to stronger constraints on the energy density at early times and the transport properties of the QCD medium. Based on (conformal) scaling properties of the Green's functions, we further obtain pragmatic bounds for the applicability of hydrodynamics in nuclear collisions.

hep-ph↗