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Urs Achim Wiedemann

Publications and source records attributed to Urs Achim Wiedemann.

At least 37 records · Page 2Linked to original sources
arXiv2019

The dark matter bispectrum from effective viscosity and one-particle irreducible vertices

Dark matter evolution during the process of cosmological structure formation can be described in terms of a one-particle irreducible effective action at a characteristic scale $k_m$ and a loop expansion below this scale, based on the effective propagators and vertices. We calculate the form of the effective vertices and compute the bispectrum of density perturbations within a one-loop approximation. We find that the effective vertices play a subdominant role as compared to the effective viscosity and sound velocity that modify the (inverse) propagators. For the bispectrum we reproduce the results of standard perturbation theory in the range where it is applicable, and find a slightly improved agreement with N-body simulations at larger wavenumbers.

astro-ph.CO↗
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

Thermal width of the Higgs boson in hot QCD matter

Following Caron-Huot and combining results for the thermal dependence of spectral functions at large time-like momenta, we write an explicit expression for the thermal width of the Higgs boson to $\mathcal{O}(α_\mathrm{s})$ for $T \ll M_H$. It is an $\mathcal{O}( α_\mathrm{s} (T/M_H)^4 )$ correction for $H\to gg$ and $H\to q\bar{q}$. We also compile corresponding results for the thermal width of the $Z$-boson, and we recall which generic structures of the field theory, accessible via the operator product expansion, fix the $T/M$-dependence of the decay of heavy particles.

hep-ph↗
arXiv2018

Collectivity in pp from resummed interference effects?

Azimuthal asymmetries $v_n$ in the soft transverse momentum spectra of hadronic collisions can result as a consequence of quantum interference and color flow which translates spatial anisotropies into momentum anisotropies via multipole radiation patterns. Here, we analyze to what extent these effects result in signal strengths $v_n\lbrace 2s\rbrace$ that can persist in higher order $(2s)$ cumulants. In a simple model of soft multi-particle production with quantum interference effects in which $m$ particles are emitted from $N$ sources and in which interference contributions appear naturally ordered in inverse powers of the adjoint color trace, $1/(N_c^2-1)$, we provide the first resummed calculation of all powers of $m^2/(N_c^2-1)$. This allows one to determine all higher order flow cumulants $v_n\lbrace 2s\rbrace$ with the same parametric accuracy. For a phenomenologically relevant range of $N$ sources emitting $m$ particles, we find that the even flow coefficients $v_n\lbrace 2s\rbrace$ decrease very mildly with increasing cumulants. This provides a proof of principle that non-vanishing higher order cumulants $v_n\lbrace 2s\rbrace$ can persist in systems that exhibit neither final state interactions nor phenomena related to high (saturated) initial parton densities.

hep-ph↗
arXiv2018

Nearly isentropic flow at sizeable $η/s$

Non-linearities in the harmonic spectra of hadron-nucleus and nucleus-nucleus collisions provide evidence for the dynamical response to azimuthal spatial eccentricities. Here, we demonstrate within the framework of transport theory that even the mildest interaction correction to a picture of free-streaming particle distributions, namely the inclusion of one perturbatively weak interaction ("one-hit dynamics"), will generically give rise to all observed linear and non-linear structures. We further argue that transport theory naturally accounts within the range of its validity for realistic signal sizes of the linear and non-linear response coefficients observed in azimuthal momentum anisotropies with a large mean free path of the order of the system size in peripheral ($\sim 50 \%$ centrality) PbPb or central pPb collisions. The shear viscosity to entropy density ratio $η/s$ of such a transport theory is approximately an order of magnitude larger than that of an almost perfect fluid. The phenomenological success of transport simulations thus challenges the perfect fluid paradigm of ultra-relativistic nucleus-nucleus and hadron-nucleus collisions.

hep-ph↗
arXiv2017

Analytic structure of nonhydrodynamic modes in kinetic theory

How physical systems approach hydrodynamic behavior is governed by the decay of nonhydrodynamic modes. Here, we start from a relativistic kinetic theory that encodes relaxation mechanisms governed by different timescales thus sharing essential features of generic weakly coupled nonequilib- rium systems. By analytically solving for the retarded correlation functions, we clarify how branch cuts arise generically from noncollective particle excitations, how they interface with poles arising from collective hydrodynamic excitations, and to what extent the appearance of poles remains at best an ambiguous signature for the onset of fluid dynamic behavior. We observe that processes that are slower than the hydrodynamic relaxation timescale can make a system that has already reached fluid dynamic behavior to fall out of hydrodynamics at late times. In addition, the analytical control over this model allows us to explicitly demonstrate how the hydrodynamic gradient expansion of the correlation functions can be Borel resummed such that the full nonperturbative information is recovered using perturbative input only.

hep-ph↗
arXiv2017

Collectivity from interference

In hadronic collisions, interference between different production channels affects momentum distributions of multi-particle final states. As this QCD interference does not depend on the strong coupling constant, it is part of the no-interaction baseline that needs to be controlled prior to searching for other manifestations of collective dynamics. Here, we introduce a model that is based on the QCD theory of multi-parton interactions and that allows one to study interference effects in the production of $m$ particles in hadronic collisions with $N$ parton-parton interactions ("sources"). In an expansion in powers of $1/(N_c^2-1)$ and to leading order in the number of sources $N$, we calculate interference effects in the $m$-particle spectra and we determine from them the second and fourth order cumulant momentum anisotropies $v_n$. Without invoking any azimuthal asymmetry and any density dependent non-linear dynamics in the incoming state, and without invoking any interaction in the final state, we find that QCD interference alone can give rise to values for $v_n\lbrace 2\rbrace$ and $v_n\lbrace 4\rbrace$, $n$ even, that persist unattenuated for increasing number of sources, that may increase with increasing multiplicity and that agree with measurements in proton-proton (pp) collisions in terms of the order of magnitude of the signal and the approximate shape of the transverse momentum dependence. We further find that the non-abelian features of QCD interference can give rise to odd harmonic anisotropies. These findings indicate that the no-interaction baseline including QCD interference effects can make a sizeable if not dominant contribution to the measured $v_n$ coefficients in pp collisions. Prospects for analyzing QCD interference contributions further and their possible relevance for proton-nucleus and nucleus-nucleus collisions are discussed shortly.

hep-ph↗
arXiv2017

Sensitivity of jet substructure to jet-induced medium response

Jet quenching in heavy ion collisions is expected to be accompanied by recoil effects, but unambiguous signals for the induced medium response have been difficult to identify so far. Here, we argue that modern jet substructure measurements can improve this situation qualitatively since they are sensitive to the momentum distribution inside the jet. We show that the groomed subjet shared momentum fraction $z_g$, and the girth of leading and subleading subjets signal recoil effects with dependencies that are absent in a recoilless baseline. We find that recoil effects can explain most of the medium modifications to the $z_g$ distribution observed in data. Furthermore, for jets passing the Soft Drop Condition, recoil effects induce in the differential distribution of subjet separation $ΔR_{12}$ a characteristic increase with $ΔR_{12}$, and they introduce a characteristic enhancement of the girth of the subleading subjet with decreasing $z_g$. We explain why these qualitatively novel features, that we establish in \textsc{Jewel+Pythia} simulations, reflect generic physical properties of recoil effects that should therefore be searched for as telltale signatures of jet-induced medium response.

hep-ph↗
arXiv2016

Renormalization-group flow of the effective action of cosmological large-scale structures

Following an approach of Matarrese and Pietroni, we derive the functional renormalization group (RG) flow of the effective action of cosmological large-scale structures. Perturbative solutions of this RG flow equation are shown to be consistent with standard cosmological perturbation theory. Non-perturbative approximate solutions can be obtained by truncating the a priori infinite set of possible effective actions to a finite subspace. Using for the truncated effective action a form dictated by dissipative fluid dynamics, we derive RG flow equations for the scale dependence of the effective viscosity and sound velocity of non-interacting dark matter, and we solve them numerically. Physically, the effective viscosity and sound velocity account for the interactions of long-wavelength fluctuations with the spectrum of smaller-scale perturbations. We find that the RG flow exhibits an attractor behaviour in the IR that significantly reduces the dependence of the effective viscosity and sound velocity on the input values at the UV scale. This allows for a self-contained computation of matter and velocity power spectra for which the sensitivity to UV modes is under control.

astro-ph.CO↗
arXiv2015

The sensitivity of R_pA to color recombination effects

In hadronization models with color recombination, partons are allowed to regroup into color singlet structures that are different from those determined by the perturbative parton shower. This aims at modeling the possibility that soft interactions of partons with the underlying event can change color connections. If such an effect is at play in proton-proton collisions, it may be expected to be enhanced in proton-nucleus collisions due to the higher color charge density in the underlying event. Here, we provide a qualitative argument that color recombination effects could lead to a multiplicity dependent hardening of single inclusive hadron spectra that dies out very weakly with increasing transverse momentum. We present results of a (conservative) model implementation in the cluster hadronization model of the SHERPA event generator. In this model, we find that color recombination effects harden indeed the single inclusive hadron spectra without affecting the jet spectra, but that this effect does not depend significantly on underlying event activity. We explain this model feature and we argue why, in general, data on proton-nucleus collisions can help to constrain hadronization models used in proton-proton event generators.

nucl-th↗
arXiv2015

Large scale structure from viscous dark matter

Cosmological perturbations of sufficiently long wavelength admit a fluid dynamic description. We consider modes with wavevectors below a scale $k_m$ for which the dynamics is only mildly non-linear. The leading effect of modes above that scale can be accounted for by effective non-equilibrium viscosity and pressure terms. For mildly non-linear scales, these mainly arise from momentum transport within the ideal and cold but inhomogeneous fluid, while momentum transport due to more microscopic degrees of freedom is suppressed. As a consequence, concrete expressions with no free parameters, except the matching scale $k_m$, can be derived from matching evolution equations to standard cosmological perturbation theory. Two-loop calculations of the matter power spectrum in the viscous theory lead to excellent agreement with $N$-body simulations up to scales $k=0.2 \, h/$Mpc. The convergence properties in the ultraviolet are better than for standard perturbation theory and the results are robust with respect to variations of the matching scale.

astro-ph.CO↗
arXiv2015

Backreaction effects on the matter side of Einstein's field equations

Recently, we have derived a novel and compact expression for how perturbations in the matter fields of the cosmological fluid can lead to deviations from the standard Friedmann equations. Remarkably, the dissipative damping of velocity perturbations by bulk and shear viscosity in the dark sector can modify the expansion history of the universe on arbitrarily large scales. In universes in which this effect is sufficiently sizeable, it could account for the acceleration of the cosmological expansion. But even if dark matter should be less viscous and if the effect would be correspondingly smaller, it may have observable consequences in the era of precision cosmology. Here, we review the origin of this backreaction effect and possibilities to constrain it further.

gr-qc↗
arXiv2015

Nonlinear evolution of density and flow perturbations on a Bjorken background

Density perturbations and their dynamic evolution from early to late times can be used for an improved understanding of interesting physical phenomena both in cosmology and in the context of heavy-ion collisions. We discuss the spectrum and bispectrum of these perturbations around a longitudinally expanding fireball after a heavy-ion collision. The time-evolution equations couple the spectrum and bispectrum to each other, as well as to higher-order correlation functions through nonlinear terms. A non-trivial bispectrum is thus always generated, even if absent initially. For initial conditions corresponding to a model of independent sources, we discuss the linear and nonlinear evolution is detail. We show that, if the initial conditions are sufficiently smooth for fluid dynamics to be applicable, the nonlinear effects are relatively small.

hep-ph↗
arXiv2015

Accelerating cosmological expansion from shear and bulk viscosity

The dissipation of energy from local velocity perturbations in the cosmological fluid affects the time evolution of spatially averaged fluid dynamic fields and the cosmological solution of Einstein's field equations. We show how this backreaction effect depends on shear and bulk viscosity and other material properties of the dark sector, as well as the spectrum of perturbations. If sufficiently large, this effect could account for the acceleration of the cosmological expansion.

gr-qc↗
arXiv2014

Statistics of initial density perturbations in heavy ion collisions and their fluid dynamic response

An interesting opportunity to determine thermodynamic and transport properties in more detail is to identify generic statistical properties of initial density perturbations. Here we study event-by-event fluctuations in terms of correlation functions for two models that can be solved analytically. The first assumes Gaussian fluctuations around a distribution that is fixed by the collision geometry but leads to non-Gaussian features after averaging over the reaction plane orientation at non-zero impact parameter. In this context, we derive a three-parameter extension of the commonly used Bessel-Gaussian event-by-event distribution of harmonic flow coefficients. Secondly, we study a model of N independent point sources for which connected n-point correlation functions of initial perturbations scale like 1/N^(n-1). This scaling is violated for non-central collisions in a way that can be characterized by its impact parameter dependence. We discuss to what extent these are generic properties that can be expected to hold for any model of initial conditions, and how this can improve the fluid dynamical analysis of heavy ion collisions.

hep-ph↗