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Berndt Müller

Publications and source records attributed to Berndt Müller.

At least 55 records · Page 3Linked to original sources
arXiv2020

Phenomenological implications of asymmetric $AdS_5$ shockwave collision studies for heavy ion physics

This paper discusses possible phenomenological implications for p+A and A+A collisions of the results of recent numerical AdS/CFT calculations examining asymmetric collisions of planar shocks. In view of the extreme Lorentz contraction, we model highly relativistic heavy ion collisions (HICs) as a superposition of collisions between many near-independent transverse "pixels" with differing incident longitudinal momenta. It was found that also for asymmetric collisions the hydrodynamization time is in good approximation a proper time, just like for symmetric collisions, depending on the geometric mean of the longitudinally integrated energy densities of the incident projectiles. For realistic collisions with fluctuations in the initial energy densities, these results imply a substantial increase in the hydrodynamization time for highly asymmetric pixels. However, even in this case the local hydrodynamization time still is significantly smaller than perturbative results for the thermalization time.

hep-ph↗
arXiv2020

Shear viscosity of classical Yang-Mills field

We investigate the shear viscosity $η$ of the classical Yang-Mills (CYM) field on a lattice by using the Green-Kubo formula, where the shear viscosity is calculated from the time-correlation function of the energy-momentum tensor in equilibrium. Dependence of the shear viscosity $η(g,T)$ on the coupling $g$ and temperature $T$ is represented by a scaling function $f_η(g^2T)$ as $η(g,T)=Tf_η(g^2T)$ due to the scaling-invariant property of the CYM. The explicit functional form of $f_η(g^2T)$ is successfully determined from the calculated shear viscosity: It turns out that $η(g,T)$ of the CYM field is proportional to $1/g^{1.10-1.88}$ at weak coupling, which is a weaker dependence on $g$ than that in the leading-order perturbation theory but consistent with that of the "anomalous viscosity" $η\propto 1/g^{1.5}$ under the strong disordered field. The obtained shear viscosity is also found to be roughly consistent with that estimated through the analysis of the anisotropy of the pressure of the CYM dynamics in the expanding geometry with recourse to a hydrodynamic equation.

hep-ph↗
arXiv2020

Issues with Search for Critical Point in QCD with Relativistic Heavy Ion Collisions

A systematic search for a critical point in the phase diagram of QCD matter is underway at the Relativistic Heavy Ion Collider (RHIC) and is planned at several future facilities. Its existence, if confirmed, and its location will greatly enhance our understanding of QCD. In this note we emphasize several important issues that are often not fully recognized in theoretical interpretations of experimental results relevant to the critical point search. We discuss ways in which our understanding on these issues can be improved.

nucl-th↗
arXiv2019

Fate of Heavy Quark Bound States inside Quark-Gluon Plasma

Transport equations have been applied successfully to describe the quarkonium evolution inside the quark-gluon plasma, which include both plasma screening effects and recombination. We demonstrate how the quarkonium transport equation is derived from QCD by using the open quantum system framework and effective field theory. Weak coupling and Markovian approximations used in the derivation are justified from a separation of scales. By solving the equations numerically, we study the Upsilon production in heavy ion collisions.

hep-ph↗
arXiv2019

Quarkonium inside Quark-Gluon Plasma: Diffusion, Dissociation, Recombination and Energy Loss

We consider the quarkonium diffusion, dissociation and recombination inside quark-gluon plasma. We compute scattering amplitudes in potential nonrelativistic QCD for relevant processes. These processes include the gluon absorption/emission at the order $gr$, inelastic scattering at the order $g^2r$ and elastic scattering with medium constituents at the order $g^2r^2$. We show these amplitudes satisfy the Ward identity. We also consider one-loop corrections. The dipole interaction between the color singlet and octet is not running at the one-loop level. Interference between the tree-level gluon absorption/emission and its thermal loop corrections cancels the collinear divergence in the $t$-channel inelastic scattering. The inelastic scattering has no soft divergence because of the finite binding energy of quarkonium. We write out the diffusion, dissociation and recombination terms explicitly for a Boltzmann transport equation and define the dissociation and recombination rates. Furthermore, we calculate the diffusion coefficient of quarkonium. We find our result of diffusion coefficient differs from a previous calculation by two to three orders of magnitude. We explain this and can reproduce the previous result in a certain limit. Finally we discuss two mechanisms of quarkonium energy loss inside quark-gluon plasma.

hep-ph↗
arXiv2018

The Chiral Magnetic Effect and an experimental bound on the late time magnetic field strength

We first compare different approaches to estimates of the magnitude of the chiral magnetic effect in relativistic heavy ion collisions and show that their main difference lies in the assumptions on the length of persistence of the magnetic field generated by the colliding nuclei. We then analyze recent measurements of the global polarization of $Λ$ and $\bar Λ$ hyperons in terms of the bounds they set on the magnitude of the late time magnetic field.

hep-ph↗
arXiv2018

Quarkonium production in heavy ion collisions: coupled Boltzmann transport equations

We develop a set of coupled Boltzmann equations to describe the dynamical evolution of heavy quarks and quarkonia inside the quark-gluon plasma. The quarkonium dissociation and recombination terms are calculated from pNRQCD. Their interplay drives the system to a detailed balance. The heavy quark energy loss term is necessary for the system to reach kinematic thermalization. By coupling the transport equations with initial particles' momenta generated by PYTHIA and hydrodynamic medium evolutions, we can describe the $R_{AA}$ of $Υ$ family at both RHIC and LHC energies. The transverse momentum azimuthal anisotropy of $Υ$(1S) in $2.76$ TeV peripheral Pb-Pb collisions is also studied.

nucl-th↗
arXiv2018

Approach to equilibrium of quarkonium in quark-gluon plasma

We calculate the dissociation and recombination rates of $Υ(1S)$ in quark-gluon plasma by using potential non-relativistic QCD. We then study the dynamical in-medium evolution of the $b\bar{b}-Υ$ system in a periodic box via the Boltzmann equation and explore how the system reaches equilibrium. We find that interactions between the free heavy quarks and the medium are necessary for the system to reach equilibrium. We find that the angular distribution of $Υ(1S)$ probes the stages at which recombination occurs. Finally, we study the system under a longitudinal expansion and show that different initial conditions evolve to distinct final ratios of hidden and open $b$ flavors. We argue that experimental measurements of the ratio could address open questions in the quarkonium production in heavy ion collisions.

hep-ph↗
arXiv2018

Doubly charmed baryon production in heavy ion collisions

We give an estimate of $Ξ_{cc}^{++}$ production rate and transverse momentum spectra in relativistic heavy ion collisions. We use Boltzmann transport equations to describe the dynamical evolution of charm quarks and diquarks inside quark-gluon plasma. In-medium formation and dissociation rates of charm diquarks are calculated from potential non-relativistic QCD for the diquark sector. We solve the transport equations by Monte Carlo simulations. For $2.76$ TeV Pb-Pb collisions with $0-10\%$ centrality, the number of $Ξ_{cc}^{++}$ produced in the transverse momentum range $0-5$ GeV and rapidity from $-1$ to $1$ is roughly $0.02$ per collision. We repeat the calculation with a melting temperature $250$ MeV above which no diquarks can be formed. The number of $Ξ_{cc}^{++}$ produced in the same kinematic region is about $0.0125$ per collision. We discuss how to study diquarks at finite temperature on a lattice and construct the anti-triplet free energy in a gauge invariant but path dependent way. We also comment on extensions of the calculation to other doubly heavy baryons and doubly heavy tetraquarks and the feasibility of experimental measurements.

hep-ph↗
arXiv2018

Why does the thermal model for hadron production in heavy ion collisions work?

The yields for hadrons and even light nuclei measured at midrapidity in relativistic heavy ion collisions are found to be dictated exclusively by their thermal Boltzmann factor for a common temperature of approximately 155 MeV. The reason for the validity of the thermal model description is widely discussed. Here, we offer a new type of argument in its favor.

nucl-th↗
arXiv2017

Hot Spaghetti: Viscous Gravitational Collapse

We explore the fate of matter falling into a macroscopic Schwarzschild black hole for the simplified case of a radially collapsing thin spherical shell for which the back reaction of the geometry can be neglected. We treat the internal dynamics of the infalling matter in the framework of viscous relativistic hydrodynamics and calculate how the internal temperature of the collapsing matter evolves as it falls toward the Schwarzschild singularity. We find that viscous hydrodynamics fails when either, the dissipative radial pressure exceeds the thermal pressure and the total radial pressure becomes negative, or the time scale of variation of the tidal forces acting on the collapsing matter becomes shorter than the characteristic hydrodynamic response time.

gr-qc↗
arXiv2017

From Strangeness Enhancement to Quark-Gluon Plasma Discovery

This is a short survey of signatures and characteristics of the quark-gluon plasma in the light of experimental results that have been obtained over the past three decades. In particular, we present an in-depth discussion of the strangeness observable, including a chronology of the experimental effort to detect QGP at CERN-SPS, BNL-RHIC, and CERN-LHC.

nucl-th↗
arXiv2017

Dynamical Screening of $α$-$α$ Resonant Scattering and Thermal Nuclear Scattering Rate in a Plasma

We use effective field theory and thermal field theory to study the dynamical screening effect in the QED plasma on the $α$-$α$ scattering at the $^8$Be resonance. Dynamical screening leads to an imaginary part of the potential which results in a thermal width for the resonance and dominates over the previously considered static screening effect. As a result, both the resonance energy and width increase with the plasma temperature. Furthermore, dynamical screening can have a huge impact on the $α$-$α$ thermal nuclear scattering rate. For example, when the temperature is around $10$ keV, the rate is suppressed by a factor of about $900$. We expect similar thermal suppressions of nuclear reaction rates to occur in those reactions dominated by an above threshold resonance with a thermal energy. Dynamical screening effects on nuclear reactions can be relevant to cosmology and astrophysics.

nucl-th↗
arXiv2016

Shear Viscosities of Photons in Strongly Coupled Plasmas

We investigate the shear viscosity of thermalized photons in the quark gluon plasma (QGP) at weak coupling and $\mathcal{N}=4$ super Yang-Mills plasma (SYMP) at both strong and weak couplings. We find that the shear viscosity due to the photon-parton scattering up to the leading order of electromagnetic coupling is suppressed when the coupling of the QGP/SYMP is increased, which stems from the blue-shift of the thermal-photon spectrum at strong coupling. In addition, the shear viscosity rapidly increases near the deconfinement transition in a phenomenological model analogous to the QGP.

hep-th↗
arXiv2016

An Effective Field Theory Approach to the Stabilization of $^8$Be in a QED Plasma

We use effective field theory to study the $\mathrmα$-$\mathrmα$ resonant scattering in a finite-temperature QED plasma. The static plasma screening effect causes the resonance state $^8$Be to live longer and eventually leads to the formation of a bound state when $m_D\gtrsim 0.3$ MeV. We speculate that this effect may have implications on the rates of cosmologically and astrophysically relevant nuclear reactions involving $\mathrmα$ particles.

nucl-th↗
arXiv2016

Probing QED Vacuum with Heavy Ions

We recall how nearly half a century ago the proposal was made to explore the structure of the quantum vacuum using slow heavy-ion collisions. Pursuing this topic we review the foundational concept of spontaneous vacuum decay accompanied by observable positron emission in heavy-ion collisions and describe the related theoretical developments in strong fields QED.

nucl-th↗