arXiv subjects

Bjoern Schenke

Publications and source records attributed to Bjoern Schenke.

At least 37 records · Page 2Linked to original sources

arXiv2020

Diffractive dijet production in impact parameter dependent saturation models

We study coherent diffractive dijet production in electron-hadron and electron-nucleus collisions within the dipole picture. We provide semi-analytic results for the differential cross section and elliptic anisotropy in the angle between dijet transverse momentum and hadron recoil momentum. We demonstrate the direct relation between angular moments of the dipole amplitude in coordinate space and angular moments of the diffractive dijet cross sections. To perform explicit calculations we employ two different saturation models, extended to include the target geometry. In the limit of large photon virtuality or quark masses, we find fully analytic results that allow direct insight into how the differential cross section and elliptic anisotropy depend on the saturation scale, target geometry, and kinematic variables. We further provide numerical results for more general kinematics in collisions at a future electron-ion collider, and study the effects of approaching the saturated regime on diffractive dijet observables.

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arXiv2020

Bulk properties and multi-particle correlations in large and small systems

Charged particle production is calculated in a hybrid framework consisting of the IP-Glasma initial state, MUSIC viscous relativistic fluid dynamics, and the UrQMD microscopic hadronic cascade. Using one set of parameters, we compute observables for a large variety of collision systems. We compare to experimental data in p+p, p+Pb, Xe+Xe, Au+Au and Pb+Pb collisions at various energies, and make predictions for potential O+O runs at RHIC and LHC.

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arXiv2019

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

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 $v_2$ 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.

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arXiv2019

Multi-particle and charge-dependent azimuthal correlations in heavy-ion collisions at the Relativistic Heavy-Ion Collider

We study multi-particle azimuthal correlations in relativistic heavy-ion collisions at a center of mass energy of 200 GeV. We use the IP-Glasma model to initialize the viscous hydrodynamic simulation MUSIC and employ the UrQMD transport model for the low temperature region of the collisions. In addition, we study effects of local charge and global momentum conservation among the sampled particles. With the exception of the lowest order three particle correlator $C_{112}$, our framework provides a good description of the existing charge inclusive azimuthal correlation data for Au+Au and U+U collisions at RHIC. We also present results for charge dependent two and three particle correlators in Au+Au and U+U collisions, and make predictions for isobar (Ru+Ru & Zr+Zr) collisions to provide a much needed baseline for the search for the Chiral Magnetic Effect at RHIC.

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arXiv2018

Photons from thermalizing matter in heavy ion collisions

We investigate the production of direct photons in heavy ion collisions within the modified bottom-up thermalization scenario, which we show to be related to the thermalizing Glasma framework. The dynamics of the parton system up to thermalization/equilibration can be described by two momentum scales, by means of which we express the photon invariant yield excluding the prompt pQCD contribution. We derive an analytic formula, which provides an estimate of photon production from thermalizing matter for a wide range of collision systems and energies. We compare the yield with that measured in the PHENIX Au+Au run04 and the combined run07+run10 data sets at root-s = 200GeV. We also make theory-data comparisons for Pb+Pb at 2760 GeV and d+Au at 200 GeV collision energies. Finally, we make predictions for the direct photon invariant yield from collisions of U+U, Cu+Au, $^3$He+Au at 200 GeV and Pb+Pb at 5020 GeV.

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arXiv2017

Color Instabilities in the Quark-Gluon Plasma

When the quark-gluon plasma (QGP) - a system of deconfined quarks and gluons - is in a nonequilibrium state, it is usually unstable with respect to color collective modes. The instabilities, which are expected to strongly influence dynamics of the QGP produced in relativistic heavy-ion collisions, are extensively discussed under the assumption that the plasma is weakly coupled. We begin by presenting the theoretical approaches to study the QGP, which include: field theory methods based on the Keldysh-Schwinger formalism, classical and kinetic theories, and fluid techniques. The dispersion equations, which give the spectrum of plasma collective excitations, are analyzed in detail. Particular attention is paid to a momentum distribution of plasma constituents which is obtained by deforming an isotropic momentum distribution. Mechanisms of chromoelectric and chromomagnetic instabilities are explained in terms of elementary physics. The Nyquist analysis, which allows one to determine the number of solutions of a dispersion equation without explicitly solving it, and stability criteria are also discussed. We then review various numerical approaches - purely classical or quantum - to simulate the temporal evolution of an unstable quark-gluon plasma. The dynamical role of instabilities in the processes of plasma equilibration is analyzed.

hep-ph↗

arXiv2017

Origins of collectivity in small systems

We review recent developments in the theoretical description and understanding of multi-particle correlation measurements in collisions of small projectiles (p/d/$^3$He) with heavy nuclei (Au, Pb) as well as proton+proton collisions. We focus on whether the physical processes responsible for the observed long range rapidity correlations and their azimuthal structure are the same in small systems as in heavy ion collisions. In the latter they are interpreted as generated by the initial spatial geometry being transformed into momentum correlations by strong final state interactions. However, explicit calculations show that also initial state momentum correlations are present and could contribute to observables in small systems. If strong final state interactions are present in small systems, recent developments show that results are sensitive to the shape of the proton and its fluctuations.

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arXiv2017

A hybrid approach to relativistic heavy-ion collisions at the RHIC BES energies

Using a hybrid (viscous hydrodynamics + hadronic cascade) framework, we model the bulk dynamical evolution of relativistic heavy-ion collisions at Relativistic Heavy Ion Collider (RHIC) Beam Energy Scan (BES) collision energies, including the effects from non-zero net baryon current and its dissipative diffusion. This framework is in full (3+1)D, which allows us to study the non-trivial longitudinal structure and dynamics of the collision systems, for example baryon stopping and transport, as well as longitudinal fluctuations. For the first time, the quantitative effect of net-baryon diffusion on hadronic observables is addressed. Finally, we propose a dynamical initialization scheme to study the importance of the pre-equilibrium stage at the RHIC BES energies.

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arXiv2016

3-D Glasma initial state for relativistic heavy ion collisions

We extend the impact parameter dependent Glasma model (IP-Glasma) to three dimensions using explicit small x evolution of the two incoming nuclear gluon distributions. We compute rapidity distributions of produced gluons and the early time energy momentum tensor as a function of space-time rapidity and transverse coordinates. We study rapidity correlations and fluctuations of the initial geometry and multiplicity distributions and compare to existing models for the three dimensional initial state.

hep-ph↗

arXiv2016

Mass ordering of spectra from fragmentation of saturated gluon states in high multiplicity proton-proton collisions

The mass ordering of mean transverse momentum $\left $ and of the Fourier harmonic coefficient $v_2 (p_T)$ of azimuthally anisotropic particle distributions in high energy hadron collisions is often interpreted as evidence for the hydrodynamic flow of the matter produced. We investigate an alternative initial state interpretation of this pattern in high multiplicity proton-proton collisions at the LHC. The QCD Yang-Mills equations describing the dynamics of saturated gluons are solved numerically with initial conditions obtained from the Color Glass Condensate based IP-Glasma model. The gluons are subsequently fragmented into various hadron species employing the well established Lund string fragmentation algorithm of the PYTHIA event generator. We find that this ab initio initial state approach reproduces characteristic features of bulk spectra, in particular the particle mass dependence of $\left $ and $v_2 (p_T)$.

hep-ph↗

arXiv2016

Initial State Quantum Fluctuations in the Little Bang

We review recent developments in the ab-initio theoretical description of the initial state in heavy-ion collisions. We emphasize the importance of fluctuations, both for the phenomenological description of experimental data from the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC), and the theoretical understanding of the non-equilibrium early time dynamics and thermalization of the medium.

hep-ph↗

arXiv2016

Novel Collective Phenomena in High-Energy Proton-Proton and Proton-Nucleus Collisions

The observation of long-range collective correlations for particles emitted in high-multiplicity pp and pPb collisions has opened up new opportunities of investigating novel high-density QCD phenomena in small colliding systems. We review experimental results related to the studies of collective phenomena in small systems from RHIC and the LHC over the past several years. Latest development in theoretical interpretations motivated by different frameworks are also reviewed, and confronted with the experimental data. Perspectives on possible future directions are discussed, with the aim of further exploring the rich emergent QCD phenomena.

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arXiv2016

Theory @ Hard Probes 2015

Overview of the latest theory developments presented at the Hard Probes 2015 conference, held at McGill University, Montreal, Canada, in July 2015.

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arXiv2015

New insights from 3D simulations of heavy ion collisions

Viscous relativistic hydrodynamics in 3+1 dimensions is applied to describe heavy ion collisions at RHIC and LHC. We present calculations of observables that are sensitive to the longitudinal structure of the created system. In particular we present pseudo-rapidity correlations and demonstrate their dependence on both the initial state and short range correlations introduced via a microscopic transport description. We further demonstrate the effect of a varying temperature dependence of the shear viscosity to entropy density ratio on rapidity dependent flow harmonics.

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arXiv2015

Moving forward to constrain the shear viscosity of QCD matter

We demonstrate that measurements of rapidity differential anisotropic flow in heavy ion collisions can constrain the temperature dependence of the shear viscosity to entropy density ratio η/s of QCD matter. Comparing results from hydrodynamic calculations with experimental data from RHIC, we find evidence for a small η/s $\approx$ 0.04 in the QCD cross-over region and a strong temperature dependence in the hadronic phase. A temperature independent η/s is disfavored by the data. We further show that measurements of the event-by-event flow as a function of rapidity can be used to independently constrain the initial state fluctuations in three dimensions and the temperature dependent transport properties of QCD matter.

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arXiv2015

Dilepton emission in high-energy heavy-ion collisions with dissipative hydrodynamics

In this contribution we study the effects of three transport coefficients of dissipative hydrodynamics on thermal dilepton anisotropic flow observables. The first two transport coefficients investigated influence the overall size and growth rate of shear viscous pressure, while the last transport coefficient governs the magnitude of net baryon number diffusion in relativistic dissipative fluid dynamics. All calculations are done using state-of-the-art 3+1D hydrodynamical simulations. We show that thermal dileptons are sensitive probes of the transport coefficients of dissipative hydrodynamics.

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arXiv2015

Probing the dissipative properties of a strongly interacting medium with dileptons

We investigate the effects of the presence of a non-vanishing net baryon number density and its diffusion on dilepton production, within a hydrodynamical description of the medium created at $\sqrt{s_{NN}}=7.7$ GeV collision energy. This energy value is explored within the Beam Energy Scan (BES) program at Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory. Particular attention is devoted to a new dissipative degree of freedom: the net baryon number diffusion ($V^μ$), and to the net baryon number conductivity ($κ$) | a transport coefficients governing the overall magnitude of $V^μ$. The effects of $κ$ on dilepton production are assessed, with an outlook on how future experimental dilepton data can be used to learn more about $κ$.

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arXiv2015

Pseudorapidity correlations in heavy ion collisions from viscous fluid dynamics

We demonstrate by explicit calculations in 3+1 dimensional viscous relativistic fluid dynamics how two-particle pseudorapidity correlation functions in heavy ion collisions at the LHC and RHIC depend on the number of particle producing sources and the transport properties of the produced medium. In particular, we present results for the Legendre coefficients of the two-particle pseudorapidity correlation function in Pb+Pb collisions at 2760 GeV and Au+Au collisions at 200 GeV from viscous hydrodynamics with three dimensionally fluctuating initial conditions. Our results suggest that these coefficients provide important constraints on initial state fluctuations and the transport properties of the quark gluon plasma.

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