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Bjoern Schenke

Publications and source records attributed to Bjoern Schenke.

At least 55 records · Page 3Linked to original sources
arXiv2015

Rapidity dependence in holographic heavy ion collisions

We present an attempt to closely mimic the initial stage of heavy ion collisions within holography, assuming a decoupling of longitudinal and transverse dynamics in the very early stage. We subsequently evolve the obtained initial state using state-of-the-art hydrodynamic simulations, and compare results to experimental data. We present results for charged hadron pseudo-rapidity spectra and directed and elliptic flow as functions of pseudo-rapidity for $\sqrt{s_{NN}}=200\,{\rm GeV}$ Au-Au and $2.76\,{\rm TeV}$ Pb-Pb collisions. The directed flow interestingly turns out to be quite sensitive to the viscosity. The results can explain qualitative features of the collisions, but the rapidity spectra in our current model is narrower than the experimental data.

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arXiv2015

A Tale of Tails: Photon Rates and Flow in Ultra-Relativistic Heavy Ion Collisions

We consider the possibility that quark and gluon distributions in the medium created in high energy heavy ion collisions may be modified by a power law tail at energies much higher than the temperature. We parametrize such a tail by Tsallis distributions with an exponent motivated by phenomenology. These distributions are characterized by an effective temperature scale that we assume to evolve in time like the temperature for thermal distributions. We find that including such a tail increases the rates for photon production and significantly delays the emission times for photons of a fixed energy. We argue that these effects are sufficiently large that they should be able to account for photon yields and flow patterns seen in LHC and RHIC experiments.

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arXiv2014

Probing the non-equilibrium dynamics of hot and dense QCD with dileptons

It is argued that, in heavy ion collisions, thermal dileptons are good probes of the transport properties of the medium created in such events, and also of its early-time dynamics, usually inaccessible to hadronic observables. In this work we show that electromagnetic azimuthal momentum anisotropies do not only display a sensitivity to the shear relaxation time and to the initial shear-stress tensor profile, but also to the temperature dependence of the shear viscosity coefficient.

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arXiv2014

Probing the early-time dynamics of relativistic heavy-ion collisions with electromagnetic radiation

Using 3+1D viscous relativistic fluid dynamics, we show that electromagnetic probes are sensitive to the initial conditions and to the out-of-equilibrium features of relativistic heavy-ion collisions. Within the same approach, we find that hadronic observables show a much lesser sensitivity to these aspects. We conclude that electromagnetic observables allow access to dynamical regions that are beyond the reach of soft hadronic probes.

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arXiv2014

The shape of the proton at high energies

We present first calculations of the fluctuating gluon distribution in a proton as a function of impact parameter and rapidity employing the functional Langevin form of the JIMWLK renormalization group equation. We demonstrate that when including effects of confinement by screening the long range Coulomb field of the color charges, the evolution is unitary. The large-x structure of the proton, characterized by the position of three valence quarks, retains an effect on the proton shape down to very small values of x. We determine the dipole scattering amplitude as a function of impact parameter and dipole size and extract the rapidity evolution of the saturation scale and the proton radius.

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arXiv2014

Collective effects in light-heavy ion collisions

We present results for the azimuthal anisotropy of charged hadron distributions in A+A, p+A, d+A, and He3+A collisions within the IP-Glasma+MUSIC model. Obtained anisotropies are due to the fluid dynamic response of the system to the fluctuating initial geometry of the interaction region. While the elliptic and triangular anisotropies in peripheral Pb+Pb collisions at root-s=2.76 TeV are well described by the model, the same quantities in root-s=5.02 TeV p+Pb collisions underestimate the experimental data. This disagreement can be due to neglected initial state correlations or the lack of a detailed description of the fluctuating spatial structure of the proton, or both. We further present predictions for azimuthal anisotropies in p+Au, d+Au, and He3+Au collisions at root-s=200 GeV. For d+Au and 3He+Au collisions we expect the detailed substructure of the nucleon to become less important.

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arXiv2014

Eccentric protons? Sensitivity of flow to system size and shape in p+p, p+Pb and Pb+Pb collisions

We determine the transverse system size of the initial non-equilibrium Glasma state and of the hydrodynamically evolving fireball as a function of produced charged particles in p+p, p+Pb and Pb+Pb collisions at the Large Hadron Collider. Our results are consistent with recent measurements of Hanbury-Brown-Twiss (HBT) radii by the ALICE collaboration. Azimuthal anisotropy coefficients v_n generated by combining the early time Glasma dynamics with viscous fluid dynamics in Pb+Pb collisions are in excellent agreement with experimental data for a wide range of centralities. In particular, event-by-event distributions of the v_n agree with the experimental data out to fairly peripheral centrality bins. In striking contrast, our results for p+Pb collisions significantly underestimate the magnitude and do not reproduce the centrality dependence of data for v_2 and v_3 coefficients. We argue that the measured v_n data and HBT radii strongly constrain the shapes of initial parton distributions across system sizes that would be compatible with a flow interpretation in p+Pb collisions. Alternately, additional sources of correlations may be required to describe the systematics of long range rapidity correlations in p+p and p+Pb collisions.

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arXiv2014

The Glasma, Photons and the Implications of Anisotropy

We introduce distribution functions for quarks and gluons in the Glasma and discuss how they satisfy various relationships of statistical physics. We use these distributions to compute photon production in the early stages of heavy ion collisions. Photon rates satisfy geometric scaling, that is, the emission rate per unit area scales as a function of the saturation momenta divided by the transverse momentum of the photon. Photon distributions from the Glasma are steeper than those computed in the Thermalized Quark Gluon Plasma (TQGP). Both the delayed equilibration of the Glasma and a possible anisotropy in the pressure lead to slower expansion and mean times of photon emission of fixed energy are increased. This delayed emission might allow for larger photon elliptic flow.

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arXiv2014

Initial state geometry and fluctuations in Au+Au, Cu+Au and U+U collisions at RHIC

We study within the IP-Glasma and two-component MC-Glauber models the effects of initial state geometry and fluctuations on multiplicities and eccentricities for several collision species at the Relativistic Heavy Ion Collider (RHIC). These include copper-gold (Cu+Au), gold-gold (Au+Au), and uranium-uranium (U+U) collisions. The multiplicity densities per participant pair are very similar in all systems studied. Ellipticities vary strongly between collision systems, most significantly for central collisions, while fluctuation driven odd moments vary little between systems. Event-by-event distributions of eccentricities in mid-central collisions are wider in Cu+Au relative to Au+Au and U+U systems. An anti-correlation between multiplicity and eccentricity is observed in ultra central U+U collisions which is weaker in the IP-Glasma model than the two-component MC-Glauber model. In ultra central Au+Au collisions the two models predict opposite signs for the slope of this correlation. Measurements of elliptic flow as a function of multiplicity in such central events can therefore be used to discriminate between models with qualitatively different particle production mechanisms.

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arXiv2013

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

The invariant mass spectrum and the elliptic flow of lepton pairs produced in relativistic heavy-ion collisions at RHIC are studied with viscous hydrodynamics. The effects of viscous corrections on dilepton observables are explored. The lepton pairs originating from charm quarks evolving in the viscous background are seen to be a good probe of quark energy loss and gain, as quantified by the dilepton spectrum and by the dilepton elliptic flow.

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arXiv2013

Multiplicity distributions in p+p, p+A and A+A collisions from Yang-Mills dynamics

We compute transverse momentum and momentum integrated multiplicity distributions consistently in the IP-Glasma model for proton-proton and proton-lead collisions at the LHC, in deuteron-gold collisions at RHIC, and in heavy ion collisions at both RHIC and LHC energies. Several sources of sub-nucleon scale contributions to the multiplicity distributions are identified. Our results, which are constrained by inclusive and diffractive deeply inelastic scattering data from HERA, are compared to measured distributions for a range of collision energies. These results are an essential first step in quantifying the relative role of initial and final state effects on multiparticle correlations in light and heavy ion collisions.

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arXiv2013

Transverse Momentum of Protons, Pions and Kaons in High Multiplicity pp and pA Collisions: Evidence for the Color Glass Condensate?

The CMS experiment at the Large Hadron Collider has recently presented data on the average transverse momentum of protons, kaons and pions as a function of particle multiplicity. We relate the particle multiplicity to saturation momentum using recently published computations of the interaction radius determined from the theory of the Color Glass Condensate. We show that the pp and the pA experimental data scale in terms of these saturation momenta. Computing transverse momentum spectra for identified particles using Boltzmann-type distributions and relating different associated multiplicities using geometric scaling, these simple distributions reproduce the observed dependence of the mean transverse momentum on particle multiplicities seen in both pp and pA interactions for pions to good accuracy, and to fair agreement for protons and kaons.

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arXiv2013

Initial state geometry and the role of hydrodynamics in proton-proton, proton-nucleus and deuteron-nucleus collisions

We apply the successful Monte Carlo Glauber and IP-Glasma initial state models of heavy ion collisions to the much smaller size systems produced in proton-proton, proton-nucleus and deuteron- nucleus collisions. We observe a significantly greater sensitivity of the initial state geometry to details of multi-particle production in these models compared to nucleus-nucleus collisions. In particular, we find that the size of the system produced in p+A collisions is very similar to the one produced in p+p collisions, and predict comparable Hanbury-Brown-Twiss radii in the absence of flow in both systems. Differences in the eccentricities computed in the models are large, while differences amongst the generated flow coefficients v_2 and v_3 are smaller. For a large number of participants in proton-lead collisions, the v_2 generated in the IP-Glasma model is comparable to the value obtained in proton-proton collisions. Viscous corrections to flow are large over characteristic lifetimes in the smaller size systems. In contrast, viscous contributions are significantly diminished over the longer space-time evolution of a heavy ion collision.

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arXiv2013

Hydrodynamic Modeling of Heavy-Ion Collisions

We review progress in the hydrodynamic description of heavy-ion collisions, focusing on recent developments in modeling the fluctuating initial state and event-by-event viscous hydrodynamic simulations. We discuss how hydrodynamics can be used to extract information on fundamental properties of quantum-chromo-dynamics from experimental data, and review successes and challenges of the hydrodynamic framework.

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arXiv2012

Initial state fluctuations and higher harmonic flow in heavy-ion collisions

A framework combining Yang-Mills dynamics of the pre-equilibrium glasma with relativistic viscous hydrodynamic evolution of the quark-gluon plasma and hadron gas phases is presented. Event-by-event fluctuations of nucleon positions and color charges are taken into account, leading to negative binomial fluctuations of gluon multiplicities. Experimental anisotropic flow coefficients v2-v5 of charged hadron distributions in heavy-ion collisions at the Large Hadron Collider are well described. Furthermore, event-by-event distributions of v2, v3 and v4 measured by the ATLAS collaboration are reproduced.

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arXiv2012

MUSIC with the UrQMD Afterburner

As RHIC is entering the precision measurement era and the LHC is producing a copious amount of new data, the role of 3+1D event-by-event viscous hydrodynamics is more important than ever to understand the bulk data as well as providing the background for hard probes. For more meaningful comparison with the experimental data, it is also important that hydrodynamics be coupled to the hadronic afterburner. In this proceeding we report on preliminary results of coupling MUSIC with UrQMD.

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arXiv2012

Event-by-event anisotropic flow in heavy-ion collisions from combined Yang-Mills and viscous fluid dynamics

Anisotropic flow coefficients v_1-v_5 in heavy ion collisions are computed by combining a classical Yang-Mills description of the early time glasma flow with the subsequent relativistic viscous hydrodynamic evolution of matter through the quark-gluon plasma and hadron gas phases. The glasma dynamics, as realized in the IP-Glasma model, takes into account event-by-event geometric fluctuations in nucleon positions and intrinsic sub-nucleon scale color charge fluctuations; the pre-equilibrium flow of matter is then matched to the MUSIC algorithm describing viscous hydrodynamic flow and particle production at freeze-out. The IP-Glasma+MUSIC model describes well both transverse momentum dependent and integrated v_n data measured at the Large Hadron Collider (LHC) and the Relativistic Heavy Ion Collider (RHIC). The model also reproduces the event-by-event distributions of v_2, v_3 and v_4 measured by the ATLAS collaboration. The implications of our results for better understanding of the dynamics of the glasma as well as for the extraction of transport properties of the quark-gluon plasma are outlined.

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