Search

We present our first results on a direct computation of the complex in-medium heavy quark potential from realistic lattice QCD simulations. Ensembles with $N_\tau=12$ from the HotQCD and TUMQCD collaboration offer unprecedented high statistics, those with $N_\tau=16$ unprecedented time resolution, making possible a robust extraction of the real part from the spectral functions of Wilson line correlators. To this end we deploy a combination of a Bayesian reconstruction (BR method), as well as a Pad\'e-like approximation. We corroborate findings made on less realistic lattices that ${\rm Re}[V]$ smoothly transitions from a confining to a screened behavior at high temperatures and its values lie close to the color singlet free energies. A finite value of the ${\rm Im}[V]$ is observed in the quark-gluon-plasma phase., Comment: 4 pages, 4 figures, talk given at the XXVIIth International Conference on Ultra-relativistic Nucleus-Nucleus Collisions (Quark Matter 2018), Venice, Italy

We present the final results of our high statistics study on the properties of bottomonium and charmonium at finite temperature. We focus on the temperature range around the crossover transition $150\leq T\leq 410$MeV, relevant for current heavy ion collision experiments. The QCD medium degrees of freedom which consist of dynamical u,d, and s quarks and gluons are captured by realistic state-of-the art ($m_\pi\approx 161$MeV) lattice QCD simulations of the HotQCD collaboration. For the heavy quarks we deploy the non-relativistic effective field theory of QCD, NRQCD. The in-medium properties of quarkonium are deduced from their spectral functions, which are reconstructed using improved and novel Bayesian approaches. Through a systematic analysis we shed light on the origin of the discrepancies in melting temperatures previously reported in the literature, showing that they are owed to underestimated methods uncertainties of the deployed spectral reconstructions. Our simulations corroborate a picture of sequential in-medium modification, ordered according to the vacuum binding energy of the states. As a central quantitative result, our study reveals how the mass of the heavy quarkonium ground state reduces as temperature increases. The observed spectral modifications are interpreted in the light of, and compared to previous studies based on the complex lattice potential for heavy quarkonium. Thus for the first time we provide a robust picture of in-medium heavy quarkonium modification in the quark-gluon plasma consistent among different non-relativistic methods. We also critically discuss the perspectives for improving on these results., Comment: 84 pages, 45 figures (t.b.p. in JHEP, submission includes ancillary datafiles for in-medium mass shifts and tentative widths)

In this paper we study the interactions among a static quark-antiquark pair in the presence of dense two-color quark matter with lattice simulation. To this end we compute Polyakov line correlation functions and determine the renormalized color averaged, color singlet and color triplet grand potentials. The color singlet grand potential allows us to elucidate the number of quarks induced by a static quark antiquark source, as well as the internal energy of such a pair in dense quark matter. We furthermore determine the screening length, which in the confinement phase is synonymous with the string breaking distance. The screening length is a decreasing function of baryon density, due to the possibility to break the interquark string via a scalar diquark condensate at high density. We also study the large distance properties of the color singlet grand potential in a dense medium and find that it is well described by a simple Debye screening formula, parameterized by a Debye mass and an effective coupling constant. The latter is of order of unity, i.e. even at large density two-color quark matter is a strongly correlated system., Comment: 14 figures, 1 table

The strong suppression of heavy quarkonia is a good indicator that one has generated a quark-gluon plasma (QGP) in an ultrarelativistic heavy ion collision. Recent advancements in first principles calculations of the heavy quark potential provide additional insight into this suppression and can be used to further pin down the in-medium properties of the QGP. Realistic 3+1d dissipative hydrodynamical models can be used to simulate the QGP and, with bottomonium as a probe, one can make inferences for the initial temperature of the QGP and other phenomenological parameters such as the shear viscosity to entropy density ratio. However, progressing to LHC $\sqrt{s_{NN}}=5.02$ TeV Pb-Pb collisions, one expects regeneration to have an increasingly important impact on the suppression observables. In this proceedings, we present a brief overview to set up the model and then provide model results for bottomonium suppression and regeneration, for ultrarelativistic heavy-ion collision experiments at RHIC and LHC., Comment: 4 pages, 6 figures; proceedings contribution to Quark Matter 2018, May 13-19 2018

We study the open system dynamics of a heavy quark in the quark-gluon plasma using a Lindblad master equation. Applying the quantum state diffusion approach by Gisin and Percival, we derive and numerically solve a nonlinear stochastic Schr\"odinger equation for wave functions, which is equivalent to the Lindblad master equation for the density matrix. From our numerical analysis in one spatial dimension, it is shown that the density matrix relaxes to the Boltzmann distribution in various setups (with and without external potentials), independently of the initial conditions. We also confirm that quantum dissipation plays an essential role not only in the long-time behavior of the heavy quark but also at early times if the heavy quark initial state is localized and quantum decoherence is ineffective., Comment: 10 pages, 7 figures, v2: published version

More than 50 years ago Bongard introduced 100 visual concept learning problems as a testbed for intelligent vision systems. These problems are now known as Bongard problems. Although they are well known in the cognitive science and AI communities only moderate progress has been made towards building systems that can solve a substantial subset of them. In the system presented here, visual features are extracted through image processing and then translated into a symbolic visual vocabulary. We introduce a formal language that allows representing complex visual concepts based on this vocabulary. Using this language and Bayesian inference, complex visual concepts can be induced from the examples that are provided in each Bongard problem. Contrary to other concept learning problems the examples from which concepts are induced are not random in Bongard problems, instead they are carefully chosen to communicate the concept, hence requiring pragmatic reasoning. Taking pragmatic reasoning into account we find good agreement between the concepts with high posterior probability and the solutions formulated by Bongard himself. While this approach is far from solving all Bongard problems, it solves the biggest fraction yet.

We reconstruct the gluon spectral function in Landau gauge QCD from numerical data for the gluon propagator. The reconstruction relies on two novel ingredients: Firstly we derive analytically the low frequency asymptotics of the spectral function. Secondly we construct a functional basis from a careful consideration of the analytic properties of the gluon propagator in Landau gauge. This allows us to reliably capture the non-perturbative regime of the gluon spectrum. We also compare different reconstruction methods and discuss the respective systematic errors., Comment: 12 pages, 10 figures

We present the first combined study of correlators and spectral properties of charmonium and open-charm-mesons at finite temperature using a fully relativistic lattice QCD approach. The QCD medium is captured by second generation anisotropic 24^3xN_t lattices from the FASTSUM collaboration, including 2+1 flavors of clover discretized quarks with m_pi~380 MeV. Two Bayesian methods are deployed to reconstruct the spectral functions, the recent BR method as well as the Maximum Entropy Method with Fourier basis. We take particular care to disentangle genuine in-medium effects from method artifacts with the help of the reconstructed correlator. Consistent with the direct inspection of correlators, we observe no significant in-medium modification for J/Psi and eta_c around the crossover, while the chi_c states on the other hand show clear changes around the transition. At the highest temperature, T=352 MeV, J/Psi and eta_c also exhibit discernible changes compared to the vacuum. For D mesons around $T_c$, no significant modifications are observed, but we find clear indications that no bound state survives at the highest temperature of T=352 MeV. Above T_c we discover a significant difference between D and D* mesons, the latter being much more strongly affected by the medium., Comment: 17 pages, many pdf figures. Matches version published in PRD

I report on the first application of a novel, generalized Bayesian reconstruction (BR) method for spectral functions to the characterization of QCD constituents. These spectral functions find applications in off-shell kinetics of the quark-gluon plasma and in calculations of transport coefficients. The new BR method is applied to Euclidean propagator data, obtained in Landau gauge on lattices with $N_f=2+1+1$ dynamical flavors by the "twisted mass at finite temperature" (tmfT) collaboration. The deployed reconstruction method is designed for spectral functions that can exhibit positivity violation (opposed to that of hadronic bound states). The transversal and longitudinal gluon spectral functions show a robust structure composed of quasiparticle peak and a negative trough. Characteristic differences between the hadronic and the plasma phase and between the two channels become visible. We obtain the temperature dependence of the transversal and longitudinal gluon masses., Comment: talk given by E.-M. Ilgenfritz at the 11th APCTP-BLTP JINR-PNPI-SPbSU Joint Workshop , Peterhof, 24 -- 28 July 2017, appears in the Proceedings (to be published in PEPAN Letters)

The computation of real-time properties, such as transport coefficients or bound state spectra of strongly interacting quantum fields in thermal equilibrium is a pressing matter. Since the sign problem prevents a direct evaluation of these quantities, lattice data needs to be analytically continued from the Euclidean domain of the simulation to Minkowski time, in general an ill-posed inverse problem. Here we report on a novel approach to improve the determination of real-time information in the form of spectral functions by setting up a simulation prescription in imaginary frequencies. By carefully distinguishing between initial conditions and quantum dynamics one obtains access to correlation functions also outside the conventional Matsubara frequencies. In particular the range between $\omega_0$ and $\omega_1=2\pi T$, which is most relevant for the inverse problem may be more highly resolved. In combination with the fact that in imaginary frequencies the kernel of the inverse problem is not an exponential but only a rational function we observe significant improvements in the reconstruction of spectral functions, demonstrated in a simple 0+1 dimensional scalar field theory toy model., Comment: 8 pages, 5 figures, Talk given at the XXXVth International Symposium on Lattice Field Theory, June 18-24, 2017, Granada, Spain

We estimate bottomonium yields in relativistic heavy-ion collisions using a lattice QCD vetted, complex-valued, heavy-quark potential embedded in a realistic, hydrodynamically evolving medium background. We find that the lattice-vetted functional form and temperature dependence of the proper heavy-quark potential dramatically reduces the dependence of the yields on parameters other than the temperature evolution, strengthening the picture of bottomonium as QGP thermometer. Our results also show improved agreement between computed yields and experimental data produced in RHIC 200 GeV/nucleon collisions. For LHC 2.76 TeV/nucleon collisions, the excited states, whose suppression has been used as a vital sign for quark-gluon-plasma production in a heavy-ion collision, are reproduced better than previous perturbatively-motivated potential models; however, at the highest LHC energies our estimates for bottomonium suppression begin to underestimate the data. Possible paths to remedy this situation are discussed., Comment: 27 pages, 6 figures

We report results on the quark spectral function in the Landau gauge at finite temperature determined from its Dyson-Schwinger equation. Compared to earlier quenched results this study encompasses unquenched $N_f=2+1$ fermion flavors in the medium. For the reconstruction of real-time spectra we deploy a recent Bayesian approach (BR method) and develop a new prior in order to better assess the inherent systematic uncertainties. We identify the quark quasi-particle spectrum and analyze the (non-)appearance of zero modes at or around the pseudo-critical temperature. In both, the fully unquenched system and a simpler truncation using a model for the gluon propagator we observe a characteristic three-peak structure at zero three-momentum. The temperature dependence of these structures in case of the gluon propagator model is different than observed in previous studies. For the back-coupled and unquenched case we find interesting modifications at and around the pseudo-critical transition temperature., Comment: 18 pages, 21 figures

We investigate the real-time evolution of quarkonium bound states in a quark-gluon plasma in one dimension using an improved QCD based stochastic potential model. This model describes the quarkonium dynamics in terms of a Schr\"odinger equation with an in-medium potential and two noise terms encoding the residual interactions between the heavy quarks and the medium. The probabilities of bound states in a static medium and in a boost-invariantly expanding quark-gluon plasma are discussed. We draw two conclusions from our results: One is that the outcome of the stochastic potential model is qualitatively consistent with the experimental data in relativistic heavy-ion collisions. The other is that the noise plays an important role in order to describe quarkonium dynamics in medium, in particular it causes decoherence of the quarkonium wave function. The effectiveness of decoherence is controlled by a new length scale $l_{\rm corr}$. It represents the noise correlation length and its effect has not been included in existing phenomenological studies., Comment: 8 pages, 4 figures

Many measurements of quarkonium suppression at the LHC, e.g. the nuclear modification factor $R_{AA}$ of $J/\Psi$, are well described by a multitude of different models. Thus pinpointing the underlying physics aspects is difficult and guidance based on first principles is needed. Here we present the current status of our ongoing high precision study of in-medium spectral properties of both bottomonium and charmonium based on NRQCD on the lattice. This effective field theory allows us to capture the physics of quarkonium without modeling assumptions in a thermal QCD medium. In our study a first principles and realistic description of the QCD medium is provided by state-of-the-art lattices of the HotQCD collaboration at almost physical pion mass. Our updated results corroborate a picture of sequential modification of states with respect to their vacuum binding energy. Using a novel low-gain variant of the Bayesian BR method for reconstructing spectral functions we find that remnant features of the Upsilon may survive up to $T\sim400$MeV, while the $\chi_b$ signal disappears around $T\sim270$MeV. The $c\bar{c}$ analysis hints at melting of $\chi_c$ below $T\sim190$MeV while some $J/\Psi$ remnant feature might survive up to $T\sim245$MeV. An improved understanding of the numerical artifacts in the Bayesian approach and the availability of increased statistics have made possible a first quantitative study of the in-medium ground state masses, which tend to lower values as $T$ increases, consistent with lattice potential based studies., Comment: 4 pages, 3 figures, oral contribution to the XXVI International Conference on Ultrarelativistic Nucleus-Nucleus Collisions (Quark Matter 2017), Chicago, USA, February 6-11, 2017

We investigate gluon spectral functions at finite temperature in Landau gauge, based on a subset of lattice QCD ensembles with $N_f=2+1+1$ dynamical twisted mass quarks flavors, generated by the tmfT collaboration. Our study uses a novel Bayesian approach for the extraction of non-positive definite spectral functions, which for each binned spatial momentum takes into account the gluon correlation functions at all available discrete imaginary frequencies. The spectral functions are extracted at three different lattice spacing, where for each of them, a scan of temperatures around the crossover transition is carried out at fixed scale. We find indications for the existence of a well defined quasi-particle peak. Due to a relatively small number of imaginary frequencies available, we focus on the momentum and temperature dependence of the position of this spectral feature. This dispersion relation reveals different in-medium masses for longitudinal and transversal gluons at high temperatures, qualitatively consistent with weak coupling expectations., Comment: 23 pages, 30 figures

Generalization performance of trained computer vision systems that use computer graphics (CG) generated data is not yet effective due to the concept of 'domain-shift' between virtual and real data. Although simulated data augmented with a few real world samples has been shown to mitigate domain shift and improve transferability of trained models, guiding or bootstrapping the virtual data generation with the distributions learnt from target real world domain is desired, especially in the fields where annotating even few real images is laborious (such as semantic labeling, and intrinsic images etc.). In order to address this problem in an unsupervised manner, our work combines recent advances in CG (which aims to generate stochastic scene layouts coupled with large collections of 3D object models) and generative adversarial training (which aims train generative models by measuring discrepancy between generated and real data in terms of their separability in the space of a deep discriminatively-trained classifier). Our method uses iterative estimation of the posterior density of prior distributions for a generative graphical model. This is done within a rejection sampling framework. Initially, we assume uniform distributions as priors on the parameters of a scene described by a generative graphical model. As iterations proceed the prior distributions get updated to distributions that are closer to the (unknown) distributions of target data. We demonstrate the utility of adversarially tuned scene generation on two real-world benchmark datasets (CityScapes and CamVid) for traffic scene semantic labeling with a deep convolutional net (DeepLab). We realized performance improvements by 2.28 and 3.14 points (using the IoU metric) between the DeepLab models trained on simulated sets prepared from the scene generation models before and after tuning to CityScapes and CamVid respectively., Comment: 9 pages, accepted at CVPR 2017

Open and hidden heavy-flavor physics in high-energy nuclear collisions are entering a new and exciting stage towards reaching a clearer understanding of the new experimental results with the possibility to link them directly to the advancement in lattice Quantum Chromo-dynamics (QCD). Recent results from experiments and theoretical developments regarding open and hidden heavy-flavor dynamics have been debated at the Lorentz Workshop "Tomography of the quark-gluon plasma with heavy quarks}, which was held in October 2016 in Leiden, the Netherlands. In this contribution, we summarize identified common understandings and developed strategies for the upcoming five years, which aim at achieving a profound knowledge of the dynamical properties of the quark-gluon plasma., Comment: 18 pages, 4 figures, Lorentz workshop document, submitted to EPJ A