XQCD 2016

1 Aug 2016, 08:00 → 3 Aug 2016, 18:55 Europe/London

Portland Square (Plymouth University)

Antonio Rago (School of Computing, Electronics and Mathematics Plymouth University), Craig McNeile (Plymouth University)

The 14th International workshop on QCD in extreme conditions, Aug. 1-3 2016

Monday, 1 August

Monday AM: 1

Convener: Prof. Heng-Tong Ding (Central China Normal University)

1 Opening

Speaker: Prof. Kurt Langfeld (Plymouth University)

2 The smallest of the Little Bangs: thermalization and collective flow in pp and pA collisions (??!)

Recent experimental data from p+Pb and high-multiplicity p+p collisions at the LHC, as well as p+Au, d+Au and He3+Au collisions at RHIC, suggest strongly that the same collective phenomena that have been attributed to hydrodynamic flow in collisions between heavy nuclei also manifest in qualitatively identical ways in collisions where one or both collision partners are as small as a proton. How can this be??! To resolve this surprise, I will discuss the conditions under which hydrodynamic behavior can be expected, and show that modern (resummed) versions of viscous fluid dynamics that account more efficiently for unavoidable strong local momentum anisotropies in rapidly and strongly anisotropically expanding systems can quite accurately describe the evolution of fireballs whose local phase-space distribution is far from thermal equilibrium. I will argue that the largest stumbling block on the way towards a quantitative dynamical understanding of the evolution of small collision systems is not a likely breakdown of the hydrodynamic framework, but our insufficient knowledge of the internal spatial structure of the proton and its quantum fluctuations from collision event to event. Some recent suggestions for improved modelling of the initial-state fluctuations in small collision systems will be reported.

Speaker: Prof. Ulrich Heinz (The Ohio State University)

No Video https://docs.google.com/document/d/1d5Nu5glSUnJeImkz9l3UL1_LUNPLsJMrSsqRHCIn15Y/edit?usp=sharing

Slides XQCD_Heinz.pdf

3 Phenomenological signals of QCD critical point in heavy-ion collisions

The properties of a QCD medium at finite density is extensively studied in the recent beam energy scan program at BNL Relativistic Heavy Ion collider. One of the main goals of the program is to understand the QCD phase structure and to verify the existence of a critical point. A difficulty in such analyses is that a signal of the critical point could be thermally washed away if its location is far from the freeze-out, unless its effects on dynamical evolution is sizable enough to modify the freeze-out hypersurface. The relativistic hydrodynamic model has been proven to be a powerful tool for heavy-ion analyses in the past decades. In this study, we propose to extend the model to lower energies at finite density and estimate thermal photon spectra and flow harmonics. Since the medium is electromagnetically transparent, there is an advantage that the critical point does not have to be near the freeze-out for its photonic signal to be detectable. We present the results from (2+1)-dimensional hydrodynamic simulation and discuss their phenomenological consequences.

Speaker: Dr Akihiko Monnai (Institut de Physique Théorique, CNRS/CEA)

No Video https://docs.google.com/document/d/1d5Nu5glSUnJeImkz9l3UL1_LUNPLsJMrSsqRHCIn15Y/edit?usp=sharing

Slides xqcd16_monnai.pdf

10:25 Coffee break

Monday AM: 2

Convener: Dr Philippe de Forcrand (ETH Zurich & CERN)

4 Anderson localisation of Dirac eigenmodes in high temperature QCD

We investigate the properties of the background gauge field configurations that act as disorder for the Anderson localization mechanism in the Dirac spectrum of QCD at high temperatures. We compute the eigenmodes of the M\"obius domain-wall fermion operator on configurations generated for the $SU(3)$ gauge theory with two flavors of fermions, in the temperature range $[0.9,1.9]T_c$. We identify the source of localization of the eigenmodes with gauge configurations that are self-dual and support negative fluctuations of the Polyakov loop $P_L$, in the high temperature sea of $P_L\sim 1$. The dependence of these observations on the boundary conditions of the valence operator is studied. We also investigate the spatial overlap of the left-handed and right-handed projected eigenmodes in correlation with the localization and the corresponding eigenvalue. We discuss an interpretation of the results in terms of monopole-instanton structures.

Speaker: Dr Guido Cossu (Higgs Centre for Theoretical Physics, School of Physics and Astronomy, University of Edinburgh)

No Video https://docs.google.com/document/d/1d5Nu5glSUnJeImkz9l3UL1_LUNPLsJMrSsqRHCIn15Y/edit?usp=sharing

Slides Anderson_Localization_in_QCD_XQCD.pdf

5 Competing order in the fermionic Hubbard model on the hexagonal graphene lattice

We study the phase diagram of the fermionic Hubbard model on the hexagonal lattice in the space of on-site and nearest neighbor couplings with Hybrid-Monte-Carlo simulations. With pure on-site repulsion this allows to determine the critical coupling strength for spin-density wave formation. We compare the standard approach of introducing a small mass term, explicitly breaking the sublattice symmetry, with a purely geometric mass, i.e. using lattices and boundary conditions such that the Dirac points fall in between the grid points inside the Brillouin zone without explicit sublattice-symmetry breaking. For the first method we extrapolate the corresponding susceptibility peaks towards zero mass and infinite volume in the usual way, while with the geometric mass only infinite volume extrapolation is needed. An added bonus is that it can be used with nearest neighbor repulsion and charge-density wave formation where the corresponding mass term would introduce a sign problem. The geometric mass thus provides a promising method to study the competition between these different types of order and the resulting phase diagram with ab-initio simulations.

Speaker: Prof. Lorenz von Smekal (Justus-Liebig University Giessen)

No Video https://docs.google.com/document/d/1d5Nu5glSUnJeImkz9l3UL1_LUNPLsJMrSsqRHCIn15Y/edit?usp=sharing

Slides von_Smekal_August2016.pdf

6 Scale-invariance of three-dimensional QED

We present results from our lattice simulation of parity-invariant three-dimensional QED with two dynamical flavors of massless two-component fermions, implemented using the overlap operator. Contradictory to the expectation from various plausibility arguments, we find evidences from the finite-size scaling of eigenvalues of the Dirac operator and from correlators of fermion bilinears, for the two flavor theory to be scale-invariant with the mass anomalous dimension close to 1, the maximum allowed value for a conformal field theory.

Speaker: Dr Nikhil Karthik (Florida International University)

No Video https://docs.google.com/document/d/1d5Nu5glSUnJeImkz9l3UL1_LUNPLsJMrSsqRHCIn15Y/edit?usp=sharing

Slides Karthik.pdf

12:30 Lunch

Monday PM: 1

Convener: Prof. Lorenz von Smekal (Justus-Liebig University Giessen)

7 Lattice constraints on the thermal photon rate

We estimate the photon production rate from an SU(3) plasma at temperatures of about $1.1~T_c$ and $1.3~T_c$. Lattice results for the vector current correlator at spatial momenta $k\sim (2-6)T$ are extrapolated to the continuum limit and analyzed with the help of a polynomial interpolation for the corresponding spectral function, which vanishes at zero frequency and matches to high-precision perturbative results at large invariant masses. For small invariant masses the interpolation is compared with the NLO weak-coupling result, hydrodynamics, and a holographic model. At vanishing invariant mass we extract the photon rate which for $k \sim 3T$ is found to be close to the NLO weak-coupling prediction. For $k \sim 2T$ uncertainties remain large but the photon rate is likely to fall below the NLO prediction, in accordance with the onset of a strongly interacting behaviour characteristic of the hydrodynamic regime.

Speaker: Dr Olaf Kaczmarek (University of Bielefeld)

Slides 1_XQCD_2016_Kaczmarek.pdf

Video https://youtu.be/oTLjHvGAGVM

8 Charm quark diffusion coefficient and relaxation time on the quenched lattice

We study the transport coefficient for the charm quark in the deconfined phase from correlator in the temporal channel with nonzero momentum on the quenched lattice. Lattice Euclidean correlator in the temporal channel with zero momentum is constant as a function of the imaginary time because of the charge conservation. However, the derivative of the correlator with respect to momentum contains information for the diffusion coefficient and the relaxation time. We measure the derivative on the lattice and give a constraint on the charm quark diffusion coefficient and the relaxation time on the lattice for 1.5 < T/T_c < 4.5.

Speaker: Mr Atsuro Ikeda (Osaka University)

Slides xqcd2016_2.pdf

Video https://www.youtube.com/watch?v=Sz4oxyzRER4

9 Temperature dependence of shear viscosity in SU(3)-gluodynamics

This report is devoted to the study of temperature dependence of shear viscosity in SU(3)-gluodynamics. To calculate shear viscosity we measured the correlation function of the energy-momentum tensor T_{12}T_{12} for a set of temperatures in the region T/T_c \in (0.9, 1.5). The measurements were carried out using multilevel algorithm which considerably improves the accuracy of the data. The results of the calculation allow to determine temperature dependence of the ratio of shear viscosity to the entropy density {\eta}/s.

Speaker: Dr Victor Braguta (ITEP)

Slides 3_braguta.pdf

Video https://www.youtube.com/watch?v=G2YlDZ2jpX8

10 QCD energy momentum tensor at finite temperature using gradient flow

We apply the gradient flow method to Nf=2+1 full QCD at finite temperature. Our main interest is on temperature dependence of: 1. energy-momentum tensor with quark contribution, 2. chiral condensate and its susceptibility. We show that these observables are well measured with the gradient flow method. Our finite temperature system is constructed by fixing the lattice spacing and varying the temporal length Nt (fixed scale method). We adopt the lattice spacing a=0.07 (fm) and cover a wide range of temperature region $174\le T\le697$ MeV. The ud quark mass is rather heavy$m_{\pi}/m_{\rho}\simeq0.63$ and s quark mass is set to almost the physical value.

Speaker: Dr Yusuke Taniguchi (University of Tsukuba)

Slides tanigchi_20160803_XQCD2016_handout.pdf

Video https://www.youtube.com/watch?v=P1BvnBB271Y

15:40 Coffee break

Monday PM: 2

Convener: Dr George Fleming (Yale University)

11 Collisions in Non-conformal Theories: Hydrodynamization without Equilibration

We numerically simulate gravitational shock wave collisions in a holographic model dual to a non-conformal four-dimensional gauge theory. We find two novel effects associated to the non-zero bulk viscosity of the resulting plasma. First, the hydrodynamization time increases. Second, if the bulk viscosity is large enough then the plasma becomes well described by hydrodynamics before the equilibrium equation of state becomes applicable. We discuss implications for the quark-gluon plasma created in heavy ion collision experiments.

Speaker: Dr Jorge Casalderrey Solana (University of Oxford)

Slides 01-jcs_talk.pdf

Video https://www.youtube.com/watch?v=U4G0SnrKjpQ

12 Lattice NRQCD study of thermal Sommerfeld factor

The thermal Sommerfeld factor, concerning the co-annihilation rate of heavy quarks in thermal equilibrium, is shown that it can be defined in terms of two-point correlator ratios. Then, we calculate non-perturbatively the co-annihilation rate of heavy quarks close to thermal equilibrium using lattice NRQCD framework. Lattice measurements indicate a modest suppression in the octet channel, in reasonable agreement with perturbation theory, and a large enhancement in the singlet channel, much above the perturbative prediction. The additional enhancement is suggested to originate from bound state formation and subsequent decay, omitted in previous estimates of thermal Sommerfeld factors, which were meant for use in Boltzmann equations governing single-particle phase space distributions. We show how bound state effects can be included in perturbative computations. The latter formalism may find applications also in specific dark matter models.

Speaker: Prof. Seyong Kim (Sejong University)

Slides xQCD2016.pdf

Video https://www.youtube.com/watch?v=rVrTXu7pSdk

13 Novel chiral structure realized by rotation

In nonrelativistic many-body systems, rotation produces interesting phenomena as well as external electromagnetic field and finite chemical potential. On the other hand rotational effects on relativistic systems have not been elucidated. Nevertheless it is known that a rapid rotation is realized in main objectives of QCD physics: heavy-ion collisions and neutron stars. In this talk, we construct the framework of the QCD in rotating systems. Based on this, we investigate the chiral structure under the presence of magnetic background and rotation. We suggest that the interplay between magnetic field and rotation leads to the “*rotational magnetic inhibition*” that is a novel phenomenon analogous to the inverse magnetic catalysis.

Speaker: Mr Kazuya Mameda (The University of Tokyo)

Slides slides.pdf

Video https://www.youtube.com/watch?v=6UmezMwJBNk

Poster session

14 $\theta$-dependence of the massive Schwinger model

Understanding the role of the $\theta$ parameter in QCD and its connection with the strong CP problem and axion physics is one of the major challenges for high energy theorists. Due to the sign problem, at present only the QCD topological susceptibility is well known. Using an algorithmic approach that could potentially be extended to QCD, we study as a first step the $\theta$-dependence in the massive Schwinger model, and try to verify a conjecture of Coleman.

Speaker: Eduardo Royo (Universidad de Zaragoza)

15 A G(2)-QCD Neutron Star

G2-QCD, i.e. QCD with the gauge group SU(3) replaced by the exceptional group G(2), shares many features with SU(3). But it is accessible at finite density on the lattice, as it has no sign problem, and at the same time has a neutron. Therefore, this theory can sustain in princple neutron star. Using the equation of state for this theory from lattice simulations, we solve the Oppenheimer-Volkoff equation and obtain the mass-radius-relation of such neutron stars. This study shows how different phases, visible in the equation of state, influence the mass-radius relation, and therefore gives guidance for the case of full QCD and true neutron stars.

Speaker: Ms Ouraman Hajizadeh (University of Graz)

16 A new method to determine the topological susceptibility at high-temperature

At $T>T_c$, the topological susceptibility quickly drops down, although the precise speed of this fall is still subject to theoretical debates. It then becomes practically impossible for lattice QCD to handle volumes large enough to allow non-trivial topology and a precise sampling of the sectors involved in the $\theta=0$ vacuum. We present a new method applicable to the computation of the decrease of the susceptibility on a wide range of situations, including in particular finite-temperature ensembles with $\chi_t\ll 1/V$. This method distinguishes itself from most other methods by not relying on the $\theta=0$ vacuum, nor any semi-classical approximation around $\theta=0$, but only on topology-fixed configurations. We will provide some encouraging preliminary results on quenched ensembles, and discuss the generalisation to any simulation.

Speaker: Dr Julien Frison (KEK Theory Center)

17 Baryons across the deconfinement transition

Unlike mesons, lattice studies of baryons at finite temperature are very limited. We present results for baryon correlators and spectral functions across the deconfinement transition, using the FASTSUM 2+1 flavour ensembles as well as results with free lattice fermions.

Speaker: Prof. Gert Aarts (Swansea University)

18 Block Solver for multiple right hand sides on NVIDIA GPUs

Analysis tasks in Lattice QCD thermodynamics often requires solving linear equations for multiple right hand sides for a constant gauge field. Recently deflation methods have become more widely used and proven to be very efficient. They however do require the calculation and storing of eigenvectors which is either not always feasible or suffer from high setup costs. We present results for an implementation of a block solver for multiple right hand sides using the QUDA library for QCD on NVIDIA GPUs. By making use of the gauge field reuse in the Dslash operator, mixed precision approaches and block Krylov space methods they do provide outstanding solver performance. We present results on NVIDIA's most recent Pascal architecture. Furthermore we provide an update on the QUDA library and overview of NVIDIA's Pascal architecture.

Speaker: Dr Mathias Wagner (NVIDIA)

19 Charmonium spectral functions from large quenched lattice QCD

We present our most updated results on the charmonium correlation and spectral functions. The correlation functions are obtained using the clover improved Wilson fermions on quenched lattices with spatial size fixed to 192. And temporal sizes of these lattices are chosen to be 96, 64, 56, 48 and 32 corresponding to 0.73, 1.1, 1.25, 1.5 and 2.2 Tc. We perform a detailed analysis of these charmonium correlators using two different stochastic approaches, namely Stochastic Analytical Inference (SAI) and Stochastic Optimization Method (SOM) to extract the spectral functions. The systematic uncertainties in spectral functions obtained from SOM and SAI will be discussed, and the comparisons with those from Maximum Entropy Method (MEM) also will be presented.

Speaker: Mr Haitao Shu (CCNU)

Poster updated_poster_haitao.pdf

20 Chiral phase transition in (2 + 1)-flavor QCD

We present our recent results on the chiral phase transition in 2+1 flavor QCD on lattices with temporal extent $N_{\tau} = 6$. We have performed lattice QCD simulations using the Highly Improved Staggered Quarks with 5 different values of pion masses, i.e. 160, 140, 110, 90, 80 MeV in a temperature window from 140 MeV to 170 MeV. We have analyzed the universal scaling behaviour of the chiral phase transition at vanishing baryon chemical potential by looking into the temperature and quark mass dependences of the chiral condensates of chiral susceptibilities. The window of criticality compared to previous studies is also discussed.

Speaker: Mr Sheng-Tai Li (Central China Normal University)

21 Complex Langevin Dynamics for a Random Matrix Model of QCD at finite density

We study a Random Matrix Model for QCD at finite density via Complex Langevin dynamics. This model has a phase transition to a phase with non-zero baryon density. We study the convergence of the algorithm as a function of the quark mass and the chemical potential and focus on two main observables: the baryon density and the chiral condensate. As expected, for simulations close to the chiral limit, the algorithm has wrong convergence properties when the quark mass is in the spectral domain of the quenched Dirac operator. Possible solutions of this problem are discussed.

Speaker: Dr Savvas Zafeiropoulos (Goethe University Frankfurt)

22 Confinement and Magnetic Fields

One of the greatest achievement of lattice QCD is the numerical determination of the confinement potential between pairs of static quarks, as well as the measurement of the shape of the color-flux tube connecting them. In this talk I will present how this is modified in the presence of a magnetic field.

Speaker: Mr Francesco Sanfilippo (University of Southampton)

23 Confinement/deconfinement phase transition in the $SU(3)$ Yang-Mills theory: center symmetry and Meissner effect

The dual superconductivity is a promising mechanism for quark confinement. We have presented a new formulation of the Yang-Mills theory on the lattice, and pointed out that the $SU(3)$ Yang-Mills theory has a new way of reformulation using new field variables (minimal option), in addition to the conventional option adopted by Cho, Faddeev and Niemi (maximal option). In the preceding works, we have accumulated the numerical evidences that support the non-Abelian dual superconductivity using the minimal option for the SU(3) Yang-Mills theory.

In this talk, we focus on the confinement/deconfinement phase transition from the viewpoint of the dual superconductivity. The confinement/deconfinement phase transition is conventionally detected by using center symmetry unbroken/spontaneous breaking. In view of the dual superconductivity, the confinemenet/deconfinement phase transition must be described by the phase transition dual Meissner effect at finite temperature. For this purpose, we measure the distribution of the chromoelectric flux connecting a quark and an antiquark and the induced magnetic-monopole current around the flux tube which is the candidate of the order parameter for the confinement/deconfinement phase transition. Finally, we will discuss the relation between center symmetry and the dual Meissner effect.

Speaker: Akihiro Shibata (Computing Research Center, KEK)

24 Electrical conductivity and light vector-meson dissociation across the deconfinement phase transition in two flavor lattice QCD

We present results on the isovector vector channel spectral functions at vanishing momentum across the deconfinement phase transition in dynamical $n_f=2$ lattice QCD. Using Wilson-Clover fermions the calculations are carried out with a vacuum pion mass of $\simeq 270$MeV at the temperatures $T/T_c=0.156, 0.8, 1.0, 1.25$ and 1.67. The spectral functions are reconstructed from local-conserved vector correlation functions by fitting a phenomenologically motivated Ansatz and enforcing exact sum rules to the data. We show results for the electrical conductivity across the deconfinement phase transition and find evidence for the dissociation of the $\rho$-meson. In addition we apply the Backus-Gilbert method to our data and compare kinetic theory predictions and previously published phenomenological spectral functions to our lattice study.

Speaker: Dr Anthony Francis (York University)

25 Mastering the effect of Fermionic Determinant Zeroes in Complex Langevin Simulations of Heavy-Dense QCD

Zeroes in the fermionic determinant induce poles in the drift of the Langevin equation. We here study their effect on Complex Langevin simulations in an effective model related to QCD in the limit of heavy quarks and large chemical potential. The latter was defined long ago by one of the authors and used thereafter in various contexts. We approach in this way a quantitative understanding of the conditions for reliability and define systematic errors for the results which could be extended to realistic lattice simulations.

Speaker: Prof. Ion-Olimpiu Stamatescu (I.T.P., University Heidelberg)

26 Observables for study of the deconfinement transition

The deconfinement transition is typically studied using the Polyakov loop, which needs renormalization. We look at the renormalization of Polyakov loop using Wilson flow, for both quenched and $N_f=2$ QCD. We also investigate some observables related to the Polyakov loop that has been suggested, and the behavior of the gluon condensate across the transition.

Speaker: Dr Saumen Datta (Tata Institute of Fundamental Research)

27 Renormalization constants of the lattice energy momentum tensor using the gradient flow

We present an update about our program for the non perturbative renormalization of the energy momentum tensor. Our strategy is based on probing suitable lattice Ward identities with observables computed along the gradient flow. This set of identities exhibits many interesting qualities, resulting from the UV finiteness of flowed composite operators, and can be used to measure the renormalization constants of the energy momentum tensor. We apply this method for a SU(3) Yang-Mills theory on the lattice, and report our numerical results.

Speaker: Mr Francesco Capponi (Plymouth University)

28 Stochastic Quantization with Colored Noise

Studying the topological properties of the QCD vacuum on the lattice, observables such as the gauge action density are blurred by short distance fluctuations at scales of the order of the lattice spacing. To extract the physical content from the mentioned configurations, those have to be treated using smoothing methods such as cooling or the Wilson flow which have been investigated extensively over the past years. We present an alternative method based on stochastic quantization with UV regulated noise enabling a direct control of the quantum fluctuations at all lattice momentum scales. Our particular interest is to investigate the correspondence between the flow time and the associated physical scale of the system. Thereto observables depending on the long-range physics such as topological quantities are investigated. The method is tested on a real scalar field theory as well as on pure SU(2) gauge theory. Effects of colored noise on observables are analyzed. Moreover, we investigate whether physical quantities become insensitive to UV fluctuations above a characteristic momentum scale. The suitability and efficiency of the method regarding the generation of smoothed fields in a Monte Carlo simulation is probed. We present and discuss our recent results.

Speaker: Mr Felix Ziegler (Heidelberg University)

29 Testing dynamic stabilization in complex Langevin simulations

Complex Langevin methods have been successfully applied in theories that suffer from a sign problem such as HDQCD. We present and illustrate a novel method that ensures that Complex Langevin simulations stay close to the SU(3) manifold, which lead to correct and improved results in the framework of HDQCD and pure gauge simulations. Applying the same technique in fully dynamical QCD simulations shows great potential to allow the determination of the phase diagram from first principles.

Speaker: Dr Benjamin Jaeger (Swansea University)

30 The $U_A(1)$ breaking in the chiral limit of two-flavour QCD

We study the strength of the anomalous breaking of the $U_A(1)$ symmetry in the chiral limit of two-flavour QCD at the transition temperature using the difference of the pseudoscalar and scalar iso-vector screening masses. To this end we use simulations with non-perturbatively $O(a)$-improved Wilson fermions for three different pion masses in the range of 200-550MeV on lattices with a temporal extent of 16 and volumes of $32^3$ to $64^3$. We compare our results to the strong breaking at zero temperature and find that the effect of the breaking in the mass difference is reduced by at least a factor of five. This indicates a weak $U_A(1)$ breaking at the transition temperature and raises the question of its influence on the nature of the chiral transition. It is generally expected that a weaker breaking strenghtens the chiral phase transition.

Speaker: Dr Bastian Brandt (University of Frankfurt)

31 The lattice CP(N-1) phase diagram

We recently proposed a new type of worm algorithm for simulating the CP(N-1) model at finite density in a dual representation based on flux-variables. Here we continue by discussing how a topological term can be incorporated into our dual formulation.

Speaker: Mr Tobias Rindlisbacher (ETH Zürich)

32 Thermodynamics of two-color matter at non-zero baryon density

We study two-color QCD with two flavours of Wilson fermion as a function of quark chemical potential and temperature. Recent results are presented and the phase diagram is discussed.

Speaker: Dr pietro giudice (University of Muenster)

33 Topological visualisation techniques to enhance understanding of lattice QCD simulations

Topology driven techniques are an important and established tool in volume visualisation computer graphics; however, they have yet to see widespread adoption in the lattice QCD community. In comparison to traditional isosurfacing algorithms they offer a number of advantages including faster rendering speeds and enhanced data exploration possibilities. The ability to summarise the topology of a scalar field is achieved using various graph based data structures and algorithms. In our work we use these techniques to visualise and quantify the topological effects experienced in SU(2) lattice gauge theory as chemical potential is varied.

Speaker: Mr Dean Thomas (Swansea University)

34 Towards the phase diagram from analytical continuation

We calculate the QCD cross-over temperature, the equation of state and fluctuations of conserved charges at finite density by analytical continuation from imaginary to real chemical potentials. Our calculations are based on new continuum extrapolated lattice simulations using the 4stout staggered actions with a lattice resolution up to Nt=16. The simulation parameters are tuned such that the strangeness neutrality is maintained, as it is in heavy ion collisions.

Speaker: Dr Szabolcs Borsanyi (University of Wuppertal)

Tuesday, 2 August

Tuesday AM: 1

Convener: Prof. Paulo Bedaque (University of Maryland)

35 A density of state approach to the sign problem

Most of the successes of lattice QCD are strictly connected with advances in numerical methods. In particular, importance sampling Monte Carlo methods have underpinned lattice QCD calculations since the early days of the field. However, it is well known that importance sampling methods fail in cases when one has to sample over configurations whose occurrence is suppressed by the Monte Carlo measure. Typical examples are first order phase transitions, in which two equally probable equilibrium states are connected by configurations with probability of occurrence exponentially suppressed with the system size, or the sign problem, whereby strong cancellations occurring for observables evaluated over highly probable configurations give a high relative weight to less recurring states. A possible way around these problems is a direct sampling of the density of states through a non-Markovian Random Walk in configuration space, as proposed by Wang and Landau a few years ago. In this contribution, I review the Logarithmic Linear Relaxation (LLR) method, a recently introduced algorithm in the Wang-Landau class that allows us to determine density of states over several orders of magnitude with the same relative accuracy. After an introduction to the ideas inspiring the algorithm and the presentation of some practical implementations, I discuss some applications to spin and gauge systems, with emphasis on the sign problem and the challenges it presents.

Speaker: Prof. Biagio Lucini (Swansea University)

Slides 01-lucini_xqcd16.pdf

Video https://www.youtube.com/watch?v=6JuPd8RU65Y

36 The density of states method applied to the Ising model with an imaginary magnetic field

Above the Curie temperature, the Ising model partition function can vanish under the effect of an imaginary magnetic field, as first shown by Lee and Yang. This system has a severe sign problem. We study it numerically, using the density of states method. In particular, we consider how the computer effort scales with the system size.

Speaker: Dr Philippe de Forcrand (ETH Zurich &amp; CERN)

Slides 02-XQCD_0816.pdf

Video https://www.youtube.com/watch?v=B7ifmmwWMHg

37 Lattice QED with dual variables

With the help of auxiliary fields, it is possible to integrate out analytically the link variables in lattice gauge theory with staggered fermions, for arbitrary values of the lattice coupling. In the case of N_f=1 QED, the subsequent Grassmann integration yields a dual representation of the partition function, which traces over an ensemble of monomers, dimers, and electron loops coupled to the auxiliary fields. We simulate this system using diagrammatic Monte Carlo methods, and explore the residual sign problem which survives after all the analytical integrations.

Speaker: Helvio Vairinhos (ETH Zurich)

Slides 03-helvio_xqcd16.pdf

Video https://www.youtube.com/watch?v=oWjReDf5tjc

10:35 Coffee break

Tuesday AM: 2

Convener: Dr Szabolcs Borsanyi (University of Wuppertal)

38 Recent progress of Lefschetz-thimble path integral and refine complex Langevin method

If the classical action takes complex values, the path integral formulation of quantum field theories suffers from the sign problem, and some artifice becomes necessary in order to study their nonperturbative properties. The idea to complexify field variables is a candidate for such tricks, and there are two methods belonging into this category: Lefschetz-thimble path integral and complex Langevin method. In this talk, I will review recent progress of Lefschetz-thimble path integral with its brief introduction, and try to reconsider about complex Langevin method from this point of view.

Speaker: Dr Yuya Tanizaki (RIKEN BNL Research Center)

Slides Tanizaki_XQCD2016.pdf

Video https://www.youtube.com/watch?v=OYQ6L2bk2N0

39 Solving the sign problem by going with the flow

We discuss an approach to solve the sign problem based on deforming the region of integration in the path integral from real fields to a manifold in complex space. It generalizes the"Lefschetz thimble" approach that received much attention lately. We discuss principles through a simple geometrical picture, discuss algorithm features required to make the idea work and present examples in simple models ranging from 0+1 D fermionic models to 3+1D bosonic models and real time dynamics.

Speaker: Prof. Paulo Bedaque (University of Maryland)

Slides 02-xqcd_2016.pdf

Video https://www.youtube.com/watch?v=MokTXfvAlHk

40 Progress in thimble regularization: a new algorithm, 0+1 QCD and beyond

In the context of the chiral random matrix model we introduced a way to compute lattice quantum field theories on Lefschetz thimbles by taking into account the contributions to the functional integral which come from complete flow lines. The latter are the steepest ascent paths attached to critical points, i.e. the basic building blocks of thimbles. While the thimble regularization was successfull in solving the chiral random matrix model, the simulating algorithm we first made use of was the roughest one could devise, i.e. static, crude Monte Carlo. The static, crude Monte Carlo was also useful in solving QCD in 0+1 dimensions at finite chemical potential, whose results we will present in the talk. We have also come up with a new algorithm, which is based on a heat bath sampling of the gaussian approximation of the thimble: this defines the proposals for a Metropolis-like accept/reject step. We present this new algorithm, showing its effectiveness in the case of the chiral random matrix model and discussing applications to other theories. In particular, we are going to address the issues and the formalism for thimble regularization of Yang Mills theory in 2 dimensions with complex coupling.

Speaker: Dr Francesco Di Renzo (University of Parma and INFN)

Slides 03-direnzo.pdf

Video https://www.youtube.com/watch?v=FdvmYNYLElI

12:40 Lunch

Tuesday PM: 1

Convener: Prof. Biagio Lucini (Swansea University)

41 Infinite dimensional/continuous compressed sensing in physics

Compressed sensing has had a considerable impact on inverse problems and sampling theory over the last ten years. However, the traditional compressed sensing setup and theory is usually discrete, a fact that may seem at odds with many of the physical models that are over the continuum. In this talk we will discuss how one can design compressed sensing over the continuum and how this approach changes the theory and also the algorithmic setup. There are several advantages that this path may bring. First, it allows for new types of applications in physics and potentially improved performance of the reconstruction techniques. Second, it allows for a better understanding of the asymptotic phenomena that govern compressed sensing. By understanding these phenomena one can taylor compressed sensing to the specific applications in order to optimise the quality of the reconstructions.

Speaker: Dr Anders Hansen (DAMTP Cambridge)

Slides 01-Plymouth_2016.pdf

Video https://www.youtube.com/watch?v=buLs7uO8mUA

42 Complex spectrum of QCD at finite density

We consider the spectrum of transfer matrix eigenvalues associated with Polyakov loops in lattice QCD at strong coupling. The transfer matrix at finite density is non-Hermitian, and its eigenvalues become complex as a manifestation of the sign problem. We show that the global symmetries of finite-density QCD ensure that the complex eigenvalues are part of a complex conjugate pair, and they lead to sinusoidally modulated decay in Polyakov loop correlation functions. We argue that the results reflect oscillatory behavior in color-charge densities reminiscent of density-density correlation functions in liquids, and it is generic in spin models for QCD at finite density, as well as phenomenological models using complex saddle points.

Speaker: Dr Hiromichi Nishimura (RIKEN BNL Research Center)

Slides Nishimura_XQCD2016.pdf

Video https://www.youtube.com/watch?v=cEOgV8bptKw

43 Gauge cooling for the singular-drift problem in the complex Langevin method

Recently, the complex Langevin method has been applied successfully to finite density QCD either in the deconfinement phase or in the heavy dense limit with the aid of a new technique called the gauge cooling. In the confinement phase with light quarks, however, convergence to wrong limits occurs due to the singularity in the drift term caused by small eigenvalues of the Dirac operator including the mass term. We propose that this singular-drift problem should also be overcome by the gauge cooling with different criteria for choosing the complexified gauge transformation. I will explain the idea of our method, what criteria allows us to avoid the singular drift problem. Applying the method to chiral Random Matrix Theory, I will show that the method indeed works for the chRMT, where the gauge cooling changes drastically the eigenvalue distribution of the Dirac operator measured during the Langevin process. This change of the eigenvalue distribution is explained by a generalized Banks-Casher relation. I will also present a result for finite density QCD in the confinement phase. K. Nagata, J. Nishimura, S. Shimasaki, arXiv:1604.07717, arXiv:1511.08580.

Speaker: Dr Keitaro Nagata (KEK, Theory Center)

Slides 03-Nagata2016_07_XQCD.pdf

Video https://www.youtube.com/watch?v=5zHToEP0w3s

15:35 Coffee break

Tuesday PM: 2

Convener: Prof. Seyong Kim (Sejong University)

44 Complex Langevin for Lattice QCD at $T=0$ and $\mu \ge 0$.

We simulate Lattice QCD with 2 light quark flavours at zero temperature and finite quark number chemical potential $\mu$. Gauge cooling is applied, along with adaptive rescaling of the updating `time' increment, to stabilize the algorithm. We see evidence for the expected transition at $\mu \approx m_N/3$ and for saturation at large $\mu$. Limitations of the method are discussed.

Speaker: Dr Donald SInclair (Argonne-Seville)

Slides 01-Sinclair-southampton.pdf

Video https://www.youtube.com/watch?v=HQOWdjF6vY4

45 Results on the heavy-dense QCD phase diagram using complex Langevin

Complex Langevin simulations have been able to successfully reproduce results from Monte Carlo methods in the region where the sign problem is mild and make predictions when it is exponentially hard. We present here our study of the QCD phase diagram in the limit of heavy and dense quarks (HDQCD) for 3 different lattice volumes and the boundary between the hadronic phase and the quark-gluon plasma. We also discuss instabilities encountered in our simulations.

Speaker: Mr Felipe Attanasio (Swansea University)

Slides Attanasio.pdf

Video https://www.youtube.com/watch?v=VpWYn7W_HW0

Panel discussion

Conveners: Dr Claudia Ratti (University of Houston), Dr Francesco Di Renzo (University of Parma &amp; INFN), Prof. Gert Aarts (Swansea University), Dr René Bellwied (University of Houston), Prof. Simon Hands (Swansea University), Dr Szabolcs Borsanyi (University of Wuppertal)

Video https://www.youtube.com/watch?v=VbtIKHntNDw

Wednesday, 3 August

Wednesday AM: 1

Convener: Dr Francesco Di Renzo (University of Parma &amp; INFN)

46 Bulk properties of strongly interacting matter: recent results from lattice QCD

I will review the most recent results on QCD thermodynamics from lattice simulations. In particular, I will focus on the equation of state at zero and finite chemical potential, the curvature of the QCD phase diagram and fluctuations of conserved charges. The latter will be discussed in relation with the experimental Beam Energy Scan program at RHIC.

Speaker: Dr Claudia Ratti (University of Houston)

Slides 01-talk_Plymouth_Ratti.pdf

Video https://www.youtube.com/watch?v=k31wswDKmhM

47 The QCD equation of state and fluctuations of conserved charges at non-vanishing temperature and density

We present results from a calculation of the QCD equation of state up to the sixth order in the baryon number, strangeness and electric charge chemical potentials. We consider various cases to express strangeness and electric charge chemical potentials as function of the baryon chemical potential and temperature. Among those, the conditions met in heavy ion collision are best reproduced by enforcing strangeness neutrality and a constant baryon number to electric charge ratio. We will further discuss how cumulant rations of conserved charge fluctuations that are available in both, lattice QCD simulations and heavy ion experiments, can be used to determine freeze-out parameters including the freeze-out curvature of the experiment. The presented results are based on lattice calculations performed with the Highly Improved Staggered Quark action (HISQ) in the temperature range 140 MeV < T < 330 MeV, with lattice sizes $24^3 \times 6$, $32^3 \times 8$, $48^3\times 12$ and $16^3\times 16$. The strange quark mass is tuned to its physical value and we use ratios of strange to light quark masses including $m_s/m_l = 20$ and 27, which in the continuum correspond to pion masses of 160 and 140 MeV.

Speaker: Dr Christian Schmidt (Universitaet Bielefeld)

Slides 02-Schmidt_xqcd.pdf

Video https://www.youtube.com/watch?v=tXNDLG7B2OI

48 Theta dependence in the deconfined phase of QCD

We will present a determination of the topological properties of QCD for temperatures up to around 600MeV, obtained by using Lattice QCD simulations with 2+1 flavours of fermions at the physical point. Numerical results will be compared with the predictions of instanton gas calculations and the implications of these results for the cosmological bounds on the axion mass will be discussed.

Speaker: Dr Claudio Bonati (Università di Pisa)

Slides slides.pdf

Video https://youtu.be/G4oNjvP2lcg

10:35 Coffee break

Wednesday AM: 2

Convener: Prof. Gert Aarts (Swansea University)

49 Finite temperature lattice QCD review

I will review recent numerical results as well as new ideas and approaches to finite temperature lattice QCD at zero chemical potential.

Speaker: Daniel Nogradi (Eotvos University)

Slides finitet.pdf

Video https://www.youtube.com/watch?v=w0ALezzF6QY

50 The phase diagram of QCD with isospin chemical potential

We investigate the phase diagram of QCD at finite isospin chemical potential using 2+1 flavours of staggered fermions with physical quark masses at different lattice spacings and volumes. For high chemical potentials and low temperatures the confined phase shows a transition to the pion condensation phase. This phase is characterised by a proliferation of low modes that slow down the simulation considerably and necessitate the use of an infrared regulator. We discuss a novel strategy to determine the pion condensate in the limit of vanishing regulators. To investigate the convergence properties and the range of validity of the Taylor expansion method at a given order, we compare our results to the Taylor expansion for finite isospin chemical potential at 4 derivative order. This can serve as an important crosscheck for the Taylor expansion method at finite baryon chemical potential, where direct simulations are impossible due to the sign problem.

Speaker: Dr Bastian Brandt (University of Frankfurt)

Slides 02-xqcd2016_brandt.pdf

Video https://www.youtube.com/watch?v=AZe_2ngH7Uo

51 Topological susceptibility in finite-temperature (2+1)-flavor QCD with gradient flow

We study the temperature-dependence of the topological susceptibility, determined in (2+1)-flavor QCD with improved Wilson quarks. To calculate the topological charge and its susceptibility, we adopt the gradient flow method. With heavy up and down quarks ($m_{\rm PS}/m_V \approx 0.63$) and approximately physical strange quark, we study in the range $T=174-697$ MeV.

Speaker: Prof. Kazuyuki Kanaya (CiRfSE, Univ. Tsukuba)

Slides 1608_kanaya-xqcd-talk-rev.pdf

Video https://www.youtube.com/watch?v=-8qUAX2zgV8

12:40 Lunch

Wednesday PM: 1

Convener: Mr Francesco Sanfilippo (University of Southampton)

52 Dark Matter from the lattice

Speaker: Dr George Fleming (Yale University)

Slides Fleming.pdf

Video https://www.youtube.com/watch?v=o06Mw-Q-s_E

53 Lattice results for SU(2) dark matter

We present lattice investigations of the SU(2) gauge theory with two fermions in the fundamental representation. This model provides a minimal template which is ideal for a wide class of Standard Model extensions featuring novel strong dynamics that range from composite (Goldstone) Higgs theories to several intriguing types of dark matter candidates. In the light of these results, we discuss models of SIMP-, symmetric- and asymmetric dark matter. We estimate the interaction of the dark matter candidate with Standard Model particles through it’s electric polarisability and discuss experimental constraints on these models.

Speaker: Dr Jarno Rantaharju (CP3-Origins)

Slides xqcd2016.pdf

Video https://www.youtube.com/watch?v=yToiLNssCX4

54 Parity doubling in two-color and two-flavor gauge theory at high temperature

Recently the two-color gauge theory with two flavors of fundamental fermions has received considerable attention in BSM model building, as it provides a minimal template for a composite Higgs theory that includes dark matter candidates. In this work, we consider the two-color model with two flavors of Wilson fermions at non-zero temperature. For a more reliable investigation of meson correlation functions at high temperature, we perform simulations on anisotropic lattices with a target anisotropic factor of a_s/a_t=6.3. The tuning of bare parameters was carried out using the pseudo-scalar dispersion relation and Wilson loop ratios. We present our preliminary results for the identification of parity doubling from the temporal and spacial correlation functions of vector and axial-vector mesons.

Speaker: Dr Jong-Wan Lee (Swansea University)

Slides XQCD2016_Lee.pdf

Video https://www.youtube.com/watch?v=ZRyxQq9E3Zg

15:35 Coffee break

Wednesday PM: 2

Convener: Benjamin Jaeger

55 Critical Phenomena in 8-Flavour QCD

Novel (quasi-)conformal dynamics associated with Infra-red fixed point is anticipated to emerge in many-flavor QCD, and advocated as a basis for strongly-interacting mechanisms of electroweak symmetry breaking. The subject is closely related to the critical phenomena of chiral phase transition in usual (3-flavour) QCD. In this regard, we show the latest lattice simulation (LatKMI) results for the scaling property of 8-flavor QCD. Also, motivated by the Baryogenesis, we discuss thermodynamic properties of 8-flavour QCD.

Speaker: Dr Kohtaroh Miura (Centre de Physique Theorique, Aix-Marseille Universite)

Slides xqcd2016_miura.pdf

Video https://www.youtube.com/watch?v=yr-cIVzPBC0

56 Critical endline of the finite temperature phase transition for 2+1 flavor QCD around the SU(3)-flavor symmetric point

We investigate the critical endline of the finite temperature phase transition of QCD around the SU(3)-flavor symmetric point at zero chemical potential. We employ the renormalization-group improved Iwasaki gauge action and non-perturbatively $O(a)$-improved Wilson-clover fermion action. The critical endline is determined by using the intersection point of kurtosis, employing the multi-parameter, multi-ensemble reweighting method to calculate observables off the SU(3)-symmetric point, at the temporal size $N_{\rm T}$=6 and lattice spacing as low as $a \approx 0.19$ fm. We confirm that the slope of the critical endline takes the value of $-2$, and find that the second derivative is positive, at the SU(3)-flavor symmetric point on the Columbia plot parametrized with the strange quark mass $m_s$ and degenerated up-down quark mass $m_{\rm l}$.

Speaker: Dr Yoshifumi Nakamura (RIKEN)

Slides 02-nakamura.pdf

Video https://www.youtube.com/watch?v=ZjvuBI6-ZDc

57 Study of the phase diagram of dense two-color QCD with $N_f=2$ within lattice simulation

In this talk we present our results on the low-temperature scan of the phase diagram of dense two-color QCD with two flavors of quarks. The study is conducted using lattice simulation with rooted staggered quarks and real baryon chemical potential. At small chemical potential we observe the hadronic phase, where the theory is in a confining state, chiral symmetry is broken, the baryon density is zero and there is no diquark condensate. At the critical point $\mu = m_{\pi}/2$ we observe the expected second order transition to Bose-Einstein condensation of scalar diquarks. In this phase the system is still in confinement in conjunction with non-zero baryon density, but the chiral symmetry is restored in the chiral limit. We have also found that in the first two phases the system is well described by chiral perturbation theory. For larger values of the chemical potential the system turns into another phase, where the relevant degrees of freedom are fermions residing inside the Fermi sphere, and the diquark condensation takes place on the Fermi surface. In this phase the system is still in confinement, chiral symmetry is restored and the system is very similar to the quarkyonic state predicted by SU($N_c$) theory at large $N_c$.

Speaker: Mr Aleksandr Nikolaev (ITEP)

Slides talk.pdf

Video https://www.youtube.com/watch?v=FAlkHs4hjJo

Closing

Video about XQCD 2017 https://www.youtube.com/watch?v=gsIDxxU2WO0